In this study, the Richmond River entrance and adjacent beaches were selected to examine the impact of the construction of major training walls, and the impact ...
Vol. 17 No. 1
CHIN. J. OCEANOL. I3MNOL.
1999
IMPACT OF R I V E R TRAINING WAIJIS ON A D J A C E N T B E A C H E S , BALLINA, NEW SOUTH WAI,ES, AUSTRALIA" HUANG Hai-jun(~i~f~:~),Andrew. D. Short*, Thomas Zeng*, David Hanslowt* ( Institute of Oceanology, CJtiaa.seAcademy of Sdenm~, Qingdao 266071, China ) ( "?CoastalStudies Unit, Department of ~ a p h z, University of Sydney, Sydney, NSW, 2006,Australia ) ( i't Detx~mnenl of Land and Water Conservation, Sydney, NSW, 2000, Australia) Received Mar. 31,1998; revisionaccepted Oct. 6, 1998 River and inlet training wails have been built at scores of locations along the NSW coast. This study examines the impact of training walls built at the R i c e d River mouth, Ballina, over the period 1889 to 1991 on the adjacent beach systems. GIS analysis of 2 bathymetric maps and photograrmnetric analysis of 6 aerial photo sets were used. The position of the shoreline and contours and their temporal changes for each profile were determined to identify accretional and erosional patterns. Spatial variations in the patlern were then used to assess the impact of training walls. The impact of training wall can be divided into three types: ( I ) accretion in up&fit and erosioa in downdrift; (2) accretion on both sides of the training ,~11, and (3) accretion on both sides near the training wall, but erosion on both sides away from the training wall. South Ballina beach and Lighthouse beach exemplify the second type. The impact of training wall varies hnearly with the distance frtma the wall.
Key words: training wall, shoreline change, GIS, accretion, erosion
INTRODUCTION The effects of human activities on shoreline processes have been considerable, particularly over the last century. The most significant effects are those from harbour construction and river mouth training wall. In many instances, major interruption of littoral drift resulted from the construction of massive entrance piers (Barrett, 1989) and the two usual.ly long and high walls. These structures have major impact on adjacent shores, and their influence must be taken into account in any assessment of shoreline change due to the structures and natural changes. Major natural changes, such as extreme coastal storm events must also be taken into account in calculating shoreline change, as they have major impacts on shoreline position and consequently affect or depress the shoreline position for some years following the events. In this study, the Richmond River entrance and adjacent beaches were selected to examine the impact of the construction of major training walls, and the impact of major storm events. GIS analysis of historical bathymetric maps and analysis of 6 aerial photograph sets from 1947 to 1991 were used to accurately determine shoreline and contour posi" ContributionNo, 3378 from the Instituteof Oceanology, Chinese Academyof Sciences.
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tion and sand volumes, and their changes in time and space, in order to assess the contribution of the structures and storms to large scale (years-decades and kilometres) beach change. The river brings minimal flow and sediments into the estuary and there is a net northward alongshore sand drift passing across the estuary. STUDY AREA AND DATA SOURCES The study area was the
Richmond River mouth at Lightho'USBloBcekChz ~ Ballina, with Lighthouse Beach to the north and RichmondRiver~ South Ballina to the south. trainingwall "x5 Six sets of aerial photographs taken on 27/5/1947 (only southern part of the training wall), 2/6/1965, 13/11/1971, 14/3/1974, 31/7/1981 and 21/7/1991 were used to analyse the Block shoreline changes at BalliX'~"N~'~~ South acif'cOcean na and Lighthouse Beach. qhe water depth data surveyed by W. Shellshear in 1884 and the 1:25 000 topographic map produced in Fig. 1 The locationof studyareasand pms I973 were used to determine the evolution of Shaws Bay downdrift of the training wall. Photogrmm~tric techniques were used to survey 58 profiles at South Ballina Beach and 32 profiles at Lighthouse Beach. The beaches were then each divided into three blocks of profiles containing 32, 26 and 32 perpendic~ar profiles at 50 m, 50 m and 20 m intervals respectively. The first two blocks were perpendicular to the beach, the third paralleled the training wall (Fig. 1). The aerial photographs covered a 44 year period (1947- 1991), the maps an 89 year period (1884- 1973). .
