Dry season hydrodynamics and sediment transport in a Mangrove Creek

Abstract

Mangroves are tropical ecosystems of great ecological significance, but the relationship between their hydrodynamics and sedimentation patterns has been given little attention in the literature. This study quantified bedload and suspendedload sediment fluxes in Gordon Creek, a 3 km long mesotidal creek and mangrove swamp system in N.E. Australia, which occupies an area of ca. 1.5 km 2. Tidal current, suspended sediment, and bedload transport data were collected in a mangrove creek and adjacent mangrove forest flat during the dry season which operate for 9-10 months of the year. Analysis of this data indicate that sediment transport is influenced by the relative elevation of the mangrove forest flat. 'Overbank' tides, which inundate the mangrove forest flat, create strong ebb currents in the creek, which flush bedload seaward at instantaneous transport rates of up to 0.3 kgs -1 per metre of creek width. Suspended sediment fluxes peak on the flood tide, also at 0.3 kgs -1 per metre of creek width. Within-creek' tides of smaller ange have weaker currents, and, because they do not inundate the mangrove forest flat, the flood and ebb currents are of similar strength. Under these conditions, sediment transport rates are greatly reduced. Dry season conditions produce a tidally-driven annual seaward discharge of bedload sediment in the creek of up to ca. 400 tonnes. Time-series data in the mangrove creek reveal no clear tendency for net transport of suspended sediment, but data from the mangrove forest flat suggest net landward movement of suspended sediment. The upper bound on the calculated accumulation rate of mud is in the order of 1.6 mm/yr. Mixing in Estuaries and Coastal Seas Coastal and Estuarine Studies Volume 50, Pages 388-404 Copyright 1996 by the American Geophysical Union