Every Coanda screen design is a system of interdependent parameters. Change one variable (tilt angle, slot width, screen inclination, curvature radius) and it affects capacity, filtration quality, self-cleaning performance, and head loss simultaneously. There is no single "best" setting for any parameter; there is only the best combination for your specific site conditions.
The US Bureau of Reclamation (USBR) Coanda Design Guide, Design Guidance for Coanda-Effect Screens (Wahl, 2003, report R-2003-03) [1], remains the foundational reference for Coanda screen hydraulics. This guide builds on that work by turning the engineering science into practical design decisions: explaining the trade-offs behind each parameter from the perspective of a manufacturer that has designed and built these screens for hundreds of different sites.
Table of Contents
- The Dual Flow Mechanism
- Parameter 1: Wire Tilt Angle
- Parameter 2: Slot Width
- Parameter 3: Wire Width
- Parameter 4: Screen Inclination Angle
- Parameter 5: Acceleration Plate Profile
- Parameter 6: Screen Curvature (Arc Radius)
- Parameter 7: Drop Height (Available Head)
- The Dimensionless Numbers That Determine Performance
- Parameter Interaction Summary
- Design Process: From Site Data to Screen Geometry
- Frequently Asked Questions
- References
The Dual Flow Mechanism
Before examining individual parameters, you must understand the two flow mechanisms that distinguish a Coanda screen from a conventional planar (flat) screen [1][2]:
Orifice flow: Water passes through each slot opening, pushed through by the hydraulic head (depth of water) above the screen. This component behaves like flow through a submerged orifice: it is proportional to the slot width and the square root of the water depth.
Sheared flow: The tilted wire geometry creates a step (offset) at each slot. This offset physically separates a thin layer of water from the bottom of the sheet of water and directs it through the slot opening. The sheared flow component is proportional to the offset height and the velocity across the screen face.
The combined effect of these two mechanisms gives Coanda screens their remarkably high capacity for such narrow slot openings. Every design parameter influences the balance between orifice flow and sheared flow, and understanding that balance is the key to optimising screen performance.
Parameter 1: Wire Tilt Angle
Definition: The angle at which each individual wedge wire is tilted relative to the screen surface, with the leading edge projecting into the flow. This tilt creates the shearing offset at each slot.
Typical range: 3° to 7°, with 5° as the industry standard. Most manufacturers offer 3° to 6°; 7° is the practical upper limit, because above about 7° the flow separates from the wedge wires and the Coanda effect is lost [1][3][4].
How Tilt Angle Affects Performance
| Tilt Angle | Shearing Offset | Capacity | Filtration Quality | Self-Cleaning |
|---|---|---|---|---|
| 3° | Small | Lower | Better (finer effective screening) | Good |
| 5° | Moderate | Moderate–High | Good (standard) | Good |
| 6° | Large | High | Reduced (wider effective slot opening) | Best |
| 7° | Largest | Highest (upper limit) | Reduced | Best |
The trade-off: Increasing the tilt angle increases the shearing offset, which increases the sheared flow component and total capacity. However, a larger offset also means that particles slightly smaller than the nominal slot width may be pulled through by the shearing action. Reducing the tilt angle improves filtration precision but reduces capacity.
The effect is amplified at steep screen inclinations. At steep slopes (45°+), flow velocity across the screen face is high, and shearing flow dominates. In this regime, tilt angle has a pronounced effect on capacity. At shallow slopes (25°–30°), orifice flow dominates and tilt angle has less influence [1][2].
ADENCO Design Practice
We use 5° as our standard for most applications: it provides the best balance of capacity, filtration, and self-cleaning. We choose 3° when filtration precision is critical (pharmaceutical water, ultra-fine sediment exclusion) and accept the capacity reduction. We choose 6° or 7° when maximum capacity is the priority and filtration requirements are moderate (coarse pre-screening, high-flow diversions); 7° is the maximum we will manufacture, since beyond it the Coanda effect breaks down [1].
Manufacturing tolerance on wire tilt is ±0.25°, which is critical: at these small angles, a 0.5° error represents a 10% change in the shearing offset height.
Parameter 2: Slot Width
Definition: The slot opening between adjacent wedge wires (the gap between one wire and the next), measured from the trailing edge of one wire to the leading tip of the next. This is the primary filtration parameter: it determines the maximum particle size that passes through the screen.
