Data are presented which provide sand fluxes and sedimentation rates in a macrotidal estuary on tidal to decadal time scales.
The dynamics of the Taf Estuary in Wales (UK) are established by a macrotidal regime with a tidal range up to 10 m. The tidal wave is asymmetric with a short flood tide of <3 h and fast spring tide currents exceeding 2 m s-1. The estuary is sandy and characterised, at low water, by intertidal sandflats and shallow channels and, at high water, by a well mixed, or partially stratified, water column. Freshwater input is small (mean discharge 7 m3 s-1 ) in comparison with saltwater input (mean tidal prism 107 m3). The estuary is exposed to large waves from the North Atlantic but much of the wave energy is dissipated across a wide shoreface before waves reach the estuary mouth.
The fast tidal currents generate intense sand transport and rapid morphological change. A unique 30 year time series of sedimentation rate has been obtained. The sand budget has been determined by periodic surveys of the sand flats at low water. Levelling of fixed transects across the estuary over a 30 year period shows that the estuary has been infilling with sand at a mean annual sedimentation rate of 0.02 m. Mineralogical analysis of the sand proves that it is sourced from offshore rather than via river input. A testable hypothesis is that the landward transport of marine sand is due to tidal pumping associated with the time-velocity asymmetry of the tidal currents rather than to storm events.
The very fine sand of the sandflats is entrained into suspension for most of every tidal cycle. Bed load transport is minimal, and total sand transport can be quantified by measurement of the suspended sediment load. The suspended sand flux has been determined by measurements of current velocity and suspended sand concentration (SSC) along 2 transects across the estuary. Measurements were made through the water column over several tidal cycles, during fair-weather conditions, using profiling transmissometers for SSC. Flood sand flux exceeded ebb sand flux on all tides; hence there was a net landward flux on all tides. Relationships between sand flux and tidal height were determined and used to extrapolate the annual flux and annual sedimentation rate. The annual sedimentation rate is 0.014± 0.036 m. This ignores any forcing other than tidal.
The long term budget and short term flux measurements provide very similar estimates of the sedimentation rate in the estuary. This provides compelling evidence that the high sand fluxes and sedimentation rates of the estuary are determined predominantly by the tidal regime. It is concluded that the hypothesis that tidal pumping, rather than storms, controls net sediment transport is correct. |
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