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New South Wales Government
Severe storms are often accompanied by extended periods of heavy rainfall. This is especially so of tropical cyclones, which typically degenerate into rainfall depressions on crossing the coast. The relationship between rainfall and beach erosion is examined by Bryant (1985).
Rainfall and its associated runoff have a number of effects on coastal processes. In many cases these are not as extreme as the primary storm induced forces of wave erosion, increased water levels, etc. Nevertheless, they can be of significance.
Many coastal areas are characterised by relatively high groundwater levels. Rainfall onto sandy beaches and dune systems quickly percolates to the groundwater table. Intense rainfall may cause significant increases in groundwater levels, thereby increasing pore water pressures. This facilitates the erosion of dune areas and increases the instability of coastal bluffs (see Appendix C7).
Extensive areas of NSW to the east of the Great Dividing Range drain to the coastline via major river systems. The management of freshwater flooding in these river systems is a significant problem in its own right.
Whilst these eastward draining rivers have steep bedslopes in inland areas, their bedslopes become progressively flatter as they approach the coast. This has two effects on hydraulic behaviour. First, flat bedslopes allow tidal effects to propagate a considerable distance upstream from the river mouth and interact with freshwater discharges. Second flat bedslopes reduce the capacity of the main channel to pass flood flows, which typically spill out over the wide floodplain areas which characterise the lower reaches of these rivers.
Flood levels in the lower reaches of coastal rivers depend in a complex way on the interaction of freshwater discharges, tidal effects and the elevation of coastal water levels through storm surge and other effects. Guidelines for the estimation of flood levels and the development of coastal areas subject to flooding are contained in the "Floodplain Development Manual" (NSW, 1986).
The discharge of flood waters into the sea can have marked effects on coastal morphology.
Extensive scour around river entrances can occur as major flood discharges debouch into coastal waters. This can result in significant changes to offshore bathymetry that may persist for months or perhaps years (see Appendix C4).
Rivers are an important source of clay and silt sized sediment. During a major flood in a large river, many hundreds of thousands of tonnes of sediment can be delivered to nearshore areas.
Unless stabilised by training walls, the mouths of many coastal rivers tend to "migrate" up and down the coast over considerable distances in response to the combined effects of freshwater discharge, tides and wave action. Often such rivers flow parallel to the coast behind the hind dune area forming elongated spits. Large floods can cause major changes in the location of the river mouth, to the location and nature of river shoal areas and to the entrances of coastal lagoons hydraulically connected to the river (see Appendix B11).
Bryant, E.A. (1985). "Rainfall and Beach Erosion Relationships, Stanwell Park, Aust. 1895-1980: Worldwide Implications for Coastal Erosion." Zeitschrift fur Geomorphology, Sup. 57, pp 51-65.
I.E.Aust., (1987). "Australian Rainfall and Runoff". Published by the I.E.Aust, editor in chief, R.P. Canterford, 2 Vols., 1987
NSW Government, (1986). "Floodplain Development Manual". Prepared by Public Works Department of New South Wales, Report No. 86010, New South Wales Government Printer, December 1986.