CIRIA C753: pre-treatment forebay sizing for sediment removal upstream of SuDS components.
1
Catchment & Inflow
2
Forebay Design
3
Results & Compliance
Project Information
Catchment & Inflow Calculation
?
Required to calculate peak inflow rate
hectares (ha)
percent (%)
mm/hr
percent (%)
Peak Inflow Rate:
-- l/s (not calculated yet)
Calculated using: Q = C × i × A / 360 (Rational Method)
Annual Sediment Loading:
-- kg/yr (not calculated yet)
Forebay Type & Strategy
Forebay Sizing Method
cubic meters (m³)
minutes
Forebay Dimensions
meters (m)
meters (m)
meters (m)
meters (m)
Length-to-Width Ratio Check:
-- (minimum 2:1 recommended)
Forebay Sizing Summary
Forebay Volume
--
cubic meters (m³)
% of Pool Volume
--
percent (%)
✓ CIRIA Compliant
Residence Time at Peak Flow
--
seconds
Length-to-Width Ratio
--
:1 (min 2:1)
✓ Adequate
Annual Sediment Accumulation
--
cubic meters per year (m³/yr)
Maintenance Interval
--
years until 50% full
Particle Capture Analysis
Indicative: Stokes' law settling velocities for clean spherical particles (ρp = 2.65 g/cm³) in still water at 20 °C (νw ≈ 1.0 × 10⁻⁶ m²/s). Real-world rates are reduced by flocculation, turbulence and short-circuiting; treat as upper bounds.
Particle Type
Size
Settling Velocity (Stokes)
Captured
Coarse sand
1000 μm
~150 mm/s
✓ Yes
Medium sand
250 μm
~31 mm/s
✓ Yes
Fine sand
100 μm
~6.0 mm/s
✓ Yes
Very fine sand
50 μm
~1.6 mm/s
✓ Yes
Coarse silt
20 μm
~0.27 mm/s
✗ Limited
Medium silt
6 μm
~0.024 mm/s
✗ Not captured
Settling Velocity Analysis:
Compliance & Standards
Standard/Requirement
Criterion
Your Result
Status
CIRIA C753
10-20% of pool volume
--
✓ Pass
CIRIA C753
Length-to-width ratio ≥2:1
--
✓ Pass
Defra 2025 S4
Sediment removal efficiency
--
✓ Pass
Recommendations
Design Optimisation
Recommendations will be generated based on your inputs