coastal protection

COASTAL PROTECTION Design of Seawalls and Dikes incl. Overview of Revetments

by Krystian W. Pilarczyk

S h o r t Course

Content

Part I :

Introduction to coastal protection

P a r t I1 :

Design of seawalls and dikes incl. overview of revetments

Appendix : Data collection and prediction methods

INTRODUCTION T0 COASTAL PROTECTION

by

K. W. Pilarczyk

Rijkswaterstaat, Road and Hydraulic Engineering Division Van der Burghweg 1, P.O. Box 5044, 2600 GA Delft, The Netherlands


l.

INTRODUCTION

Coastal communities world-wide are faced with difficult problems of shoreline erosion control. Because of the high land values inherent in the shore zones, chronic erosion as wel1 as storm erosion can be viewed as important management problems resulting in attempts to reduce erosion by coastal defence measures. As a result of natural shore processes, the boundary between land and water may undergo a shift in position with time. This shift may be seaward with advancing shorelines (accretion) and landward with receding shorelines (erosion). These processes occur along every shoreline. The rate at which shores erode or accrete depends on the composition of the shore zone and its exposure to erosive forces. Erosion result from two basic causes: forces of nature acting along the shoreline and the actions of man. Structures created by man can inter£ere with the continuing shore process in the following ways (Figure l): interruption of littoral drift patterns (i.e. the movernent of sand by wave action and currents along the shore); the deflection of shore current patterns; the removal of sediments by dredging; and, modification of wave regimes through reflection from and diffraction around structures. Natura1 causes of erosion include: ................................. n wind (there is a predominant or resultant wave direction which is usually oblique t; the coastline); waves and their longshore component of wave energy; water levels incl. tides, storm-surges and sea-level rise; fluctuations in river discharge of sediment; rain striking exposed bluff faces; ground and surface water movements; ice action; ground motion; anima1 activities; etc. By £ar, the most significant natural erosive force, along our shorelines, is wind-driven wave action in combination with water level changes (tide, wind set-up, sea-level rising).

INTRODUCTION T0 COASTAL PROTECTION K.W. Pilarczyk

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years. The type, size and location of coastal protection must be based on actual needs, benefits expected from the method, its effect on adjacent shorelines and on economy. There are many types of coastal protection (seawalls, groynes, nourishment etc.), each suitable for a definite purpose (Figure 2). However, an improper or ill-advised design may adversely affect the coast rather than remedy the situation (Per Bruun, 1963). Before starting any construction of a coastal protection a decision must be made as to what kind of coastal protection is needed. The right answer depends very much on what is expected from the coastal protection. If a beach is the ultimate aim, then there should be source on the updrift side. If there is a drain, it should be rendered harmless in one way or other, or a source of material should be established. If a coastal protection is to be constructed in an erosion area, the water depth up to which erosion takes place should be known if possible before starting construction. If erosion takes place to a limited depth only, there should be a good chance of building up and maintaining a beach. If erosion takes place up to deep water, coastal protection may have to be moved back, after a time interval, depending upon the rate of erosion and the rate of a possible artificial nourishment to the beach. If a beach is not wanted, a source on the updrift side is not important, but if there is no source, erosion will continue. In this case it will be absolutely necessary to know the depth up to which erosion takes place. If erosion takes place to only a limited depth, it should be possible to stop the erosion at that depth, but if erosion takes place up to deep water, it will be irnpossible to maintain the beach only by groins and or seawalls. After some time such constructions will have to be withdrawn and certain land area given up unless al1 the material is artificially replaced. In many cases, this will be impossible for economic reasons. Unless these important facts are taken into consideration, it will be very risky to build any coastal protection. 1.2

Review of coastal defence methods

Various measures can be used for coastal protection: direct measures which prevent or alleviate the immediate effects of the problems and indirect measures to take away the causes of the problern. For indirect measures one can think of, for instance, measures which stop the reduction of river sediment supply to the coastal system. Direct measures include, for example (Figure 2):

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an artificial sand supply used to replenish an eroding part of a coast. It may seem expensive and the need for repetition may discourage coastal managers. However, careful considerations of capital and maintenance costs frequently prove that it is, in fact, the optimal solution. An added advantage is that the recreational function of the beach is preserved. a row of groynes used on an eroding part of the coast to locally reduce the longshore sediment transport capacity and thus the coastal erosion. In this way erosion areas can be shifted to less harmful locations. a detached breakwater (parallel to and at a certain distance from the shore) used to change the transport capacities both along and perpendicular to the coast, resulting in accretion in the lee of the breakwater, referred to as a tombolo. Such structures have an effect which is comparable to groynes.


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a seawall used to fix the shoreline. In this way the shoreline erosion is replaced by erosion of the sea bed immediately in front of the wall. Stability problems may occur, unless the foundation of the seawall is wel1 below the sea bed.

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Examples of sea-protection

Design considerations and methodology (Weide, van der, 1989)

When designing coastal structures, following aspects have to be conside red : The function of the structure Coastal structures are used for many such as hi~h-water - purposes, protection, coastal-defence, wave-attenuation, flow-guidance etc.


K.W. Pilarczyk

For each of these functions different functional requirements have to be met and consequently different design-specifications are formulated. The environment ----physical ----------------As stated above, the design-loads on coastal structures are mainly determined by hydrodynamic forces. Moreover, hydrodynamic processes may induce morphological changes which may affect the performance and the integrity of structures. Finally, the geotechnical characteristics of the site are of importance as they determine the bearingcapacity of the subsoil, an important parameter for the overall-stability of the structures. The construction-method envisagei .............................. The availability of material, equipment, and labour determines to a large extent the construction procedure. This procedure in its turn determines the possibilities of the designer. In those areas where there are severe constraints in construction-procedures the designer should be aware of this and should revise his design accordingly. Operation and maintenance ......................... Once the structure is finished, the structure will be used and mainte-. nance will be required to secure its per£ormance. The designer should be aware, therefore of the maintenance- procedures and should take care that maintenance is possible. In order to make a wel1 balanced design al1 aspects have to be integrated in a framework, which describes the various aspects and the mutual interaction between these aspects. Ideally, the above aspects should be considered during al1 the stages of the designprocess. During the design-process, following stages are indentified: Conceptual design ----------------In this stage a number of different alternatives are generated, which al1 mee.t the functional requirements. Designs are general and only main dimensions are given. At this stage the relevant aspects should be identified, which determine the technical and economical feasibility. Their relative importance should be evaluated and rough figures should be obtained to quantify them. Preliminary design -----------------In this stage. a limited number of alternatives is selected after a screening procedure which focusses on the technical feasibility. I£ required the conceptual stage should be repeated when the first set of alternatives does not meet these criteria. At this stage, structural dimensions are quantified in some detail and a check is made on the economic feasibility. Again the design process may have to be repeated when the project becomes economically unfeasible. Detailed engineering ----------------At this stage the structural details are designed and detailed design drawings are made. Parallel to this the possibilities for financing


are explored, and environmental and socio-politica1 aspects are considered in greater detail. Technical, economical and socio-politica1 criteria are used in this stage to screen the feasibility of the proposed design. Construction-stage When the design stage is completed, tender documents are prepared and the tendering procedure is started, resulting in the construction. Although this stage is not part of the actual design-process, the designer should be aware of the constructional constraints imposed by this stage. Availability of material, accessibility of the area and the corresponding limitation in the use of equipment, feasibility of intermediate construction stages are some of the aspects to be considered. Especially in the detailed engineering stage these aspects should be evaluated. Operation and rnaintenance Again this stage is beyond the actual design-process, but should be considered by the designer. Especially maintenance can have an impact on the selection of the design. When possibilities for maintenance are poor, the initia1 design should be such that the structure can operate without regular maintenance. ff local labour-cost is cheap and capital investments are difficult, a cheap structure may be more appropriate, when regular maintenance can be guaranteed. The design-methodology is shown schematically on Figure 3. As shown in this Figure, a simulation-model is required to evaluate the behaviour of the structure in the various stages of the design. One of the most difficult aspects of the design is the selection of the model which should be used in the various stages of the design. In general it can be stated that in the course of the design-process more advanced methods are used. The actual choice, however, is dependent on the complexity of the problems, the size of the project and the risklevel which is acceptable. 1.4 Design process of coastal structures In Figure 4 the process of the design of a coastal project has been drawn in diagrammatic form (CUR, 1987). The starting point in this diagram is the identification of a beach erosion problem. The second phase in the design is the selection of the type of protective measure. The final phase can be the risk analysis of the project. Boundary conditions and identification of problem ................................................. The actual design of coastal protection is determined by the local conditions, like bathymetry, waves, tides, current~,morphologic processes, and the characteristics of sediment and soil. Since these loads cannot be computed accurately, they are defined in statistica1 terms. Design-conditions are therefore specified in terms of probabilities, and a probabilistic design concept is applied. Moreover, other factors may impose certain constraints on the design like environmental conditions, infra- structure, time of execution, etc. For the design of a number of tools are available, such as the mathematical models for integrated wave current sediment-transport that can be used also for the prediction of the coastal development without and with shore structures, available knowledge about the effect of sand size on coastal profile development etc. For the design a clear insight into the hydraulic boundary condition and the morphologic processes in the area of interest is necessary. The


rnorphologic p r o c e s s e s n o t o n l y a f f e c t t h e t y p e of p r o t e c t i o n t o be chos e n , b u t a l s o , i n t h e c a s e of e r o d i n g c o a s t , t h e r e q u i r e d volume of sand t o be s u p p l i e d i n o r d e r t h a t t h e p r o j e c t f u l f i l l s i t s p u r p o s e s during a s p e c i f i e d time. 1'

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FUNCTIONAL REOUIREMENTS

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CRITERIA

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The d e s i g n rnethodology

The knowledge a b o u t t h e rnorphologic p r o c e s s e s can be o b t a i n e d by a c a r e f u l a n a l y s i s of a v a i l a b l e d a t a ( i . e . soundings and l e v e l l i n g ) , supplemented by cornputations of sediment t r a n s p o r t . P o s s i b l y a d d i t i o n a l measurements may be deemed n e c e s s a r y . I n t h i s c o n n e c t i o n it i s observed t h a t t h e s e a c t i v i t i e s should be done a l r e a d y b e f o r e t o d e c i d e upon an a r t i f i c i a l beach nourishment o r a n o t h e r c o a s t a l p r o t e c t i o n measure. Not o n l y t h e p r e s e n t e v o l u t i o n s i n t h e c o a s t a l zone need t o be known, b u t a l s o t h o s e which a r e e x p e c t e d t o happen i n f u t u r e . P o s s i b l y t h e o b s e r v e d e v o l u t i o n s may change f o r i n s t a n c e i n t h e c a s e of t h e migrat i o n of l a r g e s c a l e sand waves along t h e c o a s t . I n f a c t t h e p r e s e n t c o n d i t i o n and tiìe f u t u r e development of t h e c o a s t s h o u l d be c o n f r o n t e d w i t h t h e i n t e r e s t s i n v o l v e d , i n o r d e r t o d e c i d e whether o r n o t p r o t e c t i v e measures s h o u l d be t a k e n . The r e c e n t p u b l i c a t i o n s by Goda (1985), Horikawa ( 1 9 8 8 ) , SPM (1984) can s e r v e a s good r e f e r e n c e s f o r d e t e r m i n a t i o n of t h e boundary c o n d i t i o n s f o r the coastal projects.

INTRODUCTION T0 COASTAL PROTECTION K.W. P i l a r c z y k

M U N ü M Y CONüITIONS:

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Process of design, execution and evaluation of coastal projects Example of beach nourishment project

INTRODUCTION T 0 COASTAL PROTECTION

K.W. Pilarczyk

Choice of measure ----------------In order to arrive at the decision whether or not protective measures should be taken different activities are needed. On the one hand the causes and the extent of the beach erosion should be assessed by means of a study of the morphologic processes in the area considered. The basis of this study should be formed by an analysis of available data, while for a prediction of the future development extrapolation techniques and mathematica1 models can be applied. On the other hand an evaluation should be made of the different interests, which may be related to safety, recreation, environment, economy, etc. A weighing of these interests against the rate of the beach erosion may result int0 a deciSion whether further action should be taken or not. In case it is deemed necessary t o protect the should be made of the most promising measure. procedure is presented in Figure 5, which has Kobayashi et al (1985). In this case a number beach protection have been evaluated on basis apply for the considered project at Yokohama.

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Selection of beach protection measures (Kobayashi et al. 1985)

For some coastal protection and/or reclamation projects in The Netherlands a comparison of the cost of beach nourishment and different types of permanent structures has been made, which provided important information for the selection of the protective measure. In the Dutch situation, the beach nourishment, apart from flexibility and environmental considerations, may be very cost-effective by comparison with 'fixed' structures.

INTRODUCTION T 0 COASTAL PROTECTION K.W. Pilarczyk

The costs of various beach protection measures are compared in Figure 6. For an ' average ' situation, beach nourishi50 ment appears to be most economic if the sediment deficit is of the order of 500.000 m v y r or less and the ., length of beach to be protected 5 km 100 or more. The cost rnay be reduced by a $ factor of two or three for lower defi- g cits or longer beaches. u However, from the results of the se50 lection procedure it rnay also appear that no feasible measure exist and that the zero-option has cost to be chosen.

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Figure 6 Beach protection works comparison for a 5 km beach Risk analysis (probabilistic approach) The common problem in the design of coastal structures is often being the risk of underestimating or misinterpreting the quantities involvéd in the design. This poses a potential threat to the economics of the project, particularly in areas where proper equipment is not readily available. An answer to this is provided by the probabilistic approach to beach erosion problems. In a probabilistic design, probability density function, or just minimum and maximum estimates, are defined for al1 parameters involved. The result is a specified contribution by each parameter to the total uncertainty of the design problem. Owing to the large number of statistica1 manipulations involved, mostly computers must be employed. The use of a probabilistic approach to equilibrium beach profiles, and consequently to dune erosion, and design of coastal structures is now standard practice in the Netherlands. It is also possible to extend probabilistic calculations to establish coastal developments. The probability of a certain amount of erosion or accretion as a function of time and place can easily be determined on the basis of a single-line model. The most probable outcomes, as well as the extreme high or low probabilities, can be assessed and compared for various alternat ives. Apart from an overall 'failure probability', the probabilistic approach makes it possible to quickly select the parameters of significance in terms of risk contribution and the effect of variations in these parameters. Although the risk, as such, will not be reduced, the band-width will be well defined and this prevents surprises. Computer models are also extremely helpful in simulating various execution scenarios. Again the aim is a 'no surprise' solution, based on extensive and rapid variation of parameters for beach scenarios.

1.5

Structural aspects and design procedure checklist

In the past only local usage and experience have determined the selection of the type and dimensions of the coastal protection. Often designs were conservative and too costly or were inadequate, for example revetments with insufficient strength which were damaged when exposed to wave attack.


Actually, Delft Hydraulics and the Rijkswaterstaat in the Netherlands, and many other institutions world-wide, have gained much experience with coastal structures. Sophisticated mathematica1 models and test facilities are now available in which structures can be studied or tested. In this respect, the Delta Flume should be mentioned particularly. This facility enables full-scale or near to full-scale tests to be carried out. The dimensions, stability and technica1 feasibility of coastal protection works can be actually determined on the more founded basis and supported by a bette r experience than in the past. Often, however, the solution being considered should still to be tested in a scale model, since no generally accepted design rules exist. The existing design rules wil1 be briefly reviewed in the subsequent chapters. The design procedure-checklist for beach nourishment projects can be derived from the diagram in Figure 4. The common design checklist for the rigid (fixed) structures is roughly outlined below and in Figure 7. Design schecklist for rigid structures -___ -- procedure .......................... ------------The most critica1 structural design element~are a stability of coverlayer, secure foundation to minimize settlement and toe protection to prevent undermining. Al1 of these are potential causes of failure of coastal structures. The usual steps needed to develop an adequate structure design are: a. Formulate functional requirements. b. Prepare alternative solutions. c. Select suitable solution. d. Determine the water level range for the site. e. Determine the wave heights and (eventual) currents. f. Detect suitable structure configurations (geometry). g. Review the possible failure mechanisms. h. Select a suitable armor alternatives and armor units size. i. Design the filter and underlayers. j. Determine the potential run-up to set the crest elevation. k. Determine the amount of overtopping expected for low structures. 1. Design the toe protection, transitions and crest protection. m. Design under drainage features if they are required. n. Provide for local surface run-of£ and overtopping run-of£, and make any required provisions for other drainage facilities such as culverts and ditches. o. Consider end conditions to avoid failure due to flanking. p. Provide for-firmcompaction of al1 fill and backfill materials. This requirements should be included on the plans and in the specifications, and due allowance for compaction must be made in the cost estimate. q. Make final check of your design r. Develop cost estimate for each alternative. s. Select the final design. t. Prepare specifications for materials and execution incl. quality control. The review of the key elements that must be considered in the design (dimensioning) are illustrated for some protection structures in Figure 7. More detailed design-methods for these structures are discussed in the subsequent contributions in the course. Because of the complexity of the subject it is impossible in the scope of the short course to give detailed review for al1 coastal protection structures. However most of the design principles and design formulae are also applicable for other structures.


K.W. Pilarczyk

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K.W. Pilarczyk

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As a base line for the course, the Shore Protection Manual (SPM, 1984) can be used. Supplementary information on the various topics, may be found in more specialized textbooks and publications, as referred to in the referentes and in the subsequent sections and appendices. This wil1 be sufficient for those who are involved in planning and conceptual design. The design of coastal protection is not a simple matter. In al1 cases, experience and sound engineering judgement play and important role in applying these design rules, or else mathematica1 or physical testing can provide an optimum solution (Figure 8).

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REEERENCES CUR (1987), Manual on artificial beach nourishment, report 130, Centre for Civil Engineering, Research, Codes and Specifications (CUR), Gouda, The Netherlands. Goda, Y. (1985), Random Seas and Design of Maritime Structures, University of Tokyo Press, Japan. Horikawa, K. (1988), Nearshore dynamics and coastal processes, University of Tokyo Press, Japan. Kobayashi, H., T. Tanaka and S. Shoyama (1985), Beach nourishment in Yokohama Marine Park, PIANC, 26th International Navigation Congress, Brussels Ontario Ministry of Natura1 Resources (1987), How to protect your shore properly. Ontario, Canada. Per Bruun and M. Manshar (1963), Coastal protection for Florida, Bulletin no. 113, University of Elorida, Cainesville, U.S.A. SPM (1984), Shore Protection Manual, U.S. Corps of Engineers, Vicksburg , U. S. A. Tanaka, N. (1983), A study on characteristics of littoral drift along the coast of Japan and topographic change resulted from construction of harbours on sandy beach (in Japanese), Tech. Note, Port and Harbour Res. Inst., no. 453, 148 p. Weide, van der, J. (1989), General introduction and hydraulics aspects, in short course on design of coastal structures, AIT Bangkok.

INTRODUCTION T 0 COASTAL PROTECTION

K.W. Pilarczyk

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS

Krystian W. Pilarczyk

Rijswaterstaat, Road and Hydraulic Engineering Division Van der Burghweg l', P.O. Box 5044, 2600 GA Delft, The Netherlands

CONTENT (continued) 8.

SEMI-PROBABILISTIC CALCULATION OF REVETMENTS

9. 9.1 9.2 9.3 9.4 9.5

STRUCTURE RELATED DEMANDS Slope protection Optimalisation of slope stability Scour protection Protection against overtopping Joints and transitions MANAGEMENT AND MONITORING

11.

CONCLUSIONS AND RECOMMENDATIONS

REFERENCES

DESIGN OF SEAWALLS AND SEADIKES INCL. OVERVIEW OF REVETMENTS

1.

