Assessing the Potential of Floodplain Woodland in

Assessing the Potential of Floodplain Woodland in Flood Amelioration

A dissertation submitted in partial fulfilment of the requirements for the degree of Masters of Science (MSc) in Environmental Forestry of the University of Wales

By Jerome O Connell B Tech Wood Science and Technology (1998, Limerick)

School of Agricultural and Forest Sciences University of Wales, Bangor Gwynedd, LL57 2UW, UK www.bangor.ac.uk Submitted in September, 2004.

Abstract As the intensity of extreme flood events increase across the UK and Europe, wide interest has been expressed in the use of soft engineering approaches to river flood defence. Floodplain woodland provides one of the most versatile and self-sustainable methods of reducing flood peaks. Floodwaters are allowed to spill out on to the floodplain, thereby increasing the storage capacity of the river and modifying the rivers hydrograph to a more gradual curvature. Three floodplain sites were identified in the Mawddach catchment, just north of the town of Dolgellau in mid-Wales. Various scenarios were modelled on each site using the hydrodynamic model River 2D. Each scenario had varying degrees of vegetation density, ranging from grassland to dense woodland. The basal area of each scenario was related to Manning’s roughness coefficients so that results could be linked back to the physical attributes of each site. The first two sites provided no real potential in mitigating flood peaks, although water depths were increased by up to 500mm due the presence of dense vegetation. In the third site, dense woodland, with an average basal area of 0.10071m²m³-¹, reduced peak discharge by 45m³sec-¹ when compared to grassland. This resulted in a delaying the peak discharge by more than 30 minutes less than if the floodplain was covered in grassland. The increased turbulence and backwater of the dense woodland increased water depths by more than 1.2m in some locations when compared to grassland. Two other scenarios were presented in the third site. These were, a sparse woodland with and average basal area of 0.0244m²m³-¹, and grouped woodland with and average basal area of 0.10071m²m³-¹. It was found that although the sparse woodland has just 45% of the total basal area of the grouped woodland, it preformed equally well at mitigating floodwaters.

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Declaration This work has not previously been accepted in substance for any degree and is not being concurrently submitted in candidature for any degree. Candidate: ………………………………………… Date: ………………………………………………

Statement 1: This dissertation is being submitted in partial fulfilment of the requirements for the degree of Masters of Science. Candidate: ………………………………………… Date: ………………………………………………

Statement 2: This dissertation is the result of my own independent work/investigation, except were otherwise stated. Candidate: ………………………………………… Date: ………………………………………………

Statement 3: I herby give consent for my dissertation, if accepted, to be available for photocopying and interlibrary loan, and for the title and summary to be made available to outside organisations. Candidate: ………………………………………… Date: ………………………………………………

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Acknowledgements I wish to thank Dr. Morag MacDonald for her help and guidance, and also, always being available for a chat. I would also like to thank my sister, Jenna, for her proof reading at the eleventh hour as well as my parents for all their support, especially when things got tough. I also wish to thank Mrs. Margaret Hall for her relentless enthusiasm in partaking in the surveying, especially on them rainy days, which were quiet frequent unfortunately. And finally, special thanks goes out to Mr. Graham Hall for his help in surveying as well as his expertise in the Mawddach catchment and hydrodynamic modelling.

IV

Table of Contents Section

Page

Abstract

II

Declaration

III

Acknowledgement

IV

Table of Contents

V

List of Graphs

VIII

List of Figures

IX

1.0 INTRODUCTION

1

1.1 Floodplain Forests and Woodland

2

1.2 Floodwater Amelioration and Storage in the Floodplain

5

1.2.1 New Approaches Towards Flood Defence

6

1.2.2 Hydraulic Roughness of the Floodplain

8

1.3 The Mawddach Catchment

11

1.3.1 Study Areas

11

1.3.1.1 Floodplain 1

12


13


15

1.3.2 Geology

17

1.3.3 Landuse

18

1.3.4 Flooding and the Mawddach

18

1.4 Hydraulic Modelling

21

1.4.1 River 2D

22

1.4.1.1 Data Requirements

22

1.4.1.2 Sub Programs

23

V

1.5 Objectives

27 1.5.1 Modelling Scenarios

27

2.0 METHODS

29

2.1 Surveying

30

2.1.1 Materials

30

2.1.2 Surveying Procedure for Floodplain1 and 2

31

2.1.3 Surveying Procedure for Floodplain 3

34

2.1.4 Vegetation Density Analysis and Manning’s Roughness Coefficients

2.2 Interpolation

35

37

2.2.1 Procedure

38

2.3 Computer Modelling

39

2.3.1 Input into R2d Bed

39

2.3.2 Mesh Creation in R2d Mesh

40

2.3.3 Hydrodynamic Modelling in River 2D

41

3.0 RESULTS

43

3.1 Floodplain 1

46

3.2 Floodplain 2

49

3.3 Floodplain 3

53

4.0 DISCUSSION

57

4.1 Floodplain 1

58

4.2 Floodplain 2 4.3 Floodplain 3

62

VI

5.0 CONCLUSION AND RECOMMENDATIONS 5.1 Conclusions

67 68

4.1.1

Floodplain 1

68

4.1.2

Floodplain 2

69

4.1.3

Floodplain 3

70

5.2 Recommendations

72

5.0 REFERENCES

73

7.0 APPENDICES

83

VII

List of Graphs Graph

Title

1.0

July 3rd Flood Hydrograph

Page

19

Floodplain 1 Results 3.0

Water Surface Elevations Across CS1 at 12000 seconds

46

3.1


46

3.2


47

3.3

Discharge Intensity at Point 6 (Floodplain) in CS2

47

3.4

X-Discharge Intensity Across CS4 at 12000 seconds

48

3.5

Hydrograph of Inflow and Outflow for Floodplain 1

48

3.6


49



49

3.8


50

3.9


50

3.10


51

3.11

Y-Discharge Intensity at Point 8 (River Channel) in CS5

51

3.12

Discharge Intensity Across CS5 at 12000 seconds

52

3.13


52



53

3.15

Water Surface Elevations at Point 7 in CS2

53

3.16


54

3.17


54

3.18


55

3.19

Discharge Intensity at Point 7 in CS2

55

3.20

Y-Discharge Intensity Across CS4 at 14000 seconds

56

3.21

Outflow Hydrograph for Floodplain 3

56

VIII

List of Figures Figure

Title

Page

1.0

River Drac, France

2

1.1

Schematic of Integrated Flood Management

7

1.2

1:200,000 Map of the Mawddach Catchment

12

1.3

Inflow Area of Floodplain 1

13

1.4

Upstream Section of Floodplain 2

14

1.5

Centre of Floodplain 2

15

1.6

800m Upstream of the Outflow of Floodplain 3

16

1.7

Outflow Section of Floodplain 3

16

1.8

Damage Caused by July 3rd Flood

20

1.9

Example of a Mesh Structure

24

2.0

Surveying Equipment

31

2.1

Surveying Method

32

2.2

Obtaining Elevations

33

2.3

Vegetation Plots for Floodplain 1

36

2.4

Vegetation Plots for Floodplain 2

37

2.5

Grid used as Input into R2D Bed

38

IX

Introduction

1.0 INTRODUCTION

1

Introduction

1.0 INTRODUCTION Examining the ability of floodplain woodland in flood amelioration requires knowledge from several different fields, namely ecology, hydrology and hydrodynamic modelling. Sections 1.1 to 1.4 are designed to give an introduction into such fields in an effort to make the outlined objectives (Section 1.5) more logical.

1.1 FLOODPLAIN FORESTS AND WOODLAND Floodplain woodland and indeed floodplain forests are highly dynamic ecosystems, receiving a complex but critical flooding regime (Richards et al, 2004). They occur in river valleys and therefore tend to be linear in structure, depending on the cross sectional shape of the river valley, and the extent of regular flood levels. High flood levels create areas of regeneration through deposition and erosion, and low to medium water levels are essential to maintain the floodplain water table necessary for growth. The dynamic environment, which dictates this ecosystem, means that such woodlands or forests are an intricate mobile mosaic of vegetation communities that vary with age (Hughes et al, 2003). Fig. 1.0 below shows the changing channel pattern of the River Drac over a 46year period, from a mobile braded floodplain to a mature floodplain woodland.

Fig.1.0 River Drac, France (Courtesy of Richards et al, 2004).

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Introduction A floodplain woodland with a young vegetation structure on or near the bank and a more mature developed structure future back is said to regularly experience flood events. This is so because, the young vegetation on or near the back never get a chance to develop substantially due to the erosive power of the river and the constantly changing river channel. This is one of the main reasons for the huge diversity that is present in such ecosystems. A 200 km stretch of the Tana River floodplain forests of Kenya, which have an average width of 0.75 km have approximately 175 woody plant species, over 250 species of birds and 57 species of mammals (Richards et al, 2004). As already mentioned, the species present in a floodplain woodland or forest evolve with the rivers hydrograph. For maximum diversity, intermediate levels of disturbance must be present. Too little disturbance and a mature monoculture will develop; whereas too much disturbance and only pioneer species will be present (Environment Agency, 2003). Natural examples of floodplain forests and woodland are very rare, especially in Northern and Western Europe. Floodplain forests are considered one of Europe’s most threatened natural ecosystems, and are listed in Annexe 1 of the European Habitats Directive as being a “priority forest habitat type” (Richards et al, 2004). Perhaps the main reason for the devastation of such habitats is their location and topographical characteristics. Floodplains are typically low-lying undulating topography with topsoil consisting mainly of fertile alluvial deposits, providing ideal opportunities for high production agricultural use. During the past 5000 years, clearance of many of Europe’s natural floodplain vegetation has occurred to make way for agriculture practices. As well as the presence of fertile soil, early colonisers where attracted to such sites by the presence of a ready supply of food in the form of fish, as well as the communication facilities that the river provided. More recently (in the last 200 years), modification of the hydrological regime by deforestation of up land catchments, and alteration of the rivers natural erosion and sedimentation characteristics by the use of dykes and embankments has lead to a critical change in the natural habitat of these woodlands and forests. In the last 1000 years, as human populations continue to colonise and modify the natural landscape, over 90% of

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Introduction Europe’s natural floodplain forests and woodlands have been removed (Richards et al, 2004). However, in recent years, an increasing awareness in the importance of floodplain forests and woodlands has emerged throughout Europe. A drive for habitat restoration has begun. Several factors have attributed to this, namely; increased knowledge of the implications of climatic change, hydrology and ecosystem management, pollution control, water quality, increased woodland cover, and a reduction in the use of “hard engineering” methods of flood control. Several policies and projects have been introduced, both at national and European level. Catchment Flood Management Plans (CFMPs) have just recently been introduced in the UK, with 60 to 80 CFMPs proposed for England and Wales. Such plans examine opportunities and constraints in relation to flood management, and policies are drawn up based on the whole river system, rather than on a minute scale (Richards et al, 2004). The EU Water Framework Directive gives guidelines on “good water management” forcing local authorities to look more closely at the natural flow regime of the river. Management decisions are made, based on this flow regime rather than on the surrounding infrastructure. The UK Biodiversity Action Plan has outlined a target of 3,200 ha of native wet woodland and aims to colonise and/or plant 6,750 ha of wet woodland on unwooded or ex-plantation sites by 2015 (UK Biodiversity Group, 1998). The restoration of floodplain woodland or forests is similar to the UK Forestry standard and Woodlands for Wales (Forestry Commission, 1999), as well as international policies such as Agenda 21, which emerged from the Rio Summit on sustainability in 1992 (UN Department of Economic and Social Affairs, 2003). Creation of a “sustainable” environment, which has multifunctional attributes, is the key message from all documents, therefore making floodplain forest and woodlands ideal candidates. Floodplain forests and woodlands have been identified as a key ingredient in the landscape (Richards et al, 2004). Several characteristics make such habitats an essential part of any riparian landscape. Firstly, as mentioned in previous paragraphs, there is a large species composition present. Large diversity occurs in these habitats because they provide an interface between land and water, therefore getting a mixture of certain aspects of both

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Introduction environments within the woodland or forest. In addition, the floodplain biota is shaped by the natural flow regime of the river, which varies seasonally and annually. This provides the ideal platform for a complex and diverse vegetation structure. Also coupled with this, floodplain forests and woodlands provide an important corridor from the catchment to the river estuary. Seedlings obtained from the entire river system usually agglomerate at the floodplain, building up a complex and varied seed bank, adding to the diversity of the area (Richards et al, 2004). Another benefit and characteristic of floodplain forests and woodland, is their high productivity (Kerr et al, 1996). As mentioned earlier, floodplains are very often nutrient rich environments. This is due the occurrence of regular flood events, washing nutrients from the upland catchment and river system across the floodplain; a typical example of this being the River Nile in Egypt. These nutrients greatly enhance growth rates as well as organic break down, therefore nurturing diversity of the surrounding vegetation. Other benefits and characteristics of floodplain forests and woodlands are that they add to landscape diversity. The uneven age structure and diverse species composition, coupled with the presence of water would make a floodplain forest or woodland a key recreation area in the landscape (Tir Coed, 2001). Another benefit is the fact that floodplain forests and woodlands diffuse pollution run off from other areas before it reaches the water system, acting almost like a buffer (Kerr et al, 1996). Finally, but perhaps most critically for this dissertation is the assumed ability of floodplain woodlands and forests to hold back peak river flows, and release them slowly back into the river in the form of ground water discharge. This topic is discussed in more detail in the following section.

1.2 FLOODWATER AMELIORATION AND STORAGE IN THE FLOODPLAIN When a large flood event occurs, huge volumes of water come cascading down from the upland catchment; this puts the river channel under immense pressure to contain the flood. If the event is large enough, the in channel water depth is too great and the floodwaters escape the channel and spill into the floodplain. This is the natural function of a floodplain, a wash-land to relieve the river of extreme flood events, therefore

5

Introduction protecting the down stream infrastructure by gently releasing the floodwaters as groundwater recharge and floodplain runoff. However, as mentioned in Section 1.1, modification of up to 85% of the UK’s rivers have occurred in the last 500 years (Environment Agency, 2003), especially in flood prone areas, namely floodplains. Such modifications have meant that the floodplains natural function has been removed, very often for either industrial or agricultural purposes. This has meant that “hard” engineering structures such as dams, artificial banks and dyke have to be introduced to contain the floodwaters that would otherwise be realised on to the floodplain. This in turn has resulted in huge expense for local authorities; for example the Environment Agency spends on average £260 million on costal and river flood defences every year, which amounts to over 40% of the Agency’s annual budget. Currently the Environment Agency manages £7.5 billion worth of flood defence infrastructure, and recent reports have pointed out that an additional £30 to £40 million would need to be spent each year to bring all flood defence infrastructure up to an acceptable level of safety (Friends of the Earth, 2000).

1.2.1 New Approaches Towards Flood Defence Such costs, coupled with recent low profits in agriculture, increased extreme flood events, as well as government policy on increased biodiversity has lead to a growing interest in floodplains, and more especially floodplain woodlands in flood amelioration. This new concept is deviating away from hard engineering by moving back the flood defences in key areas and reusing the floodplains as a temporary storage area for flood waters (Hughes et al, 2003). Such engineered structures as dams had previously been seen as the typical solution to flooding, however in today’s changing world such methods of flood control maybe viewed as to expensive, intrusive and anti-environmental (Smith and Ward, 1998). The concept of merging biodiversity and flood management is proving to be a very attractive prospect to government agencies, and several trial projects in both the UK (Hess et al, 2003) and throughout Europe (Poulard et al, 2003) been accessed. The Water Framework Directive (WFD), amended in by the European Parliament in 2000, is the key to this policy change. The move away from the “single problem

6

Introduction approach” to integrated river basin management is clearly outlined in Article 1 of this document (Dworak et al, 2003). Fig 1.1 below depicts a schematic of a typical river system in the UK, however the integration of biodiversity and catchment scale flood management isn’t so characteristic. The flood defences have been set back to allow floodwaters to spill on to the natural floodplain, which was previously a mine and arable land. It is now understood that such flood defence measures as vegetated floodplains provide a more sustainable and environmentally sound option than most of the typical man made solutions (Dworak et al, 2003). This “soft” approach to flood prevention has a clear bias towards floodplain woodlands as the predominant vegetation type. From a hydraulic perspective, the reasoning for this is quite simple; trees provide greater hydraulic roughness and turbulence than most other vegetation types.

Fig. 1.1 Schematic of Integrated Flood Management, (Courtesy of Richards et al, 2004).

