How to Size a Coanda Intake Screen: Flow Rate Calculations

The most common question ADENCO receives from engineers is straightforward: "I need X litres per second. How big does my Coanda screen need to be?"

The answer requires more than a simple formula. Coanda screen capacity depends on the interaction of weir width, available head, slot width, wire tilt angle, screen inclination, and water temperature. However, a reliable preliminary sizing estimate can be made with one number and one measurement, and then refined through detailed hydraulic analysis.

This guide explains the complete sizing process step by step, from initial estimate to final multi-panel array design, with worked examples at every stage.

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Table of Contents

1. The Fundamental Sizing Relationship
2. Step 1: Preliminary Sizing Estimate
3. Step 2: Adjustments for Design Parameters
4. Step 3: Head Verification
5. Step 4: Multi-Panel Array Design
6. Worked Examples
7. Safety Factors and Design Margins
8. ADENCO Sizing Reference Table
9. Frequently Asked Questions
10. References

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The Fundamental Sizing Relationship

A Coanda screen delivers approximately 140 litres per second per metre of weir width (140 l/s/m) on the US Bureau of Reclamation (USBR) reference geometry under typical operating conditions: 1.0 mm slot width, 5° wire tilt, concave screen with an arc radius of about 3 m, and 450–1,300 mm of available head, the typical range given in the USBR design guide [1][2] (ADENCO's standard screens use drop heights of 450, 700 and 1,270 mm; custom screens can go higher). This is the generic industry baseline used throughout this guide for preliminary planning, not a manufacturer's rated capacity: see the note below on ADENCO's own model capacities.

This yields the fundamental sizing equation:

Required weir width (m) = Design flow (l/s) ÷ 140

This is a preliminary estimate for planning purposes. The real capacity depends on the specific combination of design parameters: which is why the USBR developed a computer model to calculate precise hydraulic performance for any configuration [3][4].

Note on ADENCO's catalogue capacities: 140 l/s/m is a generic industry baseline for the USBR reference geometry, and this guide uses it only for preliminary planning. ADENCO's own series have rated capacities of 35 l/s (ADENCO-45), 67 l/s (ADENCO-70) and 150 l/s (ADENCO-127) per metre of screen width; the rated capacity rises with the screen's drop height. Once you move from a planning estimate to a real ADENCO configuration, size the screen using the model's own rated capacity on the products page or with the design tool, not the generic baseline.

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Step 1: Preliminary Sizing Estimate

Start with your peak design flow: the maximum instantaneous flow the screen must deliver, not the average.

Design Flow	Required Weir Width (First Estimate)
25 l/s	0.18 m
50 l/s	0.36 m
100 l/s	0.71 m
200 l/s	1.43 m
350 l/s	2.50 m
500 l/s	3.57 m
1,000 l/s (1 m³/s)	7.14 m
2,000 l/s (2 m³/s)	14.29 m

Use peak flow, not average. An irrigation system averaging 80 l/s may peak at 250 l/s during midsummer. A hydropower turbine has a set design flow. A snowmaking system operates at full capacity or not at all. Always size for the maximum.

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Step 2: Adjustments for Design Parameters

The 140 l/s/m baseline assumes standard parameters. If your design departs from standard, adjust the estimate accordingly.

Slot Width Adjustment

Screen capacity is relatively insensitive to slot width [3][5], but narrower slots do reduce flow. The figures below apply to the moderate screen slopes this planning method assumes; on a steep screen (roughly 50° and above) shearing flow dominates and slot width has almost no effect on capacity [3]:

Slot Width	Capacity Adjustment
2.0 mm	+15% (≈161 l/s/m)
1.5 mm	+5–8% (≈147–151 l/s/m)
1.0 mm	Baseline (140 l/s/m)
0.75 mm	−5–8% (≈129–133 l/s/m)
0.5 mm	−10–15% (≈119–126 l/s/m)

Wire Tilt Angle Adjustment

Tilt Angle	Capacity Adjustment
6°	+8–12%
5°	Baseline
3°	−10–15%

The tilt angle effect is more pronounced at steep screen inclinations where shearing flow dominates [3][5].

