Fish injury and mortality during passage through pumping stations

An unwanted side effect of pumping stations is that fish suffer from injury and mortality when passing through the pumps and that fish migration is hampered. In recent years, the development of so-called fish-friendly pumping stations has received increasing attention from European governmental institutions and pump manufacturers. In the Netherlands, many field studies have been conducted over the last decade to assess the chances of survival for fish passing through pumps. A clear correlation between observed injury or mortality and, for example, flow rate, shaft speed, or pump type could not be established. This paper presents a new analysis of these field studies. It uses American studies on the biological criteria for fish injury, the most important of which are pressure changes, shear forces, and mechanical injury. A blade strike model is adapted to fish passing through centrifugal pumps of radial, mixed-flow, and axial type. It reveals the relation between fish injury and the type of pump, its size, shaft speed, and pressure head. The results correlate fairly well with experiments. The flow through a typical mixed-flow pump is calculated using computational fluid dynamics (CFD). The results show that pressure fluctuations and shear forces are not likely to add much to fish mortality. Guidelines for the design and selection of fish-friendly pumps are given with the introduction of two new dimensionless numbers: the blade strike probability factor and the blade strike velocity factor. It shows that fish-friendliness of pumps decreases with increasing specific speed value.