Storm impact and evolution of a mangrove-fringed chenier plain, Shoal Bay, Darwin, Australia

Abstract

Storms are considered to have significant impacts on the development of chenier plains, particularly through the devastation of mangrove vegetation, but also in terms of winnowing sand and shell from mudflats and forming chenier ridges. Shoal Bay, in the Beagle Gulf, northern Australia, contains a small chenier plain, which was struck by a severe tropical cyclone, Cyclone Tracy, on Christmas Day 1974 as it devastated the city of Darwin. The morphology, vegetation and stratigraphy of the plain are described. The plain is underlain by lower intertidal sand with shell hash. A radiocarbon age of 6130 years BP, indicates mangrove colonisation of this flat at the time that sea level stabilised around its present level after the postglacial transgression. Subsequently, progradation of mudflats has occurred, especially around 2300 years ago, and shells of this age are found both in growth position from within the mudflats, and incorporated into the shelly chenier ridges. A further phase of build-out, and subsequent erosion is reflected by stumps and in situ bivalves being excavated presently on the foreshore, radiocarbon dating about 900–1100 years BP. Within this context of evolution, Cyclone Tracy can be seen to have had severe consequences upon the patterning of mangroves, with extensive windthrow of Ceriops, and defoliation of Rhizophora at the western end of the plain. Recovery of the mangrove vegetation has been gradual, and is still incomplete. However, the storm had little influence on the pattern of landform development. Indeed, the aerial photographic sequence indicates a trend by which sand shoals are gradually reworked landwards, in places moving through mangrove vegetation. The study suggests that rather than pronounced regional episodes of alternative mudflat buildout and erosion, both processes occur simultaneously at different points along this foreshore.