Available Head Adjustment

The 140 l/s/m figure assumes adequate available head (0.45 to 1.3 m, the typical range given in the USBR design guide). If the available head is limited:

Available Head	Capacity Effect
>800 mm	Full capacity (140 l/s/m baseline)
500–800 mm	80–100% of baseline
450–500 mm	50–80% of baseline (borderline; requires engineering review)
<450 mm	Insufficient: below the smallest screen's 450 mm drop height

Cold Water Adjustment

Surface tension increases in cold water, reducing flow through narrow slots [5]. For water temperatures approaching 0°C, expect a capacity reduction of about 10–20%, with narrower slots affected most. The correction follows the surface-tension work in Wahl et al. (2021) rather than a set catalogue figure, so the number for your slot width and temperature is confirmed during ADENCO's engineering review.

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Step 3: Head Verification

After determining the required weir width, verify that your site has sufficient available head.

Measure: The vertical distance from the normal water surface at the weir crest to the centreline of the outlet pipe connection below the screen.

Minimum required head: 500 mm for functional Coanda operation [1][2].

Recommended head: 600–1,000 mm for standard applications. This range provides good capacity while keeping the head used by the screen reasonable.

Head allowance for hydropower: At a hydropower site, every millimetre of hydraulic head used up by the screen is head not available for power generation. For a site with 50 m of head, 1,000 mm of screen head loss represents 2%, which is usually acceptable. For a site with 3 m of head, the same 1,000 mm represents 33%, which is likely unacceptable. Calculate the percentage of the total available head used up by the screen and verify it is within your project's tolerance [2].

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Step 4: Multi-Panel Array Design

When the required weir width exceeds what a single screen panel can accommodate (typically >2 m), the design transitions to a multi-panel array: multiple screen panels mounted side by side across the weir, sharing a common collection chamber below.

Array Configuration Principles

1. Equal panel widths for uniform flow. All panels in an array should be the same width so that each panel receives the same flow per metre of width and operates at the same hydraulic conditions. Unequal panels create uneven flow distribution and reduce overall efficiency.

2. Shared collection chamber. The collection chamber extends along the full width of the array beneath all panels, collecting screened water and directing it to a single outlet pipe or manifold.

3. Continuous acceleration plate. The acceleration plate spans the full weir width: it is a single hydraulic surface, not segmented by panel breaks. Panel-to-panel joints must be level with each other, with no step, to avoid flow disruption.

4. Structural support at panel joints. Each panel-to-panel joint requires a structural support bar. These bars should have a streamlined cross-section (rounded leading edge) to minimise flow disruption and avoid debris accumulation.

5. Bypass channel. Design the bypass channel to carry the full incoming flow minus the screened flow. During high-debris events, a larger fraction of flow bypasses the screen; the bypass channel must accommodate this without overtopping.

Maximum Panel Width

Standard panel manufacturing width is up to 2.0 metres [6]. Wider panels are possible but require heavier support structures and become difficult to transport and install. For weir widths above 2 m, multi-panel arrays are the standard approach.

Array Sizing Formula

Number of panels = Required weir width ÷ Panel width

Round up to the next whole number. Then recalculate the real capacity per panel to verify the array meets the design flow.

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Worked Examples

Example 1: Small Hydropower Intake

Requirements:

• Design flow: 150 l/s
• Available head: 800 mm
• Slot width: 1.0 mm (fish protection required)
• Water: Clean freshwater, temperature range 4–18°C

Sizing:

1. First estimate: 150 ÷ 140 = 1.07 m weir width
2. Slot width adjustment: 1.0 mm = baseline (no adjustment)
3. Head verification: 800 mm (adequate)
4. Cold water: minimum 4°C, so surface tension has little effect; confirmed during ADENCO's engineering review
5. Safety factor: fish protection applies, so the critical factor is used: 1.07 × 1.5 = 1.61 m
6. Array: Single panel at 1.6 m width

Result: 1 × ADENCO custom panel, 1.6 m wide, 1.0 mm slots, 304 stainless steel. Estimated capacity at 800 mm head: approximately 225 l/s, compared with the 150 l/s design flow.