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Introduction 1.2.2 Hydraulic Roughness of the Floodplain The basic concept of hydraulic roughness in the context of this dissertation is that, the more obstructions on the floodplain, the greater the turbulence and resistance to flow, therefore the larger the effect at reducing velocities. Vegetation, especially if erect but flexible will create local turbulence and so reduce the magnitude of instantaneous velocity due to a “drag force” on the moving water (Fisher et al, 2001). As flow rates decrease, infiltration rates increase, and therefore the hydrograph is modified by the attenuation of flood discharge peaks. As already mentioned, floodplain forest and woodlands offer one of the highest natural friction values in a floodplain, and therefore should be one of the most effect vegetation types at amending floods (Arcement et al, 1990). The hydraulic roughness of vegetation increases with water depth, yet once the vegetation is submerged, roughness coefficients begin to drop noticeably (Fisher et al, 2001). This submergence can occur prematurely with flexible flora such as grasses and reeds, as the velocity of the floodwaters bend the stems sufficiently to immerse the vegetation. Most woody vegetation will have both the height and stiffness to prevent submergence, and with the presence of woody debris and other smaller plant species provides the ideal environment for velocity attenuation and flood amelioration. Analysis of the hydraulic impact of floodplain woodland in a 2.5km stretch of the River Cary in Somerset by Thomas and Nisbet provided some positive results. With the floodplain covered in dense woodland, the storage capacity of the floodplain was increased by 67%, water travel time reduced by 90 minutes, and water levels increased by 190mm, with a backwash of 118mm stretching 300m upstream from the floodplain (Thomas and Nisbet, 2004). Intense husbandry practices associated with agricultural result in high soil compaction, and subsequent low soil porosity and infiltration rates. The presence of a forest or woodland in the floodplain will alleviate this problem, especially with the taprooting characteristics of most broadleaved trees. High infiltration rates on the floodplain will favourably modify the hydrograph, however it is assumed that large flood events will require a large floodplain to have a reasonable effect due to infiltration. In addition, because most extreme flood events occur in the winter months when the water table is high, low-lying topographic such as floodplains may already be saturated. Therefore, it

8

Introduction could be said that the effectiveness of floodplain forests and woodlands in flood amelioration may lie, in most circumstances, in hydraulic roughness and subsequent delaying of peak flows rather than through infiltration. Other critical factors in the ability of floodplain forests and woodlands in flood attenuation are; the planting structure, density and shape. When compared to an irregular planting pattern, the linear structure of a typical plantation may have a limited effect at retarding flow velocities during a large flood event. Flow paths will quickly be established between the rows of trees, leading to outflanking areas of high roughness. Planting patterns must be sufficiently irregular in order to create continuous wake zones (turbulence), therefore prevent any substantial volumes of water reattaching to create a substantial flow (Fisher et al, 2001). The creation of turbulence in or around tree trunks will result in the breakdown of laminar flow, therefore increasing pressure drag. The overall effect is increased transverse flow resulting in reduced velocities and peak discharge (Telionis, 1981). Similarly, but on a larger scale, the shape of the stand or group of trees must be of sufficient size to have a positive effect on reducing velocities. Long narrow strips of woodland parallel to the direction of flow will have only a partial effect, but the same strips perpendicular the direction of flow will have a considerably greater effect at attenuating flood flows and velocities (Arcement et al, 1990). Stem density, has obvious consequences to the roughness and turbulence creation of the floodplain. However there is the question of the effectiveness of woodland with a small scattering of large diameter trees or woodland with the same cross sectional area of vegetation, but distributed in the form of smaller diameter trees and shrubs. There are reservations however, as to the potential of floodplain forests and woodlands in flood amelioration. Heavy concentrations of large vegetation such as trees, on or near the riverbank have implications on bank stability. Experiments have shown (Environment Agency, 1997) that stiff woody stems will continue to retard flow up to very high velocities, but may develop local scour through convective acceleration of flow around their trunks as well as through eddy shedding. This can be especially prolific for downed or falling trees located on the river margins, resulting in often-severe damage to the solidity of the riverbank. It is also argued that large amounts of woody debris in the

9

Introduction river channel will reduce water capacity, therefore raising water depths and enhancing bank overflow (Environment Agency, 2000). However, other studies have pointed out that large scale scouring which occurs around submerged woody debris results in small depressions or pools in the riverbed, eventually counteracting the volume of the woody debris in the channel (Environment Agency, 1997). The presence of mobile woody debris can also cause problems when encountering such man-made structures as bridges and dams, and can be especially problematic after large flood events. It is clear from previous paragraphs that there are several factors that effect the flow conveyance of a floodplain, namely vegetation density and distribution, height, shape and width. Previous studies have given indications of the effect of some of these variables on conveyance, however in general, the amount of knowledge of the effect of floodplain woodlands and forests on flood control is limited, especially in a UK context. Most research in the UK has looked at the interaction of upland catchments with conifer plantations, which is not directly related to floodplain woodlands (Kerr et al, 1996). Previous studies into the effectiveness of upland plantations in flood control have shown poor results (Institute of Hydrology, 1991, 1995, 1998). This has been mainly attributed to the fact that most upland catchments are virtually saturated throughout the winter, when most extreme flood events occur. As a result, infiltration rates and holding capacity are poor and the hydrograph remains relatively unchanged regardless of the presence or absence of mature forest cover (Nisbet et al, 2003). The quest for conclusive knowledge of the potential of UK floodplain woodlands in flood amelioration continues, however it is hoped that this dissertation will shed some light on the topic.

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Introduction 1.3 THE MAWDDACH CATCHMENT The Mawddach catchment is located north of the town of Dolgellau (Grid reference SH 730 175), and within Snowdonia National Park. The population of the town of Dolgellau is 2,632. However, this increases substantially during the summer months, as Dolgellau is a popular tourist centre in North-West Wales (Barton, 2002). Due to the towns proximity to the Mawddach catchment and the flooding history of the area, flood prevention and control is a very real issue pertaining to public safety and protection of property.

1.3.1 Study Areas Fig. 1.2 below shows a 1:200,000 map centring in the Mawddach catchment. The river system compromises of the Afon Eden, Afon Gain, Afon Gamlan, Afon Mawddach, Afon Wen, Afon Wnion, Afon Cwn Mynach and the Afon Gwynant. In terms of flooding potential, the Afon Wnion and the Afon Mawddach are the most critical, with catchments areas of 116.55 km² and 164.58 km² respectively. However, the influence of the extensive network of tributaries flowing into both rivers must not be underestimated, especially during large flood events. Also pointed out in Fig. 1.2 is the location of the three floodplain areas identified for examination in this dissertation. For simplicity reasons the floodplain located at the confluence of the Afon Eden and Afon Mawddach is called Floodplain 2, the floodplain located further upstream in the Afon Mawddach is called Floodplain 1 and Floodplain 3 stretches from the confluence of the Afon Mawddach and the Afon Wen to just west of the A470 bridge.

11

Introduction

Fig 1.2 1:200,000 Map of the Mawddach Catchment Reproduced from (2004) Ordnance Survey map with the permission of the Controller of Her Majesty's Stationery Office, © Crown Copyright NC/04/32899

1.3.1.1 Floodplain 1 Floodplain 1 is located on the Afon Mawddach, in the centre of Coed y Brenin forest. The floodplain area is relatively small (approx. 4000m²), consisting of a car park and a mixture of mature and juvenile conifer woodland. The approximate length of the site is also relatively small at 350m. Fig 1.3 below gives an idea of the topography of Floodplain1; also see Appendices 1 and 19 for a 360º view, maps, as well as other pictures from various angles in the floodplain. The river channel is mainly composed of large cobblestones and boulders, with some areas of rocky outcrops changing the cross section of the river channel from typical u-shaped to a more constricted v-shaped profile. The banks are relatively high when compared to a typical floodplain area, although there are pockets along the rivers profile where the banks open out into the floodplain with a reasonably low gradient.

12

Introduction

Fig 1.3. Looking upstream at the inflow section of Floodplain 1.

Vegetation densities within the floodplain vary from 0.011m²m³-¹ to 0.265m²m³-¹ (see Appendix 6). As explained in Section 1.2, vegetation density is a critical parameter in hydraulic roughness and turbulence creation, so the distribution of vegetation density within Floodplain 1 and indeed Floodplain 2 is important. Appendix 4 shows hydraulic roughness height values for Floodplain 1, 2 and 3. These values are calculated by evaluating obstructions to flow, with vegetation density very often being one of the most significant. . 1.3.1.2 Floodplain 2 Floodplain 2 is located at the confluence of the Afon Mawddach and the Afon Eden. The approximate area of the site is 27200m², much of which is unaffected by flood events due to the steep inclines that surround the floodplain. The study area begins with a gorge section and then opens out into a flat heavily vegetated floodplain. Fig 1.4 below shows a picture of the inflow area, with the gorge section in the centre. For further images and views of Floodplain 2, see Appendices 2 and 20.

13

Introduction

Fig 1.4. Looking upstream at bridge and gorge section of Floodplain 2.

The Afon Mawddach river channel, in the foreground of fig. 1.4, is made up of small cobblestones and gravel, with a large gravel bar situated at the confluence of the Afon Eden (See Fig 1.5 on the next page). Large boulders are dominant throughout most of the Afon Eden, especially as you get closer to the confluence. The transition from river channel to floodplain is very often gradual, giving this site a more typical floodplain topography, especially when compared to Floodplain 1. The vegetation density of the area varies from 0.018m²m³-¹ to 0.082m²m³-¹, and resulting roughness heights can be seen in Appendix 4. The vegetation structure mainly compromises of juvenile conifers, ranging from 5 to 20 years. Located on the down streamside of the Afon Eden is a large area of dense bush and bramble, which is quite different to the vegetation structure of the rest of the floodplain (see Appendix 2). The hydraulic roughness of this area is likely to be quite limited, especially during peak discharges when velocities and flow depth are high.

14

Introduction

Fig. 1.5 Centre of Floodplain 2 1.3.1.3 Floodplain 3 Floodplain 3 runs for about 3.7km from the confluence of the Afon Wen and Afon Mawddach to the tidal point of the Afon Mawddach, located just west of the bridge of the A470. It has typical floodplain topography, with an area of approximately 1,007,500m², low banks and a flat undulating floodplain, varying in width from 50m to 250m. The predominant landuse of the floodplain is open pasture. Traces of riparian woodland are located along some of the riverbanks with a thin linear orientation. Due to the high proportion of grassland in the area, it was deemed unnecessary to do any vegetation density analysis of the area. Figures 1.6 and 1.7 give an idea of the topography of the area, with additional pictures and avi files available in Appendices 3 and 21.

15

Introduction

Fig. 1.6. 800m upstream of the outflow section of Floodplain 3

Fig. 1.7 Looking downstream at the outflow section of Floodplain 3. Note the bridge of the A470 in the centre of the picture. The river channel composition varies from fine alluvial sediment to reasonable sized cobblestones (60mm in diameter). Most of the deposition of finer sediments usually

16

Introduction takes place at the outflow area and around the two bridges, with the cobblestones mainly confined to the upper and middle section of Floodplain 3. The low deviation in elevation around the river channel and floodplain means that the area experience flooding regularly. Although Floodplain 3 is located downstream of the town of Dolgellau, it has large flooding importance for the Mawddach estuary and such towns as Barmouth. Due to the absence of a site-specific hydrograph and detailed flood debris lines, it was deemed unnecessary to model the existing conditions of Floodplain 3. Instead, the area is looked upon as a “test site” with four different scenarios modelled. With a high potential for flood amelioration due to low riverbanks and large floodplain area, Floodplain 3 will provide a very interesting comparison to the previous two sites.

1.3.2 Geology The main geology of the Mawddach catchment consists of hard, impermeable sedimentary and igneous rocks of the Cambrian period, overlain in several locations by sediments of the Ordovician and Silurian periods (Barton, 2002). The Cambrian era lasted approximately 200 million years and was dominated by the processes of slow subsidence and deposition. The subsidence was the result of a huge uplift that occurred due to collision of continental plates prior to this period. Succeeding the Cambrian period was an era of instability in which large earth movements occurred (e.g. Caledonian period), resulting in several fault lines, which still dominate the Mawddach catchment today. Finally, the effect of the glacial period was as influential in the Mawddach catchments as it was in the rest of North Western Europe, sculpturing the landscape with truncated U-shaped valleys and rugged mountain tops. The topographical characteristics of each of the rivers of the Mawddach system are quite similar in that they rise in flat moorlands, fall through constricted gorges and finally meander through the flat lowlands. Several strands of quartz veins also occur within the sedimentary and igneous rocks of Mawddach and surrounding regions. The formation of such veins was both post and pre-tectonic, the most famous of which is the “gold-belt”.

17

Introduction The geological factors outlined in the previous paragraphs are influential to the hydrological characteristics of the Mawddach catchment. Saturated upland catchments and constricted gorges sections results in rapid increases in discharge rates during flood events. The impervious nature of most of the underlying rock means that percolation to aquifers is only at best localised, therefore the hydrograph may increase substantially as the flood moves down through the catchment. These geological factors further enhance the importance of adequate flood control in such a catchment.

1.3.3 Landuse Looking at Fig.1.2 it is evident that the predominant Landuse of the Mawddach catchment is forestry, in particular conifer plantations. Coed y Brenin is located at the centre of the catchment and consists of 2,958 ha of conifer plantation. The forest was purchased from the Nannau Estate in 1922 by the Forestry Commission, and is almost at the end of the second rotation (http://www.forestry.gov.uk/forestry/). The other dominant landuse in the catchment is pasture, which is marginal in the uplands and stocked almost exclusively with sheep. However, the lowlands are densely stocked with sheep, dairy cows and beef cattle. The intensive agricultural practices in low-lying pastures results in high surface runoff, low hydraulic roughness and poor infiltration rates, therefore providing an almost “concrete” surface, which has obvious implications during large flood events.

1.3.4 Flooding and the Mawddach The Mawddach catchment receives and average annual rainfall of 2,000mm, ranging from 1,000mm on the coast to 2,500mm in the uplands (Barton, 2002). The implications of prolonged rainfall can be felt in less than a half an hour by increased discharge rates in the Mawddachs rivers (Hall, 2004). The topography, wet upland moors, hard parent rock and often-thin soils make the Mawddach catchment particularly susceptible to flash flooding. This type of flooding is difficult to forecast, as it is associated with atmospheric convectional activity, and often therefore causes the most loss of life and damage. Sudden bursts of precipitation mean that infiltration rates cannot possibly compensate, and as a result, the river holding capacity is quickly exceed as

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Introduction flooding of the surrounding areas occurs. Extremely high flow rates are often associated with flash flooding, with the potential to do extensive damage to infrastructure and vegetation alike. Photographs of Dolgellau dating back over a hundred years have shown the large scale damage caused by severe flood events in the Mawddach catchment. In 1964, the town council decided to construct a flood defence wall along the Afon Wnion following a severe flood that year, which caused extensive damage to the town and railway tracks. On completion of the flood defence wall at the end of the 1960s, no record Dolgellau flooding has been reported since, although flooding still occurs outside the town to this day (Barton, 2002). On July 3rd 2001, the Mawddach catchment and surrounding areas suffered a severe flood event. The storm responsible for the extensive flash flooding was tracked moving across the Irish sea early that morning, but by 4.30pm it had centred over the Northern half of the Mawddach catchment dumping up to 34mm per hour in the Afon Wnion catchment. Graph 1.0 below shows the discharge rate for the Afon Mawddach at the time of the July 3rd storm event. It is clear from looking at the hydrograph, just how flashy the deluge was, with a discharge rate of just 0.369 m³sˉ¹ prior to the storm event, and then just three and a half hours later it peaked to a massive 349.292 m³sˉ¹. To put these discharge rates into context, the initial stage level of 0.18m gives a water volume of 2.88m³ (assuming a 1m length of river, and 16m width), but at the peak of the July storm this swells to 65.3m³ (4.076 X 16 X 1), a 22.7 times increase in three and a half hours.

400 350 300 250 200 150 100 50 0

17 .0 0 18 .0 0 19 .0 0 20 .0 0 21 .0 0 22 .0 0 23 .0 0 0. 00 1. 00 2. 00 3. 00 4. 00 5. 00 6. 00 7. 00

Discahrge (cu.m/sec)

Hydrograph for July 3rd, 2001

Time

Graph.1.0 Stage Hydrograph for Afon Mawddach for July 3rd 19

Introduction It is apparent from Graph 1.0 that peak flows of almost 350m³sˉ¹ have been calculated for the July flood event, giving rise to extensive destruction of infrastructure within the surrounding area of the Afon Mawddach. Fig. 1.8, below, shows the destruction of an old stone bridge over the Afon Mawddach, taken two days after the July flood.

Fig. 1.8 Damage Caused by the July 3rd Flood (Courtesy of http://www.geologywales.co.uk/storms/storm02.htm)

Such hydrological information and pictures supports the theory of the topographical and geological characteristics of the Mawddach landscape adding to the effect of flash flooding. The huge increase in stage level and discharge rates at such short time intervals proves that the influence of steep topography, constricting gorges, thin soils and impermeable rocks are greatly influencing the hydrological responses of the catchment to large flood events. The Afon Wnion didn’t experience as extreme levels of flooding as the Afon Mawddach, however there was a 2.6 times increase from pre-flood volumes to peak flood volumes. The storm on July 3rd caused extensive damage to homes and infrastructure in the Mawddach, with costs for repair amounting to millions of pounds. No loss of life occurred but the damage caused by the flood can still be seen today and, with a calculated return period of just 17 years, such a flood event is a very real concern for the public and

20

Introduction local authorities. The hydrological data from the July 3rd flood event will be used to calibrate the River 2D Hydrodynamic model. This model will be used to assess the potential of floodplain forests and woodlands in flood amelioration (see Section 1.4 for more detail). Recent research into climatic change and global warming in Wales has show that extreme flood events are 10 to 50 times more likely by the year 2090 (Friends of the Earth, 2000). Other research into precipitation patterns in Wales over the last 97 years has show increases of 10% in winter rainfall, with decreases of 15% in summer rainfall, giving an overall increase of 3% (Barton, 2002). Further predictions for Wales include a 2-5% increase in precipitation by 2050. The importance of adequate flood defence infrastructure now appears to be even more critical to the safety of flood prone areas of Wales in the future. It could be argued that floodplains, and in particular floodplain forest and woodlands could provide a great scope to handle the ever-increasing extreme flood events, however this remains to be proven in a U.K context.

