Near-bottom hydrosedimentary data from current profiler moorings, Aulne River estuary, France

[Lien vers la fiche source] Timeseries of water level and near-bottom (1.3 m above bottom) velocity from current profilers, and near-bottom salinity and suspended sediment concentration from YSIs deployed with the current meters. Two deployments in 2013: February and September. Two locations in the estuary: Site 1 (48° 16.842’ N, 004° 16.009’ W), and Site 3 (48° 14.851’ N, 004° 10.140’ W). Water level was corrected for variations in atmospheric pressure. Moored instruments were deployed for 3 weeks at two locations in the Aulne estuary in February and September 2013. Each mooring consisted of an acoustic Doppler current profiler and a YSI 6290 V2 equipped with turbidity, conductivity, salinity, and pressure sensors. A Nortek AWAC wave and current profiler (frequency 1 MHz) was deployed along with a YSI 6290 V2 at Site 1, near the mouth of the Aulne. An RD Instruments Workhorse Acoustic Doppler Current Profiler (ADCP, 1.2 MHz) was deployed along with a YSI 6290 V2 at Site 3. The AWAC and ADCP were programmed to measure a 2-minute burst average at 1Hz every 5 minutes for profiles of currents and acoustic backscatter. The internal compasses of the current profilers were calibrated and the pressure sensors for all instruments were reset before each deployment. The turbidity sensors of the YSIs were calibrated before deployment with a 3-point calibration using distilled water and Hach Formazin Turbidity Standard at 4000 NTU, diluted to 400 and 1000 NTU. The YSI turbidity sensors were calibrated again to SSC with water samples. The YSIs were programmed to take 1 measurement every 5 minutes, and were moored approximately 10m away from the current profilers at a height of 1.3 m above bottom. Note that all salinity data are expressed according to the practical salinity scale. Water level data from the AWAC and the ADCP were corrected for post-deployment changes in barometric pressure using the inverse barometer method, and in the case of the AWAC, for a systematic pressure offset. The along-channel velocity direction was determined for each current profiler from histograms of near-surface velocity direction. During February the Site 1 YSI failed partway through the deployment, so the AWAC backscatter was converted to SSC to compensate for the lost data. The SSC timeseries from the moored YSI was used to calibrate the AWAC echo intensity to SSC. The echo intensity output by the AWAC was converted to relative backscatter intensity with the sonar equation. The backscatter intensity (BI) in the current profiler bin closest to the height of the moored YSI above bottom was regressed against the timeseries of calibrated SSC from the moored YSI. The final form of the regression equation between BI and SSC was: 10log(SSC)=a*BI + b, where a and b are correlation coefficients and SSC is the suspended sediment concentration in mg L-1. Two regression equations were used in converting BI to SSC, to improve regression accuracy for BI greater than -45 dB. Separate calibrations for flood and ebb did not result in an improved regression coefficient. It should be noted that some maximum values during the second spring tide at Site 1 in February may have been underestimated. Before the YSI failed, the calibrated AWAC SSC sometimes underpredicted the YSI SSC because of a high amount of scatter in the regression relationship. It is therefore likely that peak SSC values were underestimated during the second spring tide.