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Example 2: Municipal Water Diversion

Requirements:

• Design flow: 350 l/s
• Available head: 1,000 mm
• Slot width: 1.0 mm
• Water: River water, chloride 350 ppm, temperature 2–22°C

Sizing:

1. First estimate: 350 ÷ 140 = 2.50 m weir width
2. Slot width adjustment: 1.0 mm = baseline
3. Head verification: 1,000 mm (good)
4. Cold water: minimum 2°C, so capacity is checked for the cold-water condition during ADENCO's engineering review
5. Safety factor: the only municipal water supply, in cold water, so the critical factor is used: 2.50 × 1.5 = 3.75 m
6. Array: 3 panels × 1.25 m each = 3.75 m total
7. Material: 316L required (chloride 350 ppm > 200 ppm threshold)

Result: 3 × ADENCO custom panels, each 1.25 m wide, 1.0 mm slots, 316L stainless steel, anti-icing recommended. Estimated capacity at 1,000 mm head: approximately 525 l/s, compared with the 350 l/s design flow.

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Example 3: Large Snowmaking System

Requirements:

• Design flow: 500 l/s (30+ snow guns operating simultaneously)
• Available head: 1,200 mm (mountain stream with steep gradient)
• Slot width: 1.0 mm (nozzle protection + fish protection)
• Water: Mountain stream, clean freshwater, temperature 0.5–12°C
• Anti-icing: Required (snowmaking operates exclusively in freezing conditions)

Sizing:

1. First estimate: 500 ÷ 140 = 3.57 m weir width
2. Slot width adjustment: 1.0 mm = baseline
3. Head verification: 1,200 mm (excellent)
4. Cold water: the system operates at 0.5°C, the case the cold-water check exists for
5. Safety factor: cold climate and fish protection, so the critical factor is used: 3.57 × 1.5 = 5.36 m
6. Array: 4 panels × 1.35 m each = 5.40 m total
7. Anti-icing: Integrated screen heating for continuous operation in freezing conditions

Result: 4 × ADENCO custom panels, each 1.35 m wide, 1.0 mm slots, 304 stainless steel with integrated anti-icing. Estimated capacity at 1,200 mm head: approximately 755 l/s, compared with the 500 l/s design flow.

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Example 4: Irrigation Canal Diversion

Requirements:

• Design flow: 80 l/s (peak midsummer)
• Available head: 600 mm
• Slot width: 1.5 mm (no fish protection requirement; sediment exclusion for drip emitters)
• Water: Canal, clean freshwater, temperature 8–25°C

Sizing:

1. First estimate: 80 ÷ 140 = 0.57 m weir width
2. Slot width adjustment: 1.5 mm → +7% → 0.57 ÷ 1.07 = 0.53 m
3. Head verification: 600 mm (adequate)
4. Cold water: minimum 8°C, no cold-water correction needed
5. Safety factor: seasonal irrigation, not the only water supply, so the standard factor is used: 0.53 × 1.3 = 0.69 m

Result: 1 × ADENCO custom panel, 0.7 m wide, 1.5 mm slots, 304 stainless steel. Estimated capacity at 600 mm head: approximately 105 l/s, compared with the 80 l/s design flow.

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Safety Factors and Design Margins

The safety margin is the project engineer's decision. On average a margin of about 30% (a factor of 1.3 on the design flow) is applied; some engineers apply none, because the capacity calculation already includes a margin. Where a margin is applied, these are the typical values:

Application	Typical safety factor
Standard: clean freshwater, temperate climate, non-critical supply	1.3 × design flow
Critical: the only water supply, fish-protection compliance, cold climate	1.5 × design flow

Cold water, heavy sediment loading and narrow slots (≤0.5 mm) do not each add a separate multiplier. Any one of them moves the project from the standard factor (1.3) to the critical factor (1.5).

Example: A 200 l/s municipal intake is the only water supply and draws cold water, so it is sized at the critical factor: 200 × 1.5 = 300 l/s capacity.

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ADENCO Sizing Reference Table

This reference table provides planning-level sizing for common application scenarios. All values assume 1.0 mm slots, 5° tilt, concave screen, and adequate head (≥600 mm), with a small planning allowance included; the safety margin itself is the project engineer's decision (see Safety Factors and Design Margins).