INTRODUCTION

The low-lying countries as the Netherlands are strongly dependent on good (safe) seadefences. Driven by the necessity to withstand the water, during centuries the engineers built up their knowledge on hydraulic engineering, and particularly on constructing of dikes and the design of dikes and their protection - measures (revetments). --- .--- However - revetments was mostly based more on rather vague experience than on the general valid calculation methods. The increased demand on reliable design methods for protective structures has resulted in increased research in this field and, as a result, in preparing a set of design guidelines for various coastal structures. This guidelines are intended for technicians and organizations directly involved in the design and management of protective structures. In this contribution a brief review on genera1 design philosophy, different hydraulic and geotechnical aspects and design criteria for dikes and various types of revetments is given. The treatment is restricted -. mainly to sloping structures which are widely used in coastal engineering practice. These structures are mainly composed of soil materials such as rock, gravel, sand and clay. In addition other construction materials e.g. asphalt, man-made block/revetments and geotextiles can be applied. The typical applications of sloping coastal structures are, respectively: dikes, dams, breakwaters, dune protection, navigation channels, bottom protection and al1 kind of revetments. Seawalls and revetmentstructures are typically built in shaliow water or on land, whereas breakwaters, dams and dikes (incl. revetments for dams and dikes) are typically built in deeper water. In the present contribution, al1 such structures are conceptualised as 'Jseawalls and/or seadikes". -

This set of guidelines is not intended as a scientific work dealing exhaustively with theoretica1 fundamentals. It has been endeavoured as £ar as possible to give the genera1 practica1 design guidelines with some background information but without offering a solution for every conceivable prÓblem. For a treatment of these matters &n greater depth the reader is referred to the original reports. For the revetment, i.e. the protective covering of a water retaining structure (dike) requirements are formulated with reference to the purpose of the structure and the revetment, the technica1 features of constructing it, and possible special circumstances involved.

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K.W. Pilarczyk

VII. 1

The shape of the cross-sectiona1 profile of the dike is of influence on the type of revetment material suitable for revetment construction. The design of the shape and the height of a dike are thus also discussed. Because of the complexity of the subject there are as yet no simple-to use mathematica1 models available for dealing with various kinds of structural elements and subgrade. The actual progress in this direction is discussed. Al1 the Same, with the aid of the data yielded by theoretical/empirical research, and the available experience, it is possible t o determine approximately the necessary dimensions of dikes and the given types of revetments. 1.2

Funct ions

Seawalls are only one option for coastal defence and must be considered in conjunction with or as an alternative to beach management and other opt ions. Seawalls and/or dikes (or dams) are built to protect upland (incl. land reclamation) areas when resources become endangered by inundation and erosion due to storm surges, waves and wave overtopping. Seawalls/dikes are one of several forms of shore protection which may be used singularity or in combination with other systems. Al1 these coastal protection systems have advantages and disadvantages which should be recognized before the choice is made (Figure 1, Per Bruun 1963). The main purpose of seawall or dike is t o fix the land and.sea boundary, and it is not intended to protect either the beach fronting it or adjoining, unprotected beaches. On an eroding coast, the beach in front of a seawall may narrow and eventually disappear if there is an inadequate sediment supply (Kraus, 1988). Thus, seawalls neither promote accretion nor reduce the regional trend of the coast to erode, but it is constructed for protection of upland under extreme conditions. 1.3

Starting-points for the design

Based on the main functional objectives of the coastal structure a set of technica1 requirements has to be assessed. It is very important that these requirements should provide answers to questions concerning the acceptable risk level with respect to loss of function (for example inundation, or loss of stability) and concerning the service lifetime of the structure. Since absolute safety and infinite durability is not possible the limitations of the above mentioned requirements have to be specified in the early design phase. Several methods are available to select more or less optimum design requirements. These methods are based on risk analyses in which both the probability of disfunctioning and the possible associated damage effects (in terms of economic costs) are incorporated (see Chapter 111). When designing a seawall dike, the following requirements to be met can be f ormulated: 1. the structure should offer the required extent of protection against flooding at the acceptable risk 2. events at the seawallldike should be interpreted with a regional perspective of the coast 3. it must be possible to manage and maintain the structure 4. requirements resulting from landscape, recreational, monumental, and/or ecological viewpoint should also be met when possible


VII. 2

Figure 1

DESIGNOF

Different types of seawalls/dikes

SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K.W. P i l a r c z y k

5 . the construction-cost should be minimized to an acceptable/responsi-

ble level 6. legal restrictions. The elaboration of these points depends on specific local circumstances as a type of upland (low-land or not) and its development, availability of safety standards, availability of equipment, manpower and materials etc. The high dikes are needed for protection of low-lands against inundation while lower dikes or seawalls are sufficient in other cases. It is important, already in a design stage, to create conditions for proper management and maintenance of the dike. It may include such points as: the dike must be easy to reach under al1 circumstances the slope angle should allow good maintenance of slope protection (i.e. grass-mat) special arrangements (i£ not possible to avoid) in respect to strange objects as buildings pipes, cables etc. The starting-points for the design should be carefully examined in cooperation with the client and/or future manager of the object. The selection and the evaluation of site conditions, hydraulic factors and the design requirements are necessary for the functional planning of the structure and the selection of design conditions. The cost of construction and maintaining is generally a controlling factor in determining the type of structure to be used. In assessing the perfomance of a-sea-wal1 C m I A (k986Y fomulated four principal criteria which must be taken into account (see Figure 2). Al1 these criteria have some relevance in assessing the performance of seawall, but their relative importance varies from case to case, and a defence work rarely meets them al1 equally. A sea-wal1 should clearly be designed to meet its primary purpose, be it coast protection, flood prevention or land reclamation, under a wide range of conditions due to varying water level and wave conditions. In achieving the primary purpose, the designer may have been forced to compromise performance as judged against the other criteria.

Seawalls and/or dikes are one of various forms of coastal protection which may be used singularly or in combination with other methods. There is still much misunderstanding on the use of seawalls and their possible disadvantages related to the disturbance of the natura1 coastal processes and even acceleration of beach erosion. These disagreements on the function of the seawalls and their repercussions for the total coastal management brings alco the decision-making people in great problems. However, it should be said that in many cases when the upland becomes endangered by inundation (as in The Netherlands) or by high-rate erosion leading to high economical or ecological losses, whether one likes it or not, the dike or seawall can even be a must for surviving. The proper coastal strategy to be followed should always be based on the total balance of the possible effects of the counter measures for the coast considered, including the economical effects or possibilities. It is an 'engineering-art' to minimize the negative effects of the solution chosen.

DESIGN OF SEAWALLS'AND DIKES INCL. OVERVIEW OF REVE~~ENTS K. W. Pilarczyk

PRINCIPAL CRITERIA

1

PRIMARY

l

CHOICE OF SOLUTION

1

EFFECT ON MJOINIYO

l

STABILI~Y

AOECUAC7

. CF

STRUCTCRE

I 1

l

v AOACENT

EFiiC; ON

1

I

LINO BEHINOI

Figure 2

2.2

Performance c r i t e r i a of s e a w a l l s (CIRIA, 1986)

P h y s i c a l i n t e r a c t i o n s and consequences

A s e a w a l l forms a p e c u l i a r p r o t e c t i o n s o l u t i o n . I t p r e v e n t s f u r t h e r e r o s i o n of t h e c o a s t l i n e ( i t f i x e s t h e l a n d - w a t e r l i n e ) but d o e s n o t s t o p t h e p h y s i c a l p r o c e s s e s which cause t h e e r o s i o n ( a g r a d i e n t i n t h e l o n g s h o r e t r a n s p o r t and t h e o f f s h o r e t r a n s p o r t ) , ( B i j k e r , 1989). Theref o r e , i t does n o t s t o p t h e e r o s i o n of t h e i n s h o r e zone, and p r o b a b l y even i n c r e a s e s i t t h r o u g h r e f l e c t i o n of t h e incoming waves, s o a heavy t o e p r o t e c t i o n is n e c e s s a r y . Not o n l y d o e s s c o u r o c c u r i n f r o n t of a wal1 b u t a l s o downcoast of i t , where t h e d i f f r a c t i n g r e f l e c t e d waves combined w i t h t h e i n c i d e n t t o c o n t i n u e t h e s h o r t - c r e s t e d wave system ( S i l v e s t e r , 1978). The r a p i d removal of m a t e r i a l r e s u l t s i n a s h o a l f u r t h e r downcoast because t h e i n c i d e n t waves c a n n o t cope w i t h t h e i n c r e a s e d l o a d removed and hence s h o a l s and even s h o r e l i n e p r o t u b e r a n c e o c c u r s . Very o f t e n t h e l o c a l s o l u t i o n by a s e a w a l l l e a d s t o t h e d i s p l a cement and expansion of t h e problem downcoast. On an o r i g i n a l l y e r o d i n g

DESIGN OF-SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS Pilarczyk

K.W.

coast, the erosion in front of a seawall can even lead t o undermining the seawall or dikes, and often additional measures are needed (i.e. groyns in front of a seawall) t o prevent this. The maintenance cost of this protection can be, therefore, sometimes very high. This stresses the need for a proper studies before the decision t o construct a seawall will be taken. In genera1 can be stated that the coasts protected by a seawall will only have a beach in front of this wall if sufficient sand supply is available. This happens where no erosion takes place under normal circumstances. The seawall or dike is constructed then, for protection under extreme conditions. However, it is possible that even under these circumstances wave reflection causes so much erosion in front of the seawall that the beach will not be restored. Therefore, seawalls are generally not a very feasible solution, but under special circumstances, such as along strongly curved and heavily eroding coasts, they can be the only possibility (Bijker, 1989). By situating the defence line sufficiently landward from the eroding coast the possibility can be created for observing the natura1 developments and preparing additional measures if necessary. To minimize the effect of extra'scouring due t o reflection of waves the slope of seawalls or dikes should be not more than 1 on 3 (see also par. 8.3). That is one of the reasons that the Dutch dikes have mostly slope 1 on 4 or even milder. Also wave energy absorbing revetments as, for exarnple, rubble structures, will diminish the reflection and thus, the scouring intensity. The rather objective evaluation on the-effects of coastal armouring is presented by Dean (1986). The results are summarized in table 2.1. ~ e a n - sf inal conclusions are cited below: "~ncertainties resulting from a lack of definitive information has led to considerable speculation and claims regarding the adverse effects to coastal armoring on the adjacent shorelines. Employing sound principles and laboratory and field data, an attempt is made to evaluate the potential adverse effects of armoring. It is concluded that: 1. There are not factual data to support claims the armoring causes: profile steepening, increased longshore transport, transport of sand to a substantial distance offshore, or delayed post-storm recovery.

2. The interaction of an armored segment of shoreline with the littoral system is more of a 'geometricm or "kinematic" interaction as contrasted to a "dynamicM interaction. The interaction depends on the amount of sand in the system vis-a-vis the equilibrium beach profile for the prevailing tide and wave conditions. 3. Armoring can cause localized additional storm scour, both in front of and at the ends of the armoring. A sirnple sediment supply-demand argument is proposed to explain the scour. A rnethodology is presented to quantify the potential adverse effects of an armoring installation and appropriate periodic sand additions proposed as a means of mitigation to elevate the installation to one of neutra1 impact on the adjacent shoreline".


K. W. Pilarczyk

Concern

Assessment

C o a s t a l armoring p l a c e d i n an a r e a of e x i s t i n g e r o s i o n - TRUE a l s t r e s s causes increased e r o s i o n a l s t r e s s on t h e beaches adjacent t o t h e armoring.

By p r e v e n t i n g t h e u p l a n d from eroding, t h e beaches a d j a c e n t t o t h e armoring s h a r e a g r e a t e r p o r t i o n of t h e Same t o t a l erosional s t r e s s .

C o a s t a l armoring p l a c e d i n a n a r e a of e x i s t i n g e r o s i o n - TRUE a l s t r e s s w i l 1 cause t h e b e a c h e s f r o n t i n g t h e armoring t o diminish.

Coastal armoring i s designed t o p r o t e c t t h e upland, but d o e s n o t p r e v e n t e r o s i o n of t h e beach p r o f i l e waterward of t h e a r m o r i n g . Thus a n e r o d i n g beach w i l l c o n t i n u e t o e r o d e . I f t h e armoring h a s n o t been p l a c e d , t h e w i d t h of t h e beach would have remained a p p r o x i m a t e l y t h e Same, b u t w i t h i n c r e a s i n g t i m e , would 'have been l o c a t e d p r o g r e s s i v e l y landward

.

PROBABLY No known d a t a o r p h y s i c a l C o a s t a l armoring c a u s e s a n arguments s u p p o r t t h i s a c c e l e r a t i o n of beach e r o s i - FALSE on seward of t h e armoring concern. An i s o l a t e d c o a s t a l armoring TRUE can a c c e l e r a t e d o w n d r i f t erosion.

I f an i s o l a t e d s t r u c t u r e i s armored on a n e r o d i n g beach, the structure wil1 eventually protrude i n t 0 t h e active beach zone and w i l l a c t t o some d e g r e e a s a g r o i n , i n t e r r u p t i n g longshore sediment t r a n s p o r t and t h e r e b y causing downdrift erosion.

C o a s t a l armoring r e s u l t s i n PROBABLY No known d a t a o r p h y s i c a l arguments s u p p o r t t h i s a g r e a t l y d e l a y e d p o s t - s t o r m FALSE concern. recovery. C o a s t a l armoring c a u s e s t h e beach p r o f i l e t o s t e e p e n dramat i c a l l y . C o a s t a l armoring p l a c e d w e l l - b a c k from a s t a b l e beach is d e t r i m e n t a l t o t h e beach and s e r v e s no u s e f u l purpose.

Table 2 . 1

PROBABLY No known d a t a o r p h y s i c a l arguments s u p p o r t t h i s FALSE concern.

FALSE

I n o r d e r t o have any substantial effects t o the beaches. Moreover armoring s e t well-back from t h e n o r m a l l y a c t i v e s h o r e zone c a n provide "insurance" f o r upland s t r u c t u r e s a g a i n s t severe s t o m s .

Assessment of some commonly e x p r e s s e d c o n c e r n s r e l a t i n g t o - c o a s t a l armoring (Dean, 1 9 8 6 )

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K.W. P i l a r c z y k

2.3

Literature review

As a result of common controversy on the use of seawalls for coastal protection, the specialty technica1 conference Coastal Sediments087 had as a theme he Effects of Seawalls on the coast". The results of this conference have been evaluated and presented in a separate issue (Kraus et al., 1988). Kraus evaluates the present state of knowledge on this subject by answering (based on material presented), the following eight questions (for definitions see Figures 3 and 4). incident waves

\ ,*ev.-; ,'y

reflected waves ibar displaced?)

,*--y\,

\

' / j i

bar bar

bar

-

longshore current

s~.our.

.. . .

_

f ska!? 1

"historical shoreline"

1

impoundment

I

downdrift flanking

Figure 3

eroded unprotected shoreline

Plan view of potential impact of seawalls (Kraus et al, 1988).

1. What is the maximum scour depth at a seawall and the time scale of its development under given wave conditions, water depth, and reflection characteristics of the wall? 2. Is the amount of sand locally scoured on a seawall-backed profile equal to the amount eroded across the profile on adjacent beaches without structures? 3. Do seawalls accelerate or enhance erosion? 4. Are there systematic patterns of scour or undulatory features of the profile in front of seawalls, and which parameters determine the scour type? 5. Is the recovery pattern different for beaches with and without a seawall? 6. Is the longshore bar system in front of a seawall similar to that along neighbouring, unstructured beaches? 7. How does a seawall after the longshore current and longshore sediment transport rate? 8. Is it beneficia1 to design seawalls to be "softer1', i.e., possess lower reflection coefficients and therefore approach the hydrodynamic behaviour of a sandy beach?


Maximum depth of scour Beach profile (deflated bars, translated bars, steepened profile, undulatory profile) Beach platform (flanking, impoundment, erosion of downdrift shoreline) Waves and water level (reflection, tide, surge, setup) Circulat ion (longshore current, rip currents) Beach recovery Table 2.2

Beach processes associated with seawalls

This review is based on a comprehensive search of the literature on the physical effects of seawalls on the beach. Topics such as seawall construction, maintenance, and arrnoring against scour are not treated.

Figure 4 Profile view of potential impacts of seawalls (Kraus et al, 1988) Because of the importancy of this item for the choice of the seawall as a coastal protection structure, the short version of v raus* evaluation, including the answering the eight questions earlier mentioned, wil1 be repeated below.

h he interaction of seawall and beach is described within the genera1 framework of dimensional analysis and is offered as a potentially useful methodology for performing future research on the subject. Examples are given of seawalls that have perforrned wel1 in protecting the fastland, for which some have seen the growth of beaches in front of them. An appeal is made to understand seawalls as a t001 of coastal engineering which must be judged within the context of shore protection alternatives and intended functioning. In a previous review (Kraus, 1987), it was concluded that available quantitative information was too scarce to allow reliable assessment of the effects of seawalls on the beach. However, addition of several 01der studies and more than ten new studies, and reexamination of previously reviewed material has resulted in a sizable and coherent data set from which trends can be identified and conclusions drawn with a fair level of confidence. The present review has found that beaches with and


VII. 9

without seawalls exhibit similar behaviour and variation with regard to short-term erosion and recovery associated with s t o m s and post-storm wave conditions. The main exceptions are: (1) localized beach response at the toe and ends of a seawall in some instances, ( 2 ) denial of the sediment encased behind a seawall to the littoral system that would nourish adjacent beaches, and (3) functioning of a seawall as a groin or jetty to block sediment movement alongshore in situations where the seawall protrudes int0 the active surf zone. Case studies of long-tem changes in position of the shoreline and shape of the beach profile in front of seawalls indicate varied responses of accretion, no change, erosion, or alternating advance and retreat. It thus appears that seawalls are relatively innocuous with regard to cross-shore sediment processes and only have potential to damage neighbouring beaches if longshore processes are interrupted. Available field results indicate that the beach profile in front of a seawall does not have an automatic erosional response to s t o m s . Also, the recovery process of eroded beaches fronting seawalls can vary between extremes of rapid and complete recovery to limited or no recovery, similar to variation in beaches having no stabilization structures. Oversimplified and unqualified statements such as "seawalls cause erosion" and "seawalls -accelerate . e r o w i n n " h a v P n demonstrated to be unfounded by numerous q u a n t i t a t i v e f W & e r v * t i o R s and laboratory experiments. In this section, as a summary of results, a synthesis of what is known and what is not. known about t b effects of seawalls on the beach is given by answering the eight questions posed in the Introduction. -

(l) What is the k k - s c o u r depth at á seawall and the time scale of its development under given wave conditions, water depth and reflection characteristics of the wall? The potential maximum depth of scour at a seawall situated on either a sloping of horizontal bottom is wel1 approximated by a wave height statistically representative of the higher incident waves in deep water. The significant deep water wave height occurring during a storm appears to provide a good estimate. Maximum scour depth is related to incident wave height, duration of wave action, reflectivity of the wall, and initia1 beach morphology. However, there are no field data sets tracking development of scour depth through time during a storm. Laboratory studies and qualitative field observations indicate that scour depth decreases if the reflection coefficient of the seawall is reduced. Beach morphology exerts strong control over the amount of scour, co that the Itpotential scour depth = significant wave height" guidance may greatly overestimate scour depth at a seawall fronted by a wide surf zone or by a barred beach. (2) Is the amount of sand locally scoured on a seawall-backed profile equal t o the amount eroded across the profile on adjacent beaches without structures? Field studies indicate that the volume of material locally scoured at a seawall has similar magnitude and variation as the volume eroded from adjacent beaches not backed by walls. Variability in the data is high because numerous factors are involved, such as prestorm nearshore beach morphology, offshore bathymetry (which controls the distribution of incident wave energy alongshore), longshore sediment transport processes, and spatial and tempora1 variability of the storm. (3) Do seawalls accelerate or enhance erosion? The words uaccelerateu and "enhance" imply a comparison or referente. Seawalls are usually constructed in areas where severe eroSion already exists. Therefore, the problem is to determine whether seawalls accelerate or enhance erosion as compared to erosion on


