Hydrodynamic and geomorphological controls on suspended sediment transport in mangrove creek systems, a case study: Cocoa Creek, Townsville, Australia S. Bryce a, * , P. Larcombe a , P.V. Ridd b a Marine Geophysical Laboratory, School of Earth Sciences, James Cook University, Townsville 4811, Australia b Marine Geophysical Laboratory, School of Computer Science, Mathematics and Physics, James Cook University, Townsville 4811, Australia Received 15 February 1999; received in revised form 8 July 1999; accepted 25 February 2002 Abstract In tide-dominated sedimentary systems, close relationships exist between tidal hydrodynamics, sediment transport and geomorphology. Tropical coastlines contain many tide-dominated mangrove creeks, yet few studies to date have examined the detail of such relationships for these environments. Time-series observations of tidal height, currents and suspended sediment concentrations were taken between 1992 and 1996 in Cocoa Creek, a mangrove creek system near Townsville, NE Australia. The creek and surrounding mangrove swamps and salt flats were surveyed with an echo-sounder and total survey station, respectively. For Ôwithin-channelÕ tides, the flood tide is always the fastest, at up to 0.5 m s ÿ1 . In contrast, for overbank tides (i.e. tidal height > þ1.5 m Australian Height Datum, AHD) ebb currents are fastest in July, December and January, but flood currents are fastest in August and September, at up to 1 m s ÿ1 in both cases. The tidal asymmetry of overbank tides in Cocoa Creek is controlled by the interaction between offshore tidal forcing and the intertidal storage effect of the mangrove swamps and salt flats, with the result being that during certain periods of the year there tends to be a predominance of either faster flood or ebb velocities on overbank tides. Significant tidal suspended sediment transport in the channel is only initiated at overbank height. On overbank tides, measured net suspended sediment fluxes in the channel are mostly seaward-directed (up to 180 t per tidal cycle). However, the net flux measured over a neap–spring period may be either landwards or seawards (up to 465 and 60 t, respectively). Furthermore, on the larger overbank tides (where the maximum tidal height > þ1.85 m AHD) net sediment fluxes may be reduced because of a limited supply of available material. Thus hydrodynamic and sediment sampling durations of up to a month may not be representative of long-term trends. Given that our large dataset has not identified a clear long-term net transport direction within the creek system, we conclude tentatively that the geomorphology of Cocoa Creek may be near a long-term equilibrium. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: tidal creek; mangroves; sediment transport; Australia 1. Introduction Studies of hydrodynamics and sediment transport in mostly temperate salt marsh systems (Bayliss-Smith, Healy, Lailey, Spencer, & Stoddart, 1979; Boon, 1975; Boon & Byrne, 1981; Friedrichs & Aubrey, 1988; Pethick, 1980) have provided the initial understand- ing of hydrodynamic and geomorphological controls on sediment transport for tide-dominated creeks and estuaries. Most of these studies used tidal height data to model tidal hydrodynamics, and sediment transport equations to estimate sediment fluxes and the net direction of sediment movement. Some of this work formed the basis for early attempts to understand sediment transport processes, and calculate sediment fluxes and transport rates in their tropical equivalents, mangrove creek systems (Fisher, 1994; Lessa, 1995; Lessa & Masselink, 1995; Wolanski, Jones, & Bunt, 1980; Woodroffe, 1985a). These studies employed methods such as water sampling, applying sediment transport equations and computer modelling, and they commonly assumed that sediment transport patterns * Corresponding author. E-mail address: sonya.bryce@jcu.edu.au (S. Bryce). Estuarine, Coastal and Shelf Science 56 (2003) 415–431 0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0272-7714(02)00192-0