Suspended sediment transport processes on a wave-exposed tidal flat in the southern Firth of Thames

Abstract

Sediment accumulation rates in New Zealand estuaries have increased ten-fold following large-scale catchment deforestation that began in the mid-1800s. This increased sedimentation has led to rapidly prograding shorelines such as occur in the southern Firth of Thames (Waikato, North Island). Intertidal flats have been progressively colonised by mangroves since the early 1960s as the flats accrete above mean sea level and today mangroves occupy a 1 km-wide zone for former sand flat in the upper intertidal. Field data from an instrumented transect containing 5 sites (site 1 being the most landward and site 5 the most seaward) and spanning from the fringe of the mangrove forest, across the intertidal mudflats into the shallow subtidal waters of the Firth are used to analyse the sediments dynamics of the present-day mudflats. Suspended sediment concentration (SSC) values driven by waves and tides appeared to progressively increase from offshore to onshore as the water depth decreased. Detailed analysis of environmental variables during both calm conditions and an episodic, storm event showed a correlation between SSC and estimated significant wave height and peak period although SSC values remained non-negligible across the transect even in fair weather. Analysis of hydrodynamic measurements (e.g., current speed) allowed to infer sediment fluxes across the flat over calm and windy intervals. Results indicate a constant sediment flux seaward at site 1, while the tidal cycle dominates the fluxes at the rest of the sites. Storm conditions favour the advection of sediment seawards as the current peaks during the ebb tide, while calm conditions tend to favour onshore-directed fluxes. In the deeper waters of the transect, flux magnitudes registered during the storm event were greater than the aggregate of those registered during the rest of the experiment under calmer conditions, resulting in offshore net fluxes at site 5. Onshore net fluxes at sites 2 3 and 4, combined with the offshore flux at site 1, provide the basis for a new hypothesis of how the intertidal might be accreting; a prograding bar near the fringe of the forest that progressively moves seawards.