Over hundreds of Coanda screen projects across hydropower, municipal water supply, irrigation, and snowmaking projects, ADENCO's engineering team has reviewed thousands of intake screen specifications. We see the same mistakes repeated, and the same costly consequences on site.
These are not theoretical errors. Each one comes from real projects where an incomplete or incorrect specification led to reduced performance, premature failure, regulatory non-compliance, or complete screen replacement. Every mistake described here has cost a project owner real money and months of schedule delay.
This guide documents the five most common specification errors, explains why each one happens, shows you what happened on site as a result, and gives you the engineering solution.
Mistake #1: Specifying the Wrong Stainless Steel Grade for Your Water Chemistry
The Error
The engineer specifies AISI 304 stainless steel because it is the default, lower-cost option: without analyzing the chloride content, pH, or temperature range of the raw water. Alternatively, the engineer over-specifies 316L or duplex for a clean freshwater application, adding 30–80% to material cost unnecessarily.
Why It Happens
Material selection is often treated as a routine purchasing item rather than an engineering decision. Specifications frequently copy standard wording from earlier project specifications at different sites with different water chemistry. And because corrosion failure takes months or years to appear, the consequences are invisible during commissioning.
What We Have Seen on Site
A Coanda screen specified in 304 stainless steel was installed at a coastal intake where the raw water contained 400–600 ppm chloride. Within 18 months, pitting corrosion developed at crevice points around support bar connections. Within three years, several wedge wires had corroded through, creating slots 3–4x wider than the design width, which made the screen useless for fish exclusion and sediment removal. The entire screen panel required replacement.
304 stainless steel is susceptible to pitting corrosion in chloride concentrations as low as 200 ppm, with a critical pitting temperature (CPT) of approximately 40°C at 300 ppm chloride. 316L, with its 2–3% molybdenum content, tolerates up to 500 ppm chloride at 70°C CPT [1][2]. In seawater or brackish environments, even 316L may be insufficient: duplex 2205 or super-austenitic 904L should be evaluated.
The Fix
Include water chemistry data in your specification. At minimum, provide:
- Chloride concentration (ppm): annual range, not just average
- Water temperature: seasonal range, especially maximum
- pH range
- Whether the intake operates in estuarine or tidal zone (salinity fluctuation)
General selection rule: 304 for clean freshwater (<200 ppm chloride), 316L for brackish water (200–1,000 ppm), duplex or super duplex grades above 1,000 ppm. For the full grade decision tree with PREN values, Critical Pitting Temperature data, and a water-chemistry decision matrix, see: 304 vs 316 Stainless Steel for Water Intake Screens.
When in doubt, send a water sample analysis to your screen manufacturer. The cost of a material upgrade is a fraction of the cost of a premature replacement.
Mistake #2: Ignoring Available Head in the Hydraulic Design
The Error
The specification states the required flow rate and slot width but omits the available head: the elevation difference between the water surface at the weir crest and the outlet pipe or collection chamber. Or worse, the specification assumes the available head is "sufficient" without measurement.
Why It Happens
Many engineers are accustomed to specifying pumped intake systems where head loss through the screen is a minor consideration: a few hundred millimetres absorbed by additional pump power. Coanda screens are gravity-fed devices with no pump to compensate. If the available head is insufficient, the screen simply does not deliver the design flow.
What We Have Seen on Site
A hydropower project specified a Coanda screen with 1.0 mm slots to deliver 200 l/s. The specification provided the flow rate and slot width but did not include the available head. Site survey later revealed only 350 mm of elevation difference between the weir crest and the pipe connection. The US Bureau of Reclamation (USBR) Coanda Design Guide (Wahl, 2003, report R-2003-03) gives 0.45 to 1.3 m of hydraulic head as the typical range, with 450 mm as the absolute minimum [3][4]; ADENCO's standard screens use drop heights of 450, 700 and 1,270 mm, and custom screens can go higher. The screen was installed, but at 350 mm of head it delivered only 55% of the design flow. The project required either a weir modification (raising the crest) or a redesign to accept reduced capacity: both expensive mid-construction changes.
The Fix
Always include in your specification:
- Available gross head: measured elevation difference from normal water surface at the weir crest to the centerline of the outlet pipe.