VII. 10

n e i g h b o u r i n g beaches w i t h o u t s t r u c t u r e s o r t o t r e n d s r e c o r d e d a t t h e same beach p r i o r t o c o n s t r u c t i o n of t h e w a l l . The word 'eros i o n " must a l s o be d e f i n e d p r e c i s e l y , i n c l u d i n g p o s s i b l e l i m i t a t i o n s , a s i t c a n r e f e r t o s u b a e r i a l volume c h a n g e , movement of mean s h o r e l i n e p o s i t i o n , t o t a l volume change a l o n g t h e p r o f i l e , s h o r t t e r m change s u c h a s immediately a f t e r a s t o r m and p r i o r t o p o s t s t o r m r e c o v e r y , o r l o n g t e r m change. I n t e r p r e t a t i o n s must a l s o acc o u n t f o r r e g i o n a l p r o c e s s e s , f o r example, a v a i l a b i l i t y of s e d i ment, p r e - s t o r m beach w i d t h s and p r o f i l e s h a p e s , and wave convergence and d i v e r g e n c e caused by o f f s h o r e bathyrnetry. I f t h e beach f r o n t i n g a s e a w a l l is narrow, i . e . , narrow, t h e r e c o v e r y p r o c e s s may be a b s e n t .

t h e s u r f zone is

A p r o p e r l y f u n c t i o n i n g s e a w a l l d o e s w i t h h o l d t h e s e d i m e n t it encas e s from t h e s y s t e m t h a t would o r d i n a r i l y be a v a i l a b l e f o r t r a n s p o r t by wave and c u r r e n t a c t i e n . 4 l o n g s h o r e c u r r e n t c o u l d c a r r y t h i s sand t o t h e a d j a c e n t d o w n d r i f t beach. On t h e o t h e r hand, i f a l o n g s h o r e c u r r e n t f l o w s d u r i n g a s t o r m , s e d i m e n t removed from t h e u p d r i f t beach i s e x p e c t e d t o p a s s t h e s e a w a l l and r e a c h t h e downd r i f t beach, a l t h o u g h t h i s hyF6thësTs h a s n o t been c o n f i r m e d by d i r e c t f i e l d o r l a b o r a t o r y measurement. A t t h e p r e s e n t t i m e , t h r e e mechanisms c a n be f i r m l y i d e n t i f i e d by which s e a w a l l s may c o n t r i b u t e t o e r o s i o n o n t h e c o a s t . The most o b v i o u s is r e t e n t i o n of s e d i m e n t behind t h e w a l l which would o t h e r w i s e be r e l e a s e d t o t h e l i t t o r a l system. The second mechanisms, which c o u l d i n c r e a s e l o c a l e r o s i o n on d o w n d r i f t b e a c h e s , i s f o r t h e u p d r i f t s i d e of t h e w a l l t o a c t a s a g r o i n and impound s a n d . T h i s e f f e c t a p p e a r s t o be p r i m a r i l y t h e o r e t i c a l r a t h e r t h a n a c t u a l i s e d i n t h e f i e l d , a s a w a l l would p r o b a b l y f a i l i f i s o l a t e d i n t h e s u r f zone. The t h i r d mechanism i s f l a n k i n g , i . e . , i n c r e a s e d l o c a l e r o S i o n a t t h e e n d s of w a l l s . Three a d d i t i o n a l c a u s e s of e r o s i o n imput e d t o s e a w a l l s t h a t a r e m a i n l y s p e c u l a t i v e w i t h no f i r m q u a n t i t a t i v e s u p p o r t a r e : i n c r e a s e d t u r b u l e n c e due t o wave r e f l e c t i o n , o f f s h o r e t r a n s p o r t of sediment by r i p c u r r e n t s t h a t may d e v e l o p a t t h e ends of w a l l s and e n h a n c e l e n t of t r a n s p o r t by a s h o r t - c r e s t e d wave system composed of i n c i d e n t and r e f l e c t e d waves. Of t h e p o s i t e d s i x e r o s i v e mechanisms, o n l y t h e c a s e of a s e a w a l l p r o t r u d i n n i n t o t h e s u r f zone s u f f i c i e n t l y £ a r t o b l o c k l o n g s h o r e s e d i m e n t t r a n s p o r t a p p e a r s c a p a b l e of c a u s i n g s i g n i f i c a n t e r o s i o n of d o w n d r i f t s h o r e s , by a n a l o g y t o t h e a c t i o n of a j e t t y o r long g r o i n . The t h r e e l a t t e r , more s p e c u l a t i v e , mechanisms d e s c r i b e d above have n o t been q u a n t i t a t i v e l y documented i n f i e l d s t u d i e s . ( 4 ) Are t h e r e s y s t e m a t i c p a t t e r n s o f s c o u r o r u n d u l a t o r y f e a t u r e s i n t h e p r o f i l e i n f r o n t of s e a v a l l s , and v h i c h parameters determine t h e scour type? L a b o r a t o r y and t h e o r e t i c a l s t u d i e s i n d i c a t e t h a t a n u n d u l a t o r y prof i l e c a n be formed by r e £ l e c t i o n a t a-s e a w a l l . T h i s phenomenon h a s n o t been o b s e r v e d i n f i e l d s u r v e y s . I f a s e d i ment s u p p l y e x i s t s , f i e l d s t u d i e s i n d i c a t e t h a t a l o n g s h o r e b a r s y s t e m i n f r o n t of a s e a w a l l c a n d e v e l o p i n much t h e Same way a s a t a d j o i n i n g beaches w i t h o u t s e a w a l l s o r a s e x i s t e d a t t h e s i t e p r i o r t o s e a w a l l c o n s t r u c t i o n . I f t h e c o a s t is s e d i m e n t - d e f i c i e n t , t h e p r o f i l e may assume a lowered e q u i l i b r i u m shape o r a n e q u i l i b r i u m s h a p e governed by t h e c o a r s e r g r a i n s i z e s . ( 5 ) Is t h e r e c o v e r y p a t t e r n s i m i l a r f o r a beach w i t h a n d w i t h o u t a s e a wall? Within t h e n a t u r a 1 v a r i a b i l i t y of b e a c h change, t h e answer t o t h i s q u e s t i o n is " y e s " . The m a j o r i t y of f i e l d s t u d i e s and a l 1 r e l e v a n t l a b o r a t o r y s t u d i e s have shown t h a t i£ a s e d i m e n t s u p p l y e x i s t s , t h e beach i n f r o n t of a s e a w a l l w i l 1 r e c o v e r .


V I I . 11

Numerous laboratory and field studies have demonstrated that the position of a seawall with respect to the surf zone is a critical parameter controlling the amount of local erosion and the beach recovery process. This distance is variable because the boundaries of the surf zone shift according to tide, surge and height and period of the waves. Movable bed laboratory experiments have shown that maximum erosion occurs if a seawall is located in the middle to outer third of the surf zone, measured from the shoreline. (6) Is the longshore bar system in front of a wall similar to that along neighbouring, unstructured beaches? Field data on this point are limited by the restricted offshore extent of most post-stom surveys. Kriebel (1987) found that although there were local differences, namely scour at a wall instead of foreshore deflation on beaches away from the wall, eroded volumes (obtained from wading depth surveys) were almost equal. Under nonstom waves, field studies indicate that longshore bar systems at beaches with approximately equal numbers, positions, and volumes of bars. Two field studies for which beach profile surveys bracketed seawall construction both found that pre- and postconstruction profiles had remarkably similar longshore bars. (7) How does a seawall alter the longshore current and longshore sediment transport fate? This question could not be answered in this review. A;though arguments have been given for both increased and decreased longshore current speeds in front of a seawall, little direct evidence is available. In general, documentation of three-dimensional coastal hydraulics and beach change associated with seawalls is scarce, and results are too limited to make conclusions. Resolution of this problem may require clarification of the role of rip currents, i.e., take into account the full horizontal circulation pattern near seawalls. i.e., possess (8) Is it beneficia1 to design seavalls to be 'softer', lover reflection coefficients and therefore approach the hydrodynamic behaviour of a sandy beach? Laboratory studies, theoretica1 developments, and qualitative field observations al1 point to the conclusion that slanting seawalls and permeable, rubble mound seawalls, which have smaller reflection coefficients than vertical walls, suffer less local scour than vertical or near-vertical walls. In some cases, 'softer" structures appear to have mitigated local scour and allowed the beach to respond in a manner similar to that of a natural sand beach over the erosion and recover cycle associated with impacts of storms. Many more field data sets are required, including quantitative comparisons to control beaches with different types of structures and beaches without structures. Final conclusions: The majority of quantitative-type field studies indicates that seawalls neither accelerate nor enhance long-term erosion of beaches, and no systematic difference in port-storm recovery of beaches with and without seawalls was found if there was ample sediment or a wide surf zone existed. If beaches are deficient in sediment or if sea (or lake) level is rising at the site, erosion is more likely to occur at amore& beaches as compared to unarmored beaches. However, the magnitude of the change does not appear to be extremely greater than the variability of change in the natural, unarmored beach system. More study is needed to refine and quantify these conclusions, as wel1 as determine their limit of applicability.


K.W. Pilarczyk

VII. 12

In order to better utilize seawalls as an important instrument for shore stabilization, it is recommended that an effort be made to review historica1 data on shoreline position, beach profile, and littoral processes in the vicinity of existing seawalls, and to initiate comprehensive monitoring programs. Finally, the researcher should attempt to purge al1 preconceptions and biases in dealing with this controversial subject with the goal of defining and interpreting processes and responses at the study site within the context of that site'. It can be stated that the results of the evaluation of this conference agree firmly with the conclusions drawn by Dean. 3.

COMCEPTTJAi- DESIGN (Weide, van der, 1989)

As stated in the previous chapters, the design should be based upon the functional requirements taking into account the environmental conditions in the project area and giving due regard to constructional aspects, operation and maintenance. In the stage of conceptual design the Shore Protection Manual (SPY, 1984) can be useful reference. 3.2

The functional design

The function of the coastal structure, a s defined in the previous chapters, is mainly to protect the hinterland against the adverse effect of high-water and waves. If high-water protection is required the structure should have a height wel1 above the maximum level of wave-uprush during storm-surges. This normally calls for high crest elevations. If, however, some overtopping is allowed in view of the character of the hinterland, the design requirement is formulated in terms of the allowable amount of overtopping. In Holland a value of 2 litres per second per running metre of dike is accepted for instance. Obviously crest-elevations can be reduced considerably in this case. For structures, such as breakwaters, where wave-reduction is the main objective a further reduction in crest-height can be applied. Waveheights due to transmission and overtopping should be negligible during operational conditions but may reach values of 0.5 metres in extreme design conditions. Finally, training walls are mainly used to direct flov. The crest-elevation is mainly determined by constructional aspects, which implies that a minimum level of 2 meters above mean high water should be applied to guarantee an uninterrupted progress of work. Wave overtopping during operational and extreme conditions are of less concern in this case. Methods to compute the required crest-elevation wil1 be given in the subsequent sections. As the computational method depends to a large extent on the structural-concept, this should be selected first. 3.3

The structural design

The selection of the structural concept depends on the function, the local environmental conditions and the constructional constraints. The governing criteria are the technica1 and economic feasibility for the project under consideration. A review of possible concepts is shown on Figure 5 , in which the feasibility of various concepts is show as a .function of wave-height and water-depth.


VII. 13

Basically, a simple sheet-pile wal1 wil1 be sufficient to provide the required crest-elevation. Such a concept is feasible, only in small water-depths with moderate wave-action. In deeper areas the coffer-dam

v

caffer dam ccisrona, camparite t>pe breakwatas

Figure 5

Application coastal structures (conceptual)

concept has to be applied, which is more complicated t o build, especially in areas with frequent wave-agitation. An other method t o stabilize the sheet-pile is the use of anchors. Al1 method are particularly useful for slope and bank-protections in waters which are wel1 protected against waves. -

A . .

-

Vertical-face structures can als0 be constructed by means of gabions, block type dams, or caissons. In this case the stability is derived from the weight of the structure, which are therefor called gravitytype structure. Often, the performance of such structures in terms of overtopping, is improved by using a parapet or crown-wall. Gravity-type structures may be used in moderate to large water-depths, provided that no breaking waves occur. Due to the potential foundation problems as a result of dynamic wave-loading, such conditions should be at least twice the design wave-height. Obviously, the construction, the transport and the positioning of caissons requires knowledge and experience, which makes the method often impractical. Especially in areas where the weather-windows for construction are small, the caisson concept could be a good solution. In many areas in the world wave-heights and foundation conditions are such that n-e gravity-structures or sheet-piles can be used. Sloping structures are a solution then, as wave-loads on these structu-----------------res are more easily accounted for. Moreover, foundation-loads are more evenly distributed and differentjal_se&tlements can, to a certain extent, be accepted. The slope-type structure is most widely used since it is a versatile concept which can be constructed alco be less-experienced contractors. The stsucture can be used in moderate to deep


VII. 14

w a t e r , and can be d e s i g n e d t o w i t h s t a n d s e v e r e waves. With t h e i n c r e a s i n g s i z e of t h e s t r u c t u r e s , however, t h e l i m i t s of t h e a p p l i c a t i o n a r e more o r l e s s r e a c h e d . S t a b i l i t y of t h e a m o u r l a y e r and g e o t e c h n i c a l s t a b i l i t y of t h e c o r e a r e p o i n t s of c o n c e r n , p a r t i c u l a r l y f o r l a r g e r breakwaters. I n t h i s c h a p t e r it i s f u r t h e r assumed t h a t t h e d e c i s i o n based on t h e c o n c e p t u a l d e s i g n is i n f a v o u r of s l o p i n g s e a w a l l o r d i k e ( F i g u r e 6 ) .

S U . RIVER.LAI(E

- SIDE

HEIGHT HD

fi

LAND- SIDE

(POLDER1

ORDINARI ORDNANCE M T U M (O D 1

HEADLOSS A n

-.L-

- . -

- - - m - - -

--

. b .

TOE PROTECTION

Figure 6

HEEL

S c h e m a t i z a t i o n of a d i k e

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K . W. P i l a r c z y k

V I I . 15

OF CAPANG WIUL

SUBMIL Y T T L E M E N T

\

\

( a ) Failure modes

-----*M@

r

-

HYORAULIC BOUNûARY CONCITIONS (waves. water levets. e t c

+

C

T

REAR S U P E

*

w

OVERTOPPlNG

ARWUR CUMAGE

0

*

$ 4

.v

CAP W U L

FAIWIIL

*

h

4

FROIT YOPE ARMQiR O~MAGE

CETERIORATION CF MATESIALS

T

1

OOWN TiME

i COS7 BENEFIT ANILYSIS AMPLITUOE RECISOS

POWER SPECÏRA

(b) Simplified FAULT TREE

C H A R S E 2 l S I I C WAVE PARAMETERS

LONG TERM (EXTREME) .STAIlSTlCS

DESIGN WAVE QIMATE

MWEL E S T S OF ~ ~ E L I M I N A R Y OESiGn

--

A

.

-

icvr.,hJ.

j

4

-

- - - -WAL

DESIGN

Design wave w a v e climate and design procedure .- .. (C)

Box 3.1 Design approach acc. to Burcharth (Denmark)


VII. 16

4. DESIGN PHILOSOPHY Coastal defences are constructed to protect the population and the economical values against storm surges. However, the absolute safety is nearly impossible to realize. Therefore it is much better to speak about the probability of failure (or safety) of a certain defence system. To apply this method, al1 possible causes of failure have to be analysed and consequences determined. This method is actually under developing in the Netherlands for dike and dune design. The "fault tree" is a good t001 for this aim (Figure 7). In the fault tree, al1 possible modes of failure of elements can eventually lead to the failure of a dike section and to inundation. They can also badly influence the behaviour of the revetment even if properly designed.

GENERALLY:

I

HUMAN FAILURE

FAILURE DIKE SECTIDN Z

DIKE SECTION N i

EROSION OUTER SLOPE

I SLIDE

I

j

r

I Figure 7

OVERFLOW I

FL000 LEVEL

OIKE HEIGHT

PLANE

l

/[

OVERTOPPING I

WAVE RUN UP

l

I

OIKE HEIGHT

I

1I

I

I PIPING 1

REVETMENT FAILURE

I *y;.

.

I

I

I

ETC.

REVETMENT STRENGTH

Simplified fault tree for a dike

Although al1 categories of events, that may cause the inundation of a polder, are equally important for the overall safety, the engineer's responsibility is mainly limited to the technica1 and structural aspects. In the case of the sea-dike the following main events can be distinguished: - overflow or overtopping of the dike - erosion of the outer slope or loss of stability of the revetment - instability of the inner slope leading to progressive failure - instability of the foundation and internal erosion (i.e. piping) - instability of the whole dike. For al1 these modes of failure, the situation where the forces acting are just balanced by the strength of the construction is considered (the ultimate limit-state). In the adapted concept of the ultimate limit-state, the probability-density function of the "potential threat' (loads) and the "resistance" (dike strength) are combined. The category "potential threat" contains basic variables that can be defined as threatening boundary conditions for the construction e.g. extreme wind velocity (or wave height and period), water levels, and a chip's impact (collision). The resistance of the construction is derived from the basic variables by means of theoretica1 or physical models (e.g. theoretical or semi-empirica1 stability-model of grains). The relations that are used to derive the potential threat from boundary conditions


VII. 17

are called transfer functions (e.g. to transform waves or tides into forces on grains or other structural elements). The probability of occurrence of this situation (balance) for each technica1 failure mechanism can be found by applying mathematica1 and statistica1 techniques. The safety margin between "potential threat" and "resistance" must guarantee a sufficient low probability of failure. The different philosophies are currently available in construction practice: 1. deterministic, - - . . . 2. quasi-probabilistic and -3. probabilistic. For fully probabilistic approach more knowledge must still be acquired concerning the complete problems associated with the use of theoretical models relating loads-and strength. -

Studies on al1 these topics are still going on in the Netherlands. The present Dutch guidelines for dike and dune design follow a philosophy, that lies between the deterministic and the quasi- probabilistic approach. The u l t i m a t e - p o t e n t i a t - t h - r e a Y T o rthe Dutch dikes is derived from extreme storm surge levels with a very low probability of exceedance (1% per century settlements of deformations, etc.). However, this deterioration of constructional resistance can cause an unexpected failure in extreme conditions. These are, so called, the serviceability- and fatigue limit states which can als0 be considered as inspection and maintenance criteria. As already mentioned, the fully probabilistic approach for dikes based on the limit-state concept is rather cumbersome because a theoretical description for various failure modes is not available yet. To overcome this problem a scheme to simulate nearly al1 possible combinations of natura1 boundary conditions in a scale model of the construction and to correlate the damage done to the boundary conditions can be developed (black box approach). The full description of probabilistic approach for dikes design lies too £ar beyond the scope of this contribution. However, the detailed information can be found in the Dutch report TAW 10 (1985). 5.

SEAWALL AND DIKE DESIGN

5.1 Boundary conditions (see also CIRIA, 1986) 5.1.1

Assessment of the existing situation

A lot of relevant infomation for a seawallldike design can be drawn frorn files and existing maps. In addition to this, a field reconnaissance and a land s u r v e y ~ ~ ä r e i n d i s p e n s a b l eas , wel1 as photographic recording of the characteristic points in the area. Special attention shuld be paid to the position of the beach andlor onshore profiles, and the morphology of the area considered (erodingl accreting coast). The composition of the existing dike body and the geologic structure of the subsoil are als0 very important. When these data are not available the soil mechanica1 investigation should be considered (soundings, borings etc.). The more detailed information on these subjects can be found in TAWICUR (1990).


