In-situ investigation on real-time suspended sediment measurement techniques: Turbidimetry, acoustic attenuation, laser diffraction (LISST) and vibrating tube densimetry

Real-time measurements of Suspended Sediment mass Concentration (SSC) and Particle Size Distribution (PSD) are of prime importance for the investigation and management of fine-sediment related processes in surface water systems and hydraulic schemes. In a field study at the waterway of the hydropower plant (HPP) Fieschertal in the Swiss Alps, the real-time measurement performance of the following techniques and instruments were assessed based on measurements in the sediment seasons of the years 2013 and 2014: (1) turbidimetry, (2) single-frequency acoustic attenuation using a standard acoustic discharge measurement (ADM) installation, (3) laser diffraction (Laser In-Situ Scattering and Transmissometry, LISST), and (4) vibrating tube densimetry using a Coriolis Flow and Density Meter (CFDM). Reference SSCs were obtained from gravimetric analysis of 219 automatically pumped water samples. LISST additionally supplied PSD every minute. The median particle diameter, d50, was usually 15 µm and increased occasionally to 100 µm. The turbidimeter and the ADM underestimated the SSC when the transported particles were coarser than usual. Such temporary biases resulted from the poor correlation between d50 and SSC at this study site. The SSCs from CFDM and LISST were not or less biased by PSD variations. Mainly due to angular and flaky particle shapes, SSC from LISST needed a correction by 79% on average. With the usually prevailing silt particles, an optical path length of 5 mm and no dilution, the SSC measurement range of LISST was limited to about 1.5 g/l. The CFDM allowed measuring higher SSC than the other investigated instruments (e.g. up to 13.5 g/l). With a periodic offset correction, its relative SSC measurement uncertainty was < 20% for SSC ≥ 1.5 g/l. To reliably measure a wide range of SSC with temporarily variable PSD, a combination of instruments is recommended: e.g. a standard LISST, a CFDM, and an automatic water sampler for gravimetric reference measurements.