METHODOLOGY 1.
Photagrammetry and the formation of data sets
Photogrammetry as a science has been developing for over a century and is accepted as an alternative to traditional land survey techniques for many survey applications (Hanslow et al., 1997; Banister et al., 1994; Methley, 1986; Kavanagh and Bird, 1984, American Society of Photogrammetry, 1980). The accuracy of the methods used is
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:t 0.1 m in plan and height (Hanslow et a l . , 1997). The aerial photographs were analysed using a Wild AC1 Stereo Analytical Plotter housed at the NSW Department of "Land and Water Conservation. Each photograph was used to generate beach profiles containing the profile numbers, distances from baseline, x, y coordinates (Integrated Survey System) and height of beach. 2.
Selection of baseline for assesarnent of temporal changes
A baseline must be determined in order to compare sho~line changes in the usually more or less permanent natural features such as a bluff edge, top edge of escarpment on an eroding dune, normal high tide line (near tidal line, vegetation line, beach scarp, debris line, upper limit of wet sand), and the most seaward edge of permanent vegetation. Because of changes in many of these features over time it is essential that the photogratmr~tric operator be aware of potential changes that could introduce errors. In this study, the contours were used as baseline to avoid these problems. The 0 m, 0.5 m, 1 m, 1.5 m, 2 m, 2.5 m, 3 m, 3.5 m, 4 m, 4.5 m and 5 m contours were used for calculation of changes in beach topography, shoreline position and sand volume. 3.
Interpretation of maps and photos in GIS database
In order to extend the database to the period before the first aerial photographs (1947) the 1884 and 1973 maps were digitized. Their data and the photograminetrically derived data were used to form the beach plan and profile maps and to compare their landform, sand volume and shoreline changes. BEACH CItANGES The beach/dune systems at both beaches began to develop around 6500 years ago at the end of the post glacial marine transgression. Onshore transport of marine sands occurred following the stabilization of sea level resulting in beach and dune progradation which on many beaches continued until about 3000 to 1000 years BP (Roy, 1980). The Richmond River breakwaters were built between 1889 and 1911, with the northern wall extended 200 m between 1965 and 1968. 1.
Beach contour changes 1947 - 1991
In order to compare the changes of South Ballina and Lighthouse beaches, the contours from shoreline to 5 m, with interval 0.5 m ( 11 contours), were selected. The baseline of each block (three blocks) formed the starting point, with distances calculated between the baseline and every contour in each profile. Although they are not comparable block by block because of their different baseline, the relative position of six contours illustrates the beach behaviour in each block. Block X is located 1.3 km to 3 km south of the training wall. The trend of both the 0 m and 4.5 m contours was uniform, except in 1974, following a major erosion event. The 1991 contour was the most seaward and the 1965 contour the most landward. There
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was an overall pattern of beach accretion. Block Y exterds from 1.3 km to the south side of the training wall. It also showed few changes apart from the 1974 erosion event. The 1971 contour was the mast landward with the 1981 contour arm 1991 contour showing slight erosion, which increased toward the training wall. In block Z, located on the downdrift (northern) side of the training wall, the beaches had undergone overall progradation. They prograded between 1965 and 1991, and the 1974 erosion apparently had little or no impact on this part of the beach. In order to quantify the changes, the average contour positions at 0 m, 2 . 5 m and 4 m of all 32 profiles in block Z were calculated. The regression between contour average position and aerial photo dates was determined for the 0 m, 2 . 5 m and 4 m contours. The relationship for t h e 4 m contour is: C4m = 1903 +0.418T4m, with R = 0 . 9 2 ; for the 2.5 m contour: C25m = 1899+0.4T2.sm, with R = 0 . 9 ;
and for the 0 m contour: C0~ =
1889 + 0 . 3 7 To,,, with R - 0 . 8 4 . In the formulae above, C stands for contour average position at 0 m, 2.5 m and 4 m contour and T the aerial photo date(year). These regression lines (Fig. 2c) showed that Lighthouse Beach has been prograding since 1965 at a rate of 0.45 m/a to 3.5 m/a; and that the major 1974 erosion event had httle impact on this part of the beach. A E 240 I i
21(10 * ' ~ l a
,~ O
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....
o oO"
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1974 Aerial photo d a ~
1981
1991
120 [1947
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1974
1981
1991
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1974
1981
199~1
Ae/lal 1o~XO date
Fig. 2 Shorelinechangesin blocks
2.
