A 40‑Year‑Old Water Intake Shows the Way for Fish‑Free Irrigation Withdrawals

A 40‐Year-Old Water Intake Shows the Way for Fish-Free Irrigation Withdrawals

By Charles Coutant

A typical angled fish screen installed at the beginning of an irrigation diversion canal. Water passes through the angled screen while fish are concentrated in a small flow that is returned to the river via a fish-return ditch.

It’s no secret that water diversions from rivers in the West for irrigation feeder canals bring along fish, especially downstream-migrating juvenile salmon. No one wants to see young salmon stranded in irrigation ditches; some of the fish may be endangered or threatened stocks in need of conservation.

Many irrigation districts, with federal or state assistance, have installed elaborate screening systems to act essentially as sieves to keep the fish out of the withdrawn water and return them to the river through bypasses. The success of such systems largely depends on the features of the screen: the size of the open space in the sieve and the velocity of the water going toward the holes (approach velocity), through them (through-screen velocity), and alongside the screen (sweeping velocity). These features are the ones generally used by regulatory agencies such as the National Marine Fisheries Service as design criteria for intake screens.

The evaluation of a 40‐year-old water intake at Energy Northwest’s Columbia Generation Station on the Columbia River near Richland, Washington, has shown the benefits of a different approach. The monitoring of a cylindrical T-screen structure in the river has shown essentially no withdrawal of juvenile fall Chinook salmon and steelhead, even though it is immediately downstream of spawning areas for large numbers of both species. Up to 56 million newly emerged and rearing Chinook salmon are estimated to pass downstream by the intake. The screens have 3/8-inch pores that are capable of sucking in small fish, yet a monitoring system with near 90 percent capture efficiency caught only four fish in the 142 million cubic feet of water withdrawn over 4 years of testing during the April–June period of juvenile salmon abundance. Why? The answer seems to be the hydraulic patterns around the screen structure, which carry fish around the screen, rather than the sieving effectiveness of the actual screening material. The evaluation incorporated well-known facts about the pressure and velocity patterns around cylinders in moving water and the sensory and avoidance capabilities of fish encountering those patterns.

A cylindrical T-screen structure, manufactured by Striker Welding, that is oriented parallel with a river’s flow. Fish in the river are diverted away from the structure by the hydraulics at the nose cone, and most do not encounter the screens located at the sides of the cylinder. No fish return system is needed because the fish remain in the river.

Purely hydraulic factors at the nose cone of the T-screen structure—the commonly observed bow wave—appear to move fish away from the screens located at the side of the cylinder. Such hydraulic bypass had been identified earlier in tests of model screens in a test flume, where it was more important than the screen’s ability to physically exclude fish.

Since even small salmon are sensitive to changes in water pressure and velocity as they migrate, they can sense the hydraulic changes at the T-screen’s nose cone and avoid the structure. Both their instinctive flight response and their swimming behavior would take them away from the vicinity of the actual screens on the sides of the T-screen structure.

The takeaway for irrigation organizations is that a T-screen structure in swift river currents repels fish of all sizes and passes them in the river itself. There is no need for a fishreturn channel. As at the Columbia Generating Station, a T-screen structure could be placed midriver with a buried pipe to carry the fish-free water to a diversion canal. T-screens with wedgewire screening material are readily available commercially.

For more information on this topic, see Charles Coutant, “Why cylindrical screens in the Columbia River (USA) entrain few fish,” The Journal of Ecohydraulics, available online.

Charles C. Coutant, PhD, is the owner of Coutant Aquatics. He can be contacted at ccoutant3@comcast.net or (865) 483‐5976.