In situ monitoring of suspended sediment : development of a densimetric instrument
Calhoun, Daniel Lawrence
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Sampling methods for determining sediment concentrations can be labor intensive and may not provide sufficient temporal density during stormflow events. Optical and Doppler methods can provide necessary temporal density, but may not be representative of the water column, or of the heterogeneity of the transported material. We present a real-time densimetric technique that meets both the temporal and depth-integration needs of sediment sampling. Fluid density is determined using a precision differential pressure transducer that measures the fluid pressure change between two fixed depths. While small temporal variations in fluid density are induced by changes in temperature and dissolved solids and gasses, much larger variations are induced by suspended materials, primarily mineral sediments with densities greater than water. Laboratory and in situ tests demonstrate the principles involved, as well as the utility of the technique. Applications on tributaries to the South Fork of the Broad River in northeast Georgia, USA, show that stormflow suspended sediment concentrations obtained using the densimetric sensor vary with but lack conclusive correlation to depth-integrated observations. We also monitor bedload sediment transport by placing the sensor in a position that tracks fluid density changes in the mobile bed zone. Both suspended sediment and bedload concentrations are coupled with velocity and stage measurements to provide for the potential of real-time measurements of total sediment flux. A densimetric device to provide real-time in situ particle-size distributions is another possible application.