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Choose an irrigation-diversion fish screen only after identifying the governing fish-protection authority, the species and life stages to protect, and the diversion’s maximum flow and hydraulic conditions. Screen opening, effective wetted area, approach and sweeping velocities, bypass access, debris handling, cleaning, inspection and monitoring all have to work together. A generic mesh panel or catalog size cannot establish compliance.
Why the “right size” is not a single dimension
An irrigation fish screen is a complete fish-protection system, not simply a piece of mesh placed over an intake. It must exclude the fish covered by the permit while allowing the approved withdrawal, keep flow conditions safe at the screen face, and guide fish to a bypass or return route. A screen that blocks passage but leaves fish in a dead-end pocket can fail its purpose.
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Screen dimensions therefore follow from several project values:
- the maximum diversion flow, including the operating condition used for approval;
- the fish species and life stages present during each operating season;
- the applicable opening or slot-size requirement;
- the effective wetted screen area after structural members and inactive portions are deducted;
- approach velocity toward and through the screen;
- sweeping velocity parallel to the screen face;
- the location and hydraulic accessibility of the bypass;
- debris, sediment, flood, access and power conditions; and
- the inspection, cleaning, testing and reporting program required by the responsible authority.
The same nominal screen width can be adequate at one diversion and inadequate at another because flow, submergence, debris loading and fish requirements differ.
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Identify the governing authority before choosing equipment
Start by asking which agency has jurisdiction over the diversion. Depending on location and project type, that may be a state, federal, tribal, provincial or another fish-protection authority. The agency determines which species, life stages, seasons and screen configurations are regulated.
NOAA Fisheries’ West Coast Region Anadromous Salmonid Passage Design Manual (2022) consolidated West Coast criteria and superseded earlier regional guidance. It is a starting point for that region, not a universal standard. Washington’s fish-protection guidance likewise directs proponents to verify requirements for their particular site or facility. Confirm the applicable edition and obtain a design review before ordering a screen.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallAsk the reviewer to identify the controlling permit, design manual, variance process and acceptance tests in writing. If multiple authorities apply, design to the most restrictive compatible requirement unless the agencies approve another approach.
Define the fish you must protect
Opening size and hydraulic criteria depend on the smallest fish and weakest swimming life stage that must be protected, not merely on the largest adult species observed nearby. Request the required protection list from the agency or fisheries biologist and record:
- species and life stages;
- dates or flow conditions when each life stage is present;
- whether eggs, larvae, juveniles, adults or listed species require protection;
- whether fish must be protected during the full diversion season or only specified windows; and
- any special behavioral, debris or sediment concerns identified for the site.
Do not substitute a screen opening used at another project without confirming that its target species and life stages match yours. A screen that is acceptable for one fish community can be unsuitable for another.
Collect the site and operating data
A qualified designer needs measured or documented site data before selecting a configuration. The following checklist keeps the design conversation concrete.
| Input | What to document | Why it changes the selection |
|---|---|---|
| Design diversion flow | Maximum instantaneous withdrawal, normal operating range, start-up and shutdown conditions, and any seasonal limits | Sets the required effective wetted area and hydraulic capacity |
| Water levels | Upstream and downstream elevations, minimum and maximum operating levels, flood levels and drawdown | Determines submergence, bypass access and whether the screen remains functional |
| Fish requirements | Species, life stages, seasons and agency criteria | Controls opening size, velocities and allowable configuration |
| Channel hydraulics | Approach flow direction, turbulence, velocity distribution, bed slope and nearby structures | Affects uniformity, sweeping flow and fish guidance |
| Debris and sediment | Leaves, weeds, woody debris, ice, bed load, suspended sediment and flood-borne material | Determines screen orientation, cleaning method and maintenance frequency |
| Access and utilities | Safe work access, lifting equipment, electricity, controls, winter access and disposal route for removed debris | Limits practical cleaning and inspection options |
| Return route | Bypass entrance, conveyance, outfall, attraction flow and downstream conditions | Ensures fish can leave the screen area without injury or delay |
Choose a configuration that fits the hydraulics
Configuration is a design decision, not a cosmetic preference. Common examples described by NOAA include angled screens and rotating drum screens; horizontal screens are addressed in detail in the West Coast manual. Your designer should compare alternatives against the same approval and operating criteria.
| Configuration question | What to evaluate |
|---|---|
| Horizontal screen | Whether the channel can provide the required uniform approach flow and sweeping flow, how submergence changes usable area, and whether automated cleaning is needed |
| Angled screen | Whether the angle guides fish toward a bypass, maintains safe velocities across the full face and remains clear under local debris loads |
| Rotating drum screen | Whether rotation, seals, drive equipment, cleaning and flood conditions are suitable, and whether the bypass remains accessible at all operating levels |
These are examples, not a recommendation for an unspecified site. The approved configuration must be checked against species, flow, sediment, debris, access and agency review.
