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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix C: Coastline Hazards

Appendix C2 - Beach Erosion Hazard

1 INTRODUCTION

The large waves, elevated water levels and strong winds generated by a storm can cause severe erosion to sandy beaches. Storm wave attack can move significant quantities of sand offshore (up to 250 cubic metres per metre run of beach, as measured above MSL). Storm waves undercut the beach berm and frontal dune to form a pronounced erosion escarpment. The foredune may be cut back by up to 20m during a major storm event.

Erosion is part of the natural response of a beach to changing wave and water level conditions. Generally, the eroded sand is returned to shore and the beach is rebuilt during calmer periods of swell waves.

Buildings and facilities located within the "active" beach system, or area subject to erosion, will be undermined, and if not designed for this hazard, may collapse. Figure C2.1 shows property damage at Wamberal in 1978 due to beach erosion.

Figure C2.1

Figure C2.1 Undermining and Collapse of Buildings Caused by Beach Erosion.

2 MEASUREMENT OF BEACH EROSION

Erosion can be measured in terms of the volume of sand transported offshore or in terms of the landward advance of a significant beach feature, such as HWM or the "backbeach erosion escarpment" (erosion escarpment). Of these two features, the position of the erosion escarpment is preferred: it is a more definite and longer lasting feature than the HWM, which is subject to a much greater variation in position.

Figure C2.2 shows the landward advance of HWM and the erosion escarpment under storm conditions. During major storms, the HWM may advance 50 to 80m and foredunes may be cut back by up to 20m. The beach and foredune are rebuilt over time, but the erosion escarpment will remain clearly evident for several years. Figure C2.3 illustrates hypothetical examples of variations in the positions of HWM, incipient dune and foredune at a beach over a period of several years. The pronounced variability of the HWM is readily apparent.

Figure C2.2

Figure C2.2 Beach Erosion Under Storm Conditions.

Figure C2.3

Figure C2.3 Variation in Position of Beach Features with Time

3 FACTORS AFFECTING EROSION

The extent of beach erosion during a particular storm event depends upon a variety of factors that include:

Wave conditions and elevated water levels at a coastline depend upon the extent, intensity and duration of storm winds, and the distance of the fetch from the shoreline (see Appendix B3). The duration of peak wave and elevated water level conditions also influence the extent of erosion.

Variations in wave and current patterns along the nearshore zone also affect beach erosion. The most important of these mechanisms is rip cells, which greatly facilitate the offshore transport of sediment (see Appendix B6).

During storm conditions, most beaches attempt to establish an equilibrium profile which is largely determined by sediment size, storm wave climate and elevated water levels (see Appendix B7). The duration of the storm often determines the degree of establishment of this profile. Several storms in succession enhance the establishment of the equilibrium profile. During calm periods, the beach is rebuilt.

If a beach is in "poor" condition when a storm occurs, i.e. only partially rebuilt, a lesser volume of material will be removed than if the beach is in "good" condition.

Dune vegetation contributes to a beach and dune system being in "good" condition. "Healthy" dune vegetation speeds dune rebuilding following storms and will help to maximise the volume of sand available as a buffer to storm attack.

The presence of headlands, groynes and other features that influence sediment transport can also affect the extent of erosion. Under certain beach and storm conditions, these features can disrupt the local erosion/accretion processes and may lead to greater erosion than otherwise expected.

Dune height has a significant effect on the landward advance of erosion. In general, the higher the dune, the lesser the landward advance of the erosion escarpment. Storm waves impinging on the shore have a specific capacity to erode sand and transport it offshore. Generally, for a particular storm, there is a smaller recession of the dune escarpment for high dunes than for lower dunes, although the volume of sand eroded from the higher dunes may be greater.

If the eroding dune has a high groundwater level, it may suffer considerably more erosion than otherwise expected.

4 SLUMPING OF EROSION ESCARPMENT

The erosion escarpment formed during a storm may be nearly vertical. As the sand dries out, the escarpment will slump to a more stable slope of about 1V:1.5H. Dune rebuilding processes may further flatten the escarpment. This slumping defines a zone of "slope readjustment" in which buildings are at hazard of collapse (see Figure C2.4).

Figure C2.4

Figure C2.4 Zone of Slope Readjustment of Erosion Escarpment.

5 BEACH EROSION IN NEW SOUTH WALES

The nature and behaviour of beaches along the New South Wales coast varies considerably because of differences in sediment characteristics, degree of compartmentalisation, wave climate and evolutionary history. Consequently, there may be significant differences in storm demand from beach to beach.

Table C2.1 shows the erosive impact measured in PWD studies of some storm events at various locations along the New South Wales coast: The results for the open coast indicate storm demand during major storm events ranging from 200 to 240 m3/m of beach above AHD with the exception of the measurement for Byron Bay of 320 m3/m of beach above AHD. This stormbite was measured immediately downdrift of a seawall protecting a carpark. Downdrift erosion would have been increased by the presence of rock protection on this high longshore transport rate coast. The largest landward movement of erosion escarpments measured by PWD in NSW during major storms are 17m at Wamberal in 1978 and 18m at Byron Bay in 1989. Again, this second figure would have been influenced by the rock protection immediately updrift of the measured location.

The storm demand measured in protected embayments is up to 130 m3/m of beach above AHD.

As indicated in Appendix B2, further data re beach erosion is available from the data sets collected at Moruya and Narrabeen by members of the Coastal Studies Unit, University of Sydney (Thom and Hall (in press) and Hall, 1988).

Most post-storm volumetric measurements of beach erosion, whether made by ground level survey or by photogrammetry, usually reflect beach conditions after partial rebuilding has occurred. Thus, the full volume of beach erosion is usually underestimated in such measurements.

Table C2.1
Beach Erosion Along the New South Wales Coast
 Location  Year Beach Erosion above AHD m3/m run Method of Assessment
Open Coast Beaches      
 Narrabeen  1974  200 Photogrammetry
 Collaroy  1945  210 Ground Survey
 Wamberal  1974  240 Photogrammetry
 Forresters  1986  200 Photogrammetry
 Byron Bay  1989  320 Photogrammetry & Ground Survey
Protected Embayments      
 Pearl, Broken Bay  1986  120  Photogrammetry
  Callala, Jervis Bay  1974  130  Photogrammetry

6. REFERENCES

Hall, W.F. (1988) "Sub˝aerial Beach Profile Behaviour." MSc. Thesis, University of Sydney, Department of Geography.

Thom, B.G. and Hall, W.F. "Behaviour of beach profiles during erosion and accretion dominated periods." Earth Surface Processes and Land Forms (in press).