The effect of storms on chenier plains: the Shoal Bay chenier plain

Abstract

The southern coast of Shoal Bay, near Darwin, Northern Territory is a small, well vegetated chenier plain. Chenier plains are prograded coastal mudflats that support a number of long narrow ridges formed from sand and shell debris, that run parallel to the coast. Mangrove communities on the mudflat occur in zones, also parallel to the coastline, indicating species preference for protection, depth and duration of tidal inundation and level of salinity. Chenier plains are thought to be formed through fluctuations in sediment supply resulting in, episodic coastal progradation, and coastal erosion and ridge deposition above the level of high tide. Some of the literature suggests that progradation may be aided by the seaward mangrove zones trapping sediments, while erosion of the mudflats and subsequent ridge formation can be attributed to high energy storm activity, such as that of a tropical cyclone. This study is involved with examining the recent history of the chenier plain at Shoal Bay, in regard to the effects of cyclone Tracy, which struck the area on December 25, 1974. The storm devastated large areas of the central mangrove zones, those of Bruguiera, Ceriops and Rhizophora, mostly by windthrow. The seaward mangrove zones, such as that of Avicennia, and the landforms were mostly unaffected by the storm winds, and the storm surge associated with the cyclone was relatively small, as it occurred in conjunction with a neap high tide. The overall effects of cyclone Tracy on the chenier plain were much less than those described for other storms in the literature. All of the changes that have occurred on the Shoal Bay chenier plain over the period examined can be explained by normal wave and tide action, and do not require a high energy event.