Typical range: in the published literature the typical range is 0.2 mm to 2.0 mm [1][3], with 1.0 mm as the most common choice.
Slot Width Selection Guide
| Slot Width | Application | Fish Protection | Relative Capacity |
|---|---|---|---|
| 0.5 mm | Larval fish exclusion, ultra-fine sediment removal | Excludes all larval fish | Baseline (lowest) |
| 0.75 mm | Fine screening with eel protection | Meets UK estuarine eel regulations | +10–15% vs. 0.5 mm |
| 1.0 mm | Standard fish protection + general screening | Meets most regulatory requirements | +20–25% vs. 0.5 mm |
| 1.5 mm | General debris screening, hydropower | Excludes juvenile and adult fish | +35–40% vs. 0.5 mm |
| 2.0 mm | Coarse pre-screening, industrial | Excludes adult fish only | +50% vs. 0.5 mm |
The Capacity–Slot Width Relationship
A critical finding from the USBR research: screen capacity is relatively insensitive to slot width when other parameters are held constant [1][5]. Doubling the slot width from 1.0 mm to 2.0 mm does not double the capacity: at the moderate slopes most planning estimates assume, the increase is about 50%, and on a steep screen it is even smaller because shearing flow dominates there. This is because the orifice flow component increases with slot width, but the sheared flow component (which is determined by tilt angle and velocity, not slot width) remains unchanged.
This means that choosing a narrower slot for fish protection has a smaller capacity reduction than most engineers expect. It is one of the most important practical implications of the dual-flow mechanism.
Manufacturing Tolerance
Standard manufacturing tolerance on slot width is ±0.1 mm. For a 1.0 mm slot, this means the real slot opening ranges from 0.9 mm to 1.1 mm: precise enough to reliably exclude organisms of a specific size.
Parameter 3: Wire Width
Definition: The width of the flat top surface of each wedge wire, measured in the flow direction. Together with the slot width, wire width determines the screen's porosity (open area ratio).
Typical range: 1.5 mm to 3.5 mm, with 1.52 mm (0.060 inches) as the most common USBR test dimension [1].
How Wire Width Affects Performance
Wider wedge wires reduce porosity (the ratio of open area to total screen area), which reduces the orifice flow component. However, wider wires provide more structural strength and better resist abrasion from sediment-laden flows.
Key insight: The influence of wire width on capacity is more pronounced at shallow screen angles (where orifice flow dominates) and less significant at steep angles (where shearing flow dominates) [1][2]. For steeply inclined screens, wire width can be increased for structural benefit with minimal reduction in capacity.
The flow rate through the screen decreases as wire width increases, and this effect is more pronounced at smaller screen inclination angles [2].
ADENCO Design Practice
We select wire width based on the structural requirements of the site (span length, expected debris impact forces, sediment abrasion rate) and then verify the effect on capacity using hydraulic modelling. For most intake applications, 1.5–2.0 mm wire width provides adequate strength with acceptable capacity.
Parameter 4: Screen Inclination Angle
Definition: The overall angle of the screen panel relative to horizontal, measured on the downstream side of the weir. This is the macro-geometry of the screen: not the wire tilt angle (which is the micro-geometry of each individual wire).
Typical range: 25° to 60° from horizontal [1][3][4], within the 10° to 75° range covered by the USBR guidance. ADENCO's own screens are at the steep end of that range, typically 40° to 60°, where shearing flow keeps the screen surface free of debris.
How Inclination Affects Performance
| Inclination | Flow Velocity on Screen Face | Dominant Flow Mode | Capacity | Self-Cleaning | Head Loss |
|---|---|---|---|---|---|
| 25°–30° (shallow) | Lower | Orifice flow dominant | Higher | Moderate | Lower |
| 35°–45° (moderate) | Moderate | Balanced | Moderate | Good | Moderate |
| 45°–60° (steep) | Higher | Shearing flow dominant | Lower | Excellent | Higher |
The trade-off: Shallower screens have higher capacity because the orifice flow component is larger (more water depth above the screen per unit length). Steeper screens have lower capacity but better self-cleaning because the higher velocity on the screen face sweeps debris off more forcefully.
