Measuring suspended sediments in periglacial reservoirs using water samples, laser in-situ scattering and transmissometry and acoustic Doppler current profiler

ABSTRACT

Climate change will impact the water and sediment conveyance into periglacial reservoirs. It is therefore important to understand and forecast future reservoir sedimentation processes with regard to climate change. In the present project, particle size distribution (PSD) and suspended sediment concentrations (SSC) were measured in three reservoirs in the Swiss Alps whose catchment areas are covered by glaciers by at least 40%. The threefold combination of water sample analysis, laser in-situ scattering and transmissometry (LISST) and acoustic Doppler current profiler (ADCP) was applied and the results were compared to each other. The combination of the three measuring techniques was proven suitable for assessing PSD and SSC in periglacial reservoirs. Water sample analysis and LISST records showed that most of the suspended sediments in the reservoir are in the range of clay and silt. SSC was relatively low in the order of 100 mg/l. An increase in both PSD (e.g. median diameter d₅₀) and SSC with increasing reservoir depth could be observed in deep reservoirs. Flow velocities and Signal-to-Noise ratios (SNR) were measured with ADCP. SNR values allowed to study the mixing of inflowing river water and the evolution and decay of turbidity currents. There is evidence of dominant homopycnal flows, whereas stratified flow was restricted to the regions close to the inflow. Flocculation, influence of mica, and organic content could be neglected. This paper presents detailed information about PSD and SSC gained with water sample analysis and LISST measurements. Furthermore, flow field measurements and tracking of mixing by means of ADCP will be illustrated. Finally, application experiences and limitations will be discussed.

KEYWORDS:

• Periglacial reservoir
• laser in-situ scattering and transmissometry
• acoustic Doppler current profiler
• particle size distribution
• suspended sediment concentration
• Signal-to-Noise ratio

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. All topographical maps are reproduced by permission of swisstopo (JA100120).

2. Only velocities and SNR values outside the blanking zone and the top estimate are used in this study.

3. In stationary operation mode, the device is kept at a fixed depth at a certain location; in moving operation mode, depth, or location change.

Additional information

Funding

The project is financially supported by the Swiss National Science Foundation (SNSF) within the National Research Programme NRP 70 ‘Energy Turnaround’ Project No. 153927. It is under the umbrella of the Swiss Competence Center for Energy Research – Supply of Electricity (SCCER-SoE). The field work was technically supported by Forces Motrices de Mauvoisin / Axpo, Ofima / Kraftwerk Aegina AG, Electra Massa / Alpiq and HYDRO Exploitation SA.