Under the strong impact of climate change sediment transport in melting water draining from glacier basins, rivers and waterways, and reservoir sedi-mentation have strongly increased in both the Alpine regions and worldwide. As a consequence, three main problems arise with increasing sedimentation: the loss of storage volume for energy production, flood protection, water supply and irrigation; (2) increased hydro-abrasion at turbines and hydraulic structures; and (3) negative environmental impacts as the sediment transfer downstream is prohibited. (1) and (2) result directly in a decrease of energy production and in an increase of maintenance costs. An effective and holistic countermeasure against reservoir sedimentation is to route sediment around a dam by using a sediment bypass tunnel (SBT). A major problem affecting nearly all SBTs is severe hydro-abrasion on the tunnel invert due to the high bed load transport rates in combination with high flow velocities. Depending on site-specific operating conditions and sediment properties, i.e. size, hardness and shape, invert abrasion can cause considerable refurbishment costs.

For optimized design and operation of SBTs with respect to sustainable sediment management and cost efficiency, there is an increasing need for continuous real-time monitoring of bed load transport. Bed load transport can be monitored indirectly by using a passive sensor like geophone or hydrophone. In the present study, the so-called Swiss plate geophone system (Fig. 1) has been implemented at the outlet of Solis SBT located in Grisons in the Swiss Alps. The geophones with the sampling rate of 10 kHz are placed across the whole tunnel width of 4.40 m and have an inclination of 10° against the invert slope. Geophone sensors mounted beneath the steel plate register the oscillations caused by impacting bedload particles (Fig. 2). The number of impulses computed from the signal registered by the Swiss plate geophone correlates well with the transported bed load mass. However, this relation depends on site-specific conditions like flow velocity, particle-size and shape and hence a field calibration is required. Figure 1 Implemented geophone system at the outlet of the Solis SBT (VAW). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315623207/4fbc492d-6678-4a12-aaf6-5c2b8ea38e5f/content/fig191_1.jpg"/> Figure 2 Swiss plate geophone system (before assembly) with steel plate (1), geophone sensor encased by an aluminum hous-ing (2), and elastomer bearing (3) on the steel casing (VAW). https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315623207/4fbc492d-6678-4a12-aaf6-5c2b8ea38e5f/content/fig191_2.jpg"/>

This study deals with the field calibration of the Solis SBT geophone system. The calibration procedure consists of: (I) introducing 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 bed load transport from the reservoir into the SBT inlet, and (III) recording and analyzing of the raw geophone signals. Three particle-size classes are chosen: 16–32 mm, 32–63 mm and 100–200 mm. In this paper, the details of the geophone system at Solis SBT and the results of the field calibration are presented and discussed.