1.4 HYDRODYNAMIC MODELLING In today’s world, advances in computer capability and computational software technology are making detailed analysis of river environments common practice (Blackburn and Steffler, 2002). The application of 1D, 2D and 3D hydraulic models to engineering situations has broadened in recent years to multifunctional problems. There are limitations however to the application of some of these models, especially when dealing with complex topography. Knowing the limitations of a model will help develop a realistic and concrete hydraulic solution on which engineering or other such decisions can be based on. For the purpose of this dissertation, it was essential to choose a model that accurately reproduced the topography of a relatively small section of river and floodplain, simulate a large flood event and accurately producing velocities and water depths for such flood events. After initial analysis on the 1D hydraulic model Hec-Ras, and its ArcView extension Geo-Ras, it was decided that a 1D model wouldn’t give the desired accuracy, flexibility and presentation potential that was needed. When dealing with complex

21

Introduction topography such as a river channel with severe meandering or islands, it is widely believed that 1D models have often-poor topographical representation. This will therefore reduce accuracy and the subsequent viability of the solution (Wright, 2001). Inaccuracies may also arise by the omission of cross sections in key locations along the river channel. Considerable skill is often required when selecting correct locations for cross sections in 1D hydraulic models (Bates and De Roo, 2000). After subsequent analysis, it was felt that the 2D Depth Averaged Model, River2D, developed by Peter Steffler and Julia Blackburn from the University of Alberta, provided the adequate level of accuracy, modelling capability, presentation and useability that was necessary for this dissertation.

1.4.1 River 2D River 2D is a two dimensional depth averaged model based on the finite element method of solving governing equations. Finite element method originates from the “weighted residual method” which approximates the answers to equations by using a trial function. The trial function is specified, but has a number of degrees of freedom to achieve an estimated solution. River 2D solves the basic mass conservation equation and two (horizontal) components of momentum conservation (Steffler and Black, 2002). The conservation of mass states that the rates at which a particle of fluid movement brings it to occupy additional volume and vacate previous volume are identical (Lighthill, 1986). The conservation of momentum states that when there is a collision between two objects, the force on one of the bodies is equal and opposite to the force on the other body (http://www.mathsa.fsnet.co.uk/Maths%20A.htm). River 2D, because of its nature, assumes uniform velocity distributions in the vertical as well as hydrostatic pressure distributions.

1.4.1.1 Data Requirements In order for River 2D to perform flow simulations it requires a variety of input data. Such data includes topographical information of the river channel and floodplain, roughness coefficients, transverse eddy viscosity distributions, in and outflow parameters and boundary conditions. In order to achieve accurate and credible results from

22

Introduction simulations, the input data must be of the highest accuracy, the most critical of which is the topographical information. Topographical data can be obtained by several methods with the end result being a list of nodes with x, y coordinates and accompanying elevation values. The method of surveying in this dissertation is outlined in Section 2.1. Increasing the surveying intensity in key areas (such as river banks) is essential in order to transfer the geometry of the real world as accurately as possible into the computer model. Simplifying topography is one of the fundamental errors of hydrodynamic modelling. Adjustment of roughness values and turbulence in relation to vegetation cover will help assess the potential of floodplain woodlands in flood amelioration. In river 2D roughness coefficients are expressed as effective roughness height. This method of expressing roughness tends to be more accurate than Manning’s n, because it remains constant over a wider range of depths. Calibration of the model is achieved by adjusting the roughness values and transverse eddy viscosity distributions until the water depths and discharge of the model is equal to that of the hydrograph.

1.4.1.2 Sub Programs River 2D is divided into three sub programs each of which develops input data to a required stage so that the next program can comprehend such data and develop it further. The three programs are R2D Bed, R2D Mesh and River 2D 0.90. The topographical data acquired from the field is inputted into the sub program R2D Bed, either by using the “add fixed node” window in the program or by means of implicit or explicit code writing. The inputted nodes or points are then triangulated and breaklines and a computational boundary are specified (Steffler, 2002). Breaklines are imaginary lines, which outline a sudden change in topography, typical examples being the top and bottom of a riverbank (see Appendix 5). Roughness values are also assigned in this program, either at the input stage or by means of drawing counter-clockwise polygons around areas of similar roughness values. The information produced in the bed file is critical to the overall accuracy of the model, as this information is mirrored by the other two sub programs, and discrepancies are also mirrored, and often exacerbated.

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Introduction Once the bed file is produced, it is then exported in to R2d Mesh where a computation mesh is developed. R2d Mesh isn’t fully self-contained, and editing of the topographical data cannot take place. The primary function of this sub program is to develop a pattern of “floating” nodes, which form the computational mesh used in River 2D 0.90 to calculate water depth and velocities. The floating nodes taking their elevations from the triangulation of the bed file. The density and location of nodes is critical to the accuracy of the model. Mesh creation is a “fine art” in hydrodynamic modelling, and comparing the results of several different mesh orientations maybe necessary in order to achieve the most accurate pattern (Waddle and Steffler, 2002). Fig.1.9 below shows a typical mesh structure for a small stretch of river. Having high node density in key areas of interest is essential for model accuracy.

Fig. 1.9 Example of Mesh Structure

Creating a pattern of near equilateral triangles increases model accuracy and reduces the number of iterations per node, therefore reducing computational effort and RAM requirements. In addition, a gradual change in triangle size is desirable, with the large triangles located in areas of little importance or in even topography. Nodes located

24

Introduction on breaklines are called “sliding nodes”. The reason for this is that as the mesh is triangulated, the fixed nodes remain stationary, floating nodes move in any direction, but sliding nodes are limited to the breakline, sliding up and down along its length. This concept helps maintain high node density in key computational areas such as the bottom of a riverbank. There are several parameters in R2d Mesh to help create an accurate mesh such as QI value and elevation difference threshold. QI value is the ratio of triangle area to circum-circle area (the circle which passes through the three points defining the triangle) normalized to the corresponding ratio for an equilateral triangle. The perfect mesh would have a ratio of 1.0 (all equilateral triangles), but this is virtually impossible to achieve, so a ratio of 0.5 is more than adequate. Elevation difference threshold is the difference in elevation between the fixed nodes of the bed file and the floating nodes in R2d Mesh. Variations occur because floating node elevations are triangulated from neighboring fixed nodes, so the further the floating node is from the fixed node, the greater the elevation difference. Once the mesh has been refined to a sufficient accuracy, it is saved as a cdg file for input into River 2D 0.90. River 2D 0.90 is a two-dimensional depth averaged finite element hydrodynamic model with subcritical and supercritical capabilities (Blackburn and Steffler, 2002). Basically it is a transient model, but provides for an accelerated convergence to steadystate conditions. The conservation of mass is expressed in the St. Venant Equations, with the dependant variables of depth and discharge intensity for x and y coordinates being solved during simulation. Because the finite element method is used to solve the hydrodynamic equations, results are in a non-symmetric, non-linear form. Implicit procedures in the form of the Newton-Raphson iterative method are then used to achieve useable results. Groundwater flow equations are used in wet/dry areas, to avoid computational error that may occur by using surface water equations. River 2D has several simulation parameters including; time increment, goal time increment, maximum number of iterations, solution tolerance and implicitness. Time increment is the length of time that elapses before the next set of governing equations are calculated at each node. This parameter will increase and decrease as the simulation progresses in order to keep the model within the solution tolerance. The solution

25

Introduction tolerance is the maximum difference in values from the current to the previous iteration. If the difference in results is below the tolerance the model moves on to the next time step, if however the solution difference is greater than the tolerance, the iteration is rejected. The goal time increment is the maximum difference in time that the model will allow through the simulation. The maximum number of iterations is the maximum number of Newton-Raphson iterations per time interval. If the solution doesn’t reach convergence before it reaches the maximum number of iterations, the time step is rejected. Consequently the time step is reduced by half its current value in order to reach convergence at a more detailed level. Implicitness specifies weather the governing equation is to be solved explicitly or implicit. The stability of the model can be controlled through this parameter, i.e. fully implicitly will give a stable solution, but with a low level of detail. The model is calibrated, mainly by adjusting the turbulence parameters located in the flow options. Depth-averaged transverse turbulent shear stresses are modeled with a Boussinesq type eddy viscosity formulations. This eddy viscosity coefficient is made up of three user definable parameters, ε1, ε2 and ε3. ε1 is a constant, ε2 is a bed shear generated term and ε3 is a transverse shear generated term. The ε3 value is an important flow parameter when dealing with deep water, high transverse velocity and recirculation. Such flow characteristics will be dominant in simulations undertaken in this dissertation, especially when dealing with high roughness values such as dense woodland. In transient modeling the key is to obtain accurate spatial results throughout the duration of the hydrograph. Adjustment of the parameters outlined in the previous paragraph is critical to achieving the desired result.

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Introduction 1.5 OBJECTIVES

The main objective or this dissertation is to investigation of the potential for floodplain woodland in flood amelioration. There are several variables that affect floodplain woodlands ability to mitigate floods. All these variables will be investigated through hydrodynamic modelling, either individually or collectively. Such variables are; 1. Woodland size 2. Woodland location 3. Tree density / basal area

These sub-objectives will be tested by modelling different topographical and friction scenarios (see Section 3.0) within the hydrodynamic model, River 2D, and examining the resulting water depths and velocities for a similar flood event. Friction coefficients will be related to vegetation density through Manning’s n values. Stem density will be examined through the creation of local turbulence and its subsequent effects on velocity magnitude and direction. River 2D is a finite element model, therefore has high geometric flexibility, meaning that shape and location of the floodplain woodlands can be easily manipulated within the model. Subsequent velocities and water depths will be examined to see the effect of such parameters.

1.5.1 Modelling Scenarios To investigate the variables that effect floodplain woodlands ability to mitigate floods several different modelling scenarios were designed for each site. For Floodplain 1, three different scenarios were proposed: 1. Calibrated 2. Dense Woodland 3. Grassland “Calibrated” is the existing hydraulic conditions (n values) within the floodplain. To model grassland, Manning’s n values were obtained from previous studies with the topographical irregularities of the floodplain also being taken into account

27

Introduction (see Appendix 6). Adjustment of analyzed vegetation densities from the field and subsequent roughness coefficients gave a quantitative method of modeling dense woodland. Floodplain 2 had a similar scenario set of conditions: 1. Calibrated 2. Dense Woodland 3. Grassland The methods of modelling such scenarios are identical to that of Floodplain 1. Resulting roughness height maps of each floodplain can be seen in Appendix 4. Floodplain 3 had four different modelling scenarios: 1. Dense Woodland 2. Sparse Woodland 3. Grouped Woodland 4. Grassland Once again, the methods of modelling such scenarios are similar to that of the previous two floodplains, with the only exception being scenario 4. To model grouped woodland roughness values equal to that of dense woodland were used, but the distribution of such woodland varied throughout the floodplain (see Appendix 4). Along with varying friction coefficients, turbulence parameters with the model (River 2D) were also adjusted to reflect each scenario. For more details, see Section 2.3.3.

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Methods

2.0 METHODS

29

Methods

2.0 METHODS The experimental methods undertaken in this dissertation are divided into three sections; fieldwork, interpolation and computer modelling. Methods are described in sufficient detail so that they can be easily reproduced in similar studies, however it is assumed that documenting exact procedures is unnecessary, and therefore it has for the large part been omitted. Surveying was undertaken each morning from 9am to 12.30. After lunch interpolation of the surveyed data and subsequent input into R2d Bed was undertaken. It was felt that such a procedure had several key advantages over any other method. Such advantages are illustrated throughout the preceding sections.

2.1 SURVEYING The surveying carry out for this dissertation was completed on three sites on the Afon Mawddach (see Fig 1.2, page 12). The procedure for surveying Floodplain 1 and 2 is identical and therefore no site-specific procedure is necessary. However the procedure for Floodplain 3 is slightly different, therefore the methods used are outlined separately. The fieldwork is primarily of a surveying theme, and orientated around the input data requirements of the hydrodynamic model, River 2D. Other procedures carried out on site included, identification of flood debris lines in specified locations for model calibration, analysis of the friction values for the river channels and floodplains and vegetation density analysis for Floodplain 1 and 2.

2.1.1 Materials The materials necessary for the surveying of the two sites were; 1. Tripod 2. Optical Level (Sokkisha C3E) 3. 2m Surveying Poles 4. 30m Metric Tape 5. Drawing Board 6. Graduated Levelling Staff 7. Waders

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Methods

Fig. 2.0 Surveying Equipment

2.1.2 Surveying Procedure for Floodplain 1 and 2 As mentioned in Section 1.4.1.1, the purpose of survey the floodplains and river channels is to achieve a detailed topographical representation for triangulation in River 2D. The topographical data requirements for River 2D are x and y coordinates and a corresponding z values or elevations. The initial procedure was to us Global Positioning System (GPS) to obtain x and y coordinate points, and for these points to be cross referenced with direct linear measurement via the tape measure. However after analysing the results of both methods it was found that the GPS was on average 5 meters out when compared to the tape measure. Such inaccuracies were completely unacceptable, especially when dealing with such a minute area as was for this dissertation. It was felt that the accuracy of the tape measure, although time-consuming, was satisfactory, and so

31

Methods the GPS was made redundant. However because of the open terrain and subsequent high accuracy, GPS was used on Floodplain 3 to locate the optical level. The procedure begins (using Fig 2.1, below) by identifying a datum point. The location of such a point needs to be in an area of relatively unobstructed view for surveying purposes. The optical level is set up and levelled, with the distance from the centre of the lens to the ground being measured. Two nodes or points are then located close to (5-10meters) the level, their location maybe based on a sudden change in topography, or the fact that a clear view to the level can be obtained. Every effort was made to attain a near equilateral orientation, as this will greatly reduce the computational effort of River 2D. The horizontal distances from the level to the two node points are then measured (A and B in Fig. 2.1) using the tape. Simultaneously, the elevations are measured for each node, relative to the datum using the optical level. The procedure continues, all the time working away from the level, identifying key topographical areas.

Fig 2.1 Surveying Method When and where possible, crosschecks are made by measuring such distances as 1,2 or 3 in Fig 2.1. This means that there are three known distances locating one point in the mesh, which will help check measurement accuracy and subsequent error when the points are transferred onto a grid. Progress to a new node point can therefore only take

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Methods place when at least two known, distances linked it to other nodes of known elevation and location. As the surveying progresses a sketch of the developing mesh is drawn onto the sketchpad, with calculated distances and elevations at each node documented. Once the days surveying is complete, a triangle of three node points is marked. These points will be used to link the next days surveying, so ensuring that all nodes points remain spatially accurate on the grid. Elevations for each point are obtained by subtracting the reading on the staff from the elevation of the point located below the optical level. Using fig 2.2 below as a simple illustration to obtain the topographical elevation for point A the formulae is as follows; A= (L+d)-D or (3.34+1.45)-3.24=1.55. L is the calculated elevation of the point located underneath the optical level; d is the distance from the centre of the lens to the ground and D is the reading from the staff.

Fig. 2.2 Obtaining Elevations

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Methods When the optical level needs to be relocated to a new position the elevations of two previously calculated node points are re-measured specific to the new location of the level. This helps ensure that the ground elevation of the new location for the optical level is accurate.

2.1.3 Surveying Procedure for Floodplain 3 The procedure for surveying Floodplain 3 is similar to that of Floodplain 1 and 2, except cross sections are used instead of triangles to depict the topography. Due to the low density of large vegetation such as trees, the majority of elevation values for Floodplain 3 were obtained from a 50m Digital Elevation Map (DEM). The low variation in topography around the channel meant that the DEM provided a good accurate depiction of the landscape, with the exception of some key areas of the river channel. Due to the large resolution of the DEM (50m), some key areas of the river channel were omitted. This meant that in R2D Bed surrounding elevations were triangulated over the river channel in such areas, therefore giving the impression that the river channel merged with the surrounding floodplain. Comparing in the output map of R2D Bed with paper maps of the area identified such areas of topographical discrepancy. Due to the open topography (employing good GPS accuracy), GPS was used to locate the areas of inaccuracy. Surveying the topography wasn’t done by a series of triangles, as in Floodplain 1 and 2; instead the elevations of the floodplain and river channel were obtained by surveying in a straight line (cross section). This method of surveying is similar to that that would be used as an input in a 1D hydrodynamic model. The main advantage of such a method is speed. As node points do not have to be crosschecked from other fixed nodes for location, it saves considerably on time. Once the first point is fixed (via GPS), all that is then needed is horizontal distances for that point to the remaining points on that cross section. Elevations were taken at each point along the cross section, with the optical level being located on the highest point of the riverbank. As the elevations of the DEM are in relation to sea level, it was necessary to calibrate the elevations obtained for the optical level to that of the DEM. This was done on an excel spreadsheet (see Appendix 15).

