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Storage & TreatmentCIRIA C753 §23

Retention Pond Calculator

CIRIA C753 §23: wet pond sizing for treatment, attenuation and biodiversity.

1
Site & Design
2
Calculate
3
Compliance
4
PDF Export
Step 1 of 4: Site & Design Parameters
Project Information

Fields marked * are required

Catchment Parameters
from site plans
m²
including soft landscape
ha
5-15mm typical, check local guidance
mm
water balance check
mm
EA May 2022 Climate Change Allowances Table 3 (surface-water peak rainfall intensity). These apply to peak-flow / attenuation storage for extreme events — they are carried by the attenuation volume you enter from Calculator 2 ("includes all return periods"). They are not applied to the water-quality treatment depth above, which sizes the permanent pool (a first-flush treatment volume per CIRIA C753 and so not climate-uplifted).
default 450mm for England
mm
if applicable, else 0
l/s
Design Requirements
These values come from Calculator 1 (Discharge Control) and Calculator 2 (Volume Estimation).
from Calculator 2, includes all return periods
m³
from Calculator 1
l/s
Pond Geometry
typical 1-2m
m
shallow margin, ≤600mm
m
2:1 to 5:1
landscape design
bank stability
shallow planting zone
m
marginal vegetation zone
mm
above maximum water level
mm
Z = horizontal per 1m vertical
affects stability
sediment settling zone
Inlet & Outlet Design
1% AEP (100yr) overflow route
restricted discharge rate
Step 2 of 4: Calculate & Review
Treatment Volume Calculation
CIRIA C753 Treatment Volume
Vt = (A_imp × d_rainfall) / 1000
Where:
• A_imp = Impervious area contributing to treatment (m²)
• d_rainfall = Design rainfall depth (mm)
• Vt = Treatment volume (m³)
Permanent Pool Volume
Pool Volume Based on Facility Type
V_pool = multiplier × Vt
• Retention Pond: multiplier = 4 × (CIRIA guidance for treatment & amenity)
• Constructed Wetland: multiplier = 3 × (shallower, greater surface area)
Surface Area & Dimensions
Required Surface Area for Permanent Pool
A_surface = V_pool / d_avg
Where:
• d_avg = Average depth ≈ 2/3 × max depth (reasonable approximation for trapezoidal ponds with shallow safety benching at 1:3 side slopes per CIRIA C753 §23.5; shallower ponds with steeper sides approach 1/2 × max depth; adjust if your geometry differs significantly)
Pond Dimensions from L:W Ratio
W = √(A_surface / ratio)
L = W × ratio
Residence Time
Hydraulic Residence Time for Treatment
t_res = (V_pool × 1000) / (Q_inflow × 3600)
Where:
• Q_inflow = Average inflow (l/s) = (rainfall × area) / seasonal factor
• t_res = Residence time (hours)
• Minimum acceptable: 24-72 hours for pollutant settling
Water Balance (Annual Average)
Annual Water Inflow & Outflow
Inflow (m³/yr) = (Annual rainfall × area / 1000) + (DWF × 86400 × 365 / 1000)
Outflow (m³/yr) = (Evaporation × pond_area / 1000) + seepage + discharge
Summary of Key Volumes
Treatment Volume
—
m³
Permanent Pool
—
m³
Attenuation Storage
—
m³
Total Pond Volume
—
m³
Surface Area
—
m²
Residence Time
—
hours
Minimum 24h residence time recommended per CIRIA C753
Step 3 of 4: Design Summary & Compliance
Design Summary
Permanent Pool Volume
—
m³
Treatment Volume
—
m³
Attenuation Volume
—
m³
Total Volume
—
m³
Surface Area
—
m²
Dimensions
—
L × W (m)
Max Depth
—
m
Freeboard
—
mm
Residence Time
—
hours
Forebay Volume
—
m³
Design Cross-Section
Permanent Pool Water
Aquatic Bench Zone
Bank & Slope
Vegetation/Landscape
Compliance with Standards
Standard Requirement Design Value Status
Safety Design Notes
Amenity Design (S5)
  • Accessible pathways and viewing points (min 1.5m wide)
  • Landscaping integrated with site masterplan
  • Wildlife habitat enhancements (trees, shrubs)
  • Seating and rest areas (if space permits)
  • Interpretive signage planned
Biodiversity Design (S6)
  • Aquatic bench (2-3m wide, shallow margin)
  • Varied depth profile (shallow to deep zones)
  • Native marginal plants specified (reed-bed, willow)
  • Native submerged plants specified (pondweed)
  • Ramp or shallow entry for amphibians
  • Dead wood & brash piles for invertebrates
  • No pesticides or herbicides in maintenance
Maintenance & Access (S7)
  • Minimum 3m maintenance track around pond
  • Access for sediment removal equipment
  • Routine inspection schedule (6-12 monthly)
  • Annual vegetation management plan
  • Forebay sediment removal every 2-5 years
  • Main pool dredging every 10-20 years
Step 4 of 4: Export Report
Generate Professional Report
Export your design as a professional PDF report suitable for planning submissions, client review, or project documentation.
  • Project information & parameters
  • Design calculations with formulas
  • Volume summary & surface area
  • Compliance table (Standards S3-S7)
  • Design cross-section sketch
  • Water balance analysis
  • Safety design notes
Design Data