VII. 18

5.1.2

Hydraulic boundary conditions

In view of the function of (coastal) water defences the loads will obviously be mostly due to the actions of long and/or short waves. In broad outline the following wave phenomena can be distinguished: low-frequency water level changes, such as flood waves, tidal waves, wind set-up gradients and seiches (Box 5.1); (b) wind waves and swell (Box 5.2); (c) shipds waves in navigable waterways These water level variations stronqly influence the area which needs to . . of be protected with hard revetment. fhe flow diagram on determination hydraulic boundary conditions is given in Box 5.1. . (a)

Water level variations on canals and water-storage channels are comparatively small; probably only caused by lock-water, seepage, drainage and wind effects. Water levels on lakes can vary as a result of wind set-up, infiow or outlfow of water, and evaporation. Water levels in a reservoir can change markedly due to filling or emptying, but rainfall and wind set-up can als0 play a role. Water levels on a river are determined by the riverOs discharge regime, and in addition for the lower reaches (estuaries) by tides and also wind-set-up. For a coastal defence embankment water levels are governed by tides and winds. The most complex situation occurs at coastal shores, where water level fluctuations can assurne many forms. For this reason, and also to reduce the volume of this guide, the considerations here will be limited to coastal areas. The specification of environmental parameters are given in CIRIA (1986). The prediction methods related to hydraulic boundary conditions can be found in SPM (1984) and Goda (1985). 5.1.3

Geotechnical conditions (Quelerij, de, 1989)

For major structures a good geological analysis, based on the overall geological structures of the country, is of the utmost importance for an understanding of the geophysical and geohydrological conditions. The most important geological aspects are: geological stratification, formation and history - groundwater regime seismicity

-

The main questions which a geotechnical investigation hls t0 answer are : - what kind of soil fs found and at what depth, i.e. soft soils such as sand, clay and peat or hard soils such as limestone and calcareous sandstone, or very hard soils such as quartzite and basalt, - what are the mechanica1 properties of the various soils with respect to their strength and deformation characteristics, is the soil fissured or weathered, will the soil degrade. in (short) time.

-

The first step is-to organize and design site investigations. The field programme forming part of the site investigation is complemented by laboratory testing and geotechnical calculations.

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K. W. Pilarczyk

VII. 20

Deep Water

t ,

Lqad ,

A. S f g n i f i u n t Y.vi

hprumation

I

t

.

.

-

-

B. Highut Yvr Reprumtition C. ? r o b a b i l 1 ~Calmlrtlon

D. I m g u l a r U r n ~rfrsats

E. Spoctml Caluilation

t

E

S h a l l w Water

Box 5.2

Flow of transformation and actions of sea waves (Y. Goda)


VII. 21

The last and perhaps most difficult step is the integration of the result of the investigations and structural design, resulting in the final foundation design. At the set-up and organization of the soil-investigation program the geotechnical engineers is confront with the following questions: - which soil data have t o be collected, - at what locations (number and depths), - which site-investigation techniques and laboratory test should be per£ ormed, - when is the programme to be carried out and, - who will take care of the contracting work in the field, the laboratory tests and the interpretation of the results. The answer of these questions will depend among others on: - the boundary conditions stipulated by the client (time and money schedule); - the knowledge, judgment and experience of the geotechnical engineer; - the availability of existing data, for example topographical, geological and geotechnical maps; - the phase of the design: for a preliminary design only global information over a wide area is needed to recognize the main geotechnical problems; in the final design phase or during the construction period detailed i n f o m a t i o n on engineering soil parameters is needed; - the type of geotechnical failure mechanisms involved; - the availability and restrictions (including the terrain accessibility) of the investigation tools and the quality of the personnel to handle these tools. A high quality investigation must be economically efficient in the sense that the cost of the investigation must be money wel1 spent. The investigator must be able t o justify each and every item in the site investigation in t e m s of the value of that item in building up the geotechnical model. The investigator should be able t o show good and sufficient reason for undertaking each part of the investigation It is emphasized that there is no standard form of site investigation for a particular engineering work. Each site investigation should be regarded as a completely new venture. Several standardized investigation techniques have, however, been developed, of which the geological and geotechnical engineer can make use for obtaining the relevant data for his basic calculations and design criteria. Roughly spoken four types of site investigation methods can be distinguished: - geophysical measurements from the soil surface; - penetration tests, such as cone penetration and standard penetration tests; - borings, including sampling and installation of observation wells; - specific measurements, such as plate loading tests and nuclear dencity measurements.

5.1.4

Construction materials

A large number of materials may be used in various forms in the construction of seawalls and dikes. These can be: sand, gravel, quarry rock, industrial waste-prod-ucts (slags, minestone, silex etc.), clay, timber, concrete, asphalt, geotextile, etc. Al1 these materials have to fulfill some structural and environmental specifications which mostly are regulated by the national standards. Because of the limited space of this short course these aspects are not treated here. The useful


VII. 22

i n f o r m a t i o n s c a n be found i n v a r i o u s handbooks and g u i d e l i n e s r e p o r t s (SPM, 1984, TAW, 1998, TAWICUR, 1984, 1990, C I R I A 1986, PIANC 1987a, etc.). 5.2 5.2.1

Shape and h e i g h t of a s e a w a l l l d i k e Loading z o n e s (CURITAW, 1984)

The d e g r e e of wave a t t a c k on a d i k e o r o t h e r d e f e n c e s t r u c t u r e d u r i n g a s t o r m s u r g e depends on t h e o r i e n t a t i o n i n r e l a t i o n t o t h e d i r e c t i o n of t h e s t o r m , t h e d u r a t i o n and s t r e n g t h of t h e wind, t h e e x t e n d of t h e w a t e r s u r f a c e f r o n t i n g t h e sea-wal1 and t h e bottom t o p o g r a p h y of t h e a r e a involved. For c o a s t a l a r e a s t h e r e is mostly a c e r t a i n c o r r e l a t i o n between t h e w a t e r l e v e l ( t i d e p l u s wind s e t - u p ) and t h e h e i g h t of t h e waves, because wind s e t - u p and waves a r e b o t h c a u s e d by wind. Theref o r e , t h e j o i n e d f r e q u e n c y d i c t r i b u t i o n of w a t e r l e v e l s and waves seems t o be t h e most a p p r o p r i a t e f o r t h e d e s i g n p u r p o s e s ( a l s o from t h e economical p o i n t of v i e w ) . F o r s e a - w a l l s i n t h e t i d a l r e g i o n , f r o n t i n g deep w a t e r , t h e f o l l o w i n g a p p r o x i m a t e z o n e s c a n be d i s t i n g u i s h e d ( F i g u r e 8 ) : I t h e zone permanently submerged ( n o t p r e s e n t i n t h e c a s e of a h i g h l e v e l "f o r e s h o r e " ) ; I1 t h e zone between MLW and MW; t h e e v e r - p r e s e n t wave-loading of low i n t e n s i t y is of importance f o r t h e l o n g - t e m b e h a v i o u r of s t r u c ture; I11 t h e zone between M W and t h e d e s i g n l e v e l , t h i s zone c a n be h e a v i l y a t t a c k e d by waves b u t t h e f r e q u e n c y of s u c h a t t a c k r e d u c e s a s one g o e s h i g h e r up t h e s l o p e ; I V t h e zone above d e s i g n l e v e l , where t h e r e s h o u l d o n l y be wave runUP A bank s l o p e revetment i n p r i n c i p l e f u n c t i o n s no d i f f e r e n t l y u n d e r normal c i r c u r n s t a n c e s t h a n under extreme c o n d i t i o n s . The a c c e n t i s , howe v e r , more on t h e p e r s i s t e n t c h a r a c t e r of t h e w a v e - a t t a c k r a t h e r t h a n on i t s s i z e . The q u a l i t y of t h e sea-ward s l o p e c a n , p r i o r t o t h e o c c u r r e n c e of t h e extreme s i t u a t i o n , a l r e a d y be damaged d u r i n g r e l a t i v e l y normal c o n d i t i o n s t o s u c h a d e g r e e t h a t i t s s t r e n g t h i s no l o n g e r s u f f i c i e n t t o p r o v i d e p r o t e c t i o n d u r i n g t h e extreme storm. The d i v i s i o n of t h e s l o p e i n t o l o a d i n g z o n e s h a s n o t o n l y d i r e c t conn e c t i o n w i t h t h e s a f e t y a g a i n s t f a i l u r e of t h e r e v e t m e n t and t h e d i k e a s a whole, b u t a l s o w i t h d i f f e r e n t a p p l i c a t i o n of m a t e r i a l s and execut i o n - and maintenance methods f o r e a c h zone ( F i g u r e 9 ) .

-

A l t e r n a t i v e s have t o be g e n e r a t e d d u r i n g t h e c o n c e p t u a l d e s i g n p h a s e , t h e p r e l i m i n a r y d e s i g n phase and t h e d e t a i l e d d e s i g n p h a s e i n o r d e r t o s e l e c t t h e most s u i t a b l e d e s i g n . I t is emphasized t h a t f o r e a c h d e s i g n p h a s e t h e s e a l t e r n a t i v e s s h o u l d be worked o u t a t a comparable l e v e l of d e t a i l . The Same a p p l i e s t o t h e c o n s t r u c t i o n a l t e r n a t i v e s which may have a g r e a t i n f l u e n c e on t h e t o t a l s t r u c t u r e c o s t s .

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K. W. P i l a r c z y k

V I I . 23

.

design water Level

-

normal'water

Figure 8 ( a )

Level

Loading zones on a dike

spilling spilling

Figure 8 ( b )

Breaker t y p e s on a s l o p e

7 q -.v-

ruvotmont ot concrolo blocks Q50 x a 5 O x 0 I 0

runlayor coniirtr of clay 0.80 thick a minortono 470 thick u w e r crurhoa rtono 0.10 thick

Figure 9

dimonsionr in m lovpn ruiataa t0 N A . P

Example of d i k e p r o t e c t i o n (Oesterdam)

DESIGN OF SEAWALLS AND D I a S INCL. OVERVIEW OF REVETMENTS

K.W. P i l a r c z y k

VII. 24

5.2.2

Wave-structure i n t e r a c t i o n

The i n t e r a c t i o n between waves and s l o p e s is dependent on t h e l o c a l wave h e i g h t and p e r i o d , t h e e x t e r n a l s t r u c t u r e geometry ( w a t e r d e p t h a t t h e t o e ) , s l o p e w i t h l w i t h o u t berm, t h e c r e s t e l e v a t i o n and t h e i n t e r n a l s t r u c t u r a l geometry ( t y p e s , s i z e and g r a d i n g of r e v e t m e n t s and second a r y l a y e r s ) . The t y p e of s t r u c t u r e wave i n t e r a c t i o n is d e f i n e d by ( s e e a l s o Figure $(b)]:

where : E = breaker index = i n c i d e n t wave h e i g h t s = wave l e n g t h a t deep w a t e r ( = 1.56 T 2 i n m e t r i c u n i t s ) T' = wave p e r i o d a = s l o p e a n g l e of t h e f r o n t f a c e

L

F o r l a r g e v a l u e s of t h e wave l e n g t h o r f o r l a r g e v a l u e s of a ( s t e e p s l o p e s ) , t h e wave behaves l i k e a long wave, which r e f l e c t s a g a i n s t t h e a s o c a l l e d s u r g i n g wave. F o r s h o r t e r s t r u c t u r e without breaking waves and medium s l o p e s waves w i l 1 s h o r t and b r e a k , c a u s i n g p l u n g i n g b r e a k e r s f o r E v a l u e s i n t h e r a n g e of 1+3. T h i s f i g u r e i s comrnon along t h e Dutch c o a s t s w i t h s l o p e a n g l e s of 1 t o 3+1 t o 5 , wave p e r i o d s 6+8 s and wave h e i g h t s of 3+5 m...For m i l d s l o p e s wave b r e a k i n g becomes a more c o n t i n u o u s p r o c e s s , r e s u l t i n g i n a more g r a d u a l d i s s i p a t i o n of wave e n e r g y . T h i s t y p e of b r e a k i n g is c a l l e d " s p i l l i n g w . F o r t h e d e s i g n of s t r u c t u r e s , s u r g i n g and p l u n g i n g b r e a k e r a r e of main irnportance. The a r e a which s u f f e r s from wave-loading i s bounded by t h e h i g h e r upr u s h and t h e l o w e s t downrush p o i n t . Obviously t h i s zone i s v a r y i n g w i t h t h e t i d e . The v a l u e of maximum up and downrush i s shown i n F i g u r e 1 3 , b o t h f o r impervious and p e r v i o u s s l o p e s . I f t h e u p r u s h e x c e e d s t h e c r e s t l e v e l , f i g u r e s a r e no l o n g e r a p p l i c a b l e .

-

No r e l i a b l e f o r m u l a a r e a v a i l a b l e t o p r e d i c t t h e maximum v e l o c i t i e s d u r i n g u p r u s h and downrush. F o r s u r g i n g and s p i l l i n g b r e a k e r , n u m e r i c a l s o l u t i o n s have been o b t a i n e d , which a r e , however, n o t y e t o p e r a t i o n a l . A s f i r s t a p p r o x i m a t i o n , t h e maximum v e l o c i t y , on a smooth s l o p e 'max can be computed by t h e f o l l o w i n g f o r m u l a :

-

where : Hs s i g n i f i c a n t wave h e i g h t g = gravity a = c o e f f i c i e n t e q u a l t o about 1 f o r i r r e g u l a r waves and b exponent e q u a l r o u g h l y t o 0 . 5

-

5. 2 . 3

Dike s h a p e

The g r a d i e n t of t h e bank may n o t be s o s t e e p t h a t t h e whole s l o p e o r t h e revetment c a n l o s e s t a b i l i t y ( t h r o u g h s l i d i n g ) . These c r i t e r i a g i v e , t h e r e f o r e , t h e maximum s l o p e a n g l e . More g e n t l y ( f l a t t e r ) s l o p e l e a d s t o a reduced wave-force on t h e r e v e t ment and l e s s wave run-up; wave e n e r g y i s d i s s i p a t e d o v e r a g r e a t e r l e n g t h . By u s i n g t h e wave run-up a p p r o a c h f o r c a l c u l a t i o n s of t h e c r e s t h e i g h t of a t r a p e z o i d a l p r o f i l e of a d i k e f o r d i f f e r e n t s l o p e g r a d i e n t s , t h e minimum volume of t h e embankment c a n be o b t a i n e d .


K.W. P i l a r c z v k

V I I . 25

However, t h i s d o e s n o t n e c e s s a r i l y imply t h a t minimum earth-volume c o i n c i d e s w i t h minimum c o s t s . An e x p e n s i v e p a r t of t h e embankment comp r i s e s t h e r e v e t m e n t of t h e w a t e r s i d e s l o p e and t h e s l o p e s u r f a c e ( a r e a ) i n c r e a s e s a s t h e s l o p e a n g l e d e c r e a s e s . The optimum c r o s s - s e c t i o n ( b a s e d on c o s t s ) c a n be d e t e r m i n e d when t h e c o s t s of e a r t h works p e r m' and t h o s e of r e v e t m e n t p e r m' a r e known. C a r e f u l a t t e n t i o n i s needed however, because t h e revetment c o s t s a r e n o t a l w a y s i n d e p e n d e n t of t h e s l o p e a n g l e , e . g . f o r s t e e p s l o p e s t h e heavy p r o t e c t i o n i s nec e s s a r y while f o r t h e mild s l o p e s t h e (cheaper) grass-mat can o f t e n provide a s u f f i c i e n t protection. Another p o i n t of econornic o p t i m a l i z a t i o n c a n be t h e a v a i l a b l e s p a c e f o r d i k e c o n s t r u c t i o n o r improvement. The common Dutch p r a c t i c e is t o a p p l y t h e s l o p e 1 on 3 on t h e i n n e r s l o p e and between 1 on 3 on 5 on t h e o u t e r ( s e a w a r d ) s l o p e . The minimum c r e s t w i d t h i s 2 m. The w a t e r - s i d e berm i s a common element i n t h e Dutch d i k e c o n s t r u c t i o n . I t c o u l d i n t h e p a s t l e a d t o a r e d u c t i o n i n t h e e x p e n d i t u r e on s t o n e r e v e t m e n t s (on a v e r y g e n t l y s l o p i n g berm a good g r a s s - m a t c a n be maint a i n e d ) and it produced a n a p p r e c i a b l e r e d u c t i o n i n wave run-up. P r e s e n t p r a c t i c e i n o r d e r t o o b t a i n a s u b s t a n t i a l r e d u c t i o n i n wave run-up, is t o p l a c e t h e o u t e r berm a t ( o r c l o s e t o ) w a t e r l e v e l of t h e d e s i g n s t o r m f l o o d . I f t h e berm l i e s t o o much below t h a t l e v e l , t h e h i g h e s t s t o r m f l o o d waves would n o t break b e n e a t h o r on t h e berm and t h e run-up w i l 1 be i n a d e q u a t e l y a f f e c t e d , and t h e g r a s s - m a t on t h e u p p e r s l o p e t o o h e a v i l y loaded by waves l e a d i n g t o p o s s i b l e e r o s i o n . F o r t h e s t rm f l o o d berms a t h i g h d e s i g n l e v e l s a s i n t h e N e t h e r l a n d s ( f r e q . 10 ) t h e r e a r e i n g e n e r a 1 no problems w i t h t h e growth of g r a s s on t h e berm and t h e u p p e r s l o p e . However, t h e r e c a n be c i r c u r n s t a n c e s which r e q u i r e a l s o t h e a p p l i c a t i o n of a h a r d r e v e t m e n t on t h e berm and even on a p a r t of t h e u p p e r s l o p e i . e . when h i g h e r f r e q u e n c y of w a t e r l e v e l i s a p p l i e d , l e a d i n g t o more f r e q u e n t overwashing of t h e u p p e r p a r t by s a l t w a t e r due t o t h e run-up o r wave-spray ( a common g r a s s - m a t c a n s u r v i v e o n l y a few s a l t y e v e n t s a y e a r ) .

-z

An i m p o r t a n t f u n c t i o n of t h e berm can be i t s u s e a s a n a c c e s s road f o r d i k e maintenance. I n g e n e r a 1 c a r e s h o u l d be t a k e n t o p r e v e n t e r o s i o n of t h e grass-mat a t t h e j u n c t i o n w i t h t h e r e v e t m e n t . The a b r u p t change i n r o u g h n e s s may l e a d t o i n c r e a s e of s u r f a c e t u r b u l e n c e and more l o c a l e r o s i o n . I t i s a d v i s a b l e t o c r e a t e a t r a n s i t i o n zone by a p p l y i n g t h e c e l l blocks, g e o g r i d s o r o t h e r systems allowing v e g e t a t i o n . 5.2.4 5.2.4.1

Dike h e i g h t ; wave run-up and o v e r t o p p i n g Genera1 c o n s i d e r a t i o n on t h e h e i g h t of a d i k e

The h e i g h t of a d i k e was f o r many c e n t u r i e s based on t h e h i g h e s t known f l o o d l e v e l t h a t c o u l d be remembered. I t is e v i d e n t t h a t i n t h i s way t h e r e a l r i s k of damage o r t h e p r o b a b i l i t y of f l o o d i n g were unknown. L i t t l e was known a b o u t t h e r e l a t i o n between t h e c o s t t o p r e v e n t f l o o d i n g and t h e c o s t of t h e damage t h a t might r e s u l t from f l o o d i n g . I n t h e 2 0 t h c e n t u r y it was found t h a t t h e o c c u r r e n c e of e x t r e m e l y h i g h w a t e r l e v e l s and wave h e i g h t s c o u l d be d e s c r i b e d a d e q u a t e l y i n t e r m s of f r e q u e n c y i n a c c o r d a n c e w i t h t h e laws of p r o b a b i l i t y c a l c u l u s . However t h e c u r v e s of extreme v a l u e s , based on a r e l a t i v e l y s h o r t p e r i o d of o b s e r v a t i o n s , m o s t l y have t o be e x t r a p o l a t e d i n t o r e g i o n s f a r beyond t h e f i e l d of o b s e r v a t i o n s w i t h t h e r i s k f o r some u n c e r t a i n t i e s . A f t e r t h e 1953 d i s a s t e r , t h e frequenycy of t h e r i s k of f l o o d i n g was s t u d i e s i n t h e N e t h e r l a n d s i n r e l a t i o n t o t h e econornic a s p e c t s .