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Methods The procedure was as follows;

1. Get the grid position of the level from the GPS reading, and take this as the position of the survey point closest to the level. 2. Find the compass direction of the cross section from the OS map by taking the negative x-axis as degrees clockwise. 3. Add the distances and recorded heights along the cross section. 4. Calculate new x- and y-grid coordinates using sine/cosine. The columns “dx” and “dy” show in Appendix 7 show the changes in x and y position relative to the level. This was used as a check that the calculations were reasonable. 5. Get the approximate local elevation of the flood plain from the OS grid data, and use this to adjust the recorded heights so that the riverbanks fit correctly. Courtesy of Graham Hall

Seven cross sections were taken along the 3.7km stretch of Floodplain 3, and it was felt that this would give sufficient detail so that R2d Bed would accurately depict the landscape of this area. 2.1.4 Vegetation Density Analysis and Manning’s Roughness Coefficients Almost all predetermined Manning’s coefficient tables for dense vegetation such as woodland have been derived outside the UK, mostly in the US. The application of such tables to UK woodland would inevitably induce some inaccuracies therefore it was deemed necessary for this dissertation to complete some vegetation density analysis of Floodplain 1 and 2. Once vegetation densities had been established subsequently Manning’s n values could then be obtained using some simple formulae. The procedure outlined in the “Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Floodplains” (Arcement and Schneider) was used for both vegetation density analysis as well as calculating subsequent Manning’s n values. Four plots were identified in both Floodplain 1 and 2. Such areas were identified as an average representation for the surrounding vegetation, therefore anytime there was a visible change in vegetation density a new plot was established. Figures 2.3 and 2.4 show

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Methods the locations of the eight plots used for vegetation density analysis. Also, examining Appendices 1 and 2 will show digital pictures of each vegetation plot for both Floodplain 1 and 2. Once the vegetation densities had been determined the corresponding friction values were calculated using formulae outlined in Arcement and Schneider. Appendix 6 shows the vegetation densities and resulting friction values for each plot. Appendix 4 shows the roughness heights for Floodplain 1 and 2 (i.e. the relationship between Manning’s n and water depth). The application of these friction values to the hydrodynamic model is outlined in Section 2.2.

Fig. 2.3 Vegetation Plots for Floodplain 1

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Methods

Fig. 2.4 Vegetation Plots for Floodplain 2

2.2 INTERPOLATION Once the days surveying was completed, interpolation of the acquired sketches from the sketch board to a measure grid was undertaken. The scale used for the grids was 4:1000 with the 0,0 point (origin) orientated towards the bottom right-hand corner. Each box on the grid represents 100m² in the field or 160mm² on the grid. Fig. 2.5, on the next page, shows one of the grids used in the Floodplain 2. The x and y axes are specified as well as their numerical values located on the top and left-hand side of the grid. The numbers circled in yellow are the node numbers, with their corresponding elevations written alongside.

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Methods

Fig. 2.5 Grid used as input into R2d Bed. 2.2.1 Procedure Points were located on the grid by means of swing arcs from previously know points with measures distances from the field. Take the location of node 9 for example in Fig. 2.5. Assume that the location of node 2 and 1 has already been established. In the field the horizontal distances from node 9 to node 1 and 2 have been measured, therefore the simple procedure of swing arcs with a compass of distances to scale of that in the field (4:1000) from node 1 and 2 on the grid will give you the exact location of node 9. As already mentioned in Section 2.1.2, where possible three fixed nodes where used to locate one new point. Using the example illustrated in Fig. 2.5 the extra node used to locate node 9 would be either node 5 or 3. This ensures that the precise location of the new node is established, with the point being located by the presence of three intersecting arcs. A new grid is drawn up each day with the first new node being located from at least two fixed nodes from the previous day.

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Methods 2.3 COMPUTER MODELLING Once the surveying, interpolation and friction analysis was completed, inputting into R2d Bed was undertaken. The preceding sections outline the methods used in this dissertation to complete the hydrodynamic analysis of Floodplain 1 and 2 in chronological order.

2.3.1 Input into R2D Bed Once the nodes have been located on a measured grid (see Section 2.2), their x and y coordinates must be determined. A scaled ruler was used to convert the dimensions to field representation (i.e. 4mm = 10m) so the resulting x and y values could be inputted into R2D Bed. The nodes were inputted using the “Add Node” function in R2D Bed, with node numbers and elevations also being entered simultaneously. As the previous days surveying was entered into R2D Bed the following day, it helped to give an idea of the scale of surveying that had been done, and more importantly, how much surveying had been left to do. Any areas that the model triangulated dubiously could be resurveyed to improve accuracy. Similarly, areas that weren’t surveyed in huge detail (such as high ground above the flood levels), but was triangulated accurately by the model, meant that huge amounts of time was saved on surveying areas of low importance. Breaklines were drawn at the top and bottom of the riverbanks, as well as in other areas of significant topographical change. The bed roughness coefficients that were evaluated from the field were also inputted into R2D Bed by drawing polygons around areas of similar roughness values. Roughness values had to be converted from Manning’s n to roughness height (k) with the “Roughness Converter” provided in the model. The computational boundary was also set in R2D Bed by drawing a counter clockwise polygon around the area of hydrodynamic interest. It was found that to keep the exterior boundary on the limits of the surveyed area meant that subsequent computational meshes were unusable as some floating nodes were located outside the of boundary. This would corrupt the mesh file, and therefore mean that it could not be re-opened for further use or editing. Therefore keeping the exterior boundary in R2D Bed at least 5-10m from the limits of the surveyed area would eliminate this problem.

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Methods As the various scenarios were hydrodynamically modelled, their corresponding roughness values and shape were edited in R2D Bed. Roughness values for Floodplain 3 are superficial, with no model calibration necessary.

2.3.2 Mesh Creation in R2D Mesh Once all the surveying data had been put into R2D Bed, a computational mesh had to be designed and overlaid. As mentioned in Section 1.4.1.2, the mesh design can be critical to the overall accuracy of the model. A uniform fill of floating nodes was done first, with a spacing of 10m. Additional nodes were then placed in key areas such as on the floodplain or riverbanks. Breaklines were also bisected and sliding nodes inserted to ensure accurate modelling of the riverbanks and other key topographical features. To view the mesh maps and information for Floodplain 1, 2 and 3 go to Appendix 5. It was found during this dissertation that the RAM size and processor potential of the pc was a large limiting factor in mesh design. The temptation is to design a mesh with high node density, therefore producing a simulation of high accuracy and creditability. This was the case for the first few mesh designs of Floodplain 1, however several simulations crashed because of lack of RAM, so modifications had to be made. Keeping the Q.I value has high as possible greatly reduces the computational time of the model and also reduces the RAM allocation. This is so because as River 2D runs through the simulation it will take less iterations to complete equations at each node by having a high Q.I value (i.e. near equilateral triangles in the mesh). Achieving smooth fast simulations is based on trial and error of the mesh design. Once a mesh design was finalised, it was saved and an input cdg file for River 2D was done. Even though the bed file was edited for the various modelling scenarios, one computational mesh will suffice for all simulations, provided all nodes are located within the exterior boundary.

40

Methods 2.3.3 Hydrodynamic Modelling in River 2D Due to of the absence of outflow stage hydrographs photographs of debris lines (see Appendices 1 and 2) were matched with topographical contours in River 2D to evaluate the maximum flood line for each site resulting from the July, 2001 flood event. This method of comparing actual and computed flood extents gave and indication of the accuracy of the friction coefficients and turbulence parameters within River 2D. As Manning’s roughness values were derived from the vegetation density analysis, the resulting roughness coefficients were very close to the hydraulic conditions within each site. The only parameters used to adjust the resulting water surface elevation within the model were the turbulence parameters (see Section 1.4.1.2). The model was continuously re-run, changing the transverse shear turbulence parameter (ε3), till the peak water surface elevations in the model equalled that of recorded debris lines and pictures for each floodplain site. ε3 values of 1.2 and 1.35 calibrated Floodplain 1 and Floodplain 2, respectively. Due to the absence of a site specific hydrograph for Floodplain 3, as well as the lack of detailed evidence of peak water surface elevations for the July 01 flood, it was deemed unnecessary the calibrated the model for this site. Calibration would have been at best dubious, and it would have been difficult to have full confidence that the results of the model were identical to that experienced in July 01. As a result, Floodplain 3 is looked upon as a “test site” rather than an investigation of how the existing vegetation effect such as flood event as July 01. The ε3 value was set close to that of Floodplain 1 and 2 when modelling woodland (1.25), therefore ensuring that results were realistic and comparable. Vegetation densities were adjusted from Floodplain 1 and 2 to make them applicable to Floodplain 3 (see Appendix 6), with channel roughness obviously being site specific. Once each site was calibrated the various modelling scenarios were tested. As it was difficult to relate the relationship between vegetation density and the turbulence equations in River 2D, the ε3 value remained unchanged from that of calibration when modelling the effect of trees on flow rates and water levels. When modelling arable or grassland the ε3 value was adjusted to 0.1 to reflect the effect that such vegetation would have on turbulence creation. Manning’s roughness values were obtained from the

41

Methods relationship between vegetation density, hydraulic radius and drag coefficient (see Appendix 6). The roughness values were then converted to roughness height (ks), based on hydraulic radius and distributed across the floodplains in proportion to the observed data. Such a method meant that resulting water surface elevations and velocities could be directly linked back to vegetation density, and even tree diameter. Appendix 4 shows the roughness maps for each modelling scenario of each floodplain. Simulations were run in transient (unsteady) mode, therefore requiring inflow hydrographs. The discharge hydrograph for Floodplain 1 was obtained from an Environment Agency recording station upstream of the inflow boundary (see Appendix 16). In Floodplain 2, two hydrographs were necessary. The hydrograph for the Afon Eden was obtained courtesy of Graham Hall, and was derived from previous recordings of that river. The inflow for the Afon Mawddach was obtained by using the subsequent outflow hydrograph of the calibrated scenario of Floodplain 1. Although there is approximately 1 mile of river between the two floodplains its topography is typically rock gorges and high riverbanks so the variation in hydrographs is minimal. Looking at Appendix 8, it is evident that the shape of the Afon Eden hydrograph gives a less “flashy” flood event. The reason for this is that the Trawsfynydd reservoir is sourcing water via a canal from the Afon Eden at high water levels. This means that the peak flows for the Afon Eden are actually diverted into the reservoir rather than down through the Mawddach catchment. The inflow hydrograph for Floodplain 3 is obtained from the outflow hydrograph of Floodplain 2. Once again there is a considerable stretch of river between the two sites, as well as the discharge of the Afon Gamlan, but because Floodplain 3 is not calibrated the accuracy of the inflow hydrograph is not critical. Results were outputted by avi files (Appendix 12) and spreadsheets of water surface elevations, water depth and X and Y discharge (Appendices 13 to 15).

42

Results

3.0 RESULTS

43

Results

3.0 RESULTS As mentioned in previous sections, results are obtained using strategically placed cross sections (see Appendix 7). As River 2D can only output x and y discharge, cross-sections are orientated in such a way to allow either the x or y discharge to be graphed against time or across the cross section (see Graph 3.5). Examining discharge in floodplain areas usually requires looking at both the x and y discharge at a particular point, as flow direction is rarely one-dimensional (see Graph 3.2). The result of such a graph is a vector line (usually illustrated by a linear trend line of the data), which is representative of both direction and magnitude of flow. In relation to the legend of each graph, “Calibrated” refers to the current vegetation conditions on the site in question (for Floodplain1 and 2 only). With reference to the overall result of each modelled scenario, the outflow hydrograph is perhaps the most critical (see Graph 3.6). This clearly displays the relationship between inflow and outflow, as well as time. If a particular scenario is to be “effective” in flood mitigation the variation between the inflow and outflow curves must be unambiguous, keeping in mind the scale of the x and y-axis. Each discharge graph gives a minuscule picture of the overall flow field of the site and, conclusions must not be drawn solely on the information displayed by a few scattered discharge graphs. Avi files (Appendix 12) give a pictorial representation of the flow field of each site with either velocity of discharge being represented by both vectors and colour shading. Although such files do not give a measured result (apart from inflow and outflow), they do give an overall impression of the hydrodynamics of each site as the flood event progresses. Water surface elevations are perhaps more readily comprehendible than discharge graphs. The variations include plotting the water surface elevation of a particular point over time (see Graph 3.12) or, plotting the water surface elevation of a whole cross section at a particular moment within the hydrograph (see Graph 3.1). When the title “Location” is labelled on the x-axis it is referring to a point on the cross section in question. Looking at Appendix 7, the red arrows at each cross section represent the

44

Results orientation of the graph in relation to x values; for example point 1 on a graph is located on the opposite end to the direction of the red arrow on the maps in Appendix 7. The location points on the x-axis are essentially unit-less, due to the fact that the number of intervals that each cross section is divide up into varies. However, the bed elevation contour in each graph helps to give an idea of the topographical location of each point, especially in relation to river channel or floodplain. Nevertheless, care must be taken when looking at bed elevation, as distortion is present in almost every graph with the yaxis values (elevation) often grossly exaggerated. This phenomenon maybe especially prolific in areas of flat topography and wide cross-sections (i.e. Floodplain 3). The following sections outline the most critical graphs for each site. As most graphs are looking at only one point in time assumptions must not be made solely on these graphs. To get a distinctive pattern of the hydrodynamics of an event, examination of preceding and subsequent graphs is essential (see Appendices 8 to 10). Over and above, viewing respective avi files will give a more dynamic picture of the flood event, helping to reassure conclusions. If time isn’t a variable in the outlined graphs it can be assumed that the graph is illustrating the hydrodynamic situation at the peak or the flood (i.e. this would be about 12000 seconds for Floodplain 1 and 2). Using the inflow hydrograph from each site will help to give an idea of the hydrodynamic importance of the graph with a fixed time constraint.

45

Results 3.1 FLOODPLAIN 1

Water Surface Elevations Across CS1 at 12000sec 4 Calibrated

Water Surface Elevation (m)

3

Bed Elevation

2

Dense Woodland

1

Grassland

0 -1 0

2

4

6

8

10

12

14

16

-2 -3 -4 -5 -6

Location

Graph 3.0 Water Surface Elevations Across CS2 at 12000sec Calibrated


1

Dense Woodland Grassland

0 0

2

4

6

8

-1 -2 -3 -4 -5 -6

Location

Graph 3.1

46

10

12

Bed Elevation 14

Results

Water Surface Elevations Across CS3 at 12000sec


Grassland 1

Calibrated

0

Dense Woodland

-1 0

2

4

6

8

10

12

14

Bed Elevation

-2 -3 -4 -5 -6 -7 -8

Location

Graph 3.2 Discharge Intensity at Point 6 (Floodplain) in CS2

Y-Discharge Intensity (m2/sec)

0.25 0 -1.75

-1.5

-1.25

-1

-0.75

-0.5

-0.25

0 -0.25 -0.5

Calibrated -0.75 Grassland -1

Dense Woodland

-1.25 -1.5 -1.75

X-Discharge Intensity (m2/sec)

Graph 3.3

47

Results

X-Discharge Intensity Across CS4 at 12000sec


-12 0

2

4

6

8

10

12

14

16

18

20

22

24

-12.5 Grassland -13

Calibrated Dense Woodland

-13.5 Linear (Calibrated) Linear (Grassland)

-14

Linear (Dense Woodland) -14.5 -15 -15.5

Location

Graph 3.4

Discahrge (cu.m/sec)

Hydrograph of Inflow and Outflow for Floodplain 1 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

Inflow Hydrograph Calibrated Outflow Dense Woodland Outflow Grassland Outflow

0

5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000

Time

Graph 3.5

48

Results

Hydrograph of Inflow and Outflow for Floodplain 1 Inflow Hydrograph

380 360

Calibrated Outflow

340 320

Grassland Outflow

Discharge (cu.m/sec)

300

Dense Woodland Outflow

280 260 240 220 200 180 160 140 120 100 80 60 40 20 0 6000

8000

10000

12000

14000

16000

18000

20000

Time (seconds)

Graph 3.6 3.2 FLOODPLAIN 2 Water Surface Elevations Across CS1 at 13000sec


Calibrated 3

Dense Woodland

2

Grassland Bed Elevation

1 0 -1

0

2

4

6

8

-2 -3 -4

Location

Graph 3.7

49

10

12

Results



Calibrated

3 2 1 0 -1 0 -2 -3 -4 -5 -6 -7 -8

Dense Woodland Grassland Bed Elevation

2

4

6

8

10

12

14

16

18

Location Graph 3.8

Water Surface Elevations Across CS4 at 12000sec Calibrated

-1


0

2

4

6

-2

8

10

Dense 12 Woodland Grassland Bed Elevation

-3 -4 -5 -6 -7 -8

Location

Graph 3.9

50

Results

Water Surface Elevations Across CS7 at 12000sec 0


0

2

4

6

8

10

12

14

16

18

-1 -2


-3

Grassland -4

Bed Elevation

-5 -6 -7

Location

Graph 3.10 Y-Discharge Intensity at Point 8 (River Channel) in CS5 0


0

10000

20000

30000

40000

50000

60000

70000

-2 -4 -6

Calibrated Dense Woodand

-8

Grassland -10 -12 -14

Time

Graph 3.11

51

Results

Y-Discharge Intensity Across CS5 at 12000sec 0


0

2

4

6

8

10

-2 -4 Calibrated

-6

Dense Woodand -8

Grassland

-10 -12 -14 -16

Location

Graph 3.12

Floodplain 2 Inflow and Outflow Hydrographs 500

Inflow Calibrated Outflow Eden Inflow Mawddach Inflow Dense Woodland Outflow Grassland Outflow

450

Discharge (m3/sec)

400 350 300 250 200 150 100 50 0 0

10000

20000

30000

40000

Time

Graph 3.13

52

50000

60000

70000

Results 3.3 FLOODPLAIN 3



14 13 12

Grassland

11

Group Woodland

10

Dense Woodland

9

Bed Elevation

8

Sparse Woodland

7 6 5 0

2

4

6

8

10

12

14

16

18

20

Location

Graph 3.14 Water Surface Elevation for Point 7 (Floodplain) in CS2 8


Grassland

7.5

Group Woodland

7

Dense Woodland Sparse Woodland

6.5 6 5.5 5 4.5 4 0

10000

20000

30000

40000

Time (sec) Graph 3.15

53

50000

60000

70000

Results



Grassland

14

Group Woodland

13

Dense Woodland

12

Bed Elevation Sparse Woodland

11 10 9 8 7 6 5 4 0

2

4

6

8

10

12

14

16

18

Location Graph 3.16

Water Surface Elevations Across CS4 at 17000sec Grassland

10

Group Woodland


9

Dense Woodland

8

Bed Elevation

7

Sparse Woodland

6 5 4 3 2 1 0 0

2

4

6

8

10

Location Graph 3.17

54

12

14

16

18

Results



4.5 4 3.5 3 2.5

Grassland

2

Group Woodland

1.5

Dense Woodland Bed Elevation

1

Sparse Woodland 0.5 0 0

2

4

6

8

10

12

14

16

18

20

Location

Graph 3.18 Discharge Intensity at Point 7 (Floodplain) in CS2


0.500

-3.000

-2.500

-2.000

-1.500

-1.000

0.000 -0.500 0.000 -0.500

Grassland

-1.000

Dense Woodland Group Woodland

-1.500

Sparse Woodland Linear (Dense Woodland)

-2.000

Linear (Group Woodland)

-2.500

Linear (Grassland) Linear (Sparse Woodland)

-3.000


Graph 3.19

55

0.500

Results

Y-Discharge Intensity Across CS4 at 14000sec


0.5 0 -0.5 0

2

4

6

8

10

12

14

16

18

-1 -1.5

Grassland Group Woodland Sparse Woodland Dense Woodland

-2 -2.5 -3 -3.5 -4 -4.5

Location Graph 3.20

Outflow Hydrographs for Floodplain 3 325 300


275

Grassland Grouped Woodland Dense Woodland Sparse Woodland

250 225 200 175 150 125 100 75 50 25 0 0

5000

10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000 65000

Time (sec)

Graph 3.21 56

Discussion

4.0 DISCUSSION

57

Discussion

4.0 DISCUSSION The following sections consist of a discussion of the results of each floodplain site. Although the bulk of the discussion will be based on the graphs presented in Section 3.0, some reference will be made to Appendices 9, 10 and 11 so that a credible and accurate discussion can be achieved.

