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New South Wales Government
Many backbeach areas are drained by semi-perennial small creeks that cross the beach berm to discharge stormwater into coastal waters. During major runoff events, such creeks can cause significant erosion to the beach berm and the nearshore area around their entrance. This in turn allows larger waves to attack the creek entrance, which if unstable, may migrate along the beach.
Stormwater disposal hazard is a local problem. It should not be confused with the regional problem of freshwater flooding and the interaction of the latter with coastal inundation (these hazards are covered in Appendix C6). Similarly, the entrance behaviour of these minor creeks is essentially a local phenomenon. Although subject to the same coastal processes, the behaviour of these local creeks should not be confused with the behaviour of major coastal entrances (these hazards are described in Appendix C4).
One of the major factors affecting stormwater erosion potential is the degree of development in the catchment area draining to the creek. Urban developments such as carparks, roads, shops and other buildings result in the replacement of porous natural surfaces (very much so in the case of sandy areas) with impermeable surfaces. This has two effects on stormwater runoff. First, the volume of runoff tends to increase significantly. Second, the peak rate of runoff also increases. Both effects result in a significant increase in the erosion potential of stormwater discharges in the creek.
Creek outflows during storm events can be a triggering mechanism for the formation of rip cells. This can markedly increase the degree of berm erosion around the creek entrance (see Appendix B6).
Creek entrances on sandy beaches tend to be unstable in location. Migration of the entrance during stormwater runoff episodes may pose a threat to adjacent developments. (Creek entrances can be readily stabilised using low cost training walls, such as that recently built at Dee Why Lagoon on the Sydney coast).
The rebuilding of a beach after a major storm event may close a creek entrance, especially if the creek only flows intermittently. When subsequent rainfall occurs, this can result in local flooding in upstream areas before the blockage is flushed away.
A regional drainage study of the catchment draining to the creek is required to define the volumes and peak discharge generated by the design storm event. Once these data are to hand, various options to reduce the hazards of stormwater erosion can be investigated. These include the use of retarding basins to reduce peak discharges, the diversion of runoff into adjacent catchments where erosion problems are less critical, stabilisation of the creek entrance, protective bank works, and the use of dissipation structures to reduce beach scour.