Detention Basin Design Calculator

Step 1: Catchment & Site Data

Detention Basin Design (C753 §22): Define contributing surfaces and greenfield runoff rate. A detention basin stores peak flows from the design rainfall event and releases them at a restricted discharge rate.

Design Approach: For basins, greenfield (undeveloped) discharge rate limits the outflow, typically 4–7 l/s/ha. The basin must store the volume difference between inflow (from the design rainfall) and restricted outflow over the critical duration. Critical duration is not known in advance. The calculator tests multiple storm durations to find the one requiring maximum storage.

Project Information

Project Name

Site Reference

Engineer Name

Date

Project Reference

Basin ID / Location

1a: Contributing Surfaces

Add each surface type draining to the basin. Apply appropriate runoff coefficients per CIRIA C753 Table 24.3.

Surface Type

Area (m²) *

Runoff Coeff (C) *

Catchment Summary

Total Area: 0 m²

Weighted C: 0.00

Effective Area (C×A): 0 m²

1b: Site Area & Greenfield Runoff Rate

Fields marked * are required

Total Site Area (ha) *

Greenfield Rate Method *

Greenfield Runoff Rate (l/s/ha) *

Greenfield Discharge (Qbar)

Qbar = Greenfield Rate × Site Area = 0.0 l/s

Step 2: Design Rainfall

Design Storm for Basins: Basins must accommodate the peak inflow from the design storm and discharge it at the restricted greenfield rate. Testing multiple storm durations identifies the critical (longest) duration that requires the most storage volume. Default: 100-year return period (1% AEP) with the EA May 2022 allowance for the selected epoch/band (2070s upper = +45% national reference).

Typical Durations Tested: 15, 30, 45, 60, 90, 120, 180, 240, 360, 480, 720, 1440 minutes. Basin storage often peaks at 2–6 hour durations.
Climate Change: for most new permanent development use the 2070s upper-end allowance (1% AEP national reference +45%; confirm your management catchment on the EA map).

2a: Rainfall Source

Rainfall Entry Method *

Site Postcode

📍 UK overview map for site context

2b: Storm Parameters

Design Return Period *

Climate Change Epoch * Climate Change Band * Climate Change Allowance (%) *

Source: EA May 2022 Climate Change Allowances (peak rainfall intensity, surface water; national-reference medians across the 90 English management catchments — confirm your catchment on the EA map). The epoch + band selects pull the value via SV.getEACC(epoch, T, band) for the chosen return period. 3.3 % AEP (30-yr): 2050s central 20 % / upper 35 %; 2070s central 25 % / upper 40 %. 1 % AEP (100-yr): 2050s central 20 % / upper 40 %; 2070s central 25 % / upper 45 %.

2c: Rainfall Intensities by Duration

Enter rainfall intensity (mm/hr) for each critical duration. If using postcode lookup, values are auto-populated from FSR estimates. You can override them with FEH Web Service data.

Duration (min)	Intensity (mm/hr)	Depth (mm)

Step 3: Basin Geometry & Design

Trapezoidal Basin: Define base dimensions, side slopes (1:Z), and maximum design depth. The basin is excavated to design depth + freeboard (minimum 0.3m for safety). CIRIA C753 recommends max side slope 1:3 for maintenance access.

3a: Basin Footprint

Basin Length at Base (m) *

Basin Width at Base (m) *

Base Area (m²) *

3b: Basin Shape & Depth

Internal Side Slopes (1:Z) *

Maximum Design Depth (m) *

Freeboard Above Design Level (m) *

Settlement Allowance (m)

Embankment Soil Type

Basin Dimensions

Total Excavation Depth: 0.0 m

Basin Footprint (at top): 0.0 × 0.0 m

3c: Landscape Treatment

Landscape Finish

3d: Stage-Storage Table

Storage volume at each depth interval, calculated using prismoidal formula: V = (d/6) × (A_base + A_top + 4×A_mid)

Depth (m)	Surface Area (m²)	Storage Volume (m³)	Cumulative (m³)

Step 4: Outlet Control

Outlet Control: The basin discharge is restricted to the greenfield runoff rate (Qbar) to maintain post-development runoff equal to or less than pre-development runoff. Choose outlet device type and calculate required orifice/weir dimensions.

4a: Restricted Discharge

Restricted Discharge Rate (l/s) *

Outlet Device Type

Orifice Sizing

Sharp-edged orifice: Q = Cd × A × √(2gh) where Cd ≈ 0.61, A = πd²/4, h = design depth. Minimum diameter 75mm (below this, blockage risk). If calculated <75mm, a vortex flow control is recommended.

Required Orifice Diameter

Calculated Diameter: -- mm

Recommended: -- mm

4b: High-Level Overflow

For exceedance events beyond the design rainfall (e.g., 100yr + climate change + rainfall loss recovery). Typically a weir or pipe located at or above design water level.

Overflow Crest Level Above Design Water (m) *

Overflow Weir Width (m) *

4c: Emergency Spillway

For extreme events exceeding design capacity. Typically an unlined or lined channel at basin perimeter. Width typically 3–5m minimum to allow safe overflow.

Spillway Width (m) *

4d: Downstream Destination

Downstream Connection

Step 5: Design Summary & Results

Required Storage

--

m³

Critical Duration

--

minutes

Basin Footprint

--

m × m

Design Depth

--

m

Restricted Discharge

--

l/s

Orifice / Device

--

mm

Time to Empty

--

hours

Freeboard

--

m

Critical Storm Analysis

The following table shows storage volume required at each tested duration. The critical duration (requiring maximum storage) is highlighted.

Duration (min)	Rainfall Depth (mm)	Inflow Volume (m³)	Outflow Volume (m³)	Storage Reqd (m³)

Stage-Storage-Discharge Table

Depth (m)	Storage (m³)	Discharge (l/s)

Design Checks

Check	Requirement	Value	Status

Design Notes & Assumptions

Key Assumptions

Trapezoidal cross-section with uniform side slopes.
Staged discharge model for drain-down (200 increments).
Inflow hydrograph simplified to rectangular pulse.
No infiltration through basin base (lined basin assumed).

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

PDF Filename

Engineer's Notes Optional, included in PDF report

Detention Basin Calculator