Application	Design Flow	Weir Width	Panels	Material	Anti-Icing
Farm diversion	10–30 l/s	0.1–0.25 m	1	304	No
Small hydropower (<50 kW)	30–80 l/s	0.25–0.65 m	1	304	Optional
Irrigation canal	50–200 l/s	0.4–1.6 m	1	304	No
Medium hydropower (50–500 kW)	80–400 l/s	0.65–3.2 m	1–2	304/316L	Recommended
Snowmaking (small resort)	50–150 l/s	0.4–1.2 m	1	304	Required
Municipal water diversion	200–1,000 l/s	1.6–8.0 m	1–4	316L	Recommended
Snowmaking (large resort)	300–2,000 l/s	2.4–16.0 m	2–8	304	Required
Large hydropower (>500 kW)	500–5,000 l/s	4.0–40.0 m	2–20	304/316L	Recommended

This table is for preliminary planning only. Sizing for a real project requires site-specific hydraulic analysis accounting for available head, water temperature, slot width, and regulatory requirements. Contact ADENCO for project-specific sizing.

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Frequently Asked Questions

How do you calculate Coanda screen capacity?

The baseline capacity is approximately 140 litres per second per metre of weir width for a standard configuration (1.0 mm slots, 5° tilt, concave screen, adequate head): a generic industry baseline for the USBR reference geometry, distinct from ADENCO's model capacities of 35, 67 and 150 l/s per metre for the ADENCO-45, ADENCO-70 and ADENCO-127. Required weir width = design flow ÷ 140. This first estimate is then adjusted for slot width, tilt angle, available head, and water temperature. The USBR Coanda screen design software provides precise hydraulic calculations for custom configurations.

How many Coanda screen panels do I need?

Divide the required weir width by the maximum panel width (typically 2.0 m). Round up to the next whole number. For example, a 350 l/s project requires approximately 2.9 m of weir width: that is 2 panels of 1.45 m each. All panels should be equal width for uniform hydraulic performance.

Can I size a Coanda screen by combining several small panels side by side?

Combining several pre-built, fixed-size panels side by side, of the kind sold off the shelf for the do-it-yourself (DIY) micro-hydro market, is not recommended for professional applications. Multi-panel arrays require integrated hydraulic design: a continuous acceleration plate, shared collection chamber, structural support at panel joints, and a properly sized bypass channel. These elements must be designed as one system, not assembled from separate products.

Does slot width significantly affect the size of screen I need?

Less than most engineers expect. USBR research shows that capacity is relatively insensitive to slot width. Reducing slots from 1.5 mm to 1.0 mm reduces capacity by approximately 10–15%, not 33%. This means choosing narrower slots for fish protection adds only a modest amount to the required screen width.

What happens if I undersize my Coanda screen?

An undersized screen delivers less than the design flow during peak demand. The screen itself operates normally: it simply takes in less water. Excess water bypasses the screen and returns to the watercourse. There is no mechanical failure, but you get less water than your project requires. The solution is adding panel width, which usually requires civil works modifications to the weir and collection chamber. That is far more expensive than sizing correctly from the start.

Can ADENCO help me size a Coanda screen for my project?

Yes. Provide your design flow, available head, details of the raw water, and any regulatory requirements. ADENCO's engineering team will perform the complete hydraulic analysis and deliver a sizing recommendation within 1–2 business days. Request a sizing consultation.

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References

1. "The Coanda Effect." International Water Power & Dam Construction. Retrieved April 2026, from https://www.waterpowermagazine.com/analysis/the-coanda-effect/

2. Wahl, T.L. (2003). Design Guidance for Coanda-Effect Screens. U.S. Bureau of Reclamation, Research Report R-2003-03. Denver, CO.

3. Wahl, T.L. (2001). "Hydraulic Performance of Coanda-Effect Screens." Journal of Hydraulic Engineering, Vol. 127, No. 6, pp. 480–488. ASCE.

4. USBR. "Coanda-Effect Screens Software & Design Tools." Retrieved April 2026, from https://usbr.gov/tsc/techreferences/computer%20software/software/coanda/

5. 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.

6. Filson Filter. "Coanda Screen: Manufacturer Specifications." Retrieved April 2026, from https://www.filsonfilter.com/coanda-screen/

7. "Developing Predictive Equations for Water Capturing Performance and Sediment Release Efficiency for Coanda Intakes Using Artificial Intelligence Methods." MDPI Water, Vol. 14, No. 6, 972 (2022). DOI: 10.3390/w14060972

8. 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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Published by ADENCO: Advanced Engineering Coanda Intake Screens. ADENCO provides complete Coanda screen sizing, hydraulic analysis, and multi-panel array design for projects of all sizes. Request a sizing consultation: we deliver detailed recommendations within 1–2 business days.