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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix C: Coastline Hazards

Appendix C6 - Coastal Inundation Hazard

1 INTRODUCTION

Coastal inundation is the flooding of coastal lands by ocean waters. Unlike Holland, NSW has no significant areas below normal high tide level that are protected by artificial barriers. However, along the coast there are many low lying areas above normal high tide level that are subject to coastal inundation. These include wetland and other fringe areas of coastal lagoons and rivers, and the areas behind beach and dune systems. The inundation of these areas can be caused by large waves and elevated water levels associated with severe storms.

Severe coastal inundation is an infrequent event and is normally of short duration (peak flooding usually persists for several hours). Nevertheless, it can cause significant damage to public and private property: crops and livestock can be lost; the contents of flooded buildings can be damaged; problems can be caused by the breakdown of transport and communications. In addition, coastal inundation can be a threat to the life and limb of inhabitants of flood prone areas.

2 MECHANISMS OF INUNDATION

Elevated Water Levels

Storms increase Still Water Level (SWL) along the coast by the mechanisms of wind setup, barometric setup and wave setup (see Appendix B4). This increase in coastal water level can inundate low lying beach areas and low lying backbeach areas that are hydraulically connected to coastal waters.

Wave Runup and Overtopping

Wave runup is another mechanism whereby coastal inundation occurs (see Appendix B4). Waves can runup and overtop a coastal barrier, such as a dune or seawall, thereby inundating landward areas protected by the barrier. The vertical height of runup can be very high. During the storm event of August 1986, wave runup to a level of 7.3m AHD (i.e. about 7.3m above MSL) was measured at Narrabeen (PWD, 1987).

The height of wave runup on beaches depends upon many factors that include wave height and period, the slope, shape and permeability of the beach, the roughness of the foreshore area and wave regularity. Similar factors affect runup on rocky coasts and seawalls. Wave runup is difficult to predict accurately. Charts are available which indicate runup for specified beach slopes, seawall designs and wave characteristics (CERC, 1984).

3 AREAS AT RISK

Foreshores of Estuaries, Lagoons and Waterways

In general, the foreshores of estuaries, lagoons and coastal waterways are not protected from coastal inundation by natural or artificial barriers (e.g. sand dunes or seawalls). Flooding in these areas tends to be a gradual process. In those instances where the coastal entrance is relatively wide and deep, the variation of water levels in the waterbody closely mirrors the variation in coastal waters. In other instances where the coastal entrance is a narrow, shallow and tortuous channel, peak water levels in the waterbody caused by the ocean will be less than those of the open coast (see Appendix B10), however rainfall derived problems may be greater.

Low Lying Land Protected by Coastal Barriers

Low lying land behind coastal barriers, such as dune systems and seawalls, tends to remain completely protected until a certain critical combination of waves, elevated water levels and possibly beach erosion occurs. Inundation then commences, typically by wave runup and overtopping. The inundation of protected areas can also occur via backflow through stormwater pipes. This is often a gradual process. However, if the barrier is completely overtopped or breached, inundation of back areas can be rapid. If back areas are poorly drained, flooding behind a breached or overtopped barrier may persist for some time when coastal water levels fall.

4 INTERACTION WITH FRESHWATER FLOODING

Many areas within the coastal zone can be affected by a combination of coastal inundation and freshwater flooding, e.g. the "big" rivers of northern NSW. The extent and behaviour of such flooding is complex and depends upon the magnitude, variation and relative timing of freshwater inflows and coastal water levels.

In general, flooding along areas close to the coast is dominated by coastal water levels. The contribution of freshwater effects to flood levels becomes greater with distance upstream. Nevertheless, coastal water levels can influence river levels for a considerable distance upstream along major coastal rivers, e.g. flood levels at Ulmarra on the Clarence River are influenced by coastal water levels at its mouth, which is 30km downstream at Yamba.

5 EXTENT OF INUNDATION

Identification of the levels of coastal inundation is an essential starting point in the management of this hazard. This requires information concerning design storms and their associated storm surge, wave setup and wave runup effects. Computer models may be required to simulate coastal water levels and the extent of coastal inundation.

6 REFERENCES

CERC, 1984 "Shore Protection Manual", 2 Vols, Coastal Engineering Research Center, US Corps of Engineers, Department of the Army, Waterway Experiment Station, (US Govt. Printing Office, 1984, Fourth Edition).

PWD, (1987). "Collaroy/Narrabeen Beaches, Coastal Process Hazard Definition Study". Report prepared by Coastal Engineering Branch, Public Works Department of New South Wales. Report No. 87040.