PNNL

Abstract

We reviewed results of research conducted by engineers and biologists over the past 50 years related to development of surface flow outlets (SFOs) for juvenile salmonids that migrate downstream past hydropower dams. An SFO is a non-turbine, water-efficient passage route with an overflow structure through which flow and fish pass over a dam. Our review covered 69 SFOs in Europe and North America. We identified five main types of SFOs — low-flow bypass/sluices, high-flow sluices, forebay collectors, powerhouse retrofits, and surface spills. Most low-flow bypass/sluices are sited in Europe and on the east coast of North America, where mean annual project discharge and hydropower production for the dams we reviewed were 95 m3/s and 15 MW, respectively. The other four SFO types are found at dams on the west coast of North America with 2184 m3/s mean annual discharge and 788 MW mean output. A conceptual framework based on fish behavior and hydraulics for different regions of a hydropower project was developed to evaluate SFO performance. For all SFO types, fish collection efficiency averaged 54%, with an average effectiveness ratio of 17:1 (fish to inflow). Surface flow outlet technology can meet the goal of concurrent anadromous fish protection and hydropower generation.