Design implication: At shallow angles, slot width and wire width become more important design variables (because orifice flow is sensitive to these parameters). At steep angles, tilt angle becomes the dominant variable (because shearing flow is sensitive to tilt but insensitive to slot and wire width) [1][2].
A concave curved screen naturally transitions from steep at the top to shallow at the bottom, combining the advantages of both regimes.
Parameter 5: Acceleration Plate Profile
Definition: The shaped transition surface between the weir crest and the upstream edge of the screen panel. The acceleration plate receives the water flowing over the weir and accelerates it into a thin, uniform sheet that is delivered tangent to the screen surface.
The Ideal Shape: Ogee Profile
The ideal acceleration plate shape is an ogee-shaped (S-curved) profile: the theoretical trajectory of a free-falling jet passing over a weir under gravity [1][3][7]. This shape is described by a power-law equation where the slope increases continuously in the downstream direction, matching the parabolic path water would follow if it were freely falling.
The ogee shape is optimal because:
- No flow separation: The water remains in contact with the plate surface throughout the transition. If the plate surface is flatter than the natural trajectory, water separates from the plate, creating an air gap and turbulence. If the surface is steeper, the water is artificially compressed, creating pressure buildup.
- Uniform velocity: The accelerating flow develops a uniform velocity distribution by the time it reaches the screen, ensuring consistent water distribution across the first wedge wire.
- Tangential delivery: The flow arrives at the screen surface at the correct angle, so that the Coanda effect starts immediately, rather than impacting the screen at an angle that would cause splashing and debris disturbance.
Why the Profile Must Be Site-Specific
The correct acceleration plate shape is different for each flow rate per metre of width [1]. A weir operating at 100 l/s per metre of width has a different free-jet trajectory than one operating at 200 l/s per metre. The USBR software [8] generates the specific acceleration plate shape for each design condition.
A generic acceleration plate is the single most common cause of underperforming Coanda screens. When the plate does not match the weir geometry and design flow, the water arrives at the screen surface with the wrong velocity, wrong angle, or with flow separation, and the screen operates far below its calculated capacity.
Alternative: Circular Arc
When the ogee shape is impractical (retrofit situations, very short transitions), a simple circular arc can substitute [1]. The radius is selected to approximate the ogee shape as closely as possible. Performance is slightly reduced compared to a true ogee shape, but far better than no acceleration plate or a poorly matched profile.
Parameter 6: Screen Curvature (Arc Radius)
Definition: The radius of curvature of the screen panel, typically configured as a concave arc (curving toward the collection chamber below).
Typical range: approximately 3.0 m (10 feet) for standard concave screens, the USBR reference dimension [3][4]. Zero radius = planar screen. Negative radius = convex screen (rare).
How Curvature Affects Performance
A concave screen provides a critical advantage: the screen inclination transitions from steep at the top to shallow at the bottom. This means:
- At the top: Steep slope → high velocity on the screen face → excellent debris rejection and self-cleaning → shearing flow dominant
- At the bottom: Shallow slope → deeper water layer → strong orifice flow → maximum water intake in the lower section where debris concentration is lowest
This natural transition optimises both self-cleaning (top) and capacity (bottom) within a single panel.
Increasing curvature (smaller radius) increases capacity but also increases the head loss across the screen [4]. The designer must balance the capacity gain with the head loss the site can accept.
Planar screens are used when the site geometry is restrictive: for example, when the screen must fit against the existing downstream side of the weir without projecting far from the weir face. Planar screens lack the inclination transition advantage and may accumulate debris in the upper portion of the panel [4].
Parameter 7: Drop Height (Available Head)
Definition: The vertical distance from the weir crest (water surface) to the bottom edge of the screen panel, representing the total hydraulic head available to push water through the screen.
Typical range: the USBR design guide gives 0.45 to 1.3 m as the typical range [1][9]; ADENCO's standard screens use 450, 700 and 1,270 mm, and custom screens can go higher.
How Drop Height Affects Performance
Greater drop height means:
- More screen length (more slots for water to pass through)
- Higher flow velocity across the screen face (better self-cleaning)
- Greater shearing flow component (higher capacity)
- More total head used up (less head available for downstream use)
Each additional length adds less: The first 500 mm of screen length takes in the majority of the flow. Each additional 100 mm takes in progressively less, because the sheet of water becomes thinner as flow is removed through upstream slots. Beyond approximately 1,300 mm of screen length, the USBR design guide shows that additional length adds very little capacity [1].