Beach response to major erosion events
Temporal variations in beach conditions are mainly a consequence of the highly variable deep water wave climate and corresponding breaker climate(Thorn et a l . , 1991 ). According to Thorn (1974) and Chapman et al. (1982) a series of storms leading to periods of sustained beach and foredune erosion resulting in significant damage to property occurred in 1883,1912,1950,1967,1974 and 1978. The 1974 storms in the study area were due to three tropical cyclones (in January 24 - 26, February 4 - 7 and March 12 - 13, 1974) with wave height of up to 6 m. Lighthouse Beach lost about 500000 m3 of sand (DLWC, 1986). This event is recorded in the 1974 aerial photographs and serves as a good indicator of storm damage. The relative change between the shoreline in the photo in 1974 and that in 1965,1971, 1981 and 1991 are plotted in Figure 3. Profiles falling below the 1974 baseline (0 m) lie landward of the 1974 shoreline, while those above represent seaward relative to 1974. There are two points which highlight the difference between the downdrift and updrift of the train-
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ing wall. The updrift side of the 1974 shoreline represents the most severe shoreline erosion (the beach fully recovered to its 1971 shoreline by 1991 ). On the downdrift side, however, the 1965 shoreline and some pIaces in 1971, experienced progradation relative to the I974 shoreline. In other words, the updrift shoreline, eroded partly but as a whole, prograded gradually; while on the downdrift the erosion could have been lessened by the training wall. The simple relationship between the average contour position of all 32 profiles and aerial photo dates can be considered as confirmation of the big storm erosion on block X and block Y. The formula of the 4 m contour in block X includes 1974 data and is: C4,, = 1916 + 0. 2 2 7 T ~ , with R = 0 . 1 3 ; for the2.5 m: R = 0 . 4 8 ; for theOm: R = 0.49. Mter removing the 1974 data, the same analysis result yielded Rom = 0.62; R2.sm = 0.94; R4m = 0. 97. The difference means the South BaUina beach has been prograding since 1965; but the shoreline was even pushed back by the 1974 big storm (Fig. 2).The effect of the Richmond training wall was opposite to that of other training walls.
J
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- 1991
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numbers
Profile numbers
0.5 m contour position relative to 1974
Fig. 4
Average shoreline changes(4 m) relative to 1974
Figure 4 is a profile by profile plot of average of 5 shoreline positions relative to the 1974 shoreline. Overall there was a 35 m difference between the updrift side and the downdrift side of the training wall in the 4 m contour which explains the different behaviour between the two sides of the training walls during the big storm. 3.
Long and short term shoreline changes
A beach undergoes long term fluctuations which may involve short term erosion evextts followed by an extended recovery period, during which the beach/formhme system progrades. The long term trend may consist of short term storm fluctuations. The rate and direction of shoreline change is therefore partly dependent on the period over which it is mexsured. Lighthouse Beach is an example. The beach has been accreting since the training wall's construction. The rate of shoreline change between surveys varied with the actual survey date and the length of time between surveys and photos. The longest term gap is the 89 years between the 1884 local survey data and the 1973 topographic map. There are two profiles with shoreline change rate of 3.8 m/a and 5.5 m/a. The mean rate is used to represent this period. The second rate of this beach is an average rate of all 32 profiles in
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block z from aerial photo data. It is 2 . 2 m/a between 1991 and 1965. The third is 3.3 m/ a from the same source between 1981 and 1965. The fourth is 3.5 m/a between 1974 and 1965. The most recent is - 0 . 0 3 m/a, 0.5 m/a and 0.87 m/a at 0 m, 2.5 m and 4 m contours respectively between 1991 and 1981, or average rate of 0.45 m/a. The rates suggest the following for Lighthouse Beach. Between 1894 and 1973 the beach experienced its most rapid rate of progradation (4.65 m/a). Subsequently the rate slowed to 3.5 m/a (1965 - 1974), then 3.3 m/a (1965 - 1981), to 2 . 2 m/a (1965 1991) and finally 0.45 m/a (1981 - 1991). These figures suggest that the construction of the training wall led to substantial shoreline accretion at the first stage. The rate of accretion was initially rapid ( > 4 . 5 m/a) over the first 80 years. Over the past 20 years the rate has continued to decrease, with parts of the 1981 - 1991 rates negative, and suggesting an equilibrium shoreline position is being approached.
