A Movable Bed Physical Model Yields an Innovative Solution for Replacing a Fish Pass Suffering from Clogging

An experimental study yielded an innovative solution to replace a fish pass suffering from clogging on the Arve River in Geneva. Due to intense suspended, bed and driftwood load of the river, investigations were carried out on a movable bed physical model at the Laboratory for Applied Hydraulics of HEPIA. The planned bypass river answers to Suisse federal revitalization requirements and can be implemented on the Arve River.

Fish migration in the Vessy meander is altered by a 250 m long and 3 m high spillway conveying a part of the river discharge to a low head hydraulic power station. The Suisse Federal Law on Fishing (RS 923.0) requires from all hydraulic power station operators to guarantee a free fish migration. Further Federal laws (RS 814.20; OFEV 2012) and Cantonal acts (Genève-Vision 2030, 2014) state that the impacts of power plants on bed load regime, driftwood dynamics and river discharge have to be mitigated.

The Service Industriel de Genève (SIG) has an obligation to replace the Vessy fish pass. Because of the complex hydraulic conditions, the SIG assigned a detailed analysis on a movable bed physical model to the Laboratory for Applied Hydraulics of HEPIA.

The main goals of the investigations were a) to understand existing hydro- and morphodynamics of the Vessy meander; b) to identify the most relevant location of the entrance and the water intake of the future fish pass; c) to implement and analyse alternative hydraulic structures on the model; d) to inventory the benefits and inconvenience of alternatives; e) to provide a sustainable and ecological solution.

The analyses pointed out that a bypass river would gather the largest number of benefits. A concrete fish pass would suffer of clogging. A riprap ramp, although less exposed to clogging, would only offer favourable hydraulic conditions by weak and medium discharges of the Arve River.

The bypass river which entrance and water intake will be ideally located, will run on Arve’s right overbank thus protected from flooding of the latter as well as from clogging thanks to riprap structures to be implemented in the Arve. The bypass will offer optimal flow and ecological conditions for all fish species and can therefore be constructed.