This is why the USBR design software is essential: it calculates the optimal screen length for a given set of conditions, identifying the point where additional length no longer justifies the additional head loss and material cost.
The Dimensionless Numbers That Determine Performance
Three dimensionless numbers describe the physics of Coanda screen flow [1][2][5]:
Froude Number (Fr)
The Froude number is the ratio of inertial forces to gravitational forces. It determines the flow velocity across the screen face. Higher Froude numbers correspond to faster, thinner sheets of water: more shearing, better self-cleaning, but less orifice flow.
Weber Number (We)
The Weber number is the ratio of inertial forces to surface tension forces. Surface tension affects the ability of water to pass through narrow slots: at very small slot widths or low temperatures (high surface tension), surface tension resists flow through the slots. Wahl's 2021 research demonstrated that screen capacity depends strongly on Weber number and is largely independent of Reynolds number and viscosity [5].
Practical implication: In cold water (high surface tension), Coanda screen capacity is lower than in warm water, even with the same incoming flow over the weir and the same head. This must be accounted for in cold climate designs. A Weber number above approximately 130 may trigger flow skipping across slots, reducing capacity [5].
Reynolds Number (Re)
The Reynolds number is the ratio of inertial forces to viscous forces. The 2021 research showed that the flow rate through a Coanda screen is independent of Reynolds number within the normal operating range [5]. This simplifies design calculations: viscosity and temperature affect performance only through their influence on surface tension (Weber number), not through viscous effects directly.
Parameter Interaction Summary
This table summarises how each parameter affects the key performance metrics. Use it to understand which parameters to prioritise for your design objectives.
| Parameter | Capacity | Filtration Quality | Self-Cleaning | Head Loss |
|---|---|---|---|---|
| Tilt angle ↑ | ↑ Increases | ↓ Decreases | ↑ Improves | Neutral |
| Slot width ↑ | ↑ Increases (modest) | ↓ Decreases | Neutral | Neutral |
| Wire width ↑ | ↓ Decreases | Neutral | Neutral | Neutral |
| Screen inclination ↑ (steeper) | ↓ Decreases | Neutral | ↑ Improves | ↑ Increases |
| Curvature ↑ (smaller radius) | ↑ Increases | Neutral | ↑ Improves (top) | ↑ Increases |
| Drop height ↑ | ↑ Increases (diminishing) | Neutral | ↑ Improves | ↑ Increases |
Design Process: From Site Data to Screen Geometry
Step 1: Define Requirements
Start with the non-negotiable requirements:
- Design flow (Q): peak instantaneous flow the screen must deliver
- Slot width: determined by fish protection regulations or particle exclusion needs
- Available head: measured elevation difference from weir crest to outlet
Step 2: Select Material and Wire Tilt
- Material grade: based on water chemistry (chloride, pH, temperature)
- Wire tilt angle: default 5°; adjust to 3° for fine filtration priority or 6° to 7° for maximum capacity
Step 3: Run Hydraulic Analysis
Using the USBR methodology [1] or equivalent design tools, calculate:
- Required screen width (weir length) for the design flow
- Optimal screen length for the available head
- Acceleration plate shape for the site-specific weir geometry
- Screen curvature radius
- Flow distribution across the screen (verify uniform performance)
Step 4: Verify Cold Climate Performance
If water temperature may drop below 4°C, recalculate capacity using Weber number corrections for surface tension effects [5]. A cold-climate site uses the safety factor for critical applications (1.5 × design flow instead of the standard 1.3 ×) and, depending on how far the air temperature falls below 0°C, anti-icing measures.
Step 5: Generate Engineering Documentation
Produce the complete design package: general arrangement drawings, hydraulic performance curve (flow vs. head), material specification, acceleration plate coordinates, and regulatory compliance data.
ADENCO performs this complete design process for every custom screen order. Request a design consultation with our engineering team.
Frequently Asked Questions
What tilt angle should a Coanda screen have?
The standard wire tilt angle is 5 degrees, which provides the best balance of capacity, filtration quality, and self-cleaning for most applications. Use 3° when filtration precision is the priority (accepting reduced capacity), and 6° to 7° when maximum flow capacity is the priority (accepting slightly coarser effective screening); 7° is the practical upper limit before the Coanda effect is lost. The effect of tilt angle is more pronounced at steep screen inclinations where shearing flow dominates.
