Field measurements of suspended sediment using several methods

David Felix, ISMAIL Albayrak, Robert M. Boes

Thursday 2 july 2015

11:30 - 11:45h at Africa (level 0)

Themes: (T) Special session, (ST) Acoustic monitoring of flow, turbulence and river discharge

Parallel session: 11F. Special session: Acoustic monitoring of flow, turbulence and river discharge

Measuring concentration and size of suspended sediment is important to understand and manage fine-sediment related processes in rivers, lakes and seas as well as at hydraulic schemes for hydropower, irrigation and flood protection. There is an increasing demand for field measurements of suspended sediment concentration (SSC) and particle size distribution (PSD) with good accuracy and high temporal resolution, preferably in real-time. Various types of turbidimeters, a ‘Laser in-situ Scattering and Transmissiometry’ (LISST) device and a single-frequency acoustic attenuation method were used for continuous suspended sediment measurements (SSM) at the waterway of the hydropower plant Fieschertal located at a tributary of the upper Rhone River in the Swiss Alps. In addition, water samples were taken by an automatic bottle sampler at least every three days. From the collected water samples, SSCs were determined by the gravimetrical method (reference SSCs). The data obtained from the devices for continuous measurements were converted to SSC time series based on the gravimetrical SSCs (calibration). Furthermore, PSDs were obtained by LISST every minute. The results showed a high temporal variability of SSC and PSD as well as a weak correlation between them. At several events with increased sediment transport, time shifts between the peaks in SSC and the median particle size, d50 were observed. The SSCs obtained by the various methods were similar except for periods of increased SSC. During a sediment transport event with a SSC peak of almost 12 g/l, the particles were temporarily coarser than usual and the turbidimeters as well as the acoustic method underestimated SSC by up to 80%. These temporary biases are attributed to mainly effects of variations in particle size. However, the SSCs obtained by LISST agreed reasonably well with the reference SSCs despite occasional PSD variations. Based on these findings, the LISST technique in combination with automatic bottle sampling is recommended among the tested methods for SSM in sediment environments where PSD varies independently of SSC.