4.1 FLOODPLAIN 1 Looking at Graph 3.0, it is evident that there is a clear distinction in water surface elevations between the three scenarios. Calibrated is on average, 400mm higher than grassland. Dense woodland is typically 550mm above the grassland and 150mm above the calibrated scenario. These trends are reasonably consistent across the whole cross section of Graph 3.0, which extends from some 110m from the inflow to the opening of the main floodplain area (see Appendix 7). Such results would indicate a significant “backwater” effect, which is mainly due to the high basal areas and therefore high turbulence with the floodplains of dense woodland and calibrated. The effect of high turbulence levels means that stream tubes are disrupted and subsequent velocities reduced. As the flood progresses, the backwater swell migrates further and further upstream, and subsequent water levels increase. Looking at Appendix 8 it is evident that this backwater effect continues right through the whole flood event, although always abating once past the flood peak (12000 seconds). It must be noted that, taking water surface elevations along the river channel it is unlikely to be closely representative of the accompanying floodplain. The presence of stream tubes and higher velocities will result in significant deviations from that of the floodplain. This concept is clearly evident in Graph 3.1, with an average of 500mm increase in water levels from the river channel to the floodplain. Once again water surface elevations are as expected, with dense woodland producing the highest elevations followed closely by the calibrated scenario. The relatively low water levels produced by grassland are plausible, since such vegetation is likely to create less turbulence and higher through flow rates. The cross section in Graph 3.1 runs across the widest stretch of the

58

Discussion floodplain. This shows increased deviation in waters levels, especially when comparing the heavily vegetated scenarios to grassland. Such results are also plausible as the influence of vegetation on flow velocities will inevitably be greater in the floodplain than in the river channel. Graph 3.2 is taking some 10m upstream of the outflow boundary in Floodplain 1. For this reason the results obtained from this cross section are likely to be critical in relation to the overall effect of each scenario on flow rates and water levels. All three scenarios at the flood peak produce identical water surface elevations. Such a result is not just restricted to the peak of the flood as Appendix 8 shows similar results for water levels taking throughout the Floodplain 1 hydrograph. CS3 (Cross Section 3) is located at the closing of the main floodplain area of the site, therefore the effect of the various scenarios on water surface elevations is not likely to have dissipated by the time this cross section has been reached. Examining discharge graphs such as Graph 3.3 will help give a more concrete picture of the hydrodynamic patterns of Floodplain 1. Graph 3.3 shows x and y-discharge intensity at the centre on CS2. Magnitude and direction are close to scale, with some small distortion present. Assuming that the densely vegetated scenarios should produce the smaller discharge vector would be technically incorrect. The direction of the vector is just as critical as its length, especially in the floodplain. In the case of Graph 3.3, no real justifiable pattern has emerged. Dense woodland has the shortest vector, with calibrated producing the longest and most horizontal vector. Flow fields within the floodplain are likely to be very complex and variable, therefore comparing the three different scenarios at just one point in a cross section will give a very minuscule picture of the whole event. Graph 3.4 gives x-discharge across the whole of CS4 with linear trend lines being plotted to help illustrate the development of discharge. The shape of the curves are sensible, with discharge increasing in intensity as one progresses towards the river channel from the floodplains on both sides. The calibrated scenario shows the smallest discharge intensity (all be it in the negative x direction) with no real distinction being evident between dense woodland and grassland. However looking at the discharge graphs in Appendix 8 it is apparent that once the flood has peaked, dense woodland does show a closer correlation with the calibrated scenario, with grassland producing the highest

59

Discussion discharge intensities. Therefore it could be said that once the floodplain is at its maximum water depth (i.e. at 12000 seconds) the discharge curves develop into a more plausible pattern. The avi files in Appendix 12 give a good pictorial representation of flow direction, especially when vectors rather than colour shading show velocity. From such files it is evident that the flow field its too dissimilar for the three different scenarios. Graphs 3.5 and 3.6 are critical to measuring the overall ability of Floodplain 1 in flood amelioration. For the site to be effect in abating the flood, the outflow hydrograph for calibrated and dense woodland should show a marked deviation from that of grassland. Such a deviation clearly isn’t present in either graph. The three scenarios are closely correlated throughout the flood event with only slight anomalies occurring at various stages. Further more the scale of the site is illustrated in both graphs with the discharge time from inflow to outflow being some where in the region of 200 seconds. The hydrographs (graphs 3.4 and 3.5) reflect the discharge graphs previously discussed, with no real distinction emerging between the three scenarios. Although the water surface elevations did show promise, the topography of the site may have exacerbated such results (i.e. narrow floodplain giving rise to large water levels).

4.2 FLOODPLAIN 2 Graphs 3.7 and 3.8 in Section 3.2 echo the water surface elevation trends which occurred in Floodplain 1. Once again dense woodland produces the highest water levels followed closely by the calibrated scenario. Graph 3.7 shows water surface elevations contained within the river channel at the flood peak. CS1 is taking across the Afon Eden, and as mentioned in Section 1.3 this river has an outflow canal linking it to the Trawsfynydd reservoir. It is for this reason that the Afon Eden never really exceeds it banks, especially upstream of the confluence with the Afon Mawddach. In Graph 3.8 all three scenarios project water levels within 400mm of each other. This may seem quite a small margin, but looking at the cross sections in Appendix 7 it is evident that CS3 is reasonably long section (1800m), covering the most open part of Floodplain 2. It is for this reason that large quantities of water are required for there to be a considerable difference in water surface elevations. As well as this, the x-axis scale is quite large, giving the resulting curves a constricted x-value variation. Looking at

60

Discussion Appendix 9, it is clear that the water surface elevations almost diverge once the flood peak has been breached. Graph 3.9 shows water surface elevations 15m upstream from the outflow boundary. Once again the results are a reflection of Floodplain 1, with no distinct departure occurring between the three scenarios. Appendix 9 complements this, with almost identical water levels being projected for the three scenarios throughout the flood event. CS7 is taking further into the floodplain than CS3 and Graph 3.10 shows water surface elevations across this section. It is evident from this graph that the floodplain isn’t completely submerged at this section. On first impressions it appears that there is reasonable deviation between the three scenarios, but on closer examination of the x-axis values it would seem that this variation is not as extreme at it first seemed. The bed contours in Graph 3.10 are quite clearly exaggerated with the river channels appearing much deeper than in reality. The sudden rise in water levels in the Afon Mawddach (right channel in Graph 3.10) may be as a result of substantial stream tubes in that area of the river. Such stream tubes have occurred in constricted fashion because of the presence of the gorge section located just a few meters upstream from point 16 in CS7. This gives rise to increased velocities and water surface elevations at that point in the cross section, a phenomena that it only temporary lived as no sudden rises in water level occur in CS3 which is just 40m downstream. Graph 3.11 illustrates the x-discharge at the centre of CS5. Point 8 was picked, as this was the point with the smallest total y-discharge (see Appendix 14). Picking a point with minuscule discharge in one of the axes means it is possible to plot a discharge hydrograph with reasonably high accuracy. Graph 3.11 shows that grassland produces slightly high discharge about the peak of the flood, but nothing of a substantially nature. Again, looking at discharge in the river channel may give false impressions of the overall hydrodynamics of the site, so analysis of Graph 3.12 is necessary before any conclusions can be drawn. Looking at Graph 3.12, it is very apparent that grassland does produce much higher x-discharge, especially in the upper end of the cross section. The reasoning for this may however be due to turbulence, or lack of it in the case of grassland. The creation of transverse flow by grassland due to turbulence is likely to be very low, enhancing the parallel flow field that will be

61

Discussion predominantly in the negative x-direction in this case. Therefore, the fact that both dense woodland and the calibrated scenario has smaller x-discharge, does not necessary mean that they are flowing slower, rather in a different direction. The avi files in Appendix 12 don’t really give any clear indication of a variation in flow direction between the three scenarios about the location of CS5. None the less, if the transverse flow is prolific, discharge rates will be reduced at the outflow boundary. However, looking at the outflow hydrographs in Graph 3.13, it is evident that the correlation between the outflow curves of all three scenarios is high. Therefore, it appears that the outflow hydrograph is independent of vegetation type in this site.

4.3 FLOODPLAIN 3 As mentioned in previous sections, Floodplain 3 has no calibrated scenario, rather four different vegetation scenarios. Graph 3.14 shows water surface elevations across CS1 at the peak of the flood. There are clear differences between the four scenarios with dense woodland producing the highest water levels and grassland the least. There is over 1.2m of a difference between grassland and dense woodland at the peak of the flood, and this is likely to reflect a large quantity of floodwater due to the width of the floodplain at this point in the site (approx. 220m). Due to the wide floodplain at this point, velocities are likely to be locally slower than normal. It is for this reason that water levels appear so flat in CS1, and indeed other cross sections with wide spans. The order in which each scenario appears in the Graph 3.14 is perhaps somewhat surprising. Even though sparse woodland has half the total basal area of grouped woodland (see Appendix 11) it has produced higher water surface elevations, which employs high traverse flow and turbulence. Such a result may not be surprising further down river where the continuous effect of sparse woodland would have accumulated a reasonable backwater swell. However the fact that between the inflow boundary and CS1 the grouped woodland scenario provides a continuous stretch of dense woodland makes such a result all the more puzzling. Based on vegetation analysis (see Appendix 11), grouped woodland has over four times the basal area of sparse woodland from the inflow boundary to CS1. In fact looking at Appendix 10, it is evident that

62

Discussion grouped woodland produces the smallest water levels prior to the peak of the flood (10000 seconds). A water level hydrograph in Appendix 10 for a point on CS1 produces similar results to that of Graph 3.14. Graph 3.15 shows water surface elevations for a fixed point in CS2, which is located on the floodplain. As the variable of location is eliminated, water levels can be graphed against time. This gives a good picture of just how the floodplain is reacting to the inflow hydrograph. The steep incline from 10000 seconds to 14000 seconds reflects the flashy nature of the July 3rd flood, with water levels increasing from 4.55m to almost 7m in just over 1 hour. The difference between grassland and the other three scenarios is quite staggering; over 1.2m higher for dense woodland. Such an increase in water level would have obvious implications for anybody living in the floodplain, as an extra 1.2m in water level is likely to result in a significant increase in horizontal flood extent. Graph 3.16 is a reflection of 3.14, with a difference of almost 1m between dense woodland and grassland at the peak of the flood. Once again, the appearance of the bed elevation line is distorted in this graph, however it is evident that the river channel is located about point 9 in the x-axis. The fact that sparse woodland is still producing higher water levels than grouped woodland is not surprising this far down the floodplain. There has been a continuous 3500m stretch of sparse woodland from the inflow to CS2, where as the grouped woodland scenario has only distributed two large clumps of woodland for the same distance. None the less, the fact that the grouped woodland has over four times the basal area per m², means that the results that are portrayed in Graph 3.16 are still quite remarkable. Graph 3.17 also produces some interesting results. The fact that this cross section is located in an area of relatively flat ground means that water levels between the four scenarios are going to be much closer together. However, having grouped woodland and grassland, plotting almost identical water levels around the peak of the flood certainly wasn’t anticipated. By the time CS4 has been reached the total basal area of the grouped woodland is 54133.37m². Prior to the simulations, the effect of such a density of vegetation would have been predicted to be nowhere near the same water levels as grassland.

63

Discussion The reasons for such a phenomenon aren’t quite apparent yet, and analysis of discharge rates will need to be undertaken before any conclusions can be drawn. However one un-quantified theory is the fact that whenever a cross section is taking outside an area of woodland in the grouped scenario (see Appendix 7), a marked drop in water level is produced. This could be attributed to the fact that the floodwaters are suddenly “free” to disperse across the floodplain relatively unobstructed, therefore resulting in sudden drops in water level in the grouped woodland scenario. Again, Graph 3.18 paints the same picture as 3.17, with close correlation between grassland and grouped woodland been shown. No additional pockets of woodland will have been encountered by floodwaters from CS4 to CS5, so perhaps such results aren’t surprising. The first two graphs for CS5 in Appendix 10 clearly illustrate the time difference in flood surges between the various scenarios. By 13000 seconds, water levels for grassland are some 2m higher than all other scenarios. However, adding on an additional 16 minutes (1000 seconds) sees grouped and sparse woodland quickly draw near. Graph 3.19 shows discharge at point 7 in CS2. The curves are varied in direction throughout the hydrograph so linear trend lines are used to give average direction and magnitude. Sparse and dense woodland show the lowest discharge rates with grassland producing the highest. Such results are plausible, however not exactly quantifiable in relation to the hydrodynamics of the whole site. Y-discharge rates are shown for the whole of C43 at 14000 seconds in Graph 3.20. As CS4 is nearing the outflow boundary, the results obtained from this cross section are likely to be critical in relation to the overall hydrodynamics of the site. As expected, grassland shows the highest discharge rates at the peak of the flood. Looking at the rest of the graphs for CS4 in Appendix 10, it is evident that once the peak has been breached the remaining three scenarios begin to reduce the gap. Such a phenomena is very plausible under the circumstances, as the more heavily vegetated scenarios (i.e. dense, grouped and sparse woodland) reduce peak discharge and flatten out the hydrograph to a less “flashy” flood event. As the flood progresses, grouped woodland exceeds grassland in y-discharge (16000 seconds), and by 23000 seconds sparse woodland and grassland are outputting almost equal y-discharge. It should be noted however, that some errors are likely when

64

Discussion only analysis discharge across one of the axes. However such errors will be minuscule as discharge in the x direction along CS4 is negligible (see Appendix 15). It has already been illustrated by several water surface elevation graphs that dense woodland, as expected, is creating the highest water levels, and therefore the greatest water depths. On looking at the various avi files in Appendix 12 illustrates this phenomenon more dynamically. Comparing the grassland and dense woodland avi files of water depth simultaneously reveals that darker red colour extends much further upstream in the dense woodland than the grassland. Subsequently, it is much easier to trace the shape of the river channel in the middle and upper section of the dense woodland file, as this is the location in the site where the water depth equals 5m. What is also evident from all of the avi files of this floodplain is just how long it takes for the floodwaters to reside, testament to the flat topography of the area. As well as that, looking at the avi files of velocity, it is very apparent that grassland is producing much higher velocities throughout the site when compared to the other scenarios. Such trends are predictable, but perhaps not on such a scale as is illustrated in Appendix 12. Graph 3.21 shows the outflow hydrographs for the various scenarios in Floodplain 3. The results are plausible and fit the theories expressed in Section 1.2.2. Grassland plots the highest discharge rates from the beginning of the hydrograph to some 40 minutes after the peak of the flood. The difference between grassland and either grouped or sparse woodland about the peak of the flood is about 32m³/sec. This reduction is increased to 45m³/sec when comparing grassland and dense woodland. As expected the floodwaters that were retained by the heavily vegetated scenarios (dense, grouped and sparse woodland) were gently released when the peak of the flood had been breached. Grouped woodland, despite producing much smaller water surface elevations, reduced the flood peak by marginally less than sparse woodland (2m³/sec). When compared to grassland, time to peak discharge was reduced by some 30 minutes for dense woodland, and 15 minutes for grouped and sparse woodland. The difference between the outflow discharge of the various scenarios and the inflow hydrograph for Floodplain 3 can be seen in the final graph in Appendix 10. As the extent of flooded floodplain increases, River 2D subsequently increases groundwater storage in proportion. Therefore, as dense woodland produced the highest water surface elevations, it incurred the greatest loss of

65

Discussion floodwater to ground storage, approximately 1168.504m³ or 21.17% of the total inflow. In comparison, grassland loss to ground storage was 1017m³ or 18.42% of the total inflow.