V I I . 26

Finally it was decided to base the design of al1 sea dikes-gundamentally on a water level with a probability of exceedance of 10 per annurn. In the Netherlands the wind set-up is mostly incorporated in the estimated storm-surge level. If it is not a case, the wind set-up should be calculated separately and added to design water level (see Appendix I and 11). Besides the design flood level several other elements also play a role in determining the design crest level of a dike (Figure 10):

--- dike after construction itoe protection Figure 10 Determination of dike height

-

-

Wave run-up (2% of exceedance is applied in the Netherlands) depending on wave height and period, angle of approach, roughness and permeability of the slope, and profile shape, An extra margin to the dike height to take into account seiches (oscillations) and gust bumps (single waves resulting from a sudden violent rush of wind); this margin in the Netherlands varies (depends on location) from O to 3 m for the seiches and O to 0.5 m for the gust bumps, A change in chart datum (NAP) or a rise in the mean sea level in the Netherlands: til1 now assumed roughly 0.25 m per centuary), Settlernent of the subsoil and the dike-body during its lifetirne (Fi~ure11).

+,-

construction

,,stage

LOG time

-

i.e.30 years l

A

l

i.e. sond-fill

I fl

I

I

settlement

-'----

I I

-

.

prirnary settlement (execution stage)

1

I

secundary settlernent 1-

Figure 11

Settlements as function of time

The combination of al1 these factors mentioned above, defines the freeboard of the dike (called in Dutch as wake-height). The recommended minimum freeboard is 0.5 m.


VII. 27

5.2.4.2

Wave run-up

The effective run-up (R), on an inclined structure can be defined as

where

Rn yR

= run-up on smooth plane slopes, defined as the vertical height above still water level, and n = index of exceedanca percentage,

reduction factor due t o slope roughness and permeability, reduction factor due to berm reduction factor due to oblique wave attack, end = breaker index = = =

For random waves Rn can be expressed by

where

tana

T

P

tana

for ao/ks

s 1.57

1.57

where ks = bottom roughness. However, to avoid problems with definition of the representative roughness, the diagram as proposed by Kamphuis (1975), shown in Fig. 26, is mostly recomrnended for the practica1 use. As indication, the average bottom shear stress for a current with waves is 3 to 5 times greater than that without waves. Experiments have shown good agreement between critica1 shear stress in oscillatory and steady flow. Therefore, the modified hield's curve as shown in Figure 25 can still be used for determination the critica1 shear stress for initial motion in oscillatory flow and in combination of currents and waves. Additionally, there is a large number of formulas allowing direct calculation of conditions for initial motion. However, the discrepancy in the results is very high. As an example, two reasonable formulae can be mentioned: Komar and Miller (1974), probably as a lower limit of critical Uo;

PP" g 113 )

for (-

p,

v

u.

D 1 12.5

PS-PW g 113 for (--) D v "w

ps-pw,2/3

(7 >

"s-'w

(-

P

4/7

1

(7.16a)

= 1.05

(7.16b)

g

u.

12.5

0.24

=

213 D1/3 T1/3

417 D3/7 T1/7 g

Dingler (1975), probably as an upper limit of critica1 U

where D = media8 grain size of the sediment and ty (i.e. v = 10- m2/s at 20"~).

v =

o*

kinematic viscosi-

.

Transport When the actual values of the bed-shear stress exceed the critica1 values for initiation of motion the sediment transport will take place and deformation of the bed will occur. As the deformation is alco timedependent and nature is always unsteady, an equilibrium situation will be hardly found in practice. Because of the complexity of this phenomena, especially for the combination of currents and waves, this aspect


VII. 59

w i l l n o t be t r e a t e d h e r e . More d e t a i l e d i n f o t m a t i o n on c u r r e n t s and sediment t r a n s p o r t can be found i n s p e c i a l i s t i c t e x t b o o k s and manuals ( i . e . S l e a t h 1984, Horikawa 1989, Shore P r o t e c t i o n Manual 1984 and others).

.

I n f l u e n c e of bed s l o p e ( s l o p e f a c t o r ICs) For a p a r t i c l e on a s l o p e t h e v a l u e of ( c r i t i c a l ) s h e a r s t r e s s w i l l be reduced a s f o l l o w s (RWS, The Closure T i d a l B a s i n s , 1987):

-

For a bed s l o p e i n t h e f l o w d i r e c t i o n w i t h a n g l e $

-

For a s i d e s l o p e w i t h a n g l e a ( F i g . 2 7 ) ; f l o w p a r a l l e l t o t h e bank

i n which t ( o ) is t h e s h e a r s t r e s s f o r a h o r i z o n t a l bed a s a r e f e r e n c e , and 8 i s t h e a n g l e of r e p o s e ( i n t e r n a l f r i c t i o n ) of t h e m a t e r i a l cons i d e r e d ( F i g u r e 28).

-

For a combination of l o n g i t u d i n a l and s i d e s l o p e t h e r e d u c t i o n f a c t o r K ( a , $ ) becomes

I n above d e r i v a t i o n of s l o p e f a c t o r s is assumed t h a t t h e g r a i n s a r e l y ing £ r e e a n d , i n c a s e of motion, w i l l be t r a n s p o r t e d away. I n t h e c a s e of r e v e t m e n t s ( r i p r a p , b l o c k s e t c . ) t h e u n i t s a r e m o s t l y embedded between o t h e r u n i t s and supported by t h e t o e - c o n s t r u c t i o n . That means t h a t t h e u p l i f t i n g of t h e u n i t s p l a y s a more predominant r o l e t h a n i n t h e c a s e of s t a b i l i t y of sand, and t h a t t h e s e c r i t e r i a can o f t e n be t o o conservative. For f l o w ( i . e . o v e r f l o w ) and wave a t t a c k p e r p e n d i c u l a r t o t h e s l o p e r e v e t m e n t s , t h e s l o p e f a c t o r is d e f i n e d by ( s e e F i g u r e 27b):

-

f o r uprush c o n d i t i o n s Kup(a) = (cosa-f

sina)

-

cora (l-f

tana)

(7.19)

where f = f r i c t i o n f a c t o r (= t h e developed a n g l e of f r i c t i o n ) ; f o r r i p r a p f m t a n ü ( a s a f i r s t approximation f m 1 ) .

I t s h o u l d be n o t e d t h a t t h e s e s l o p e f a c t o r s a r e a l r e a d y i n c o r p o r a t e d i n t h e f o m u l a on wave a t t a c k , eq. 71 1 (K i n i m p l i c i t w á y and K in down e x p l i c i t way) UP

.

-

f o r downsurge c o n d i t i o n s ( i . e .

run-down,

overtopping, overflow):

For t h e downsurge, it is assumed t h a t t h e r e w i l l be w a t e r r u s h i n g o u t between t h e b l o c k s so t h a t ' f ' may be t a k e n z e r o . The s l o p e a n g l e I a 1 may r e f e r t o t h e o u t e r - s l o p e ( i . e . wave a t t a c k ) o r t o t h e i n n e r - s l o p ei (overtopping, overflow). Coarse s e d i m e n t s , s t a b i l i t y of rock and r o c k r e l a t e d u n i t s An a l t e r n a t i v e f o r t h e u s e of t h r e s h o l d v a l u e s f o r s t r e s s o r v e l o c i t y is t h e a p p l i c a t i o n of e m p i r i c a 1 formulae.

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K.W.

Pi larczvk

Pilarczyk (1989) presented the overview of some practica1 design formulae for rock and rock related units against current attack in various civil engineering applications. He has combined these formulae to one genera1 form. A genera1 formula for rock is:

(strength)

load)

in which: = thickness of protection unit [ml relative density of protection system [-l 4m = stability factor for current [-l KT = turbulence and/or shear stress adjustment factor [-l = depth (or velocity profile-) factor [-l siope factor (i.e. K ~ Kd, , K S Kdown 1 [-l = critica1 shear stress parame?&r [-l "'Cr u = mean velocity (depth averaged) [m/s 1 g = acceleration of gravity [m/s21

-

in -

The Same foxmula can be use for rock related units or systems as blocks, blockmats and gabions. The strength parameters A and Dn can be calculated with: m

.

. .

for rock: Dn

=

(MS0/ps)

or Dn

-- 0.85 DS0 and Am

for placed blocks and blockmats: Dn =

A

-

D

=

-A

(ps-pw)/pw

thickness of block and A

m

for mattresses (gabions, stone- and asphalt-mattresses, etc.): D = d ( = average thickness of mattress) and A = (l-n)A; the size oe 1 o D c a be calculated as for rock Eut with a higher the minimum thlckness of mattress is equal to d 1.8 Dn.

-

with: MS0 = 50% value of the mass distribution of the stones [kg], = density of rock [kg/m3], S ' density of water [kg/m3], APW a relative density of protection unit [-l, n = porosity of stones or sand [-] (approximately 0.4).

-

The values of equal to = 0.035 +Cr = 0.050 +Cr 0.070

-

+Cr

5 0.10

the critica1 shear stress parameter

+C r

can be assumed

for loosely rock, for £ree blocks, for blockmats, asphaltic mattresses, geotextile sandmattresses, gabions/stone mattresses for fill-rock in gabions/stone mattresses.

Some information on permissible velocities for bitumineus materials are als0 mentioned in section 7.2. The various correction factors K are estimated as follows: Kh = Z/(l~g-b?tf/k~)~ = 6k8/c2 for a logarithmic velocity profile, or Kh (h/Dn) for a not-fully gezeloped velocity profile Ks = Kd cosa (l-tan2 a/tan2 8 ) ' for banks and Ks Kdown = cosa i for downsurge on slopes; KT = 1.0 (normal turbulence, rivers); KT a 1.5 (very common case: non-uniform Plow with increased turbulence as below stilling basins, outer bends, r/B > 2, etc. E

-

-


VII. 61

C_---------_--_-----------Figure 27a

Aeduction factors for slope

l

-

(-1 u p r u s h

downrush f,, f 2 = f r i c t i o n (f,%. t a n e ) (+)

submerged w e i g h t : G

= k I(p,-p,)

g d

drag force : u p l i f t force :

Figure 27b

F C = k2pg0'H F, = k 3 p g ~ ' H H : : u2/g H = dup (cos a- f s i n c d

uprush

E

downrush

H %adown cosa

(f&)

AD

Schematization o f s t a b i l i t y o f blocks on a slope

8 40

30 *rOUnded OrOUnded + angular eangular

20

I

i

i

I

l l l i i

l Figure 28

i

l

l i l l i

10

Angle o f repose of sand and gravel

l

i

I

i t i l

l

L

I

100 ( a f t e r Simons and Albertson.


I I i I I .

1000 19601

VII. 62

KT KT

-

-

2.0 (high turbulence as in hydraulic jump, local disturbances, sharp outer bends, r/B 5 2) 3.0 (jet impact, screw race velocity)

-

with h water depth [m], r = center-line surface width at upstream end of bend, k crete blocks Dn = D = thickness of blockf units, i.e. rock), a = slope angle [O], 8

radius of bend, B = water(smooth units, i.e. conor fis = (2 t o 5)Dn ( m u g h = angle of internal friction = D

["l.

The following remarks can be maie. Firstly, the factor K h can be neglected (K = l ) if instead of u the bottom velocity u is substituted or when h/Bn5 5. Secondly, the mentioned factors Ks onPy holds for parallel flow to the bank (K ) and for perpendicular loading on slopes d (K ). For other situations a proper choice of K should be made. ~ i t a y ? ~the , value of K = 1.5 or 2 for (sharp) oufer bends should only be applied if due to diTficuities in defining the local mean velocity, the average mean velocity in the cross-section is applied. The following values of the stability factor 4c are recommended: = (1.00) + 1.50 for exposed edges and/or transitions (depending also 4c on flow direction), and = 0.75 can be treated = (0.50) + 0.75 for continuous protectionj 4c 4c a common reference value for rock. as The values of stability factor are strongly affected by composition of the system and execution. In general, the upper figures of $c are recommended as safe values. The lower +c-values refer to the systems with higher integrity, i.e. (grouted-) cabled blockmats, gabions/stone mattresses, grouted blocks etc., proper choice of permeabilities of the toplayer, and/or when a certain (limited) movement is allowed. For a continuous protection with blocks laying on properly equalized bed the 4 = 0.5 can be used while C 0.75, usually applied for rock, is also a proper 4 -value for bfocks on unequal bed. Stability of £ree placed blocks Ean be improved by washing in the interspaces between the blocks by a granular grout. The washing out of this material can be prevented when D (grout) > 0.3 d. In such case the stability can be similar to the cablgd blockmat. The blocks should be placed in a chess-pattern to limit the length of interspaces. For the exposed edge with rock on fascine mattress the $c = 1.0 can be used while for the rock direct on geotextile 4 = 1.50 is C recommended. Examples of exposed edges are: bed protection at scour holes (particularly in the case of two-directional current i.e. ebb and flood), edges of a toe protection, transitions between adjacent revetment systems, connections between mats or mattresses. When the edges can be adversely attacked, fot example, from the direction of the scour hole (i.e. during ebb) the more conservative (higher) I$ -values are recommended. Because of practical problems with edges an8 transitions, very often the whole protective system is designed based on stability criteria for edges. In other cases special measures should be taken to avoid overturning the mats at edges and transitions. Due to al1 these uncertainties it is difficult to give a more sharp indication on 4 -values. The best engineering judgement wil1 always be a C decisive factor in each particular case. For example, because of various practical reasons the minimum thickness of protective elements is defined as 0.08 m for blockmars and prefabricated open stone asphalt mats (insitu 0.10 m), 0.10 m for £ree blocks, 0.15 m for (basket-) stone mattresses and sand mattresses. Additional information can be found in Flexible Armoured Revetments (1984).

-

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW- OF REVETMENTS

K.W. Pilarczyk

VII. 63

Cohesive sediments The physical-chemica1 interaction between these particles plays a significant role. At present the approach to the determination of critica1 velocity still relies heavily on empirica1 data based on various experiment~and in-situ observations. The existing knowledge on the correlation of critical shear stress with mechanica1 properties of the soil (siltcontent), plasticity index, vane shear stress, etc.) is still not sufficient for presenting a genera1 approach. Cohesive materials such as clay generally have higher resistance to erosion than non-cohesive materials. As an indication the following values may be used, - fairly compacted clay (voids ratio = 0.50): Ucr = 0.80 m/s : U = 1.50 m/s - stiff clay (voids ratio = 0.25) Cr - grassed clay : Ucr = 2.00 m/s - grassed clay-banks adequately designed : up to 3.00 m/s and/or reinforced The information provided in this section gives a first approximation to the erosion resistance of various subsoils. On large projects it is recornmended to either check the approximation in a laboratory or construct a prototype for testing. Some additional informations can be found in Ven te Chow (1959), Sleath (1984) and in The Closure of Tidal Basins (1987). The Dutch guidelines on application of clay for dike construction and protection (incl. grassmats), (Technica1 Advisory Committee for Waterdefences, 1990) and CIRIA publications on grassed spillways (CIRIA 1976, 1987) can also be useful sources for solving some practica1 problems. 7.4 7.4.1

Filter constructions General

Stability of top layers strongly depends on the type and composition of sublayers and the structure must therefore be regarded as a whole. Instability (erosion) of sublayers and/or the subsoil can lead to failure of a toplayer. A good tuning of the permeability of the toplayer and sublayers (including geotextiles) is an essential condition for the design. The permeability of the different parts of the construction must increase from underneath to top. Examples óf filter structures are given in Figures 21 and 33. 7.4.2 Granular filters Granular filters are designed in successively coarser layers proceeding outward from the underlying finer soil. The first layer should hold the base material (i.e. subsoil), while each following layer has to be able to hold the underlying one ( = stability or piping criterion). The outer layer must also be stable under the prevailing open boundary conditions (hydraulic loading). The basic principles of the filter stability can be derived from the geometrical (idealized) conditions assuming uniform composition of the successive layers.


VII. 64

l0 densely-packed grains (spheres); D

= diameter W = weight o f stone

2O loosely packed grains

This last approach is applied in the Shore Protection Manual (SPM, 1984) for determination the sizes of the successive breakwater-layers (Figure 33), namely.

w1

W to 2 1O 15

=

(because of size range allowed)

(7.24a)

and for successive underlayers

The SPM-approach is rather conservative and only responsible for the rubble breakwaters with narrow size gradation (mostly 0.75 W < W mean < 1.25 W) under heavy wave attack. The optima1 filter especially related to protection of the wide-graded subsoil against various forms of erosion should satisfy more criteria than mentioned above. This criteria are known as the Mfilter-rules". 7.4.3

Filter rules

To define the filter requirements, sieve diameters D , DS0 and D are generally appl ied. These correspond to part icle diamh5ers where l!z, 50% and 85% of the material (by weight) is finer than the sieve size. For specific requirements, however, sieve diameters D 10' D60 and D90 are also used. The best sublayer is that which is designed (and executed) accordingly to the (geometrical-filter rules. A granular filter between subsoil and coverlayer has to meet the following requirements (related to the representative grain sizes of the basis/subsoil D and the filter D f: b criterion: - stability (piping) D15f/D85b < 4 to 5 - permeability >4to5 - segregation D15f ;:15b50b < 20 to 25 D50f - internal stability CU D /D < 10 - no migration > 20 - migration

-

Figure 29

Filter-spoil interface; definit ions

SOIL

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS U V

D;lrrc7vk

The s t a n d a r d d e s i g n method f o r g r a n u l a r f i l t e r s i s i l l u s t r a t e d i n F i g u r e 31. The r e c e n t i n v e s t i g a t i o n a t t h e DELFT HYDRAULICS h a s p r o v i d e d a new f i l t e r c r i t e r i o n which i n v o l v e s t h e p o r o s i t y of t h e f i l t e r m a t e r i a l . The maximum a l l o w a b l e g r a i n s i z e of t h e f i l t e r , is de£ ined 15 n

where n = p o r o s i t y of f i l t e r m a t e r i a l . T h i s c r i t e r i o n h a s been v e r i f i e d f o r 0.1 < D < 0.5 mm, 0 . 2 < n < 0 . 4 and c t n a 2 !I5 O he g r a p h i c a l r e p r e s e n t a t i o n of t h i s c r i t e r i o n is g i v e n i n F i g u r e 30. F i g u r e 30

o1

I

1

0.2 grain size D,, 0.1

l

I

0.3

0.4

I

0.5

(mm1

D e t e r m i n a t i o n max. a l l o w a b l e g r a i n - s i z e

More s o p h i s t i c a t e d a p p r o a c h e s r e q u i r e i n f o r m a t i o n a b o u t t h e g r a d i e n t s i n t h e s u b s o i l ( B e z u i j e n e t a l , 1987). The t h i c k n e s s of a g r a n u l a r f i l t e r l a y e r s h o u l d be a t l e a s t (minimum) e q u a l t o 0 . 1 m f o r s a n d , 0 . 2 m f o r g r a v e l and 2 t o 3 t i m e s DSOf f o r coarser materials. G r a n u l a r f i l t e r s a r e e x p e n s i v e and d i f f i c u l t t o r e a l i z e ( e s p e c i a l l y u n d e r w a t e r ) w i t h i n t h e r e q u i r e d l i m i t s . A s u b s t i t u t i o n a l s o l u t i o n is a g e o t e x t i l e ( w i t h f i l t e r f u n c t i o n ) w i t h e v e n t u a l l y a l a y e r of g r a d e d s t o n e ( w i t h f u n c t i o n t o damp t h e i n t e r n a l h y d r a u l i c l o a d s ) . A good and o f t e n c h e a p s o l u t i o n c a n a l s o be r e a l i z e d by a p p l y i n g a t h i c k l a y e r of b r o a d l y graded n a t u r a 1 o r w a s t e p r o d u c t s a s m i n e s t o n e , s l a g s , s i l e x , e t c . ( r a n g e 0.5 m t h i c k n e s s f o r h i g h h y d r a u l i c l o a d s , compacted and c o m p o s i t i o n c o n t r o l l e d a c c o r d i n g t o c r i t e r i a on i n t e r n a l s t a b i l i t y .