- Minimum operating head: lowest water level condition when the screen must still deliver acceptable flow.
- Flood level: maximum water level to verify the screen and the concrete structures (weir, collection chamber, bypass) are not overtopped or damaged during extreme events.
The USBR Coanda screen design software [5] calculates the relationship between hydraulic head, screen dimensions, and flow capacity. Provide the head data and let the manufacturer optimize the screen geometry: do not assume standard dimensions will work at your specific available head.
Mistake #3: Sizing the Screen for Average Flow Instead of Design Flow
The Error
The specification uses the annual average flow demand to size the screen, rather than the peak design flow. The screen delivers adequate water most of the year but is undersized during the critical high-demand period: exactly when maximum capacity is needed.
Why It Happens
Engineers sometimes use average water abstraction (withdrawal) data from water balance studies rather than peak instantaneous demand from the hydraulic design. In irrigation projects, the difference between average and peak demand can be 3–5x. In hydropower, the turbine design flow is the figure to use for sizing, not the average river flow.
What We Have Seen on Site
An irrigation project specified a Coanda screen based on the average seasonal water demand of 80 l/s. The real peak demand during midsummer irrigation was 250 l/s. The screen delivered the required flow from April through May, then became a bottleneck from June through August: precisely when crops needed the most water. The project had to add a second screen panel, requiring additional concrete structures, a wider collection chamber, and a shutdown of the irrigation supply for construction work during the growing season.
A typical Coanda screen delivers approximately 140 l/s per metre of weir width on the USBR reference geometry [4][6], the generic industry baseline; ADENCO's own series are rated at 35, 67 and 150 l/s per metre for the ADENCO-45, ADENCO-70 and ADENCO-127. Whichever figure you use for sizing, it must be applied to the maximum instantaneous flow the screen must deliver, not the average.
The Fix
Specify these flow parameters:
| Parameter | What to Provide | Why It Matters |
|---|---|---|
| Design flow (Qdesign) | Maximum instantaneous flow required | Screen must deliver this at minimum operating head |
| Minimum operating flow (Qmin) | Lowest flow at which the screen must function | Ensures Coanda effect and self-cleaning are maintained |
| Bypass flow | Flow that travels over the screen without passing through the slots | Must be returned to the watercourse; affects bypass channel design |
Include a safety factor. ADENCO sizes to 1.3 × the design flow for standard applications and 1.5 × for critical ones (sole water supply, fish-protection compliance, cold climate). The margin allows for partial clogging during extreme debris events, surface tension effects in cold water [7], and for long-term wear that widens the slots.
Mistake #4: Not Accounting for Cold Climate Conditions
The Error
The specification does not mention operating temperature range or ice conditions: even for sites where winter temperatures regularly drop below freezing. No anti-icing measures are included, and no strategy for frazil ice (small ice crystals carried in flowing water) is specified.
Why It Happens
Intake screens are often specified during design work carried out in warm weather. The engineer visits the site in summer, sees a gently flowing watercourse, and designs accordingly. Winter conditions (frazil ice formation, supercooled water, atmospheric icing) are not visible during the site visit and are easy to overlook at the design stage.
What We Have Seen on Site
A Coanda screen for a small hydropower plant was installed at an alpine site at 1,800 metres elevation. The specification made no mention of ice conditions. During the first winter, supercooled water formed frazil ice crystals that adhered to the wedge wire surfaces. Within hours, the entire screen was blocked. The turbine shut down due to low flow, and the site technician had to clear ice from the screen by hand: on a steep, icy weir structure, in winter, at a remote mountain location.
Research on Coanda screen performance in cold climates [8] has documented two distinct ice clogging mechanisms: frazil ice adhesion to the wedge wire surfaces from supercooled water, and atmospheric ice buildup from freezing spray and ambient air temperatures. Both can completely block a screen in hours.
Prevention of water intake blockage by ice during supercooling events is a well-studied subject [9], and solutions exist, but they must be specified at the design stage, not retrofitted after the first freeze.
The Fix
For any site where water temperature may drop below 4°C or air temperature drops below 0°C, include in your specification:
- Operating temperature range: minimum and maximum water temperature, minimum air temperature.
- Ice condition assessment: frazil ice risk (supercooled water in turbulent river sections), surface ice, atmospheric icing.
