Calibration of Swiss Plate Geophone System for bedload monitoring in a sediment bypass tunnel

Abstract

Sediment bypass tunnels (SBTs) are one of the effective countermeasures against reservoir sedimentation for small to medium-sized mountainous reservoirs. They route sediment-laden flows around the dam, reduce sedimentation and improve morphology and ecology of downstream river reaches. However, high bedload transport rates in combination with high flow velocities cause severe hydroabrasion on the inverts of nearly all SBTs and hence considerable refurbishment costs. For optimized operation of SBTs with respect to sustainable sediment management and cost efficiency, continuous realtime monitoring of bedload transport is necessary. Bedload transport can be monitored indirectly by using passive sensors like geophones or hydrophones. However, these techniques require a site-specific calibration since they are affected by hydraulic conditions, particle size and shape. This study deals with the field calibration of the Swiss Plate Geophone System (SPGS) at the Solis SBT, Switzerland. The calibration procedure consisted of: (I) depositing 10 m3 of bedload material for each particle-size class inside the tunnel, (II) running the SBT with surplus inflow after a flood event when discharge is high, thereby keeping a high reservoir level to avoid bedload transport from the reservoir entering the SBT, and (III) recording and analysing the raw geophone signals. Three particle-size classes were chosen: 16 - 32 mm, 32 - 63 mm and 0 - 400 mm. In this paper, the details of the SGPS at Solis SBT and the results of the field calibration are presented, compared and discussed with the results from two previous laboratory calibrations.