7.4.4

Geotextile f i l t e r

The g e o t e x t i l e f i l t e r must be permeable enough t o a l l o w £ r e e f l o w of w a t e r w i t h o u t i n d u c i n g u p l i f t p r e s s u r e s and must have a n opening s i z e smal1 enough t o p r e s e n t s o i l p a r t i c l e m i g r a t i o n . The r e t e n t i o n c r i t e r i on and t h e p e r m e a b i l i t y c r i t e r i o n needed f o r t h e s e l e c t i o n of t h e p r o p e r g e o t e x t i l e depend on t h e g r a i n - s i z e d i s t r i b u t i o n of t h e s o i l and t h e t y p e of f l o w ( h y d r a u l i c g r a d i e n t s ) . Because of t h e c o m p l e x i t y of t h i s approach o n l y some g e n e r a 1 i n d i c a t i o n s w i l l be mentioned below. The g e n e r a 1 r e t e n t i o n c r i t e r i o n c a n be d e f i n e d a s :

-

e f f e c t i v e opening s i z e of f a b r i c which c o r r e s p o n d s t o t h e where OgO a v e r a g e sand d i a m e t e r of a s a n d f r a c t i o n 90% of which rernains on t h e f a b r i c a f t e r a s i e v i n g t i m e of 5 m i n u t e s , and Dgob = c h a r a c t e r i s t i c s i z e of s u b s o i l which c o r r e s p o n d s t o t h e s i e v e s l z e t h r o u g h which 90% of t h e t o t a l sand mass w i l l p a s s .

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REVETMENTS K . W. P i l a r c z y k

V I I . 66

O.CC5

0.02

0.2

0.C5

SIEVE. S i Z E

0.5

Z

6

in3 l-

F i g w e 31

Standard design method f o r g r a n u l a r f i l t e r s

Figure 32

Grading envelope f o r rip-rap

SEA SIDE

-1.5H W l i m l o W ~ - e ,

---.-

-liLi--.

Figure 33

..

.

.-

-

- . 3

- 7

.

E

Application of filter rules in breakwater design


K. W. Pilarczyk

VII. 67

Note: There are various methods of determining the size of geotextile openings. Details of the test varies from country to country, the chief differences being that some use dry sieving and others wet sieving, with either one-directional flow or alternate flow. The international standardization on this subject is being expected in the coming years. Experience in Holland has proved that for bank protection with limited wave attack (H < 1 m) the criterion mentioned above can be extended S to:

Because of various uncertainties involved (i.e. clogging, blocking etc.), the required permeability of the fabrics (k) has to be much higher than the permeability of soils. Some indicative criteria are given below: k fabric > (2 to 20) k soil - for uniform soil, and k fabric > (5 to 50) k soil - for wel1 graded soil The lower values refer to more idealistic cases while the higher values to the more practica1 cases. Special attention should be paid to the execution of filters (especially under water). In the case of geotextiles the damage due to placing of cover layers should be avoided. For this reason the fall-height during dumping of stones normally should be restricted to 0.5 m. Higher fall-heights are only acceptable when supported -by special tests for specific geotextiles with max. equal to 2 m. An alternative is to cover the geotextile with a protective layer of gravel. More detailed information and specifications for various applications can be found in the handbook on geotextiles and geomembranes (Veldhuijzen van Zanten, 1986) and in PIANC (1987a). 7.4.5 Materials demands ..

Materials and subsoil characteristics must-fulfill specific demands to obtain adequate coastal and river training works. With respect to the subsoil information is need about: - grading of the material - potential cohesion - permeability Loose stone should fulfill grading criteria. In Figure 32 these criteria are summarized (PIANC, 1987a). Also hardness, density, shape and resistance to abrasion must be known. In the case of concrete elements (blocks, blockmattresses) a minimum density of 2300 kg/m3 must be available. For geotextile demands are specified in (PIANC, 1987a and Veldhuijzen van Zanten, 1986). They are related to WL-resistance, tensile strength, permeability, sand tightness, etc. Depending on the construction method (the placement in dry conditions or sinking) geotextiles applied in mattresses or blockmats have to fulfill special requirements with respect to their strength.


VII. 68

7.5

Investigation on grass-slopes

Some of the existing dikes along the Wadden Sea (northern part of the ~ e t h e r l a n d s )still need reinforcement as these do not yet meet the specific safety requirements. One of the options for reinforcement is a slope protection of grass on a bed of clay, rather than stone, concrete or asphaltic protection. This option is feasible because vast mud-flats (high foreshore) and grasslands stretch away on the seaside of the existing dikes and are inundated only during storm surges. Moreover, the wave action in the Wadden Sea is much reduced by a row of barrier islandc. Due t o these factors the design wave height does not exceed 2 m. The Delft Hydraulic Laboratory was commissioned t o access the stability of such a grass dike by means of a full scale model study which was an absolute requirement as grass cannot be scaled down. The investigations have been performed. In the Delta Flume, a five metre wide section of the grass dike was reproduced on full scale. The model consisted of a sand core covered with a clay layer on a slope 1 on 8. Sods of grass with the depth of the roots of approximately 40 cm were laid on top of the clay layer (the grass was taken from an existing dike that was reinforced ten years ago). During the tests, the wave heights and periods and water levels (tidal cycles) were varied continuously according to predetermined boundary conditions during the design storm surge. The maximum H s was equal to 1.85 m with T = 5.6 sec. (plunging breaker falling on a water cushion). The meagured maximum velocity on the slope (1: 8) was about 2 m/s. After 30 hours of continuous random wave attack the condition of the grass dike was still exceptional well. The surface erosion speed of clay protected by grass was not more than 1 mm per hour. In a number of additional tests, the durability of the grass and the enlargement of holes, previously dug in the grass, were studied. Although wave action considerably enlarged some of these holes, the residual strength of the dike was such that its collapse was for from imminente (Delft Hydraulics Laboratory, 1984). The second investigation was carried out in a large (site) flume on slope l on 4. Special equipment was used to simulate the run-up and run-down velocities on this slope. Two qualitatively different grassmats on clay were used. The grass-mats were tested with the average velocity of 2 m/s (average over 40 hours of test) and the thickness of a water layer of about 0.6 m. The maximum velocity was about 4 m/s. Erosion speed of the clay surface was 1 to 2 mm per hour up t o 20 hours depending on quality of grass-mat. After 20 hours of loading the erosion speed started to grow 'much progressively for a bad quality grass-mat. similar process took place for a good quality grass-mat but after 40 hours of loading. The detailed information on the results and grass-mat specification can be found in (Delft Hydraulics Laboratory, 1984b). Some additional information on resistance of unprotected clay-surface (slope 1 on 3.5) were obtained during the investigation carried out for the Eastern Scheldt dikes (DHL, 1985). Also in this case two qualitatively different clays were used (fat and lean clay). The surgingbreaker conditions were applied to eliminate the effect of wave impact (Hs = 1.05 m, T 12 s, max. velocity 3 m/s). The erosion on ehe upper part of slope was for both clay-types the same and equal-to about 2 3 cm after about 5 hours of loading. After the same time, the erosion below S.W.L. was about 7 cm for a good clay, while for a lean clay a cavity of about 0.4 m depth was created. This latest probably because of the local non-homogeneity of clay. Also during this investigation a nurnber of additional tests on the erosion

-

-


VII. 69

of different sublayers (incl. clay) at locally damaged top layers (some protective units were removed) were performed. Al1 the tests mentioned above indicated that the strength of the grass slopes is strongly affected by the quality of clay and the condition of grass and its rooting. The genera1 design rules cannot be defined yet. However, these informations can be of a great value for the designing of grass dikes at the present time. Some additional information on this subject can be found in (CIRIA, 1976, 1987) and TAW (1990). 8.

SEMI-PROBABILISTIC -CALCUIATION--S

S

The deterministic approach is the most traditional design method. The designer selects values of load parameters that are assumed to be adequately high and thus safe. The choice of load and strength parameters is often subjective, based on traditional practice or the designer's personal experience. The design method is based on the assumption that the structure will not fail if the loads are less than the strength, provided a good (and verified) theoretica1 model is available. A factor of safety is used to cover uncertainties. The probabilistic method is a systematic approach using statistica1 techniques. For constructional design the use of probabilistic calculations is preferred. A probabilistic procedure for revetments is currently under development, (see PIANC, W.G.3, 1987b). The reliability function Z may be defined as

-

-Z R (x )----s-(x 7 -(8.1) i i ' where R = resistance function, S = load function and Xi= basic variables. The limit state of the considered component occurs at z = 0; the failure state is related to z < 0. There are three internationally agreed levels on which the limit state equations may be solved: ----- --------------approach: ----- present construction design Level I : quasi-probabilistic methods with relevant partial safety factors. semi-probabilistic approach: ----- approximation methods are apLevel I1 : ----plied in which normal probability distributions are assumed for both strength and loading: 1. First order mean value approach. 2. First order design-point approach. 3. Approximate full-distribution approach. Level 111: Full-distribution approach; this method accounts for the exact joint probability distribution functions including the correlations among the parameters. It usually requires a considerable computational effort.

It goes beyond the scope of this course to deal with al1 the methods in detail, but the mean value approach will be discussed because of its simplicity and its illustrative value for studying the effect of the value of various strength and load parameters involved. In this method the reliability function Z is linearized about the expected mean value of the parameters involved. Mutually independent normally distributed variables are assumed. The mean value p and standard deviation a can z z be evaluated as:

and


VII. 70

Z

i-l

1

The the distance reliability between indexZ0 ==OpZ/aZ and the is

z

mean value, measured in standard deviation units and is as such a measure of the probability that Z ,!& p! I wil1 be less then zero. z Assuming the normal distribution O Clz for Z: JN(z-p/o), then for Z = Fig. 34 Reliability index O : +N ( - 6 ) . Now the probability of fallure can be read from tabulated normal distribution. Thus the probability of failure is now: P (Z < 0) = This method is less accurate then a more detailed elaboration and better approximations of the reliability function but, it is illustrative for becoming aware of the most important parameters. It can be illustrated by the next example. We use ~i1arcz~k.sstability formula for block revetments (also valid for rip-rap on relatively impermeable sublayer):

where; bs==(;ignificant wave height, A = relative density of block- pw)/pw, a = angle of slope, T = wave period (L = wave units, length), D = Shickness of block, 5 = breaker index and 4 = stab?lity factor. The limit state function is:

Z

=

R

-

4.A.D

S

-cosa

The derivative of Z according to each variable:

aZ

I

-

aHs

3

8 cosa

aZ a ( ctga) aZ

-

aT

I

-

-1

K 1

S

H . 2 cosa

2 ctga

+ cosa

s ina

-I\

cos2a (l+ctgza)

-1 4T

I

S

The following steps are taken to calculate the mean value of Z ( p ) and the standard deviation a as a result of the weighed partial stan2ard deviation of each stochagtic parameter. The assumed values of input variables are as follows:


VII. 71

2.0 m 1.4 3 (cosa=0.95) 5 s 5

a(x. 1 0.23 m or (0.10 m) 0.05 0.25 1 s or (0.5 s) 0.50

0.45 m

0.01 m

variable

3 ctga

T

4 ................................... D (assumed)

N.B. The deterministic calculation provides in this case D = 0.36 m with (per definition) 50% probability of failure (in this case the mean value for Z is O: p = O, so B = yz/oz= O). When the uncertainties are taken into account, e. g. H = 2 + 0.25 = regarding the H an8 2.25 m and = S - 0.5 = 4.5, the deterministic calcufation provides D = 0.45 m.

+

+

Assuming, as a first approximation, D = 0.45, the probability of failure wil1 be calculated in the following way:

and the probability of failure is about 0.13 (13%). From this table it appears where is the most relevant impact on the value of u thus als0 on B and consequently on the probability of failure. gives the relative contribution of different variables (Xi) 1 on a . ~ u r t g e rresearch for lowering the probability of failure may then focussed on the characteristics of these parameters. In this case the variability (or uncertainty) about the actual wave conditions (wave height and wave period) is most important (assuming that the accuracy of the formula, thus, can not be improved). Of course if one takes a larger thickness of block, a more safe situation and thus, a lower probability of failure can be expected. Assuming that in this case the prediction of the actual wave conditions can be improved namely. aHs = 0.1 m and UT = 0.5 sec, the repeating of this calculations provides B = 1.495 and the probability of failure equal to about 7%. The lay down of the criterion of acceptable probability of failure is mostly left to the responsible authorities. However, the best way is to calculate the probability of failure for various design alternatives in combination with some economical studies regarding the execution and maintenance costs, and economical consequences of failure. Such an


VII. 72

approach can easily be used for decisional analysis, where the costs of each decision and its consequences are weighed by the probability of these events. The genera1 outlines of the probabilistic approach are shown in Fig. 35. One may relate this to the design of slope protection by loosely materials (i.e. riprap). First of al1 one should be able to "predict" the frequencies of occurrence of hydraulic loads during the lifetime of construction (Fig. 35a). Secondly, the response function should be obtained from hydraulic model tests or by apprying known 'transport" relationships (Fig. 35b). The resultant damage (S ) during the lifetime of the construction is obtained as s h o w in Big. 35c.

where: T = lifetime of the construction, f = frequency of occurrence of a given load intensity, s = damage per unit time and p = intensity of load. If more than one type of loading-ts acting, the summation of the damage should be computed by integration over the various load combinations and their probabilities. The resulting total damage is a measure of the expected maintenance of the slope protection works for a given size of the top layer and revetment composition.

t 9!.E:- 3

(a) t m u e n c y ol laiding

5y '= 2 r0 tas 0 sa 52 u uS -i g 2 -s ~ C

-

T

intrnsity of loading (P)

a

a

.-E H

e

,

n0 dornage

t

3

\

(b) response tunctton

m

g

E,

u

intensity of loading (P)

P

damage S

Zs.f.1 I

intensity of loading (P)

Figure 35

Outlines of a probabilistic design approach


VII. 73

A process of economical optimalization, based on the costs of construction and maintenance, can further be carried out, leading to the selection of the optimum size of the revetment. Besides this minimum integral cost criterion, one should also restrict the "expected total damI1 age The maximum acceptable damage depends on: - the relationship between "expected total damage", which is an average over the total protection length and the "maximum possible damageM that may take place a t a certain. location, - the type of construction and the vulnerability of the subsoil, - the risk of progressive damage if repair in time is impossible for technical, organisational or financial reasons. The actual state of he knowledge allows to apply this approach only for slope protection by loosely materials where the adequate transfer-functions (transport formulae) have been developed in the recent years. However in general, there is still a lack of data and insight in many of the above aspects. Therefore the research programmes in the Netherlands for the coming years are being systematically directed towards economically justified design criteria for different protective structures and different applications.

.

9. 9.1

STRUCTURE RELATED DEMANDS Slope protection

Banks, breakwaters, and groynes may often be attacked by waves and currents with an oblique direction to the structure. In the case of a groyne this situation will even be a n o m a 1 circumstance, directly following from its function. As a consequence, the presented design formulae for current and wave attack for the various revetment types requires a safer approach. This can be obtained by multiplying the resulting diameter D (or thickness d) with a factor 1.3. At the top zone of the upper sectfon of some coastal and river training works, revetment types allowing vegetation may be considered. In the case of approach embankments and dikes the complete section may even be protected with this type of protection. Revetments consisting of cellular concrete blocks with holes running entirely through the blocks allow the growth the vegetation, provided that granular material in the holes does not erode completely. Relevant design criteria on this subject are very scarce. Scour depends on characteristics of filling material, flow velocity or wave height and the ratio of opening size of holes and block thickness. Preliminary results of physical model tests carried out at DELFT HYDRAULICS with respect to this subject indicated that if the block thickness is 1.5 times the opening size of the holes, sufficient material in the holes will remain. 9.2

Optimization of slope stability

The wave forces on a plane (continuous) slope are distributed rather unequally (the high wave-impact area near the water level, the intermediate uprush area and the low-attacked area beneath the point of breaking). The wave action on relatively fine materials indicates that the nature try to distributed the forces equally providing equilibrium S-slopes (see Chapter VI, dynamic stability). The same principle can be applle-d-1~he designing of the shape of seawalls and dikes leading to application of smaller protective units than in the case of plane slope.


