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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix C: Coastline Hazards

Appendix C3 - Shoreline Recession Hazard

1 INTRODUCTION

Shoreline recession is the progressive landward shift of the average long term position of the coastline. Recession is a different phenomena to beach erosion, although they both may be caused by the same processes.

Consideration of recession hazard is essential if coastal buildings and facilities are not to be ultimately subject to the threat of collapse. For example, on a receding coastline it is misleading to establish "setback" limits to cater for other coastline hazards. The receding coastline moves shorewards to eventually threaten the supposedly "safe" buildings and facilities.

2 CAUSES OF RECESSION

The two causes of shoreline recession are sediment loss and an increase in sea levels.

Sediment Loss

Recession of a sandy beach is the result of a long term and continuing net loss of sand from the beach system. According to the sediment budget concept, this occurs when more sand is leaving than entering the beach compartment (see Appendix B7). Recession tends to occur when:

Sea Level Rise

A progressive rise in sea level will result in shoreline recession through two mechanisms: first, by drowning low lying coastal land, and second, by shoreline readjustment to the new coastal water levels. The second mechanism is probably the more important: deeper offshore waters expose the coast to attack by larger waves; the nearshore refraction and diffraction behaviour of waves will change; a significant volume of sediment will move offshore as the beach seeks its new equilibrium profile.

Major changes in ocean levels and shoreline position have occurred over the ages, the most recent ending some 6,500 years ago when the MSL rose by some 140m (see Appendix B2). Present day shoreline recession can be viewed as the continuing long term readjustment of the coast to these changed ocean levels.

The postulated cause of significant sea level rise in the immediate future is the "Greenhouse Effect". To the year 2100, one authority has provided scenarios for planning purposes of an increase in sea level of the order of 0.5 to 1.5m (NAS,1987).

3 MEASUREMENT OF SHORELINE RECESSION

Shoreline recession is typically a long term process which in some cases is imperceptible. Its effect on a beach is often masked by the more rapid and dramatic erosion and accretion that accompanies storm events. Consequently, it can be difficult to identify recession from historical data, even if it extends over many years. Shoreline recession on undeveloped beaches may be completely unnoticeable: there are no fixed man-made features to act as reference points.

Trends in shoreline recession can be estimated in two ways:

Sediment Budget Approach

The best indication of shoreline movement is obtained from a volumetric sediment budget survey which includes the total volume of sand in the active beach system. By measuring sand volumes in both offshore and onshore areas, the short term effects of storm erosion and beach rebuilding under swell wave conditions can be taken into account. However, such investigations are expensive because of the need for offshore survey data. This approach is not usually practical on a routine basis, especially for long sections of coastline.

Sediment budget surveys are more commonly limited to the volume of sand in the active beach and dune system above MSL. This can be relatively inexpensively determined by conventional ground survey or by photogrammetry. Such an approach does not give a direct measure of total beach change. Nevertheless, with care and careful interpretation, acceptable estimates of long term recession trends can be obtained. Such an approach is shown in Figure C3.1.

Figure C3.1

Figure C3.1 Long Term Recession Estimates Based on Changes in the Volume of Beach Sediments

Position of Beach Features

Figure C2.3 of Appendix C2 depicts the change in the position of three beach features: HWM, the incipient dune and the backbeach erosion escarpment. Long term trends in the position of these features can be used as a measure of shoreline recession. The backbeach erosion escarpment is an obvious beach feature that is not affected by minor storms. Figure C2.3 shows the estimate of long term recession based on the position of this feature.

4 RELIABILITY OF RECESSION ESTIMATES

The time span, reliability of data and type of method used to determine shoreline recession all influence the reliability of the final estimate.

Most recession estimates are based on a relatively short period of historical data (typically 20 to 50 years). It is implicitly assumed that coastal processes over this period are representative of the long term situation. This may or may not be the case.

Shoreline recession progresses in a series of steps, as indicated in Figure C2.3. It only becomes evident as the landward advance of the backbeach erosion escarpment during major storm events. Thus, it is important to check the frequency and severity of storms over the time span of the data on which recession estimates are based.

Even the best recession estimates are only accurate to about 30 %.

5 RECESSION INDICATORS

Beaches undergoing significant shoreline recession usually display the following characteristic features or "recession indicators":

6 SHORELINE RECESSION IN NSW

Over the past 15 years or so, coastal process studies have been undertaken at a number of sites along the New South Wales coast. The results of these studies are shown in Table B7.1 of Appendix B7. Although the quality and extent of available data were limited in many cases, the results show a general recession trend at most beaches of 0.2 m/year or greater (up to 1.4 m/year).

7 FACTORS AFFECTING RECESSION

Shoreline recession along the NSW coast is caused principally by an imbalance in longshore or onshore/offshore sediment movement or windblown sand losses. The dominant cause depends upon the nature of the beach.

Pocket Beaches

Typically, pocket beaches experience little if any net loss of sand through longshore transport. Their bounding headlands tend to protect pocket beaches from longshore drift. Any recession is generally caused by wind losses or by an imbalance in onshore/offshore transport (loss to offshore sinks).

Extended Beaches

On extended beaches with significant longshore drift, recession is typically caused by an imbalance between incoming and outgoing longshore transport. This may be caused by the natural and continuing evolution of the coast following the post-glacial sea level rise, or by interference with sand movement along the coast e.g. by groynes, training walls, etc.

Coastal Features

Coastal features such as headlands, reefs and nearshore islands can also influence recession at adjacent beaches. Where longshore drift occurs past such features, the beach will tend to experience greatest recession immediately downdrift of the feature, i.e. coastal embayments with a net northerly longshore drift will tend to experience greatest recession at their southern ends.

8 REFERENCES

Gordon, A. D., (1987). "Beach Fluctuations and Shoreline Change - NSW". Proc. 8th Australian Conf. on Coastal and Ocean Engineering, I.E.Aust. November, 1987.

NAS, (1987). "Responding to Changes in Sea Level, Engineering Implications". (National Academy Press, Washington, 1987).