Effect of bypass layout on guidance of downstream moving fish at bar rack bypass systems

Abstract

Hydropower plants act as a barrier to fish moving in rivers. Fish guidance structures like angled bar racks with horizontal or vertical bars are used to protect downstream moving fish from turbine passage, guiding them to a bypass as an alternative passage corridor. The function of the bypass system is to safely collect, carry and reintroduce the fish to the river downstream of the barrier. An effective bypass design is essential to ensure both fish protection and guidance to the tailwater. Most importantly, a bypass must be designed according to the swimming capacity and behavior of the target fish species. A gradual velocity increase into the bypass leads to high acceptance, being a prerequisite for high fish guidance efficiencies. The bypass entrance should be easily found, accepted and passed by all fish species without harm, delay, flight or exhaustion. The findings of two recent studies on bar rack bypass systems with (i) curved vertical and (ii) horizontal bars of different spacing indicate, amongst others, that an open channel bypass was quickly accepted by fish for mild velocity gradients at the inlet. In contrast, restrictor and pipe bypasses led to backwater with low flow velocities and high local velocity gradients in front of the bypass opening(s), leading to an avoidance reaction of fish. Although most available guidelines recommend a bypass discharge as a percentage of the water intake discharge, the present results suggest that the spatial velocity gradient in front of the bypass is the decisive parameter for an efficient bypass design to be quickly accepted by fish.