K.W. Pilarczyk

VII. 74

F o r p r a c t i c a 1 r e a s o n s t h e ' o p t i o n a l ' s h a p e w i l l be s c h e m a t i z e d t o t r a p e z o i d a l p r o f i l e ( i . e . s e a w a l l w i t h a berm). By s e l e c t i o n a p r o p e r p o s i t i o n o f a berm below t h e d e s i g n w a t e r l e v e l and a p r o p e r w i d t h of a berm, t h e wave f o r c e s w i l l be d i s t r i b u t e d i n s u c h u n i f o r m way t h a t t h e same m a t e r i a l c a n be u s e d a l o n g t h e whole p r o f i l e . Some r e s u l t s on p r o f i l e o p t i m i z a t i o n f o r r i p r a p a r e shown i n F i g u r e 3 6 . I n t h i s c a s e t h e i n c r e a s e of s t a b i l i t y ( 5 0 % o r more) c a n b e r e a l i z e d by a berm, w i t h a w i d t h e q u a l t o 0.15 t i m e s wave l e n g t h , s i t u a t e d ( 0 . 5 + 1 . 0 ) t i m e s wave h e i g h t below d e s i g n w a t e r l e v e l . Based on r e s u l t s of v a r i o u s s t u d i e s , t h e i n d i c a t i v e d e s i g n g u i d e l i n e s have been p r e p a r e d f o r r i p r a p bermed s l o p e s and t o e - p r o t e c t i o n , a s shown i n t h e box 9.1. F o r d e e p w a t e r wave c o n d i t i o n s t h e r e f e r e n c e o f bermed ( s t e p p e d ) s l o p e s i s t h e s t a b i l i t y of a s t r a i g h t s l o p e , d e s c r i b e d i n s e c t i o n 7 . The i n c r e a s e i n s t a b i l i t y , c a l l e d f a c t o r $., w i l l b e 1 . 0 i f t h e bermed s l o p e h a s t h e same s t r a i g h t s l o p e (berm w i d t h , B = O). The r e q u i r e d s t o n e s i z e f o r s p e c i f i c p a r t of bermed p r o f i l e is e q u a l t o :

(bermed p a r t )

(straight slope)

F o r s h a l l o w - w a t e r wave c o n d i t i o n s (wave-height d e p t h l i m i t e d , H 0.5 h ) , t h e berm and down s l o p e c a n be c a l c u l a t e d a s g i v e n u n d e r p.So i n t h e box 9 . 1 . I n t h i s c a s e t h e d e p t h on a berm ( d B ) i s r e l a t e d t o t h e d e p t h ( h ) i n f r o n t of t h e s t r u c t u r e . I n t h e c a s e of h i g h e r p o s i t i o n of t h e berm ( d g / h 5 0 . 4 ) t h e down s l o p e w i l l be d e c i s i v e f o r s t a b i l i t y of t h e berm a s a whole. I t i s o b v i o u s t h a t t h e g u i d e l i n e s p r e s e n t e d i n t h e box 9 . 1 c a n a l s o be used, a s a f i r s t approximation, f o r p r o t e c t i o n of t o e p r o t e c t i o n . When a p p l y i n g t h i s d e s i g n - c o n c e p t , t h e s t a b i l i t y of t h e c h o s e n p r o t e c t i v e m a t e r i a l s h o u l d a l s o be checked f o r c o n d i t i o n s l o w e r t h a n d e s i g n o n e s . I n some c a s e s t h e w a t e r - l e v e l may d e c r e a s e q u i c k e r ( i . e . d u e t o t i d a l p e r f o r m a n c e ) t h a n t h e waves, p r o v i d i n g h i g h e r wave a t t a c k on l o w e r p a r t s of p r o f i l e . The model i n v e s t i g a t i o n may p r o v i d e t h e b e s t solution i n a p a r t i c u l a r case. 9.3

S c o u r p r o t e c t i o n (SPM 1984)

Toe p r o t e c t i o n i s s u p p l e m e n t a l a r m o u r i n g o f t h e beach o r bottom s u r f a c e i n f r o n t of a s t r u c t u r e which p r e v e n t s waves f r o m s c o u r i n g and underc u t t i n g i t . F a c t o r s t h a t a f f e c t t h e s e v e r i t y of t o e s c o u r i n c l u d e wave b r e a k i n g (when n e a r t h e t o e ) , wave run-up and backwash, wave r e f l e c t i o n , and g r a i n s i z e d i s t r i b u t i o n of t h e b e a c h o r b o t t o m m a t e r i a l s . Toe s t a b i l i t y is e s s e n t i a l b e c a u s e f a i l u r e of t h e t o e w i l l g e n e r a l l y l e a d t o f a i l u r e t h r o u g h o u t t h e e n t i r e s t r u c t u r e . Toe s c o u r i s a complex p r o c e s s . S p e c i f i c ( g e n e r a l l y v a l i d ) g u i d a n c e f o r s c o u r p r e d i c t i o n and t o e d e s i g n based on e i t h e r p r o t o t y p e o r model r e s u l t s h a v e n o t been d e v e l o p e d y e t , b u t some g e n e r a l ( i n d i c a t i v e ) g u i d e l i n e s f o r d e s i g n i n g t o e p r o t e c t i o n a r e g i v e n below. The maximum s c o u r f o r c e o c c u r s where wave downrush on t h e s t r u c t u r e f a c e e x t e n d s - t o t h e t o e a n d / o r t h e wave is b r e a k i n g n e a r t h e t o e ( i . e . s h a l l o w w a t e r s t r u c t u r e ) . These c o n d i t i o n s may t a k e p l a c e when t h e w a t e r d e p t h a t t h e t o e i s l e s s t h a n t w i c e t h e h e i g h t o f t h e maximum


V I I . 75

L

1,

-

A HS

h Definitions

V

Figure 36

Principle of optimization (examples) (Dns0 = const. over the whole structure)


VII. 76

expected unbroken wave that can exist in that water depth. The width of the apron for shallow water structures with a high reflection coefficient, which is generally true for slopes steeper than about 1 on 3, can be planned from the structure slope and the expected scour depth. The maximum depth of a scour trough due to wave action below the natural bed is about equal to the maximum expected unbroken wave at the site. To protect the stability of the face, the toe soil must be kept in place beneath a surface defined by an extension of the face surface int0 the bottom to the maximum depth of scour. This can be accomplished by burying the toe, where construction conditions permit, thereby extending the face int0 an excavated trench the depth of the expected scour. Where an apron must be placed on the existing bottom or only can be par.tially buried, its width should not be less than twice the wave height. Based on guidance in Engineer Manual (1110-2-1614 and Shore Protection Manual (both published by U.S. Corps of Engineers) the possible toe configurations are illustrated as shown in Figure 37. If the reflection coefficient is lower than the limit (slopes milder than 1 on 3), andlor the water depth is higher than twice the wave height much of the wave force will be dissipated on the structure face and a less apron width may be adequate, but at least equal to the wave height (minirmun requirement). Since scour aprons generally are placed on very flat slopes, quarrystone of the size (diameter) equal to 112 or even 113 of theprirnary cover layer probably will be the heaviest required unless the apron is exposed above the water surface during wave action. Quarrystone of primary cover layer size may be extended over the toe apron if the stone will be exposed in the troughs of waves, especially breaking waves. The minimum thickness of cover-layer over the toe apron should be two quarrystones. Quarrystones is the most favourable material for toe protection because of its flexibility. If geotextile is used as a secondary layer it should not be extended over the whole width of the apron to provide the flexible edges (at least 1 m) against undermining or it should be folded back, and then buried in cover stone and sand to forrn a Dutch toe. The size of toe protection against waves can also be roughly estimated by using the cornmon forrnulae on slope protection and introducing mild slopes (i.e. 1 on 8 to 1 on 10) and local wave height. The results in Figure 36 and in box 9.1 can also be used for this purpose. Sorne alternative designs of toe protection are shown in Box 9.2. Hales & Houston (1983) considered the stability of a rock blanket extending seaward from a pemeable rubble slope on a 1:25 slope foreshore. They tested with regular waves to deterrnine the conditions at which the rock in the scour blanket was just stable. To these conditions they fitted a mean trend given by:

(coefficient 17.5 represents a more conservative line)


VII. 77

d

down s l o p e

berdbermed s l o p e

l

0.15

'berm

21.1

+

I n d i c a t i o n o f optimal value f o p whole p r o f i l e at

dE3 "s

-

1 and

(-6 H

B ~ L0

-

B dg 5 L0

L0

= const.)

d

-

s t r a i g h t slope slope

B For -

down slope

-

"s

( ) Mn

slope

=

f o r berm and down s l o p e

8.6

(2)

1/4

s h o r t waves

5 L 0:

B.S.

0.015

l o n g waves H 3 0.015

0 . 1 5

'-0

(2'

Hs/h

~ 0 . 5

(Hs Z 0.5h = d e p t h l i m i t e d )

f o r 0.4

Wa:-M.L S N B A G CAMAGE MACE GO00 6Y HANO

IN EETWEEN E X H WAVE ANO FULLY RE-AlJGNED €ACH LOW rio€. ALL BAGS BURLAP GR HESSIAN @R MORE I N I m L O C K THAN RASTIC

P L A N Cf SANOBAG LAYING PATTEFIN

FfGURE 3:- GOLD COAST TEMPORARY TOP ARMOUR FOR REVETMENT WALLS.

1.5-LTONNE ROCK FACE ARWOUR

L 2ûû-L00 *g

ROCK

GOLD COAST STANDARD

(Al

0

1

2

3

L

I

1

1

5 1

WETRES

TQPSOIL ANO CRASS THIS ZONE

4

/

200- ~ 0 0 k gS K O N W

ARM^

(B) RECOMMENDED REVISIONS Figure 40

~ o u l d e rwal1 design (Gold Coast Australia (Smith & Chapman, 1982) ( H = 3.5 m, T = 8 to 18 sec, slope 1 on 1.5) S


VII. 85

j o i n t s o n t o t h e sarne r n a t e r i a l and o n t o o t h e r r e v e t m e n t m a t e r i a l s , and t r a n s i t i o n s onto o t h e r s t r u c t u r e s o r revetment p a r t s . A genera1 d e s i g n g u i d e l i n e is t h a t t r a n s i t i o n s s h o u l d be avoided a s much a s p o s s i b l e . I f t h e y a r e i n e v i t a b l e t h e d i s c o n t i n u i t i e s i n t r o d u c e d s h o u l d be minimized. T h i s h o l d s f o r d i f f e r e n c e s i n e l a s t i c and p l a s t i c b e h a v i o u r and i n t h e p e r m e a b i l i t y o r t h e s a n d t i g h t n e s s . P r o p e r e x e c u t i o n is e s s e n t i a l i n o r d e r t o o b t a i n s a t i s f a c t o r y j o i n t s and t r a n s i t i o n s . When t h e s e g u i d e l i n e s a r e n o t f o l l o w e d t h e j o i n t s o r t r a n s i t i o n s may i n f l u e n c e l o a d s i n t e r m s of f o r c e s due t o d i f f e r e n c e s i n s t i f f n e s s o r s e t t l e m e n t , m i g r a t i o n of s u b s o i l from one p a r t t o a n o t h e r ( e r o s i o n ) , o r s t r o n g p r e s s u r e g r a d i e n t s due t o a c o n c e n t r a t e d ground w a t e r f l o w . I t i s d i f f i c u l t t o f o r m u l a t e more d e t a i l e d p r i n c i p l e s a n d l o r s o l u t i o n s f o r j o i n t s and t r a n s i t i o n s . The b e s t way is t o combine t h e l e s s o n s from p r a c t i c e w i t h some p h y s i c a l u n d e r s t a n d i n g of s y s t e m s i n v o l v e d . Examples t o i l l u s t r a t e t h e problem of t r a n s i t i o n s a r e g i v e n i n F i g u r e 41. A s a g e n e r a 1 p r i n c i p l e t h e t r a n s i t i o n s h o u l d be of a s t r e n g t h e q u a l o r g r e a t e r t h a n t h e a d j o i n i n g s y s t e m s . Very o f t e n it n e e d s a r e i n f o r c e m e n t i n one of t h e f o l l o w i n g ways, i n c r e a s e t h e t h i c k n e s s of t h e c o v e r l a y e r a t t h e t r a n s i t i o n g r o u t r i p r a p o r b l o c k c o v e r l a y e r s w i t h bitumen u s e c o n c r e t e e d g e - s t r i p s o r boards t o p r e v e n t damage p r o g r e s s i n g along t h e s t r u c t u r e .

BASALT (tmdttionalhid Outch solution)

concrete blocks

PLACED BLOCKS 4

I

Fig. A

Penetration of sand into the mine-waste itone geotextile between sand dnd minestone/gravel is neressary ,

-

5-1Ocm gravel 5 2Omm (broken stanel

concrete

p s p a c e d piles ond wooden plank (baard1

blocks with penetration

Fig. 8

concrete

y

Transition from basalt columns to concrete blocks separation board t00 short .

.

F i g u r e al l l l u s t r a t i o n o f t r a n s i t i o n p r o b l e m s

-

F i g u r e 41

F i g u r e .-b) E x a m p l e s o f t r a n s i t i o n s (toe protection)

-

T r a n s i t i o n s i n revetments

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW

K.W. P i l a r c z y k

OF REVklTlENTS V I I . 86

Top edge and flank protection are needed to limit vulnerability of the revetment to erosion continuing around its ends. Extension of the revetment beyond the point of active erosion should be considered but is often not feasible. Care should therefore be taken that the discontinuity between the protected and unprotected areas is as smal1 as possible (use a transition roughness) so as to prevent undermining. In some cases, open cell-blocks or open blockmats (eventually vegetated) can be used as transition (i.e. from hard ~rotectioninto grasmat). The flank protection between the protected and unprotected areas needs mostly a thickened or grouted coverlayer, or a concrete edge-strip with some flexible transition i.e. riprap. 10.

MANAGEMENT AND MONITORING

Coastal zone management involves management and decision-making regarding: * a coastal protection plan, that is a coherent set of measures, specified in time and space, to achieve a certain extend of protection against existing or anticipated damage; * a monitoring and control system, (inspection system, measurements). Coastal zone management is characterised by its integral nature. Firstly, an integral approach to the coastal problems is required because of the interrelationship between land use, coastal protection measures and the daily management and control. Secondly, an integral approach is required since various disciplines and techniques are involved in the analysis of the coastal problems and their potential solutions, for example, coastal engineering, economics, land use planning, environmental science, mathematica1 and physical modelling techniques, etc. Thirdly, a certain spatial integration is required because of the potential physical interactions between adjacent coastal sect ions. In generating and analysing a coastal protection plan, the following steps can be distinguished: 1. definition of coastal sections; 2. creation of 'basic alternatives; 3. identification of coastal protection measures; 4. screening of measures, by section; 5. creation of alternative coastal protection plans; 6. impact assessment (full specification of al1 relevant effects); 7. evaluation (by the decision-makers). Information about the actual state of the coastal area including coastal structures is indispensable for optima1 coastal management (Figure 42). Coastal management, is therefore intimately connected with monitoring activities and the design of routine monitoring networks andlor specific field surveys. To reduce the, often high, costs of the monitoring system, its design should yield an optima1 system which provides the responsible agencies with sufficient information at minima1 costs. A generally applicable method for the design and optimization of monitoring networks being actually developed in the Netherlands consists of the five main steps: 1. identification and quantification of the objectives; 2. identification of the relevant process dynamics; 3. determination of the effectiveness of the information provided by the network;


K.W. Pilarczyk

/

1

C

observation limit

v' ~mfninum)

I

acceptable standard

I, , , , , , , , , , 1 , , , , , ,--, , , , , G

1

1 I

u

-

I t

I

m

n

1

---.

- - ,.4

,

I

*=inspection Figure (al

t1

time

ti

-

lnspection graph ,-

1

FUNCTION

I

/

I

MAINTENANCE SCENARIO

AS BUILT quality conkd

INSPECTION

STATE OF

14

Ii

BOUNDARY coNoITIoNs ILoAos I

Figure (b) flanagement model

Figure 42

Management approach to coastal structures

,

,

.,


VII. 88

4. calculation of the costs of the monitoring network; 5 . execution of a cost- effectiveness analysis. Based on the results of analyse done in the second step, the necessary instruments for monitoring can be defined. It will lead very often to development of the new types of monitoring-instruments. A new philosophy in coastal monitoring involves the combination of mathematica1 simulation models and measurements. In this approach, which is similar to that applied in the control of industrial processes, the results of measurements are compared with the forecast of the mathematica1 model.

11.

CONCLUSIONS AND RECOMMENDATIONS

The limitation of this short course does not allow to prepare a fully (detailed) evaluation of the available Dutch data on dike and coastal protection. The problem is too wide and too complicated for that. The details and/or background informations can be found in the Guidelines reports as mentioned in-the references, However, this brief evaluation seems to be sufficient for the designers and institutions involved in this problem to find a way to the detailed informat ions. The guidelines presented will bring designers closer to the solution of the typical problem of the design of dikes and/or seawalls, and the proper choice of revetments in respect to design hydraulic load, ability of materials and skill, and desired function of construction. The local conditions in respect to availability and price of manpower, materials and equipment will be decisive for the final choice of construction. It has to be stressed that, whatever calculation method and protective system is adapted, the (local) experience and sound engineering judgement play an important part in a proper design of protective structures. The research on dike constructions (sea and river dikes and other sea and bank defenc-e systems) is still going on in the Netherlands (Figure 43). Research is now being directed towards a better probabilistic description of the design, better understanding the failure mechanisms, application of new or alternative materials (e.g. waste products of industry: minestone, slags, etc.), monitoring of damage, economical aspects of design and optima1 choice of constructions applied incorporating future maintenance aspects. Because of the worldwide interest and the complexity of the proper design and management of the water defence systems the international cooperation in this field should be stimulated. It will not only save money, but it will increase the reliability of the design and in this way, it may guarantee more safety for the population and the economical values to be protected al1 over the world.


I BOUNDARY

-

SEA DIKES 1

I

I

I

Y

- -

.

MODELS

FLUME (S) I

l

p-

BANKS

I

I

I

DESK STUDIES

I

NATURE

CONDITIONS 1I

I

MODEL (S) I

I

ANALYTICAL SOLUTION

SEMI BLACK BOX

-

APPLICATIONS

v W Y

4

SEMI BLACK BOX

-

APPLICATIONS

MATHEM= MODEL(^) /

NATURE

\

VERIFICATION O D I F I C A T I O N ~ ~ ~ PROPERTIES

SUBSOIL

EXPERIENCE FUNCTIONAL REQUIREMENTS

CONSTRUCT. / COSTS MAINTENANCE .

.. .

~

~.

F i g u r e 43

S e a - d i k e s and bank p r o t e c t i o n r e s e a r c h approach i n t h e Netherlands

DESIGN OF SEAWALLS AND DIKES INCL. OVERVIEW OF REYETMENTS K.W. P i l a r c z y k

V I I . 90

REFERENCES (continued) Horikawa, K. (1988), Nearshore dynamics and coastal processes, University of Tokyo Press, Japan Jansen, P. Ph. (ed.) (1979). Principles of River Engineering. Pitman Publ. Ltd., London. Jensen O.J. and J. Juhl (1987), Wave Overtopping on Breakwaters and Sea Dikes, 2nd Inter.conf. on Coastal and Port Engng. in Developing Countries, Beij ing, China Kamphuis, J.W. (1975). Friction factors under oscillatory waves. Proc. ASCE, J. Waterways, Harbours and Coastal Eng. Div. 101 (WW2): 135144. Knauss, J. (1979), Computation of maximum discharge at overflow rock fill dams, 13th Congr. des Grands Barrages, New Delhi, 450, R.9, pp.143-160. Komar, P.D. and Miller, M.C. (1974). Sediment threshold under oscillatory waves. Proc. 14th Conf. Coastal Eng. Copenhagen, pp. 756-775. Kraus, N.C. and O.H. Pikley (eds. (1988). The Effects of Seawalls on the Beach, Journal of Coastal Research, Speciaal issue no. 4 Lane, E.W. (19552. Design of stable channels. Trans. A.S.C.E. 120. Meer, J.W. van der, and Pilarczyk, K.W. (1984), Stability of rubble mound slopes under random wave attack, 19th International Conference on Coastal Engineering, Houston, D.H.L. Public. no. 332 (see also Breakwaters 1985 Conference, October 1985, London). Meer, J.W. van der, and Pilarczyk, K.W. (1986). Dynamic stability of breakwaters, rock slopes and gravel beaches, 20th International Conference on Coastal Engineering, Taipeh. Ontairo (1987), How to Protect Your Shore Property, Ontario Ministry of Natura1 Resources, reprint rnarch 1987, Ontario, Canada Paintal. A.S. (1971). Concept of critical shear stresses in loose bounary open channels, Journal of Hydraulic Research, 9 (no. 1). Per Bruun and Madhav Manshar (1963), Coastal Protection for Florida, Bulletin Series no. 113, University of Florida, Gainesville. PIANC (1987a). Guidelines for the design and construction of flexible revetments incopr. geotextiles for inland waterways. Supplement of PIANC Bulletin no. 57, Brussel. PIANC (1987b), P i s k consideration when determining bank protection requirements, supplement to PIANC Bulletin no. 58, Brussel. Pilarczyk, K.W. and Boer, K. den, (1983), Stability and profile devel oprnent of coarse materials and their application in coastal engineering. Internationa Conference on Coastal and Port Engineering in Developing Countries, Sri Lanka, D.H.L. publ. no. 293 (see also r ra vel beaches'': D.H.L. publ. no. 274). Pilarczyk, K.W. (1985), Pilarczyk, K.W. (1985), Stability of revetments under wave and current attack, 21st International Association for Hydraulic Research Congress (IAHR), Melbourne. Pilarczyk, K.W. (1987), Sea defences: Dutch guidelines on dike protection, Rijkswaterstaat, Report UB-No-87110, Delft, The Netherlands Pilarczyk, K.W. (1989), Design of coastal protection structures, Short Course, AIT, Bangkok Quelerij, de (1989), Geotechnical aspects, in Short course on design of coastal structures, AIT, Bangkok RWS (1984), The Closure of Tidal Basins, Delft University Press, The Netherlands Shore Protection Manual (1984). Coastal Engineering Research Center U.S. Army. Silvester, R., (1978), Some facts and fancies about beach erosion, 16th Coastal Engineering Conference Simons, D.B. and Aiberton, M.L. (1960). Uniform water conveyance channels in alluvial material. Proc. A.S.C.E. J. Hydraulic Div. 86 (HY5).


