Bedload Transport
The Sedimeter® was deployed on an ooid ridge on the Bahamas during 48 hours, in order to measure the quantity of sand that was moved each day. It was put down at the crest of a 1 metre high and about 10 metres wide ridge, ontop of an oolitic bank. The variation of sediment level in the record thus results from the crest being moved back and forth by the tide. The current velocity data also comes from the Sedimeter®, while the tide level data comes from a tide gauge nearby.
Procedure
The instrument was deployed before 1300 hours on 12 Feb 1992, and retrieved after 1300 on 14 Feb 1992. The sampling interval was 1 hour, and the current was recorded during 24 seconds each hour. The site is on the southern end of Tongue of the Ocean, not far from Exuma.The data from the 16 optical back-scatter detectors were interpreted manually (if necessary by plotting the curves in a spreadsheet program) to find the level of the bottom. The tide gauge was used to infer the direction of the current (rising or falling tide), and the current measurements were given a positive or negative sign. The sea-bottom level, current, and tide, were plotted in the above diagram (the last two parameters should be read on the right - overlay - y-axis).
Results
Current velocities below 50 cm/s hardly affects the bottom elevation. Currents above 60 cm/s causes significant sediment transport, moving the crest of the sand ridge several decimetres. Currents between 50 and 60 cm/s cause smaller variations in the elevation, corresponding to migration of the about one centimetre high ripples ontop of the crest. Together with other field observations and satellite data, it could be calculated that about 70 cubic metres of sand was in motion every day, per square kilometre. This quantification would not have been possible without the Sedimeter® measurements.
