Where It Works
The Farmers Screen requires specific site conditions in order to operate properly. Put in simple terms, the site must have elevation differential in both the conveyance and the source river or stream. Constantly available by-pass flow, in addition to the screened flow and in-stream flow, is also required.
Screen Location: The site must be off-channel with flow to the screen regulated by a headgate. There must be adequate space to accommodate the screen structure in a place that is protected from high flow events.
Adequate Flow: There must be adequate flow in the source river or stream to ensure that the proper amount of by-pass flow (flow returning to the river or stream along with fish and debris) in addition to the screened flow and any necessary in-stream flow is available 100% of the time that the screen is operating. The necessary by-pass flow quantity is a function of screen design and is determined during the design process.
Elevation Differential: Also referred to as required head or fall. The Farmers Screen requires a drop in elevation from the point of diversion to the end of the screen structure. Typically, a total of 1 foot in elevation differential from the point of diversion to the end of the screen structure is more than adequate. The head generated by the elevation differential is needed in two places: the entrance to the flume, and at the leading edge of the screen. Head is required to drive the water into the flume, and more head is required as water volume increases. There must be enough head to induce velocities of 3 to 7 feet per second in the flume. Head is also required to drive water through the screen structure itself. Approximately 0.3 feet is necessary to overcome the head loss through the screen (measured from the flume water surface elevation to the attenuation bay water surface elevation).
Stream or River Gradient: The slope of the source river or stream must exceed the slope of the diverted water conveyance such that the elevation differential between the screen surface elevation and the stream (at the point where the by-pass water return pipe enters the stream) is sufficient to prevent any backwater influence in the pipe and to meet NMFS criteria regarding by-pass flow hydraulics.
Operator: The screen owner/operator must be willing to agree to operate the screen as designed and as specified in the Operation Manual.