What slot width is best for a Coanda screen?
The best slot width depends on what you need to exclude. For the fish protection level most regulations require, choose 1.0 mm; where larval fish must be excluded, choose 0.5 mm. For general screening without fish requirements, 1.0–1.5 mm is standard. For coarse pre-screening, 2.0 mm. An important finding from USBR research is that capacity is relatively insensitive to slot width: at moderate slopes, reducing from 1.5 mm to 1.0 mm costs only about 10–15% of capacity rather than the 33% that a simple proportional calculation would suggest, and on a steep screen the difference is even smaller.
What is the acceleration plate on a Coanda screen?
The acceleration plate is the curved transition surface between the weir crest and the screen panel. Its purpose is to smoothly accelerate the water and deliver it tangent to the screen surface. The ideal shape is an ogee (S-curve): the natural parabolic trajectory of a free-falling jet. The correct acceleration plate shape is specific to each site's weir geometry and design flow, which is why it must be designed for each project.
What screen inclination angle should I use?
Typical inclination angles range from 25° to 60° from horizontal. Shallower angles (25°–35°) provide higher capacity because orifice flow dominates. Steeper angles (45°–60°) provide better self-cleaning. Most designs use a concave curved panel that starts steep at the top (good debris rejection) and transitions to shallow at the bottom (maximum water intake). The USBR design software calculates the optimal inclination for your specific conditions.
Does water temperature affect Coanda screen performance?
Yes. Cold water has higher surface tension, which resists flow through narrow slot openings. Wahl's 2021 research showed that capacity depends on Weber number (the ratio of inertial to surface tension forces) and is independent of Reynolds number. In practical terms, a Coanda screen in 2°C water delivers less flow than the same screen in 20°C water. Cold climate designs must account for this with safety margins or larger screen dimensions.
Where can I download the USBR Coanda screen design software?
The USBR Coanda screen design software is available for free download from the Bureau of Reclamation Technical Service Center at https://usbr.gov/tsc/techreferences/computer%20software/software/coanda/. It runs on Windows and calculates screen capacity, generates acceleration plate shapes, and produces flow-versus-head curves for custom screen designs.
References
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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. https://www.usbr.gov/tsc/techreferences/rec/R-2003-03.pdf
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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. DOI: 10.1061/(ASCE)0733-9429(2001)127:6(480)
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Filson Filter. "Coanda Screen: Manufacturer Specifications." Retrieved April 2026, from https://www.filsonfilter.com/coanda-screen/
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Gap Technology Ltd. "Coanda Screens: Water Treatment Separation Technology." Retrieved April 2026, from https://www.gaptechnology.co.uk/wedge-wire-industry-applications/coanda-screens/
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Wahl, T.L., Shupe, C.C., Dzafo, H., & Dzaferovic, E. (2021). "Surface Tension Effects on Discharge Capacity of Coanda-Effect Screens." Journal of Hydraulic Engineering, Vol. 147, No. 8. ASCE. DOI: 10.1061/(ASCE)HY.1943-7900.0001902
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Wahl, T.L. et al. (2000). "Laboratory Testing and Numerical Modeling of Coanda-Effect Screens." ASCE Proceedings. DOI: 10.1061/40517(2000)72
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"An Experimentally Validated CFD Code to Design Coanda Effect Screen Structures." MDPI Applied Sciences, Vol. 13, No. 9, 5762 (2023). DOI: 10.3390/app13095762
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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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"The Coanda Effect." International Water Power & Dam Construction. Retrieved April 2026, from https://www.waterpowermagazine.com/analysis/the-coanda-effect/
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Wahl, T.L. "New Testing of Coanda-Effect Screen Capacities." USBR PAP-1097. Retrieved April 2026, from https://www.usbr.gov/tsc/techreferences/hydraulics_lab/pubs/PAP/PAP-1097.pdf
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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.
Published by ADENCO: Advanced Engineering Coanda Intake Screens. ADENCO's engineering team applies the USBR design methodology with refinements confirmed at operating sites to optimise every parameter for your specific site conditions. Request a design consultation to discuss your project requirements.