66

Conclusions & Recommendations

5.0 CONCLUSIONS & RECOMMENDATIONS

67

Conclusions & Recommendations

5.0 CONCLUSIONS & RECOMMENDATIONS Conclusions for this dissertation are based on the requirements of the objectives and the information portrayed by the results. Recommendations are made based on the experience and information gained from this dissertation.

5.1 CONCLUSIONS

5.1.1 Floodplain 1 The main requirement of the objectives (Section 1.5) is to assess the potential of each floodplain site in flood amelioration. After detailed analysis of Floodplain 1, it has become very apparent that this site offers no real potential in flood mitigation. It is evident however; that water surface elevations did diverge by an average of 350mm between dense woodland and grassland. As well as this, a backwater swell of 400mm high and 110m occurred between the inflow and the opening of the floodplain. However, it is most likely that the backwater swell was exaggerated by the upstream topography of the site, with its relatively high backs and restricted floodplain area. In fact, the topography and scale of the whole site is likely to be the limiting factor in flood mitigation. The general topography of Floodplain 1 is almost like a deep bowl, with steep sides and a small floodplain area (approx. 4000m²) in the centre. Although there are other pockets of floodplain dotted around the site, such areas don’t provide any significant influence on reducing water velocities. The high banks mean that water depth in the floodplain at the peak of the flood rarely exceeds 1m. Such a depth would be sufficient in a site with a large expanse of floodplain, an attribute that Floodplain1 does not have. It takes some 35% (or 125m³/sec) of the July 01 flood to exceed the riverbanks of this site, meaning that a considerable quantity of the flood has already passed the site without the floodplain even becoming saturated. As well as the site being to narrow and steep, its length is also of a minute scale for flood amelioration, at approximately 350m long. High velocities (topping 7m/sec) on a short stretch of floodplain mean that stream tubes that were separated from the main river flow by the floodplain can be quickly reattached once the topography reverts back

68

Conclusions & Recommendations to steep high channel sides, as it does around the outflow boundary. The ability of Floodplain 1 to create continuous wakes zones of turbulence within the floodplain are limited, this is evident by the correlation of the various scenarios in the discharge graphs in Section 3.1. However, the most critical piece of information on the ability of Floodplain1 in flood amelioration is the outflow hydrograph (Graph 3.5) in Section 3.1. Not only are the curves plotted by the three scenarios inter-twined, but there are also in close correlation with the inflow hydrograph. This proves that Floodplain 1 is far to small a site to have any positive affect on flood mitigation.

5.1.2 Floodplain 2 Although the size of Floodplain 2 (approx. 27200m²) is considerably bigger than Floodplain 1, the amount of area that is actually floodplain itself is not as significant. The majority of the flooding only occurs about the confluence of the Afon Eden and Afon Mawddach, leaving the allot of the site relatively untouched by the floodwaters. Once again this can be attributed to the topography of the area, although not as steep as Floodplain 1, none the less inhibiting in the sites ability to mitigate floods. The sudden drop from the gorge section to the confluence means that floodwaters are funnelled for some distance before being able to spread out across the floodplain. The left- hand bank (looking down stream) offers no real significant floodplain area, leaving the mitigation to be done primarily on the right-hand side. Results were pretty much a reflection of Floodplain 1, with water levels behaving as expected, but discharge proving to be less predictable. Due to of the close correlation between the three scenarios, it is difficult to distinguish the effect that each has on the overall hydrodynamics of the site. It could however be said that increasing the basal area does have significant effects on peak water levels. In Floodplain 2, increasing the average basal area from 0.04 m²/m³ to 0.1m²/m³ increased the water surface elevations by anything from 50 to 350mm. The outflow hydrographs show no significant variation in the three scenarios. In fact, at the peak of the flood there was only 10m³/sec of a difference between the inflow hydrograph and the dense woodland outflow. As the Afon Eden has a less flashy

69

Conclusions & Recommendations hydrograph, due to the outflow canal to the reservoir, the extent of floodplain that is flooded is limited. If the Afon Eden hydrograph was restricted to a shorter time scale, more of the site would get flooded, and therefore increase the mitigation potential of Floodplain 2. However it is unlikely that such measures would produce in a significant change in the resulting outflow hydrograph.

5.1.3 Floodplain 3 Floodplain 3 is some 37 times bigger than Floodplain 2, with a more typical floodplain topography. Good divergence is shown between the various scenarios in relation to water surface elevations with dense woodland producing waters levels 1.2m higher than grassland in some locations. Although sparse woodland had less than half the basal area of grouped woodland, it produced on average, water levels some 250mm higher. The most likely reason for this is the fact that all the cross sections are located outside any areas of grouped woodland. Therefore, any increases in water surface elevation that may have been induced within the grouped woodland would have dissipated or at least been reduced by the time the cross section hand been breached. The outflow hydrographs show close correlation between sparse and grouped woodland, therefore it must be assumed that significant increases in water surface elevation must have occurred within the pockets of woodland in the grouped scenario for the two to be eventually equal. This would also attribute to the fact that very often grassland and grouped woodland produced similar water surface elevations. Because the vegetation type around the cross section was similar for both (i.e. grass), comparable water levels were achieved, however if cross sections were located within the pockets of grouped woodland, quite a different graph would have been produced. Analysis of the outflow hydrographs showed that dense woodland has reduced the peak discharge by over 45m³/sec, when compared to grassland. This equated to a 30minute delay in the peak of the flood and an overall reduction of 21.17% of the total inflow. The overall effect of grouped woodland was a reduction in peak discharge of 32 m³/sec, when compared to grassland, and a reduction of 15 minutes to peak discharge. Sparse woodland had similar results, with 2m³/sec less of a reduction in peak discharge.

70

Conclusions & Recommendations The fact that sparse woodland had less than half the basal area, lead to some interesting theories. If sparse woodland had the same basal area as grouped woodland, the hydrodynamic benefits would almost certainly exceed that of grouped woodland. Therefore it can be said that having the woodland scattered throughout the floodplain will provide better flood amelioration potential than having the same woodland in dense pockets. The location of the grouped woodland is likely to be critical, as the effects of outflanking and river topography (meanders) will play a critical part in flow direction, and the subsequent effectives of the floodplain woodland.

To conclude on the three floodplain sites, it appears that a site of a similar scale and topography to Floodplain 3 is needed to have a significant effect on outflow hydrographs. Pockets of woodland are unlikely to have the same effect as a continuous stretch of woodland of similar basal area. The discontinuous ability of groups of woodland to disrupt stream tubes and create turbulence means that their overall effect is, for the most part inadequate. Critical to floodplain woodland’s ability to mitigate floods, is it ability to maximise the delay of floodplain flows reattaching to the main river flow. The only sure method of achieving this is continuous woodland cover. Increasing the density with this continuous woodland is likely to significantly increase flood amelioration, however significant backwater maybe created, as well as reducing the capacity of the floodplain. Although the use of floodplain woodland in flood amelioration does have considerable potential, it is unlikely to provide adequate protection without additional flood defences. However, attributes such as high biodiversity and habitat, recreation, increased landscape, timber production and an environmental friendly method of flood defence make floodplain woodland an convincing candidate for flood defence in the UK in the near future.

71

Conclusions & Recommendations 5.2 RECOMMENDATIONS 

Scientific verification of the relationship between vegetation density and the turbulence equations in River 2D.



Further hydrodynamic analysis with River 2D on a site large than 1,007,500m², investigating in more detail on the effect of location of pockets of woodland on outflow hydrographs.



Evaluating Manning’s n values for UK floodplain woodland.



Modelling the effect of using a combination of floodplain woodland and some other form of hard engineered flood defence.



Under taking an assessment of the cost benefits of floodplain woodland in flood amelioration.



Investigating the implications of using floodplain woodland in flood amelioration. Such an example would include the problem of floating woody debris.

72

Reference List

6.0 REFERENCE LIST

73

Reference List

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Barton, J. (2002).Flooding: Past, Present and Future. A Case Study of Dolgellau and the Mawddach Catchment. MSc. Dissertation, School of Agricultural and Forest Sciences, University of Wales, Bangor.

Bates, P.D. and De Roo, A.P.J. (2000). A Simple Raster-Based Model for Flood Inundation Simulation. Journal of Hydrology, Volume 236, 2000, 54-77.

Bates, P.D. and Horritt, M.S. (2002). Evaluation of 1D and 2D Numerical Models for Predicting River Flood Inundation. Journal of Hydrology, Volume 298, 2002, 87-99.

Blackburn, J. and Steffler, P.M. (2002). River 2D. Two- Dimensional Depth Averaged Model of River Hydrodynamics and Fish Habitat. Introduction to Depth Average Modelling and User’s Manual. University of Alberta. http://www.river2d.ualberta.ca/. Accessed on the 10/05/2004.

Brouns, G., De Wulg, A. and Constales, D. (2001) Multibeam Data Processing: Adding and Deleting Vertices in a Delaunay Triangulation. The Hydrographic Journal, No.101, July 2001. Brown, P. (2004). Its too Late. Climate Change Floods are inevitable – No Matter What We Do. Environment Correspondent, The Guardian. Guardian Newspapers Limited 2004.

Bryant, T., Sellin, R. and Loveless, J. (2002). An Improves Method of Roughening Floodplains on Physical River Models. Journal of Hydraulic Research, Volume 41, 2003, No.1.

74

Reference List

Dworak, T., Hansen, W. (2003). The European Flood Approach. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 6-13 September, 2003. . Accessed on the 15/04/2004.

Environment Agency. (1997). Bank Protection Using Willows. EPSRC/EA Scoping Study. http://www.geog.nottingham.ac.uk/~thorne/riverbank/contents.html. Accessed on the 15/04/2004.

Environment Agency. (2000). Guidelines for Planting beside Rivers and Within the Floodplain. Flood Defence Information Sheet No.22, Environment Agency, Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32 4UD.

Environment Agency. (2003). Wetlands, Land Use Change and Flood Management. A Joint Statement prepared by English Nature, the Environment Agency, the Dept. for Environment, Food and Rural Affairs (Defra) and the Forestry Commission. Environment Agency, Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32 4UD.

Ergenzinger, P., Bolscher, J., and Jong de, C. (2003). The Case of the Upper Rhine: Unravelling the Past and Wrapping up the Future. Geophysical Research Abstracts, Volume 5, 13746, 2003.

Fisher, K. and Dawson, H. (2001). Scoping Study on Reducing Uncertainty in River Flood Conveyance, Parameters Affecting Conveyance (Vegetation). . Accessed on the 25/03/2004.

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Reference List Forestry Commission. (1999). Woodlands for Wales. The National Assembly for Wales Strategy for Trees and Woodlands. Forestry Commission, National Office for Wales, Victoria Terrace, Aberystwth, Ceredigion.

Freeman, G., Rahmeyer, W., Derrick, D. and Copeland, R. (1998). Mannings n Values for Floodplains with Shrubs and Woody Vegetation. U.S. Army Corps of Engineers at the Waterways Experiment Station. Vicksburg, MS 39180. Friends of the Earth. (2000). Flood Alert – Climatic Change in the UK. Friends of the Earth, 26-28 Underwood Street, London, N1 7JQ. Green Consumer Guide. (2004) UK Floods to Rise With Global Warming – Report. . Accessed on the 3/05/2004.

Griva, A., Baltas, E.A., Varanou, A. and Mimikou, M.A. (2003). GIS-Based Floodplain Mapping in Greece. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 6-13 September, 2003. . Accessed on the 15/03/2004.

Haase, D., Weichel, T. and Volk, M. (2003). Analysis and Assessment of the Floods of the Mulder River in Germany 2002, Based on remote Sensing Data as a Contribution for Spatial Planning and Management. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 6-13 September, 2003. . Accessed on the 05/03/2004.

Hammond, K. and Brown, A.G. (1993). Change in Channel and Overbank Velocity Distribution and the Morphology of Forested Multiple Channels. Earth Surface Processes and Landforms, Volume 18, 741-748.

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Reference List Heiskanen, J. and Wall, A. (2003). Water-Retention Characteristics and Related Physical Properties of Soil on Afforested Agricultural Land in Finland. Forest Ecology and Management, Volume 186, 2003, 21-32.

Hess, T.M., Morris, J., Gowing, D.G., Leeds-Harrison, P.B., Bannister, N., Vivash, R.M.N. and Wade, M. (2003). Integrated Washland Management for Flood Defence and Biodiversity. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 6-13 September, 2003. Accessed on the 25/04/2004.

Hughes, F. and Rood, S. (2003). Allocation of River Flows for Restoration of Floodplain Forest Ecosystems: A Review of Approaches and Their Applicability in Europe. Environment Management Volume 32, No.1, pp 12-33.

Institute of Hydrology. (1995). Effects of Upland Afforestation on Water Resources, The Balquhidder Experiment, 1981 - 1991. Report No. 116, Institute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, UK.

Institute of Hydrology. (1998). From Moorland to Forest: The Coalburn Catchment Experiment. Report No. 133, Institute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, UK.

Institute of Hydrology. (1991). Plynlimon Research: The first Two Decades. Report No. 109, Institute of Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, UK.

Kerr, G. and Nisbet, T. (1996). The Restoration of Floodplain Woodlands in Lowland Britain, R & D Technical Report W15. Environment Agency, Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32 4UD.

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Reference List Kreis, N. (2003). Re-Entering River Waters onto Floodplains Requires Hydrological and Hydraulics Modelling. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 6-13 September, 2003. . Accessed on the 05/03/2004.

Lighthill, J. (1986). An Informal Introduction to Theoretical Fluid Mechanics. The Institute of Mathematics and its Applications Monograph Series. Clarendon Press, Oxford. LMNO Engineering, Research and Software. (2004). Manning’s n Coefficients for Open Channel Flow, The Fluid Mechanics Calculation Website. . Accessed on the 3/05/2004.

Newson, M.D. (1994). Hydrology and the River Environment. Imprint Oxford : Clarendon Press ; New York : Oxford University Press, 1994.

Nisbet, T. (2001). Can Forestry Stem the Flood? Forestry and British Timber, 18-21, June 2001.

Nisbet, T., Robinson, M., Cognard-Plancq, A-L., Cosandey, C., David, J., Durand, P., Fuher, H-W., Hall, R., Hendriques, M.O., Marc, V., McCarthy, R., McDonnell, M., Martin, C., O’ Dea, P. Rodgers, M. and Zollner, A. (2003). Studies of the Impact of Forests on Peak Flows and Base Flows: a European Perspective. Forest Ecology and Management, Volume 186, 2003, 85-97.

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Reference List Poulard, C. Ghavasieh, A. R. Gamerith, V. Szczesny, J. Witkowska, H. Dynamic Slowdown: From Integrated Management to Flood Mitigation. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 6-13 September, 2003. Accessed on the 14/05/2004.

Quinlan Consulting (2003). The Effects of Deforestation on Hydrology Regime. . Accessed on the 20/02/2004.

Richard, L. (1926). Forest Hydrology. Imprint New York Guildford Columbia University Press 198.

Richards, K., Girel, J., Moss, T., Muller, E., Nilsson, C. and Rood, R. (2004). The Flooded Forest: Guidance for Policy Makers and River Managers in Europe on the Restoration of Floodplain Forests. Edited by Francine Hughes. FLOBAR2, Funded by The European Commission.

Smith, K., Ward, R. (1998). Floods, Physical Processes and Human Impacts. John Wiley and Sons, Baffins Lane, Chichester, West Sussex, PO19, England.

Steffler, P. (2002). R2D_Bed. Bed Topography File, User Manual. University of Alberta. http://www.river2d.ualberta.ca/. Accessed on the 10/05/2004.

Steffler, P.M., Blackburn, J. (2002). River 2D. Two- Dimensional Depth Averaged Model of River Hydrodynamics and Fish Habitat. River 2D Tutorial – The Basics, University of Alberta. http://www.river2d.ualberta.ca/. Accessed on the 10/05/2004.

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Reference List Szoszkiewicz, K., Kaluza, T. Lesny, J. and Chojnicki, B.H. (2003). Remote Sensing Analysis of the Floodplain Vegetation Structure Within a Section of the Middle Vistula River. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 6-13 September, 2003. . Accessed on the 15/03/2004.

Talyor, A. (1998). Does detailed channel form information significantly increase the accuracy of 2D numerical river inundation models? Final Degree of B.Sc. at the University of Bristol, Department of Geography, December 1998. . Accessed on the 10/052004.

Telionis, D. P. (1981). Unsteady Viscous Flows. Springer Series in Computational Physics. Springer-Verlag, 175 Fifth Avenue, New York, New York 10010, U.S.A.

Thomas, H. and Nisbet T. R. (2004). An Assessment of the Hydraulic Impact of Floodplain Woodland. Final Report to the Forestry Commission, August, 2004.