4.
Sand dune volume changes
Natural sand dunes are formed by winds blowing onshore over the beach, transporting sand landward. Grass and sometimes shrubs grow on sand dunes, creating a natural barrier against erosion. The dunes provide a reservoir of beach sand during severe storms and thus help prevent wave damage to adjacent property. So, the volume of sand dune represents the store of excess beach material accumulated over a period of time. As such it is an important indicator of longer term beach behaviour. In order to estimate the longer term changes in the beach and the dune volume and to assess the impact of the training wall, the total sand dune volume updrift (block X and block Y) and downdrift (block Z) were calculated, using the 0 m and 4 . 5 m contours. (1) Total sand dune volume change The total sand dune volume above 0 m increased 10.4% from 1965 to 1991. The change above 4.5 m was similar to that above 0 m, except that the fluctuations were a little larger. (2) Updrift sand dune volume change Block X and block Y were located on the updrift of the training wall and were combined for estimating the updrift beach behaviour. At the 0 m contour, sand accumulated between 1947 and 1965, decreased in 1974. In 1991 sand accummulation recovered to about that before the 1974 storm. The rate of accumulation was low from 1947 to 1965, when increase in sand volume was 1 . 7 % , and was higher from 1965 to 1971, when increase in sand volume was 4 . 7 % . At the 4.5m contour the pattern was different. There was substantial increase of 18% from 1947 to 1965, then a decrease to 1.6% from 1965 to 1991. (3) Downdrift sand dune volume change Figure 5 is the plot of the volume of sand in the downdrift block Z above 0 m against the aerial photo dates for the period 1965 - 1991. The increase was almost linear ( Vom = -39829 + 8321 Tom with Rom = 0.91 above 0 m contour and V4.sm = -- 32444 + 1393T4.sm with R4.sm = 0 . 8 7 ) . The total sand dune volume increased by 4 6 . 7 % from
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1965 to 1991, average increase rate was about 2% per year. The impact of the 1974 erosion was not apparent, suggesting the training wall protected the dune during the severe 1974 erosion event. In summary, updrift of the training wall, the largest shoreline changes occurred v 610000 farthest from the wall, with the rate of change decreasing toward the wall, indicating the 510000 4600O0 wall protected the adjacent up&ift shoreline. 65 71 74 81 91 On the northern downdrift side however the Measure time highest rate of change occurred immediately Fig. 5 Downdrift sand volume above 0 m, 1965 1991 adjacent to the wall and reduced rapidly in size. northwards. DISCUSSION AND CONCLUSION The data on South Ballina Beach and Lighthouse Beach showed that the 1974 storm erosion had major significance. This point is evident from the profile average positions, the volumes of sand dune changes, and their change rates, although they have some differences profile by profile. The beach evolution was so strongly influenced by big storm erosion that it can be divided into two periods: one was the storm erosion period, a short period, when onshore/offshore transfers were active; the large scale beach erosions occurred when subaerial beach volume was moved offshore to form surfzone bars. The t~achface changed greatly too. The other is the period of recovery. It is a subsequent period of relatively low wave power favoring the accretionary phase. The beach volumes returned to prestorm values during the period. Accretion is a much less rapid process than erosion(Thorn et al., 1991, Wright et a l . , 1985). There were four beach behaviour patterns in response to the impact of the training wall. The first was the progressive accretion in the downdrift. Lighthouse Beach is the example. Under the protection of the training wall, the average rate of the accumulation of total sand dune was about 2% per year from 1965 - 1991. The rate of shoreline accretion was 3.8 - 5 . 