1
Catchment & Site
2
Rainfall
3
Geometry
4
Check Dams & Features
5
Results

Step 1: Catchment Area & Contributing Surfaces

Project Information

Rational Method (Q = C × i × A): Define all impermeable and permeable areas draining to the swale. Apply runoff coefficients per CIRIA C753 Table 24.3 to calculate effective contributing area.
Typical Runoff Coefficients (C753 Table 24.3): Asphalt: 0.90  |  Concrete: 0.85  |  Block Paving (Imperm.): 0.85  |  Block Paving (Perm.): 0.60  |  Pitched roof: 0.95  |  Flat roof: 0.90  |  Gravel: 0.70  |  Grass: 0.25
Design Tips: Grass swales work best with upstream areas within 50–100m  |  Typical swale gradients 1–5%  |  Residential/commercial car parks commonly 50–200m² upstream area

1a: Contributing Surfaces

Add each surface type draining to the swale. Multiple surfaces can contribute (e.g., car park + roof areas).
Catchment Summary
Total Area: 0 m²
Weighted C: 0.00
Effective Area (C×A): 0 m²

1b: Ground Slope

Swale Gradient Guidance: Typical longitudinal slope is 1–5%. Steeper slopes (>5%) increase erosion risk and reduce residence time. Flatter slopes (<1%) risk sediment deposition. CIRIA C753 recommends ≥0.5% for self-cleansing.

Step 2: Design Rainfall

Design Storm: For swales, typical critical duration is 10–30 minutes depending on swale length and gradient. C753 recommends minimum 1 in 10 year for most SuDS, 1 in 30 year for highways/critical infrastructure.
Typical UK Rainfall Intensities (mm/hr):
10yr return period: 5 min ≈ 80–100 | 10 min ≈ 55–75 | 15 min ≈ 45–60 | 30 min ≈ 30–45
30yr return period: 5 min ≈ 100–130 | 10 min ≈ 70–95 | 15 min ≈ 55–75 | 30 min ≈ 40–55
Climate Change: EA May 2022 recommends +40% upper end for all new development. Regional variation significant, so use FEH Web Service for site-specific data.

2a: Rainfall Source

📍 UK overview map for site context

2b: Storm Parameters

Design Flow Calculation (Rational Method)
Q = C × i × A / 3600
where i includes climate change uplift
Design Intensity: 0 mm/hr
Peak Flow (Q): 0.000 l/s

Step 3: Swale Geometry & Cross-Section

Trapezoidal Channel Design (Manning's Equation): Define the swale cross-section (base width, side slopes) and longitudinal profile (gradient). The calculator solves for normal depth under design flow conditions using Manning's n for grass or vegetated surfaces.
Typical Swale Dimensions: Base width: 0.5–2.0m (common 1.0m)  |  Side slopes: 1:3–1:4 horizontal:vertical (typical 1:3)  |  Gradient: 1–5% (design: 2%)  |  Total depth: 0.3–0.6m  |  Manning's n (grass): 0.035–0.060 depending on vegetation density
Design Checks: Freeboard ≥ 0.1m above design water level  |  Velocity 0.3–1.0 m/s (non-erosive, self-cleansing)  |  Residence time > 9 min (water quality per C753)

3a: Cross-Section Geometry

3b: Manning's Roughness & Vegetation

Manning's n Guidance: Values range from 0.03 (short grass) to 0.15 (long meadow grass). Typical vegetated swales use 0.03–0.06. Lower values (0.025–0.035) apply only to gravel/stone surfaces.

3c: Design Water Level & Freeboard

Step 4: Check Dams, Infiltration & Downstream

Check Dams: Low structures placed in the swale to reduce gradient locally, improve water quality residence time, and reduce velocity. Optional feature. Infiltration: If the swale is on pervious soil, a proportion of the storm runoff may infiltrate through the base, reducing the required detention volume.

4a: Check Dams (Optional)

4b: Infiltration Through Base

4c: Downstream Connection

4d: Design Notes

Fields marked * are required

Key Assumptions

  • Trapezoidal cross-section with uniform gradient.
  • Manning's equation for steady-state open channel flow.
  • Minimum 0.5% gradient per CIRIA C753.
  • Velocities 0.3-1.0 m/s acceptable; lower velocities preferred for water quality.

Refer to the Help panel (?) for full standards references and glossary.

Step 5: Design Results & Compliance Checks

Design Summary: Below are the calculated hydraulic parameters for your swale under the design storm. Check all compliance boxes before proceeding to detail design.

5a: Calculated Parameters

Design Flow
0.00
l/s
Swale Length
0
m
Base Width
0.0
m
Side Slopes
1:3
H:V
Normal Depth
0.00
m
Flow Velocity
0.00
m/s
Top Width at Design
0.0
m
Freeboard
0.00
m

5b: Manning's Equation Calculation

Hydraulic Calculation Details
Equation: Q = (1/n) × A × R^(2/3) × S^(1/2)
Cross-section Area: A = (b + z×d) × d  |  -- m²
Wetted Perimeter: P = b + 2d√(1+z²)  |  -- m
Hydraulic Radius: R = A/P  |  -- m
Slope: S = -- (gradient in decimal)
Manning's n: -- (surface roughness)
Calculated Flow Capacity: -- l/s

5c: Performance Metrics

5d: Design Compliance Checks

Design Check Requirement Value / Status Result
Run calculation first

5e: Minimum Design Recommendations

Parameter Minimum / Maximum Your Design Compliant
Run calculation first

5f: Downstream Information

5g: Design Notes