Near-bottom hydrosedimentary data from current profiler moorings, Aulne River estuary, France
|Author(s)||Moskalski Susanne1, Floc’h France1, Verney Romaric2, Fromant Guillaume1, Le Dantec Nicolas1, Deschamps Anne1|
|Affiliation(s)||1 : Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, 29280 Plouzané, France
2 : Ifremer Centre Bretagne, 29280 Plouzané, France
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.
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The Teledyne/RDI Workhorse Monitor is an acoustic current profiler, measuring currents and acoustic backscatter using sound and the Doppler effect. The instrument available for the deployment was a 1.2 MHz instrument, with a maximum profiling range of 20 m, bin sizes from 0.25 to 2 m, a 1 cm/s velocity resolution, and velocity accuracy of 0.3% of the measured velocity. The echo intensity precision is ± 1.5 dB. This instrument has internal battery and data storage, enabling it to be deployed independently. The Workhorse Monitor can be mounted facing up, down, or sideways, but in this study it was mounted facing upward in a mooring frame. Before deployment the internal compass must be calibrated and the pressure sensor reset, but no pre-deployment calibration is required. The Workhorse Monitor was programmed to measure a 2-minute burst average at 1Hz every 5 minutes, recording data in Earth coordinates. The internal compass was calibrated and the pressure sensor reset before deployment, according to manufacturer instructions.
The Nortek AWAC is an acoustic current profiler operating on the same principles as the Teledyne/RDI instruments, but also includes a wave module that can measure wave statistics. The instrument available for this deployment was a 1 MHz profiler with a maximum profiling range of 30 m, bin sizes from 0.25 to 4 m, a 1 cm/s velocity resolution, and velocity accuracy of 1% of the measured velocity. The echo intensity precision is ± 1.5 dB. For wave measurements, the maximum measuring range is 35 m and the maximum wave period range is 0.5 – 100 second. The accuracy of the significant wave height is less than 1% of the measured value and the resolution is 1 cm. For wave direction the accuracy is 2 degrees and the resolution is 0.1 degrees. The AWAC contains a battery and internal data storage and so can be deployed independently. For this study the AWAC was deployed facing upward in a mooring frame. Before deployment the internal compass must be calibrated and the pressure sensor reset, but no pre-deployment calibration is required. The AWAC was programmed to measure a 2-minute burst average at 1Hz every 5 minutes. Velocity data were recorded in Earth coordinates. The internal compass was calibrated and the pressure sensor reset before deployment, according to manufacturer instructions.
YSI6920 multiparameter sondes can measure a variety of variables, including temperature, salinity, pH, depth, dissolved oxygen, and turbidity simultaneously. For this study only conductivity, turbidity, temperature, and depth were used. The temperature thermistor sensor has a range of -5° - 50° C, and accuracy of ±0.15°C, and a resolution of 0.01°C. The depth sensor is a stainless steel strain gauge with a range of up to 200 m, accuracy of ±0.3 m, and resolution of 0.001m. The depth sensor is not vented to the atmosphere, so post-programming changes in atmospheric pressure must be subtracted from the depth output dataset. Conductivity is measured in mS/cm and has a range of 0-100 mS/cm with an accuracy of ±0.5% of the reading and a resolution of 0.001 - 0.1 mS/cm. Salinity is calculated from conductivity and temperature, and has a range of 0-70 ppt with an accuracy of ±1% of the reading or 1 ppt, and a resolution of 0.01 ppt. The turbidity sensor is optical and records data in NTU. It has an accuracy of ±2% of reading or 2 NTU, whichever is greater. It has a range of 0 to 1000 NTU and a resolution of 0.1 NTU. The YSI must be calibrated regularly to turbidity standard solutions to ensure accurate reporting. The NTU’s can also be calibrated to SSC using the same procedures as for an OBS. The two moored YSIs were programmed to take 1 measurement every 5 minutes, and the YSI used for collecting profiles was programmed to measure continuously at a rate of 1 Hz. Conductivity and turbidity were calibrated to standard solutions available from the manufacturer by metrology personnel before deployment, yielding salinity in parts per thousand (ppt) and turbidity in nephelometric turbidity units (NTU).