5 m/a from the beginning (1889) of the building of the training wall to 1973. The 1974 storm apparently did not disturb the special beach behaviour just described above. The protection of the training wall, the small, semi-closed bay, and rich supply of sediments were the special environment factors favoring this type. The second was beach accretion in the updrift of the training wall, generally characterized by accumulation of sand dune and shoreline accretion. Although there were impacts of big storm erosion, the total amount of sand exchange and range of contour change were minor. The accreting area dunes provided a reservoir of beach sand during severe storms and thus helped prevent wave erosion. In areas where substantial dunes existed, the poststorm beach width being
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greater than the prestorm width is proof. The third type was observable at the downdfift of the training wall too, where natural storm erosion and training wall induced serious erosion occured. The fourth type is a special one when there was no net littoral drift along the beach. The wall functioned as an accumulator of sediment. There was deposition near the training wall and erosion away from the wall at both sides of the walls. The walls on the Oregon coast comprise an example of this condition(Komar, 1992). The beach reaction to the training wall differed with time. At the beginning of the building of the training wall, the rates of erosion or accretion at the downdrift and accretion at the updrift were high but with time they became lower and lower. At last, the accretion and erosion ceased and total sand by-passing was established, and a stable equilibrium beach will form unless a random storm event interrupts the sequence. At this moment, they serve to stabilise a region of beach on the up&fit side and on the downdfift side. And for the same reason, the recovery was observed to be initially rapid and of great magnitude following a major storm or series of storms. After some period, the dynamic environment readjusts to the new beach form and there is a new balance between the beach alignment and tidal current and wave energy. There is total sand by-passing near the training wall area. Then the change of sand dune volume will be small and most functions of the training wall will cease.
References American Society of Photogrannnctry, 1980. Manual of Phntogrammetry. Fourth Edition, American Society of Photogranm~try, Falls Church Va, 1056 p. Banister, A., Raymond, S., I-Idn~,R., 1994. Surveying. Sixth Edition. ~ Scientificand Technical. p. 11-43. Barrett, M.G., 1989. What is ~ ~ ? In: Ctmtal 1 ~ , edited by qhtm~s Telford, ~ , p. 1-10. Chapman, D. M., George, M., Roy, P . S . , 1982. Coastal Evolution and Coastal Erosion in New South Wales. Coast Council of New South Wales, Sydney. 341p. DLWC, 1986. Summary of Storm Damage and Associated Sea and Weather Conditions on the N. S. W. Coast - 1876 to 1981, DLWC Report No. 86014, ISBN 724026126, July, 1986. Hanslow, D.J., Bob Clout, Peter Evam, 1997. Monita'ing coastal change using p h o t o . Second Joint Inst. of Aust. ~ arm N.Z. G e ~ c a l Society Conference, Jan. 1997, ~ , Tasamnia, ~ , p. 1 - 10. Komar, P. D. , 1992. Oeean ~ and hazards along the Oregon coast. In: CoastalNatural Hazards, Sdence, Enoneering and Public Policy. Edited by Jarrt~ W. Goed et al., Orngon Sea Grant. Oresu- B-92-001. p. 38-73. Kavanagh, B . F . , Bird, S. J . , 1984. Surveying: Principles and Applications. Reston Publishing Co., p. 10-58. Methley, B. D. F., 1986. Computational Models in Surveying and Pliotogrammetry. Dlackie & Son Ltd., 58p. Roy, P. S., 1980. Holocene sequences on an embayed high - energy coast: an evolutionary model. Sed/mentary C,eo/ogy 26:1 - 19. rIlaom, B. G., Hall, W., 1991. Behaviour of beach profiles during accretion and erosion dominated periods. Earth Surface Processes and Landforms. 16:113- 127. Thorn, B. G., 1974. Coastal Erosion in Eastern Australia. Search. 5:198 - 209. Wright., L. D., 1985. Short term changes in the m o r p h ~ c state of beaches and surf zones: an empirical predictive model. Mar/ne GeoLogy6 2 : 3 3 9 - 364.