Tir Coed. (2001). Woodland, Water and Flooding. Expert Workshop Held at the National Assembly for Wales, 5th of July, 2001. Report of Proceedings and Supplementary Information. New Woodlands for Wales.

Townsend, P.A. (2000). Relationships Between Forest Structure and Detection of Flood Inundation in Forested Wetlands Using C-Band SAR. Int. J. Remote Sensing, 2002, Volume 23, No.3, 443-460. Accessed on the 25/02/2004. UK Biodiversity Group (1998). Habitat Action Plan, Wet Woodland. Tranche 2 Action Plans - Volume II: Terrestrial and freshwater habitats. Accessed on the 20/05/2004.

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Waddle, T. and Steffler, P. (2002). R2D_Mesh. Mesh Generation Program for River 2D Two Dimensional Depth Averaged Finite Element. Introduction to Mesh Generation and User’s Manual. U.S. Geological Survey, University of Alberta. http://www.river2d.ualberta.ca/. Accessed on the 10/05/2004. Ward, R.C. and Robinson, M. (1990). Principles of Hydrology. Edition 3rd ed Imprint London ; New York : McGraw-Hill, c1990.

Witkowska, H., Szczesny, J., Radzicki, K., Watroba, R. and Poulard, C. (2003). Dimensioning of Dynamic Slowdown Hydraulic Structures Located in a Catchment. International Conference “Towards Natural Flood Reduction Strategies”, Warsaw, 613 September, 2003. . Accessed on the 21/02/2004.

Wright, N.G. (2001). Conveyance Implications for 2-D and 3-D Modelling, Scoping for Reducing Uncertainty in River Flood Conveyance. Prepared for HR Wallingford and the Environment Agency, March 2001. Environment Agency, Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32 4UD.

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82

Appendices

7.0 APPENDICES

Appendix 1

Appendix 1 Appendix 1(A) Panoramic view looking downstream in Floodplain 1.

Appendix 1 Appendix 1(B) Picture looking down stream, showing right bank at the centre of Floodplain 1.

Appendix 1(C) Picture looking down stream, showing left bank at the centre of Floodplain 1.

Appendix 1 Appendix 1(D) Picture looking upstream at the outflow section of Floodplain 1.

Appendix 1(E) Picture showing flood debris on right bank in Floodplain 1.

Appendix 1 Appendix 1(F) Picture showing the beginning of the floodplain on the right bank in Floodplain 1.

Appendix 1(G) Vegetation Plot A, Floodplain 1.

Appendix 1

Appendix 1(H) Vegetation Plot B, Floodplain 1.

Appendix 1(I) Vegetation Plot C, Floodplain 1.

Appendix 1 Appendix 1(J) Vegetation Plot D, Floodplain 1

Appendix 1(K) Flood Debris Line 1, Floodplain 1

Appendix 1 Appendix 1(L) Flood Debris Line 2, Floodplain 1

Appendix 1(M) Flood Debris Line 3, Floodplain 1

Appendix 1 Appendix 1(N) Flood Debris Line 4, Floodplain 1

Appendix 2

Appendix 2 Appendix 2(A) Panoramic picture of Floodplain 2.

Appendix 2 Appendix 2(B) Picture looking upstream at inflow area of Floodplain 2. Gorge section in the foreground.

Appendix 2(C) Picture looking upstream at gorge section in Floodplain 2.

Appendix 2 Appendix 2(D) Picture looking upstream at bridge and gorge section of Floodplain 2.

Appendix 2(E) Picture looking upstream at bridge and gorge section in Floodplain 2. Gravel bar with surveying poles in the foreground.

Appendix 2 Appendix 2(F) Picture looking from the bridge, downstream at centre of Floodplain 2.

Appendix 2(G) Picture looking from centre of the bridge downstream at outflow section of Floodplain 2. Surveying poles located on the gravel bar in the left centre.

Appendix 2 Appendix 2(H) Picture taking from gravel bar looking upstream at the Afon Eden.

Appendix 2(I) Picture looking from centre of the bridge, looking downstream at Floodplain 2.

Appendix 2 Appendix 2(J) Picture looking downstream in the Afon Mawddach at the outflow section for Floodplain 2.

Appendix 2(K) Flood debris Line 2, Floodplain 2.

Appendix 2 Appendix 2(L) Vegetation Plot A, Floodplain 2.

Appendix 2(M) Vegetation Plot B, Floodplain 2.

Appendix 2 Appendix 2(N) Vegetation Plot C, Floodplain 2.

Appendix 2(O) Vegetation Plot D, Floodplain 2.

Appendix 3

Appendix 3 Appendix 3(A) Panoramic picture of inflow section of Floodplain 3.

Appendix 3 Appendix 3(B) Looking downstream at the inflow section of Floodplain 3.

Appendix 3(C) Looking upstream at the inflow section of Floodplain 3. Note the footbridge in the centre as the line for the inflow boundary in the model.

Appendix 3 Appendix 3(D) 1000m downstream of the inflow section of Floodplain 3.

Appendix 3(E) Midsection of Floodplain 3, showing the typical river channel and bank topography for the whole site.

Appendix 3

Appendix 3(F) Looking upstream from the “old bridge” in Floodplain 3.

Appendix 3(G) Taking cross sections in the inflow section of Floodplain 3.

Appendix 3 Appendix 3(K) 1:50000 map of Floodplain 3 showing inflow and outflow boundaries for River 2D.

Reproduced from (2004) Ordnance Survey map with the permission of the Controller of Her Majesty's Stationery Office, © Crown Copyright NC/04/32899

Appendix 4

Appendix 4 Channel and floodplain roughness coefficients for the various scenarios within each floodplain. Note that the contours in Floodplain 1 and 2 are topographical, each contour representing a 0.5m change in elevation. Floodplain 1

Appendix 4

Floodplain 2

Appendix 4

Appendix 4

Floodplain 3

Appendix 4

Appendix 5

Appendix 5 Floodplain 1 Mesh Details: 

1333 Nodes



2551 Elements



Q.I = 0.359


573 Nodes



1055 Elements



Q.I = 0.449


1955 Nodes



3674 Elements



Q.I = 0.376

Appendix 6

Appendix 6 The following tables show the resulting vegetation density analysis of the various plots for Floodplain 1 and 2. The location of these plots can be seen on pages 36 and 37 with pictures in Appendices 1 and 2. The formulation used the determine vegetation density and Manning’s roughness (n) was obtain from Arcement and Schneider, 1990. The principal formulae are: 

Basal Area (BA) = h∑nіdi / hwl where h = hydraulic radius (or water depth) (m) ni = number of trees di = tree diameters (m) w = width of the sample plot (m) l = length of the sample plot (m)



Manning’s Roughness (n) = no√(1+Vegd(C*)(1/no)²(1/wlh)(h)0.75 where BA = Basal Area C* = the effective drag coefficient for vegetation no = summation of roughness values for the floodplain and w, l and h are as specified in the previous equation. C* is defined as the relationship between Manning’s n values and hydraulic radius

(or water depth). A C* values is obtained by using the drag coefficient graph (below) in Arcement and Schneider, 1990.

Appendix 6

(Courtesy of Arcement and Schneider, 1990) The “Manning’s Values” located on the right-hand side of each table are the estimations of the n values of the various elements of the floodplain. The acronyms are as follows: 

nb = base value of n for the floodplains natural bare soil surface



n1 = a correction factor for the effect of surface irregularities on the floodplain



n2 = a value for variation in cross section along the floodplain (assumed to be 0)



n3 = a value for obstruction on the floodplain



n4 = a value for the vegetation on the floodplain (note this is vegetation that wasn’t recorded in the vegetation density analysis, such as bushes and grasses) Although such values are unit less, they are determined base on

examples given in the tables at the end of this appendix.

Appendix 6

Floodplain 1 - Plot A No. of trees Circumference (m) Diameter (m) 10 0.07 0.02228169 4 0.145 0.04615493 1 1.115 0.35491552 1 0.185 0.05888733 1 0.32 0.10185916 3 0.13 0.04138029 1 0.103 0.03278592

TreesXDia 0.22281692 0.18461973 0.35491552 0.05888733 0.10185916 0.12414086 0.03278592 1.08002544

Manning’s Values nb= 0.025 n1= 0.005 n2= 0 n3= 0.045 n4= 0.01 No= 0.085

h= 0.75 w= 5 l= 5 C= 12.3 BA = Manning’s n =

0.043201018 m²/m³ 0.146270072

Basal Area and Manning’s n Value for Plot A in Floodplain 1.

Appendix 6

Floodplain 1 - Plot B No. of trees Circumference (m) Diameter (m) 1 0.272 0.08658029 1 0.825 0.26260566 1 0.647 0.2059465 1 0.107 0.03405916 1 0.132 0.0420169 1 0.452 0.14387607 1 0.466 0.14833241 1 0.547 0.17411551 1 0.07 0.02228169 1 0.8 0.25464791

TreesXDia 0.08658029 0.26260566 0.2059465 0.03405916 0.0420169 0.14387607 0.14833241 0.17411551 0.02228169 0.25464791 1.37446209


h= 2 w= 5 l= 5 C= 15.5 BA = Manning’s n =

0.054978484 m²/m³ 0.163381271

Basal Area and Manning’s n Value for Plot B in Floodplain 1.

Appendix 6

Floodplain 1 - Plot C No. of trees Circumference (m) Diameter (m) 1 2.256 0.7181071 1 1.692 0.53858033 1 1.848 0.58823667 1 2.349 0.74770992 1 1.746 0.55576906 1 1.749 0.55672399

TreesXDia 0.7181071 0.53858033 0.58823667 0.74770992 0.55576906 0.55672399 3.70512708


h= 1.5 w= 15 l= 15 C= 4 BA = Manning’s n =

0.016467231 m²/m³ 0.049032308

Basal Area and Manning’s n Value for Plot C in Floodplain 1. Floodplain 1 - Plot D No. of trees Circumference (m) Diameter (m) 2 0.24 0.07639437 5 0.09 0.02864789 2 0.07 0.02228169 2 0.148 0.04710986 3 0.13 0.04138029 4 0.712 0.22663664

TreesXDia 0.15278875 0.14323945 0.04456338 0.09421973 0.12414086 0.90654656 1.46549872

Manning’s Values nb= 0.025 n1= 0 n2= 0 n3= 0 n4= 0.001 No= 0.026

h= 2 w= 5 l= 5 C= 2 BA = Manning’s n =

0.058619949 m²/m³ 0.061663581

Basal Area and Manning’s n Value for Plot D in Floodplain 1.

Appendix 6

Floodplain 1- Dense Woodland No. of trees Circumference (m) Diameter (m) 22 0.24 0.07639437 5 0.09 0.02864789 10 0.07 0.02228169 10 0.148 0.04710986 0 0.13 0.04138029 0 0.712 0.22663664

TreesXDia 1.6806762 0.14323945 0.22281692 0.47109863 0 0 2.5178312



0.100713248 m²/m³ 0.207246617

Basal Area and Manning’s n Value for Dense Woodland Scenario in Floodplain 1.

Floodplain 1- Grassland Manning’s Values nb= 0.025 n1= 0.005 n2= 0 n3= 0 n4= 0.005 No= 0.035 Manning’s n Value for “Grassland” Scenario in Floodplain 1.

Appendix 6

Manning’s Roughness for the River Channel in Floodplain 1 Upper Section Manning’s Values nb= 0.025 n1= 0.015 n2= 0 n3= 0.018 n4= 0 No= 0.058 Lower Section Manning’s Values nb= 0.025 n1= 0.02 n2= 0 n3= 0.027 n4= 0 No= 0.072 Manning’s n Value for the River Channel in Floodplain 1.

Appendix 6

Floodplain 2 - Plot A No. of trees Circumference (m) Diameter (m) 1 0.742 0.23618594 2 0.158 0.05029296 1 0.388 0.12350424 1 0.423 0.13464508 2 0.649 0.20658312 1 0.22 0.07002817 1 0.512 0.16297466 1 0.255 0.08116902 1 0.428 0.13623663 2 0.007 0.00222817 1 0.772 0.24573523

TreesXDia 0.2361859 0.1005859 0.1235042 0.1346451 0.4131662 0.0700282 0.1629747 0.081169 0.1362366 0.0044563 0.2457352 1.7086875



0.017086875 m²/m³ 0.065209988

Basal Area and Manning’s n Value for Plot A in Floodplain 2.

Appendix 6

Floodplain 2 - Plot B No. of trees Circumference (m) Diameter (m) 1 0.127 0.04042536 2 0.152 0.0483831 1 0.195 0.06207043 1 0.106 0.03374085 1 0.377 0.12000283 4 0.07 0.02228169 1 0.122 0.03883381 1 0.195 0.06207043 1 0.258 0.08212395 1 0.102 0.03246761

TreesXDia 0.0404254 0.0967662 0.0620704 0.0337408 0.1200028 0.0891268 0.0388338 0.0620704 0.082124 0.0324676 0.6576282



0.026305129 m²/m³ 0.101004358

Basal Area and Manning’s n Value for Plot B in Floodplain 2.

Appendix 6

Floodplain 2 - Plot C No. of trees Circumference (m) Diameter (m) 3 0.131 0.0416986 1 0.162 0.0515662 1 0.192 0.0611155 1 0.119 0.03787888 1 0.154 0.04901972 1 0.125 0.03978874 1 0.205 0.06525353 7 0.07 0.02228169 1 0.102 0.03246761

TreesXDia 0.1250958 0.0515662 0.0611155 0.0378789 0.0490197 0.0397887 0.0652535 0.1559718 0.0324676 0.6181578



0.024726312 m²/m³ 0.146883944

Basal Area and Manning’s n Value for Plot C in Floodplain 2.

Floodplain 2 - Plot D Manning’s Values nb= 0.025 n1= 0.01 n2= 0 n3= 0.045 n4= 0.05 Manning’s n = 0.13 Manning’s n Value for Plot D in Floodplain 2

Appendix 6 Floodplain 2 - Dense Woodland No. of trees Circumference (m) Diameter (m) 5 0.742 0.23618594 3 0.158 0.05029296 8 0.388 0.12350424 1 0.423 0.13464508 2 0.649 0.20658312 1 0.22 0.07002817 2 0.512 0.16297466 2 0.255 0.08116902 2 0.428 0.13623663 10 0.007 0.00222817 1 0.772 0.24573523

TreesXDia 1.1809297 0.1508789 0.9880339 0.1346451 0.4131662 0.0700282 0.3259493 0.162338 0.2724733 0.0222817 0.2457352 3.9664595

Manning’s Values nb= 0.025 n1= 0.001 n2= 0 n3= 0.02 n4= 0 No= 0.046


0.15865838 m²/m³ 0.208755175

Basal Area and Manning’s n Value for “Dense Woodland” Scenario in Floodplain 2.

Floodplain 2 - Grassland Manning’s Values nb= 0.025 n1= 0.005 n2= 0 n3= 0 n4= 0.005 No= 0.035 Manning’s n Value for “Grassland” Scenario in Floodplain 2.

Appendix 6 Manning’s Roughness for the River Channel in Floodplain 2 Gorge Section m= 1 nb= 0.025 n1= 0.015 n2= 0.015 n3= 0.025 n4= 0 Manning’s n = 0.08 Afon Eden m= 1 nb= 0.025 n1= 0.009 n2= 0.001 n3= 0.035 n4= 0 Manning’s n = 0.07 Lower Mawddach m= 1 nb= 0.025 n1= 0.009 n2= 0.001 n3= 0.004 n4= 0 Manning’s n = 0.039 Manning’s n Value for River Cannel in Floodplain 2.

Appendix 6 Floodplain 3 - Dense & Group Woodland No. of trees Circumference (m) Diameter (m) TreesXDia 22 0.24 0.0763944 1.680676 5 0.09 0.0286479 0.143239 10 0.07 0.0222817 0.222817 10 0.148 0.0471099 0.471099 0 0.13 0.0413803 0 0 0.712 0.2266366 0 2.517831 h= 2 w= 5 l= 5 C= 9.7 BA = Manning’s n =


0.100713248 m²/m³ 0.207246617

Basal Area and Manning’s n Value for Dense Woodland Scenario in Floodplain 3.

Floodplain 3- Grassland Manning’s Values nb= 0.025 n1= 0.005 n2= 0 n3= 0 n4= 0.005 No= 0.035 Basal Area and Manning’s n Value for Grassland Scenario in Floodplain 3.

Appendix 6

Floodplain 3 - Sparse Woodland No. of trees Circumference (m) Diameter (m) TreesXDia 1 0.127 0.0404254 0.040425 1 0.152 0.0483831 0.048383 1 0.195 0.0620704 0.06207 3 0.106 0.0337408 0.101223 1 0.377 0.1200028 0.120003 1 0.07 0.0222817 0.022282 1 0.122 0.0388338 0.038834 1 0.195 0.0620704 0.06207 1 0.258 0.082124 0.082124 1 0.102 0.0324676 0.032468 0.609882

Manning’s Values nb= 0.025 n1= 0.005 n2= 0 n3= 0.05 n4= 0 No= 0.08


0.02439527 m²/m³ 0.112739714

Basal Area and Manning’s n Value for Sparse Woodland Scenario in Floodplain 3. Manning’s Roughness for the River Channel in Floodplain 3 nb= 0.025 n1= 0.005 n2= 0.005 n3= 0.004 n4= 0 Manning’s n = 0.039 Manning’s n Value for River Channel in Floodplain 3.