Size effective wetted area for the maximum diversion flow
Use the area that actually passes water at the approved velocity, not the screen’s outside dimensions. Frames, stiffeners, seals, inactive edges, blocked panels and portions above the waterline reduce effective wetted area. A designer should document the net open area and the calculation for the maximum diversion flow.
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Submergence is especially important. A screen can have a large nominal surface but too little usable area when water level falls, when a section is blocked, or when the intake operates at a different elevation than assumed. Check the minimum and maximum water levels, not only the design drawing’s typical level.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsVelocity should be evaluated across the screen, because a compliant average can conceal a high-velocity corner or a blocked panel. NOAA’s technical manual discusses near-screen hydraulic evaluation and calls for uniform approach velocity. Require the design to show how structural components and expected fouling are included in the area calculation.
Apply approach and sweeping velocity criteria in context
Approach velocity
Approach velocity is the component of water movement toward and through the screen. It must remain within the criterion adopted by the responsible authority and must be reasonably uniform over the active face. Excessive or uneven approach flow can press fish against the screen or draw them into openings.
For a horizontal-screen criterion in NOAA Fisheries’ 2022 West Coast manual, approach velocity should be less than 0.25 ft/s. The same section states that a horizontal screen with an automated mechanical cleaner should have approach velocity less than 0.4 ft/s. Those figures belong to that manual and screen context; they are not universal values for every diversion or jurisdiction.
Sweeping velocity
Sweeping velocity is the component parallel to the screen face. It should move fish toward a bypass rather than allowing them to linger against the screen. NOAA’s horizontal-screen section states: “Sweeping velocity should never be less than 2.5 ft/s or an alternate minimum velocity approved by NMFS that is based on an assessment of sediment load in the water diversion system.” Treat the 2.5 ft/s value as the cited criterion for that manual and configuration, not as a general rule outside it.
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Canadian criteria are separate
Fisheries and Oceans Canada’s end-of-pipe screen standard uses its own approach-velocity framework. Where fish-community or habitat information is unavailable, it gives a default maximum design approach velocity of 0.055 m/s, with up to 0.12 m/s when ambient flow is at least twice the design approach velocity under the standard’s stated condition. These values must not be substituted for United States or other local criteria.
Specify openings, porosity and materials
Openings must be small enough to prevent the protected fish from passing through or becoming wedged, while retaining enough porosity to pass the required withdrawal without exceeding velocity limits. Specify the opening or slot dimension in the agency-approved orientation and account for manufacturing tolerances, wear and fouling.
Porosity is the fraction of the screen area available for flow. A panel with a high gross open percentage may have a lower effective porosity after frames, support bars, seals or accumulated debris are included. Material selection also affects durability, corrosion, abrasion, biological growth and cleanability. The Canadian standard expressly considers screen or slot size, porosity, effective area and material; equivalent considerations should be documented wherever your local program requires them.
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Ask for drawings identifying clear opening, wire or bar dimensions, structural members, seals, attachment details and the condition used for hydraulic calculations. “Fine mesh” is not a sufficient specification.
Design the bypass as part of the screen
The bypass is the route that returns fish from the screen face to the river or canal reach approved by the agency. Its entrance must be located where fish can find it under the full operating range, and its conveyance and outfall must avoid drops, impacts, predation concentration and damaging turbulence.
NOAA’s plain-language guidance warns that “Dead ends where fish cannot find their way back to the river are almost as bad as no screen at all.” Use that principle during layout review: trace a fish’s path from the first contact with the screen to the return point, including low-flow, high-flow, shutdown and partially fouled conditions. Confirm attraction flow, entrance geometry, capacity, outfall location and downstream water levels with the designer and agency.
Select passive or automated cleaning
Cleaning must be matched to the debris and sediment actually reaching the intake. A passive screen may be appropriate where the site has manageable debris, stable hydraulics, safe access and an operating program that can inspect and clear it as required. Automated mechanical cleaning is more appropriate where debris arrives continuously, access is hazardous, the diversion must run unattended, or fouling would quickly reduce effective area.
Do not treat automation as a substitute for maintenance. Drives, brushes, seals, sensors and controls can fail, and a stopped cleaner can turn a compliant screen into a high-velocity or blocked screen. Specify alarm states, manual override, safe isolation, spare parts, debris disposal and a response time.
NOAA’s manual limits passive screens to specific conditions and requires inspection and maintenance consistent with its criteria. Ask the reviewer to state the required inspection interval, cleaning trigger, records and corrective-action deadline for your project.
Use a step-by-step selection process
- Map jurisdiction. Identify every agency with authority over fish passage, water diversion, habitat, listed species and construction. Obtain the current design manual, permit conditions and review contact.
- List protected fish. Record species, life stages, seasons and any site-specific biological findings supplied by the agency or qualified biologist.