- Anti-icing requirements: specify whether heated screen panels, warm water recirculation, air bubble systems, or ADENCO's proprietary anti-icing systems are required.
- Emergency bypass: specify an alternative water path if the primary screen becomes ice-blocked during extreme events.
Including cold-climate measures at the specification stage adds 15–30% to screen cost. Adding them after the first winter shutdown typically costs 3–5x that amount, plus lost revenue during downtime.
Mistake #5: Omitting Fish Protection Requirements from the Specification
The Error
The specification covers hydraulic performance (flow rate, head loss, slot width) but makes no mention of fish protection regulations, target species, life stages present at the intake site, or maximum allowable approach velocity.
Why It Happens
Fish screening regulations are fragmented across multiple agencies, jurisdictions, and species-specific rules. An engineer designing a hydropower intake may not know about the Eels Regulations 2009 in the UK [10], the Section 316(b) requirements in the U.S. [11], or Australia's Murray-Darling screening standards [12]. The hydraulic design is completed first; the regulatory requirements are discovered later: often during the permit process, when redesign is expensive.
What We Have Seen on Site
A European hydropower project specified a Coanda screen with 2.0 mm slots, optimized for maximum flow capacity. The screen was manufactured, delivered, and partially installed. During the environmental permit review, the regulatory authority identified that the intake was located in a river section with eel protection requirements, where a 1.0 mm maximum slot width applies under national implementation of the EU Water Framework Directive. The 2.0 mm screen panels had to be scrapped and replaced with 1.0 mm panels: doubling the screen procurement cost and delaying the project by four months.
This scenario is preventable. Fish screening regulations exist in virtually every developed country, and they apply to most types of water withdrawal, not just power plants. The regulatory trend across all jurisdictions is toward stricter requirements, narrower slot openings, and lower approach velocities [13].
The Fix
Before finalizing any intake screen specification, answer these questions:
- What fish species are present at the intake site? Contact the local environmental authority or fisheries agency for species surveys and critical habitat designations.
- What life stages are present during the operating season? Eggs, larvae, fry (newly hatched fish), juveniles, and adults have different screening requirements. The smallest life stage dictates the slot width.
- What regulations apply? Identify all applicable regulations: federal, state/provincial, and local. In the UK, check both general Environment Agency (EA) screening guidance and Eels Regulations. In the U.S., check both federal 316(b) and state-specific fish screening rules.
- What is the maximum approach velocity? Specify this in the screen design alongside the flow rate. For Coanda screens, approach velocity from the waterbody is effectively zero (gravity-fed, no suction), but the regulatory authority may still require documentation.
- Include the regulatory references in your specification. State the specific regulation, the required maximum slot width, and the required maximum approach velocity. This gives the manufacturer clear compliance targets.
ADENCO recommendation: When multiple species with different screening requirements are present, specify the smallest (safest) slot width required for any of the species: typically 1.0 mm for sites with larval fish or eel sensitivity. The USBR Coanda Design Guide (Wahl, 2003, report R-2003-03) [3] shows that on shallow screen slopes capacity falls as the slot narrows, so a narrower slot means more screen width for the same flow: allow for this when specifying 1.0 mm instead of 2.0 mm.
Avoid These Mistakes: Use a Complete Specification Checklist
Every mistake in this article stems from the same root cause: missing information in the original specification. If the water chemistry had been included, the material grade would have been correct. If the head measurement had been provided, the hydraulic feasibility issue would have been identified at the design stage.
ADENCO has developed a comprehensive 15-parameter specification checklist that covers every item of data needed to design, quote, and manufacture a Coanda intake screen, including the parameters most commonly omitted. For the complete checklist with a ready-to-use RFQ template, see: How to Specify a Coanda Intake Screen for Your Next Project.
Frequently Asked Questions
How do I know if my Coanda screen specification is incomplete?
Five warning signs indicate a gap in the specification: (1) no chloride or pH data for the raw water, (2) no available head measurement, (3) flow rate listed as "average" rather than peak demand, (4) no mention of operating temperature range or anti-icing, and (5) no reference to fish protection regulations even though the intake is on a natural watercourse. If any of these are missing, the specification will lead to rounds of questions from the manufacturer that delay your project. For a complete 15-parameter checklist, see: How to Specify a Coanda Intake Screen.