Appendix 6

(Courtesy of Arcement and Schneider, 1990)

Appendix 6


Appendix 6


Appendix 7

Appendix 7

Floodplain 1 Cross Sections (CS)

Appendix 7


Appendix 7


Appendix 8

Appendix 8 Floodplain 1 Water Surface Elevations



Grassland

2

Calibrated

1


0 -1

0

2

4

6

8

10

12

14

16

18

-2 -3 -4 -5 -6

Location

Water Surface Elevations Across CS1 at 12000sec Calibrated


4

Bed Elevation

3

Dense Woodland

2

Grassland

1 0 -1 0

2

4

6

8

-2 -3 -4 -5 -6

Location

10

12

14

16

Appendix 8



Grassland 1


0 -1

0

2

4

6

8

10

12

Bed16 Elevation 18

14

-2 -3 -4 -5 -6

Location



1 0 0

2

4

6

8

-1 -2 -3

Grassland -4 -5

Dense Woodland Calibrated Bed Elevation

-6

Location

10

12

14

16

Appendix 8



1 0 0

2

4

6

8

10

12

14

12

14

-1 -2 -3 Dense Woodland

-4

Grassland Calibrated

-5

Bed Elevation

-6

Location



1 0 0

2

4

6

8

-1 -2 -3

Dense Woodland -4 -5

Grassland Calibrated Bed Elevation

-6

Location

10

Appendix 8

Water Surface Elevations at CS2 at 12000sec 1 0 2

Water Surface Elevation

0

4

6

8

10

12

14

12

14

-1 -2 -3 Calibrated

-4


-5

Bed Elevation

-6

Location



1 0 0

2

4

6

8

-1 -2 -3 Dense Woodland -4 -5

Grassland Calibrated Bed Elevation

-6

Location

10

Appendix 8



1 0 0

2

4

6

8

10

12

14

-1 -2 -3

Dense Woodland

-4

Grassland Calibrated

-5

Bed Elevation -6

Location



0 -1

0

2

4

6

8

10

12

14

-2 -3 -4 Calibrated

-5

Dense Woodland

-6

Grassland

-7

Bed Elevation

-8

Location

Appendix 8



0 -1

0

2

4

6

8

10

12

14

-2 -3 -4 Grassland

-5

Calibrated

-6


-7 -8

Location



0 -1

0

2

4

6

8

10

12

14

16

-2 -3 -4

Calibrated

-5

Dense Woodland

-6

Grassland -7

Bed Elevation

-8

Location

Appendix 8 Discharge

Discharge Intensity at Point 6 (Floodplain) in CS2


0 -1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2 -0.2 0 -0.4 -0.6 -0.8

Calibrated

-1

Grassland

-1.2

Dense Woodland -1.4 -1.6 -1.8

X-Discharge Intensity (m/sec)

Discharge Intensity at Point 10 (River Channel) in CS2


0 -12

-10

-8

-6

-4

-2

0 -2 -4 -6

Calibrated

-8

Grassland Dense Woodland

-10 -12


Appendix 8



-10 -10.5

0

2

4

6

8

10

12

14

16

18

20

22

24

-11 -11.5 -12 -12.5 -13

Calibrated Dense Woodland Grassland Linear (Grassland) Linear (Dense Woodland) Linear (Calibrated)

-13.5 -14 -14.5

Location



-12 0

2

4

6

8

10

12

14

16

18

20

22

24

-12.5 -13 -13.5 -14 -14.5 -15 -15.5

Location

Grassland Calibrated Dense Woodland Linear (Calibrated) Linear (Grassland) Linear (Dense Woodland)

Appendix 8



-10 -10.5 0

2

4

6

8

10

12

14

16

18

20

22

24

-11 -11.5 -12 -12.5 -13 -13.5 -14

Grassland Calibrated Dense Woodland Linear (Calibrated) Linear (Dense Woodland) Linear (Grassland)

-14.5 -15 -15.5

Location



-4 -4.5 0 -5 -5.5 -6 -6.5 -7

2

4

6

8

10

12

Grassland Calibrated Dense Woodland Linear (Dense Woodland) Linear (Calibrated) Linear (Grassland)

-7.5 -8 -8.5 -9 -9.5 -10

Location

14

16

18

20

22

24

Appendix 8



-3 -3.5

0

2

4

6

8

10

12

14

16

18

20

22

24

-4 -4.5 -5 -5.5

Calibrated Dense Woodland Grassland Linear (Grassland) Linear (Dense Woodland) Linear (Calibrated)

-6 -6.5 -7

Location



0 -0.2

0

2

4

6

8

10

12

14

16

18

20

22

24

-0.4 -0.6 -0.8 -1

Calibrated Dense Woodland Grassland Linear (Calibrated)

-1.2 -1.4 -1.6

Linear (Dense Woodland) Linear (Grassland)

-1.8

Location

Appendix 8

X-Discharge Intensity at Centre of CS4 0


-2

0

10000

20000

30000

40000

50000

60000

-4 -6 -8 -10

Calibrated

-12

Dense Woodland

-14

Grassland

-16 -18

Time (sec)

70000

Appendix 9




3 2 1 0 -1

0

2

4

6

8

10

12

Calibrated Dense Woodland Grassland Bed Elevation

-2 -3 -4

Location



3 2 1 0 -1

0

2

4

6

8

10 Calibrated

-2


-3

Bed Elevation

-4

Location

12

Appendix 9



3 2 1 0 -1

0

2

4

6

8

10

12


-2 -3 -4

Location



3 2 1 0 0

2

4

6

8

10

-1

Calibrated -2


-3

Bed Elevation

-4

Location

12

Appendix 9



2 1 0 -1 0

2

4

6

8

10

12

14

16

18

-2 -3


-4 -5 -6 -7 -8

Location



2 1 0 -1 0

2

4

6

8

10

12

14

16

18

-2 -3 -4

Calibrated

-5

Dense Woodland

-6

Grassland

-7

Bed Elevation

-8

Location

Appendix 9



2 1 0 -1 0

2

4

6

8

10

12

14

16

18

-2 -3 -4


-5 -6 -7 -8

Location



2 1 0 -1 0

2

4

6

8

10

12

14

16

18

-2 -3 Calibrated Dense Woodland Grassland Bed Elevation

-4 -5 -6 -7 -8

Location

Appendix 9

Water Suface Elevations Across CS4 at 10000sec 4


3 2 1 0 -1 0

2

4

6

8

10

12

-2 -3

Calibrated

-4

Dense Woodland

-5

Grassland

-6

Bed Elevation

-7 -8

Location



3 2 1 0 -1 0

2

4

6

8

10

12

-2 -3

Calibrated

-4

Dense Woodland

-5

Grassland

-6

Bed Elevation

-7 -8

Location

Appendix 9



3 2 1 0 -1 0

2

4

6

8

10

12

-2


-3 -4 -5 -6 -7 -8

Location



3 2 1 0 -1 0

2

4

6

8

10

12

-2 -3


-4 -5 -6 -7 -8

Location

Appendix 9



0

2

4

6

8

10

12

14

16

-1 -2 -3 Dense Woodland Grassland Bed Elevation Calibrated

-4 -5 -6

Location



1 0 0

2

4

6

8

-1 -2 -3 Dense Woodland -4 -5

Grassland Bed Elevation Calibrated

-6

Location

10

12

14

16

Appendix 9



2 1 0 -1

0

2

4

6

8

10

12

14

16

-2 -3 Dense Woodland

-4

Grassland

-5

Bed Elevation Calibrated

-6

Location



1 0 0

2

4

6

8

-1 -2 -3 -4 -5

Dense Woodland Grassland Bed Elevation Calibrated

-6

Location

10

12

14

16

Appendix 9



2 1 0 0

2

4

6

8

10

12

14

16

-1 -2 -3

Dense Woodland Grassland Bed Elevation Calibrated

-4 -5 -6

Location

Water Surface Elevations Across CS7 at 7000sec 0 0

2

4

6

8

10

Water Suface Elevation (m)

-1 -2 -3 -4 -5 -6


-7

Location

12

14

16

18

Appendix 9

Water Surface Elevations Across CS7 at 10000sec 0 0

2

4

6

8

10

12

14

16

18


-1 -2 -3 -4


-5 -6 -7

Location



0

2

4

6

8

10

-1 -2 -3 -4 -5 -6


-7

Location

12

14

16

18

Appendix 9



1 0 -1

0

2

4

6

8

10

12

14

16

18

-2 -3 -4


-5 -6 -7

Location



0

2

4

6

8

10

-1 -2 -3 -4 -5

Calibrated Dense Woodland Grassland

-6

Bed Elevation

-7

Location

12

14

16

18


Y-Discharge Intensity Across CS4 at 7000sec 0.50

0.00 2

4


0

6

8

10

12

-0.50

-1.00

-1.50


-2.00

-2.50

-3.00

Location



1 -1 0

2

4

6

-3 -5 -7 -9 -11


-13 -15 -17

Location

8

10

12

Appendix 9



2 0 -2

0

2

4

6

8

10

12

-4 -6 Calibrated Dense Woodland Grassland

-8 -10 -12 -14 -16

Location

Y-Discharge Intensity Across CS5 at 10000sec -4


0

2

4

6

8

-5 Calibrated

-6

Dense Woodand Grassland

-7 -8 -9 -10 -11

Location

10

Appendix 9



0

2

4

6

8

10

-2 -4 -6 Calibrated -8


-10 -12 -14 -16

Location



0

2

4

6

8

-2 Calibrated

-4

Dense Woodand -6

Grassland

-8 -10 -12 -14 -16

Location

10

Appendix 9



0 0

2

4

6

8

10

-2 -4

Calibrated -6


-8 -10 -12 -14

Location



0

2

4

6

8

-2 -4

Calibrated Dense Woodand

-6

Grassland

-8 -10 -12

Location

10

Appendix 9

Y-Discharge Intensity at Point 8 (River Channel) in CS5 0


0

10000

20000

30000

40000

50000

60000

-2 -4 Calibrated

-6

Dense Woodand -8

Grassland

-10 -12 -14

Time (sec)

70000

Appendix 10




15 14

Grassland

13

Group Woodland

12

Dense Woodland

11

Bed Elevation

10

Sparse Woodland

9 8 7 6 5 0

2

4

6

8

10

12

14

16

18

20

18

20

Location



15

Grassland Group Woodland Dense Woodland Bed Elevation Sparse Woodland

14 13 12 11 10 9 8 7 6 5 0

2

4

6

8

10

Location

12

14

16

Appendix 10



14


13 12 11 10 9 8 7 6 5 0

2

4

6

8

10

12

14

16

18

20

Location

Water Surface Elevations Across CS1 at 12000sec Grassland Group Woodland Dense Woodland Bed Elevation Sparse Woodland


15 14 13 12 11 10 9 8 7 6 5 0

2

4

6

8

10

Location

12

14

16

18

20

Appendix 10



15


14 13 12 11 10 9 8 7 6 5 0

2

4

6

8

10

12

14

16

18

20

Location



15 Grassland Group Woodland Dense Woodland Bed Elevation Sparse Woodland

14 13 12 11 10 9 8 7 6 5 0

2

4

6

8

10

Location

12

14

16

18

20

Appendix 10

Hydrogrpah for Point 10 (Floodplain) on CS1 11 Grassland


10.5

Group Woodland 10


9.5 9 8.5 8 7.5 7 0

5000

10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000 65000

Time (sec)



14


13 12 11 10 9 8 7 6 5 4 0

2

4

6

8

10

Location

12

14

16

18

Appendix 10




13 12 11 10 9 8 7 6 5 4 0

2

4

6

8

10

12

14

16

18

16

18

Location




13 12 11 10 9 8 7 6 5 4 0

2

4

6

8

10

Location

12

14

Appendix 10



14 Grassland

13

Group Woodland

12

Dense Woodland

11

Bed Elevation

10

Sparse Woodland

9 8 7 6 5 4 0

2

4

6

8

10

12

14

16

18

Location




13 12 11 10 9 8 7 6 5 4 0

2

4

6

8

10

Location

12

14

16

18

Appendix 10

Water Surface Elevation for Point 7 (Floodplain) in CS2 8 Grassland


7.5

Group Woodland 7


6.5 6 5.5 5 4.5 4 0

10000

20000

30000

40000

50000

60000

70000

Time (sec)



10 9


8 7 6 5 4 3 2 0

2

4

6

8

10

Location

12

14

16

18

Appendix 10




9 8 7 6 5 4 3 2 0

2

4

6

8

10

12

14

16

18

Location



9


8 7 6 5 4 3 2 0

2

4

6

8

10

Location

12

14

16

18

Appendix 10




9 8 7 6 5 4 3 2 0

2

4

6

8

10

12

14

16

18

Location




9 8 7 6 5 4 3 2 0

2

4

6

8

10

Location

12

14

16

18

Appendix 10



9


8 7 6 5 4 3 2 0

2

4

6

8

10

12

14

16

18

Location

Hydrograph for Point 6 (Floodplain) on CS4 6


5.5 5 4.5 4

Grassland Group Woodland Dense Woodland Sparse Woodland

3.5 3 2.5 2 0

5000

10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000 65000

Time (sec)

Appendix 10



4.5 4 3.5 3 Grassland Group Woodland Dense Woodland Bed Elevation Sparse Woodland

2.5 2 1.5 1 0.5 0 0

5

10

15

20

Location



5 4.5 4 3.5 3


2.5 2 1.5 1 0.5 0 0

5

10

Location

15

20

Appendix 10



4.5 4 3.5 Grassland Group Woodland Dense Woodland Bed Elevation Sparse Woodland

3 2.5 2 1.5 1 0.5 0 0

2

4

6

8

10

12

14

16

18

20

Location



4.5 4 3.5 3


2.5 2 1.5 1 0.5 0 0

2

4

6

8

10

Location

12

14

16

18

20

Appendix 10



4.5 4 3.5 3 Grassland Group Woodland Dense Woodland Bed Elevation Sparse Woodland

2.5 2 1.5 1 0.5 0 0

2

4

6

8

10

12

14

16

18

20

Location

Hydrograph for Point 13 (Floodplain) on CS5 3.75


3.5


3.25 3 2.75 2.5 2.25 2 1.75 1.5 1.25 1 0

10000

20000

30000

40000

Time (sec)

50000

60000

70000


Discharge Intensity of Point 7 (Floodplain) in CS2


0.500

-3.000

-2.500

-2.000

-1.500

-1.000

0.000 0.000

-0.500

0.500

-0.500

Grassland -1.000

Dense Woodland Group Woodland

-1.500

Sparse Woodland -2.000

Linear (Dense Woodland) Linear (Group Woodland) Linear (Grassland)

-2.500

-3.000

Linear (Sparse X-Discharge Intensity (m2/sec) Woodland)



0

2

4

6

8

10

12

-2.5

-3

-3.5

-4


-4.5

-5

Location

14

Appendix 10



0

2

-2.5

4

6

8

10

12

14


-3 -3.5 -4 -4.5 -5

Location



0.5 0 -0.5 0

2

4

6

8

10

12

14

16

-1 -1.5


-2 -2.5 -3 -3.5 -4 -4.5

Location

18

Appendix 10



0.5 0 -0.5

0

2

4

6

8

10

12

14

16

18

-1


-1.5 -2 -2.5 -3 -3.5 -4

Location



0.5 0 -0.5

0

2

4

6

8

10

12

14

16

-1 Grassland Group Woodland Sparse Woodland Dense Woodland

-1.5 -2 -2.5 -3 -3.5 -4

Location

18

Appendix 10



0.5 0 -0.5

0

2

4

6

8

10

12

14

16

18


-1 -1.5 -2 -2.5 -3 -3.5

Location

Outflow Hydrographs for Floodplain 3 325 300


275

Grassland Grouped Woodland Dense Woodland Sparse Woodland

250 225 200 175 150 125 100 75 50 25 0 0

5000

10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000 65000

Time (sec)

Appendix 10

Inflow and Outflow Hydrographs for Floodplain 3 500


450 Inflow Grassland Grouped Woodland Dense Woodland Sparse Woodland

400 350 300 250 200 150 100 50 0 0

10000

20000

30000

40000

Time (sec)

50000

60000

70000

Appendix 10

Total Discharge in Floodplain 3 6000

Total Discahrge (cu.m/sec)

5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0

1

Grassland

4501.735

Grouped Woodland

4386.715

Dense Woodland

4350.197

Sparse Woodland

4482.671

Inflow

5518.701

Appendix 11

Appendix 11 Average Basal Area in Floodplain 1 0.04331667

Calibrated Dense Woodland 0.100713248

Average Basal Area for Floodplain 2 0.022706105


0.15865838

Average Basal Area for Floodplain 3 0.02439527

0.100713248

0.100713248

Dense Woodland Group Woodland Sparse Woodland

Appendix 11

Total Basal Areas for Floodplain 3 120000

Total Basal Area (m2)

100000 80000

Dense Woodland Group Woodland Sparse Woodland

60000 40000 20000 0

Average Basal Area Floodplain 1 Calibrated Dense Woodland Floodplain 2 Calibrated Dense Woodland Floodplain 3 Dense Woodland Group Woodland Sparse Woodland Total Basal Area Dense Woodland Group Woodland Sparse Woodland Total Areas (Floodplain3) Sparse/Dense Woodland Group Woodland

0.04332m2/m3 0.10071m2/m3 0.02271m2/m3 0.15866m2/m3 0.10071m2/m3 0.10071m2/m3 0.0244m2/m3 101468.6m2 54133.37m2 24578.23m2 1007500m2 537500m2