- Confirm the maximum design flow. Include the largest permitted withdrawal and operating transients. Do not size only for the average flow.
- Survey hydraulics. Measure or model water levels, approach direction, turbulence, sediment movement, flood conditions and available bypass alignment.
- Screen configurations. Compare horizontal, angled, drum or other approved options for effective area, opening, velocity, bypass, debris behavior, cleaning and access.
- Calculate effective area. Deduct frames, supports, seals, inactive portions and expected fouling. Check minimum submergence and the full operating range.
- Verify velocities. Demonstrate approach-velocity limits, uniformity and required sweeping flow at the screen and bypass. Keep jurisdiction-specific units and criteria together with the calculation.
- Detail the bypass. Show entrance, conveyance, outfall, attraction flow and fish-safe conditions at low, normal and high water levels.
- Write the maintenance plan. Define cleaning method, inspection frequency, debris and sediment response, access, safety, spare parts, alarms and recordkeeping.
- Submit for review before purchase. Provide drawings, calculations, materials, operating rules and monitoring commitments. Wait for written agency acceptance where required.
Plan construction, testing and monitoring
Construction acceptance should verify that the installed system matches the approved design. Check clear openings, panel alignment, seals, structural members, bypass geometry, elevations, controls and access before commissioning.
NOAA’s horizontal-screen criteria call for post-construction velocity testing, daily inspection and maintenance when required, and at least one year of post-construction monitoring for new horizontal-screen facilities. Confirm whether those obligations apply to your project and what test method, reporting format and corrective limits the reviewer will use.
Velocity testing should cover representative operating flows and identify localized high-velocity areas, blocked sections and nonuniform approach conditions. Monitoring records should connect observations to actions: debris removed, panels repaired, cleaner faults reset, bypass conditions checked and agency notifications made when thresholds are exceeded.
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Common selection and installation failures
Ordering by pipe or intake diameter alone
Pipe diameter describes the conveyance, not the fish-safe screen area. Recalculate effective area for maximum diversion flow and the approved velocity criterion.
Using a neighboring project’s opening size
Different species, life stages and agencies can require different openings. Obtain a site-specific approval instead of copying a nearby installation.
Checking average velocity only
Average values can hide a high-velocity corner, a blocked panel or a dead zone that prevents fish from reaching the bypass. Require a distribution check across the full active face.
Ignoring water-level variation
Insufficient submergence can reduce active area or strand the bypass entrance. Evaluate minimum, normal, flood and shutdown levels.
Putting the bypass at a hydraulic dead end
Fish need a continuous, findable route away from the screen. Recheck entrance location, attraction flow and return conditions under every operating mode.
Assuming a cleaner eliminates inspections
Mechanical equipment can fail or overload. Keep manual inspection, alarms, safe access and a contingency procedure.
Buying before agency review
A physically attractive product can still have the wrong opening, area, material, configuration or maintenance requirements. Submit the complete design package before committing to fabrication or installation.
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Questions to take to the designer and agency
- Which state, federal, tribal, provincial or other authority governs this diversion?
- Which species and life stages must be protected, and during what operating seasons?
- What is the maximum diversion flow, including start-up, shutdown and seasonal conditions?
- What effective wetted area is required under the applicable approach-velocity criterion?
- Which screen configuration and opening size are approved for this location?
- How are frame members, supports, seals, fouling and submergence included in the area calculation?
- What approach-velocity limit, uniformity test and sweeping-flow requirement apply?
- Where does the bypass return fish, and can fish reach that exit across the full operating range?
- What debris and sediment loads occur, and is automated cleaning required?
- What inspections, maintenance logs, velocity testing and post-construction monitoring must be completed?
- What happens if the screen clogs, the cleaner fails, water levels fall or the bypass is unavailable?
Decision checklist before specifying a screen
- Jurisdiction and current approval criteria are identified.
- Protected species, life stages and operating seasons are documented.
- Maximum diversion flow and water-level range are established.
- Effective wetted area, clear opening and porosity are calculated for the approved condition.
- Approach and sweeping velocities are demonstrated across the screen face.
- Bypass entrance, conveyance and outfall are fish-accessible and agency-approved.
- Debris, sediment, flood and ice conditions are addressed.
- Cleaning, inspection, alarms, access and spare parts are practical for the site.
- Construction testing and monitoring obligations are included in the project budget and schedule.
- Written agency or design-review acceptance is obtained before purchase.
Bottom line
The correct fish screen is the one that an approved hydraulic and biological design shows will protect the required fish at the diversion’s maximum operating flow, while reliably guiding them to a safe bypass. Start with jurisdiction and species, then size effective area, openings and velocities for the actual site. Treat cleaning, inspection, testing and monitoring as part of the screen—not as optional accessories—and verify the complete installation with the responsible agency.
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