What is the most common Coanda screen specification mistake?
The most costly mistake is specifying the wrong stainless steel grade for the water chemistry. Using 304 stainless steel in water with chloride levels above 200 ppm leads to pitting corrosion and screen failure within 2–5 years. For the complete material selection decision tree including PREN values and chloride thresholds for all six grades, see: 304 vs 316 Stainless Steel for Water Intake Screens.
How do I determine the right slot width for my Coanda screen?
Slot width is determined by the smallest organism or particle you need to exclude: not by flow capacity. Check applicable fish protection regulations first: 1.0 mm maximum for sites with larval fish or eel sensitivity, 1.0–1.5 mm for general debris screening. For the complete slot width selection guide with application-specific recommendations, see: The Engineer's Guide to Coanda Screen Design.
How much head does a Coanda screen need?
A Coanda screen requires a minimum of about 450 mm of available head (the drop height of the smallest standard screen model) between the weir crest and the outlet pipe. ADENCO's standard screens have drop heights of 450, 700 and 1,270 mm, and custom screens can be built with a larger drop; the head required at your site depends on screen length, slot width, and design flow. If your site has less than 450 mm of available head, a Coanda screen may not be the right technology: consider a passive wedge wire screen or a pumped drum screen instead.
Can ADENCO review my specification before manufacturing?
Yes. ADENCO provides free engineering review of intake screen specifications before quotation. Our team will point out any missing parameters, identify potential design issues, and recommend optimizations based on the USBR design methodology. This review has identified every type of mistake described in this article before manufacturing began. Submit your specification for review.
References
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"Susceptibility of Type 304/304L and 316/316L Austenitic Stainless Steels to Chlorides in Cooling Water." Digital Refining. Retrieved April 2026, from https://www.digitalrefining.com/article/1002873/
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British Stainless Steel Association. "Selection of 316, 304, and 303 Types of Stainless Steels for Seawater Applications." Retrieved April 2026, from https://bssa.org.uk/bssa_articles/selection-of-316-304-and-303-types-of-stainless-steels-for-seawater-applications/
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Wahl, T.L. (2003). Design Guidance for Coanda-Effect Screens. U.S. Bureau of Reclamation, Research Report R-2003-03. Denver, CO.
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"The Coanda Effect." International Water Power & Dam Construction. Retrieved April 2026, from https://www.waterpowermagazine.com/analysis/the-coanda-effect/
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USBR. "Coanda-Effect Screens Software & Design Tools." Retrieved April 2026, from https://usbr.gov/tsc/techreferences/computer%20software/software/coanda/
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Wahl, T.L. (2001). "Hydraulic Performance of Coanda-Effect Screens." Journal of Hydraulic Engineering, Vol. 127, No. 6, pp. 480–488. ASCE.
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Wahl, T.L. et al. (2021). "Surface Tension Effects on Discharge Capacity of Coanda-Effect Screens." Journal of Hydraulic Engineering, Vol. 147, No. 8. ASCE.
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Gebre, S. et al. (2014). "Performance of Coanda-Effect Screens in a Cold Climate." Journal of Cold Regions Engineering, Vol. 28, No. 4. ASCE.
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Daly, S.F. et al. (2023). "Prevention of Water Intake Blockage by Ice during Supercooling Events." Journal of Cold Regions Engineering, Vol. 37, No. 1. ASCE.
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UK Environment Agency. "Screening at Intakes and Outfalls: Measures to Protect Eel." https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1097095/Withdrawn-Screening-at-intakes-and-outfalls-measures-to-protect-eel.pdf
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U.S. EPA. (2014). "Final Regulations for Cooling Water Intake Structures at Existing Facilities." Federal Register, 79 FR 48300.
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Baumgartner, L. et al. (2025). "Protecting Larval Fish at Water Intakes." Biology Open, Vol. 14, No. 12. PMC12755069.
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Meister, J. et al. (2026). "Barriers for Fish Guidance: A Systematic Review." MDPI Water, Vol. 18, No. 2, 225.
Published by ADENCO: Advanced Engineering Coanda Intake Screens. With hundreds of projects delivered worldwide, ADENCO's engineering team reviews every specification to prevent these mistakes before manufacturing begins. Submit your specification for free engineering review.