VII. 92

REFERENCES (continued) Sleath, J.F.A. (1984). Sea Bed Mechanics, John Wiley & Sons, New York. Smith, A.W. and Chapman, D.M. (1982)) The behavior of prototype boulder walls, 18th Coastal Engineering Conference, Capetown, vol. 111: 1914-1928 Swart, D.HI (1976).talsedimenttrancport.Computation of longshore trans port. Delft Hydraulics Lab. Report R968(1). TAW/CUR (1984), Guide to concrete dike revetments, Netherlands Centre for Research, Codes and Specifications for concrete and Technical Advisory ctmuittee on Water defences (TAW), Report 119, 1984 (in Dutch; English translation available) TAW/CUR (1989), Guide to the judgement of the safety of dunes as a sea defence system. Technical Advisory Committee on water defences (TAW), The Netherlands TAW/CUR (1990), Guide for design of river dikes. Part I: Upperriverreaches. Technical Advisory committee oa Water defences, The Netherlands TAW/RWS (1985), The use of asphalt in hydraulic engineering. Technical Advisory committee on Water defences (TAW), Rijkswaterstaat Communications, no. 37, 1985, The Hague TAWlTechnical Advisory Committee on Waterdefences (1991). Guidelines on dimensioning of block revetments. CUR (Center for Civil Engineering Research) Gouda, The Netherlands (in preparation). TAWlTechnical Advisory Committee on Waterdefences (1991). Guidelines on use of clay in dike construction. Delft, The Netherlands. TAW 10 (1985), Probabilistic design of sea defences, Technical Advisory Committee on Water defences (TAW), Internai report T A W 10 (in Dutch), the Netherlands TAW (1988)) Guidelines on selection revetment materials for dikes and shores, Technical Advisory Committee on Waterdefences, The Netherlands The Closure of Tidal Basins (1987). Delft University Press, The Netherlands. Velden, E.T. van der (1989). Coastal Engineering, Delft University of Technology, The Netherlands. Veldhuijzen v a n Zanten, R. Editor (1986), Geotextiles and Geomembranes in Civil Engineering (Handbook), A.A. Balkema, Rotterdam/Boston Ven te Chow (1959). Open Channel Hydraulics. McGraw-Hill Book Comp., New York. Weide, van der (1989), General introduction and hydraulic aspects, in Short course on design of coastal structures, AIT, Bangkok


VII. 93

APPENDIX I DATA COLLECTION ANTI

PREDICTION METHODS

-

an overview

-

by Krystian W. Pilarczyk

Rijkswaterstaat, The Netherlands

Data collection and prediction methods

A basic input to the effective planning of any shoreline protection scheme is a reliable set of statistics which describe the environment against which such protection is necessary. Factors, which may include wind, waves and still-water levels, must be defined both in terms of normal conditions, and in terms of the extreme values which can be expected, with some stated probability, during the lifetime of the protection system. Inevitably, the problem is to estimate these long-tem statistics with sufficient accuracy, from the limited data available. The basic data required for the design of coastal works is derived from two principal sources (CIRIA, 1986): . - - (i) existing archived data; (ii) additional site-specific measured data. In each category there is a further important sub-division int0 short duration records and long duration records. The distinction is especially important in coastal engineering, where many of the physical processes involved are complex and not wel1 understood. Much reliance therefore has to be placed on statistica1 methods of analysing recorded data, in order to predict the likely occurrence of various design conditions during the life of the structure. The confidence which a designer can have in these predictions depends upon the quality and extent of the data. 2.

Collection of existinn data

The principal items under consideration are: - Climate - Water levels - Wind climate - Wave climate - Coastal processes - Geotechnical data - Construction constraints Many of the most important design parameters, such as the prabability of occurrence and magnitude of surges and secular changes in sea level, are dependent on long term records for their evaluation. If these are not availab.le from existing data banks, it is unlikely that there will be adequate time available during the design stage to make good this deficiency. However much depends on the extent to which the designer or his advisors have been involved in a particular area in the past. They may already have extensive experience and knowledge of the area, in which case the amount of additional data required could be minimal. But if the designer is unfamiliar with a particular coastline or the type of problem to be dealt with, the data gathering will need to be much more extensive. Primarily the requirement is for long term data, much of which 1 s readily obtainable from various national data banks, international specialistic organisations (i.e. World Meteorological Organisation), publications, and local authorities. The genera1 flow-diagram on hydraul ic boundary condit ions is given in Figure 1.

DATA COLLECTION AND PREDICTION METHODS

K.W. Pilarczyk

shoa li n g

---nófié-3hänöi-f ö~-eesTö,7ëTTT; deep w a t e r d

t

o t h e r loads: r i v e r f lood, ships loads

f wave g r o w t h (predictionl H s (011 Tp (0)

wind-

breaking (Hs (br) " 0 . 5db)

wavegrowth

w i n d set-up

v

1

Mean Sea L e v e l change in astran. tide, seiches oscillations windgusts/ bumps. e t c .

boun'd yar

+

water levei

shallow water. db

+

4

I

- j o----------------int distribution V ( p r a b a b i l i t y ) lI hydraulic conditions*

v

Y

wave c l i m a t e / pattern

I I I I I

I----'

Figure 1 Flow diagram on hydraulic boundary conditions

3.

Climate and meteorolo~ical conditions

Meteorologic conditions are of importance for the coastal engineer, as they determine the wind-fields which are of importance for the design and operation of marine structures. Moreover, wind is the important factor which determines the local wave-climate. Genera1 information on marine meteorology and climatology may be found in the Oceanographic publications (Fairbridge, 1966). More detailed information on local climatology can be obtained from the climatic atlasses, issued by the national meteorologic offices and the United Navy (i.e. see Oceanographic Atlasses). On a global scale, different meteorologic areas can be distinguished, related to the high and low pressure areas, caused by solar radiation. Apart from the genera1 pattern of wind, which are created by largescale airpressure distributions, and storms generated by depressions, violent tropical storms may occur, known as hurricanes or typhoons. Hurricanes occur mainly in the Chineese sea, the Indian Ocean and in the Atlantic Ocean near the United States.

The water-level is determined by the mean sea level, the astronomic tide, barometric pressures and wind-effects ( = meteorogically induced surges). A review of these aspects is given by Lisitzen (1974). Detailed information on Tides can be found in Dronkers (1964), whilst tidal data are given in tide-tables such as presented for instance by the British Admirality. As a result of climatologic changes due to the greenhouse effect, the average sea-level may rise at a higher rate than observed ( O to 20 cm per centuary) in the past. For structures with lifetimes in the order of a century or more, this effect should therefore be taken int0 account (eventually in combination with vertical land movement/subsidence).

DATA COLLECTION AND PREDICTION METHODS

K.W. Pilarczyk

4.1

Tides

Tides are regular, periodic, vertical water movements, driven by the gravitational forces of the moon and sun. . . Tidal information for most areas is available in tide-tables. Tables include normally a list of the so called tidal constituents - period, phase and amplitude of the various tidal components - and predictions of the tidal elevations based upon these constituents. For practica1 purposes a reference level is defined, known as ChartDatum. Most depths and elevations in coastal areas are referred to this datum. The Chart-Datum (C.D.) is selected in such a way that the actual waterlevel fails very seldom below this datum. Al1 tidal data are referred to this datum. Mean High and Mean Low water (MHW and MLW respectively) are the averages of al1 possible high and low waters. Mean High Water Spring and Mean Low Water Spring (MHWS and MLWS respectively) are the averages of al1 high and low waters during spring-tides. Finally, a Highest Astronomie Tide (H.A.T.) can be defined, which is the theoretica1 tide which occurs if al1 components have their maximum at the Same time. If no tide-information is available, tides should be maximum at the Same time. If no tideinformation is available, tides should be recorded for a period of at least one month. The tidal constituents can then be computed from a harmonic analysis. This method is discussed in Dronkers (1964). 4.2

Meteoroloaic effects: sur~es/windset-up

- pressures the water-level may As a result of a drop in barometric increase temporarily. Such phenomena are observed during the passage of a cyclone. More important, however, is the effect of the wind shearstress associated with strong winds and stoms. Due to this shearstress storm-surges may occur, which are particularly dangerous in shallow areas as the associated wind set-up is inversely proportional to the water-depth. For steady-state conditions wind set-up (S) can be computed easily from the static equilibrium between wind-shear, surface-shear and waterlevel gradient (i) (see also Shore Protection Manual). The basic equilibrium provides:

-

l p

where : 2 -=p1 " w .c1 S = wind set-up (m) e--F = fetch length (m) --i = gradient Öf water-level ( - ) h-d g = gravitational acceleration (m/sZ) A1 h = water depth = wind velocity n ar water surface Sm/s) . . . --- . = factor; =314.10-g (-1 (the value of 'C' can be strongly affected by the geometry/topography of the area considered). For moving wind-fields, however, this method gives erroneous results. More sophisticated flow-models are then used to compute the watermotion and associated water-levels for the combined effects of tides and stormsurges (v.d. Weide, 1989). The prediction of the surges (wind set-up) can be done by using (see also Figure 2): . . (a) statistics of wind-fields, or (b) statistics of the measured wind set-up.

DATA COLLECTION AND PREDICTION METHODS K.W. Pilarczyk

The l a s t method g i v e s m o s t l y t h e b e s t r e s u l t s . The Gumbel p r o b a b i l i t y p a p e r c a n be used f o r s t a t i s t i c a 1 a n a l y s i s . Surge s t a t i s t i c s a r e n o t y e t s u f f i c i e n t l y w e l 1 d e f i n e d t o a l l o w US t o i d e n t i f y long-term changes: w h i l e a p a r t i c u l a r decade may have subs t a n t i a l l y more e x t r e m e s u r g e s t h a n a n o t h e r , s u c h v a r i a t i o n would be e x p e c t e d on t h e b a s i s of random sampling a l o n e . With r e g a r d t o t h e measurement of s u r g e s , much depends on t h e l o c a t i o n o f t h e s i t e and i t s d i s t a n c e from a s u i t a b l y c a l i b r a t e d t i d e gauge. The example of t h e combined e f f e c t of a s t r o n o m i c t i d e and wind s e t - u p is g i v e n i n F i g u r e 3.

A waterlevel design waterlevel

I

Figure 2

I

stormsurge + wind set-up)

tide de

I

C

frequency

D e t e r m i n a t i o n of d e s i g n s t o r m - s u r g e l e v e l .-

- - e

..

--

astronomical tide + wind set-up

wind set-up

Figure 3

4.3

Example of combined e f f e c t of t i d e and wind s e t - u p

O t h e r changes

B e s i d e s t h e main components a l r e a d y m e n t i o n e d , t h e w a t e r l e v e l c a n be i n f l u e n c e d by a number of a d d i t i o n a l e f f e c t s a s : wave s e t - u p due t o waves b r e a k i n g i n s h a l l o w w a t e r ; t h i s e f f e c t i s m o s t l y of l e s s i m p o r t a n c e f o r t h e d e s i g n of c o a s t a l s t r u c t u r e s . However, t h e wave s e t - u p may induce c o a s t a l c u r r e n t s which a r e o f g r e a t i m p o r t a n c e f o r t h e morphologic p r o c e s s e s . - s e i c h e s , o s c i l l a t i o n s w i t h a p e r i o d of 15 t o 45 m i n u t e s , c a u s e d by sudden m e t e o r o l o g i c a l c o n d i t i o n s (macroscopic t u r b u l e n c e c h a n g e s ) s u c h a s t h e p a s s a g e of a l i n e s q u a l l . The p r o p a g a t i o n speed i s

-

DATA COLLECTION AND PREDICTION METHODS K. W. P i l a r c z y k

a).

-

similar to the long waves (C = The appearance of a seiche results mostly in a few decimeters water-level fluctuation along the coastline. Accordingly to Thorn and Roberts (1981), the seiche that occurred on July 3, 1946 on the south coast of England caused the sea level to drop 1.25 m in a few minutes and then to rise rapidly 2.5 m. wind gusts (gust bumps), single waves resulting from a sudden violent rush of wind; it may als0 result in a few decimeters waterlevel fluctuation. Because both, seichess.and wind gusts, propagate as long waves, they may introduce some resonance problems in the harbours and estuaries. tsunamis are waves of very long period excited by seismic activity (i.e. earthquake). They are potentially extremely dangerous in certain geographical regions (i.e. Japan). bore is a pecular form of a tidal wave. It may occur in the mouth of a river when the propagation speed of the top of the tidal wave (C = Jgh) is larger than this of the dale of the tidal wave. It results in a vertical water-wal1 similar to a hydraulic jump propagating with a high speed (C = other effects locally/geographically dependend.

m).

Al1 these additional effects, if relevant, should be incorporated in prediction of design water-level. 4.4 Determination of desinn water-level Shorelines/structures are attacked by the total compound sea level, and the approach must be capable of some resynthesis of the component statistics to give usuable total level statistics (Figures 1 and 2). For extreme levels, a combination of the separate tidal and surge level probability distributions, to produce joint probability distribution, can give more stable estimates of levels having a particular annual exceedance probability than is possible using traditional methods of ranking the annual maximum values obtained over the Same periode. Design estimates may incorporate projected changes in mean sea level, land subsidence and other changes by simple addition of the levels cornputed by joint probability. Undoubtedly the problem of environmental design parameters and their stability becomes much more complicated in shallow-water and particularly in estuaries. A single measuring point in an estuary is unlikely to give statistics representative of its whole length: the nature of spatial and tempora1 changes in these design statistics for estuaries needs special consideration.

5. Wind climate For practica1 applications in civil engineering a more detailed description of the local wind-clirnate is required. Wind-recordings are used as basis for this description. Records should be made in a representative area outside the influence of disturbing topographic features or buildings. Data should be recorded and processed in accordance with the procedures of the World Meteorologic Organization. Data are normally presented in tabular form, showing the percentage of occurrence of wind-speeds per direction. Separate tables are presented in case the wind regime shows a distinct seasonal variation. In general, direction intervals of 30 degrees are applied, wind-speeds are given in Beaufort. For easy comparison, data are plotted as wind-roses. An explanation of the


Beaufortscale and typical example of a wind-rose are given in Table 1 and Figure 4, respectively. Force (Beaufort Scale)

Sea rniles, 6080 ft/h (Knots)

b/ h

o

0- 1

0- 2

1-2

2-6

4-1 1

3

7-10

13-18

4

11-16

20-30

5

17-21

32-39

6

22-27

41-50

7

28-33

52-61

8

34-40

63-74

9

41-47

76-87

1O

48-55

89-102

11

56-65

104-120

12

over 65

2 120

Wind

(indication of probability)

light moderate

strong

gale

storm (2 1/10)

2 Table 1

hurricane ( 2 1/100)

Wind strength (The Beaufort Scale)

D a t a c o l l e c t i o n : windO

direction in

% SC

p G ö q wind v e l o c interv Figure 4 Examples of wind- and wave-rose


waves

Wind data.is also essential to design calculations because of the major deficiency in routine wave data collection - the absence of information on wave direction. The directional distribution of winds of different intensities, usually presented in the form of a "wind rose" is a help in assigning directional properties to wave climate. In the case of wind data, which is predominantly used to develop the local wave climate, the designer is frequently restricted by the absence of good data specific to his site. The number of coastal weather stations, especially in a certain geographical regions, is less than adequate for this purpose. Wind data in the offshore area where waves are generated is largely non-existent. Thus, the local windmeasurements are often hardly needed. 6.

Wave climate (Definitions and information sources)

As a result of the wind shear-stress, waves are generated on the watersurface. In the immediate vicinity of the wind-field the waves show an irregular and random pattern, due to the turbulent variation of the wind-speed. This is called wind-generated sea. Their appearance is more regular as these waves move outside the area where they have been generated. Due to the difference in propagation-velocity of waves with different period, a separation occurs between different wave-periods. Waves with roughly the Same period will cluster together. Such waves are known as swell. Waves contain a large amount of energy, for that reason they are of paramount importance for the design of coastal structures. In general, design-conditions are defined in two stages: Firstly, the wave-climate is defined in terms of the probability of occurrence of specified wave-height, wave-period combinations as a function of their direction of incidence. Visual observations, waverecordings and wave-hindcasting are possible sources from which the information can be obtained. Once the wave-climate is known, a design-wave condition can be determined, normally a condition is selected which will occur with an average probability of once every hundred years. The corresponding seastate is then described in a way which is consistent with the design-method. When simple design-graphs are used, significant waveheights and periods are sufficient to describe the wave-field. When more advanced spectra1 computations are performed, the sea-state should be characterized in terms of the variance-density spectrum. Finally, when fluid-structure interactions are described, the physics of the wave and the internal hydrodynamic processes should be known. In this course the various aspects will be discussed briefly. Reference is made to text books for more detailed information. More information on the physics of waves and the various wave-theories is given by Newman and Pierson (1966) and by Kinsman (1965). Based upon these theories Tables have been prepared for practica1 use. Examples are given in Shore Protection Manual (see also Figure 6). A review of statistica1 properties of waves and an explanation of the concept of wave-spectra can be found in various references. The statistical aspects of wave climatology and the corresponding procedures for the determination of design conditions are treated by Muir (1986). For detailed information on wave-conditions in specific areas, Sea and Swell atlasses are available (Oceanographic Atlasses). Also the wavedata given by Hogben and Lumb (1967) can be used, although the geographic resolution is often not sufficient. Recently, the IAHR has presented a list of symbols to be used for the analysis and the description of waves (IAHR, 1986), which replaces earlier definitions as given by PIANC.


A short state-of-the-art on design wave conditions for maritime structures is given by Y. Goda in Appendix I11 and Goda ( 1 9 8 5 ) .

f l 9 .mve ~ heibt r e l a t i m based LM #e Ryleifl distribution

extreme

local negative maxima

lal Dfinition of ~ind- ave pafaneters

~ a height n

A, I

crest

significant height average of tenth highest waves

H=H~ Hs

1/3

G

o.626

1.416

4.005

1.000

1.80

5.091

1.271

t .

trough

12) Refioition of Have parameters Figure 5

7. 7.1

of hundreth highest waves

Definitions and some wave height relations

Wave prediction and wave related parameters Dee~-waterconditions; h 2 L /2. (Lo = nT2/2n) O

The techniques used for forecasting and hindcasting models are the same, the main difference lies in the infomation available on the wind conditions. Wave forecasting refers to the calculation of wave conditions at a particular site using forecast wind conditions, whereas wave hindcasting uses recorded wind conditions to calculate wave conditions. A wide range of wave prediction techniques are in common use, varying from complex numerical models that attempt to simulate as many of the physical processes as possible, to simple charts or graphs relating wind speed, fetch length, and possibly water depth, to resulting wave conditions. The simplest of the mathematica1 models assumes that the waves being considered are due entirely to a.wind blowing at constant speed and direction for a given duration (Figure 6). The deep-water wave conditions are characterized by the significant wave height (H ) and the peak spectra1 period (T ) or the zero-crossing period ( T Z ) Figure 5 P

IZ

DATA COLLECTION AND PREDICTION METHODS K. W. Pilarczyk

Figure 6 Prediction of wave conditions for deepwater some typical wave height inter-relations based on the Rayleigh distribution are als0 given. 7.2

Shallow-water effects; h