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Fish attraction:
Migratory fish instinctively seek the faster part of a stream. A unique downstream spillover is used to lure them into the horizontal first
section of the system. (Details of the spillover are not shown in the simplified diagram.)
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After fish enter, a very slowly turning rotary valve within the in-stream passageway works to prevent the full force of the stream from thrusting against them. At the same time, a bypass directs a regulated flow around the inside curve of the chamber to orient the fish and motivate them to continue their passage.
Fish migrating upstream enter the fish attraction section and swim toward the current. This brings them to the opening of the rotary valve chamber. Pursuing the current, they enter the chamber and are gently guided around to the exit in a sort of "revolving door" effect, at the rate
of 1 RPM or slower. Upon leaving the chamber, they are free to continue unimpeded through the rest of the fishway.
The system permits easy and safe passage for adult fish migrating upstream and smolts migrating downstream.
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A set of electronically controlled, fish-friendly gate valves within the instream passageway works in sequence to prevent the full force of the stream from thrusting against the fish. The velocity is adjustable, set to the speed fish biologists believe best for the specific species found in the waterway. At the same time, screened bypass conduits allow a steady stream flow, such as 1 to 2 feet per second, within the system to orient the fish and motivate them to continue upward.
The fish gather at the first valve and pass through it when it opens.
They proceed to the second valve, which opens after the first valve closes slowly, allowing nearby fish to stay clear of it. When the second valve opens, the fish located between the valves swim through and upward over the obstruction. After a pre-established period of time, the second valve closes slowly and the first valve opens again, allowing a new group of fish to enter the system. The two gate valves alternate in the closed position at all stages of the cycle.
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Fish protection:
The fish swim in the reduced current with minimum exertion and no anticipated injuries. Their natural buoyancy within the fishway permits them to rise without serious effort, in contrast to the struggle they face ascending fish ladders or being captured and trucked around obstructions. While traveling within the fishway, they are safe from predators.
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The main passageway tube is heavy walled polyethylene plastic. Its diameter is determined by the size and number of fish using the system. The normal diameter may be in the range of 8 to 18 inches. The fish friendly rotary chamber (about four feet in diameter) can be plastic or aluminum. The Swim-Thru Fishway is powered only by the natural force of the stream, controlled by a brake. The system is designed for minimal maintenance and includes shut off valves to stop all flow through, if necessary.
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The main passageway tube can be currogated metal, concrete, steel or plastic (Plastic resists the interior buildup of organic growth.) It can be semitransparent in sections and can include attraction lighting. Its diameter is determined by the size and number of fish using the system. The normal diameter may be in the range of 24". The valves and controls are industry standard and can be pneumatic or electric, located for safety, convenience and easy access. The system is designed for minimal maintenance and includes a shut off valve not shown in the diagram that can be positioned near the top of the system to stop all water flow through the system.
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Debris-free exit:
At the upper end of the system, the submerged exit is curved around 180 degrees to face downstream, preventing stream-borne debris from flowing in while allowing migrating fish to pass freely. (Details of the exit design are not shown on the simplified diagram.)
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Portability:
Although the system is designed for long life in permanent installations, it may also be used as a portable unit at construction sites and other situations to meet temporary fish passage requirements.
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Hydrostatic Pressures:
While in the passageway, the fish experience some hydrostatic pressure, depending on the head created by the obstruction. A study done by a recognized fisheries expert has determined that the pressures found at any site at which the Swim-Thru Fishway system would be used are minimal and create no risk of harm to fish.
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