Abstract
Since 1998, ferry observations have been carried out in the Marsdiep tidal inlet (Dutch Wadden Sea), using amongst other instruments a vessel-mounted acoustic Doppler current profiler (ADCP). Besides 32 cross-sections a day of current velocity data, the instrument also records the echo intensity, which has been interpreted in terms of suspended sediment concentration (SSC) before (Thorne and Hanes, Cont Shelf Res 22:603–632, 2002). However, we show herein that the random phase scattering model as outlined by Thorne and Hanes (Cont Shelf Res 22:603–632, 2002), predicts unrealistically high values of SSC if the depth-averaged current velocity exceeds approximately 0.7 m/s. Therefore, we extended the random phase scattering model by including scattering by turbulence-induced variability in SSC. The important mechanism is that when SSC fluctuations are present at length, scales of the order of the acoustic wavelength, the phase of the returned signal is no longer random and causes stronger backscatter. Such SSC fluctuations occur as a result of turbulent eddies in combination with an SSC gradient. The extended model was compared with data of two field surveys carried out in the Marsdiep inlet. The extended model, when compared with the classical random phase model, showed a large improvement of accuracy of the estimated SSC, which allows us to apply the model to the ferry data set to analyse suspended sediment transports through the Marsdiep tidal inlet.
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Notes
The acoustic intensity is expressed in decibels, derived from the ADCP’s internal unit counts. The offset of the decibel scale is arbitrary, as the system’s specifications (Nortek 1998) provide the proportionality factor between counts and decibels only.
In fact, the Nortek ADCP used during the survey(s) AS26 (and AS27) was replaced by a new Nortek ADCP operating at the same frequency for AS29. During a test survey when the old and new ADCPs were deployed in 20-min shifts, it turned out that backscatter intensity of the new ADCP was systematically lower by an amount of 8 dB. This is probably due to different internal electrical circuits, resulting in a different offset in the relationship between counts (the ADCP’s internal parameter) and decibels. The effect of the systematic lower echo intensity has been accounted for by adjusting the offset in the counts to decibels relationship, so that the value for K R can be assumed to be constant.
A typical frequency of a mid-range ADCP, applicable for coastal waters is about 1 MHz, which corresponds to a Bragg length of about 0.7 mm.
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Acknowledgements
We thank Margriet Hiehle for her help with the data processing and the crew of the R.V. Navicula for their help in collecting the data during the two surveys.
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Responsible editor: Paulo Salles
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Merckelbach, L.M., Ridderinkhof, H. Estimating suspended sediment concentration using backscatterance from an acoustic Doppler profiling current meter at a site with strong tidal currents. Ocean Dynamics 56, 153–168 (2006). https://doi.org/10.1007/s10236-005-0036-z
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DOI: https://doi.org/10.1007/s10236-005-0036-z