In hydro terms, the tailrace is the water conveyance that connects the turbine outflow to the river or reservoir downstream of the powerhouse.  For an underground powerhouse, this takes the form of a tailrace tunnel, usually designed for low velocity (and therefore low head loss). For a surface powerhouse, the tailrace is typically a short channel.

The stability of the tailrace boundary must be considered in the design – if the channel bed scours and lowers the tailwater level, it could reduce the hydraulic head downstream of the turbine runner, raising the risk of cavitation. If the channel agrades (silts up), the tailwater level will rise, resulting in less net head on the turbine and therefore less power.

Fish may be attracted to tailraces, especially salmon following the signature characteristics of their home waters. Nets are sometimes used to exclude fish. Sometimes the tailrace is used as a collecting station for transporting the salmon by fish lift or truck up to the reservoir.

White sturgeon like to hide in dark places, and are attracted to turbine draft tubes. To keep this Listed Species* from harm by the turbine runners, the draft tube outlets at Waneta Expansion Project in British Columbia, Canada, were equipped with “Sturgeon exclusion screens” on the draft tube outlets. These screens open once the turbine flow is high enough to prevent entry of sturgeon, and close again when the turbine flow is reduced prior to shutdown.

* Species and ecosystems at risk are assigned to the red, blue or yellow list according to their conservation status rank. This helps to set conservation priorities and provide a simplified view of the status of a region’s species and ecosystems. The lists also help to identify species and ecosystems that can be considered for designation as “Endangered” or “Threatened.”

Published by hydrogray

Hydropower specialist

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