Rainwater Attenuation & Infiltration Calculator

Attenuation crates & soakaways - volume, unit count, footprint, emptying time

Drainage Strategy

Attenuation Tank

Throttle discharge to a permitted rate - store peak flows and release slowly to sewer or watercourse

Soakaway / Infiltration

Discharge surface water to ground via percolation through the soil - no connection to sewer required

Storage Type

Modular Crates

Polypropylene interlocking crate system with ~95% void

Precast Tank

Concrete or GRP underground tank - bespoke volume

Oversized Pipe

Large-diameter pipe storage (600 mm dia)

Design Storm

Rainfall values below are approximate UK figures based on FEH/DEFRA data. For planning submissions use site-specific values from your drainage consultant or design software.

Return Period

1 in 2 year

1 in 10 year

1 in 30 year

1 in 100 year

Storm Duration

15 min

30 min

1 hour

2 hours

6 hours

♦ 13 mm Use manual rainfall value

Catchment Areas

Permitted Discharge Rate (l/s) Max flow rate to sewer / watercourse as set by planning or the water authority. Use 0 for full attenuation with no discharge.

Crate Capacity

300 L

400 L

450 L

600 L

1,000 L

Void Ratio

95%

90%

85%

Safety / Freeboard Factor

× 1.10

× 1.15

× 1.20

× 1.30

Multiply required storage volume by this factor - accounts for uncertainties in rainfall estimates and site conditions.

Job Costing (optional)

Cost per crate

Labour per m³ excavation

Delivery (flat fee)

Attenuation Results

Which Strategy?

Strategy	Use when...
Attenuation	Ground is unsuitable or planning requires controlled discharge to a sewer or watercourse
Soakaway	Permeable soil is present; preferred by SuDS regulations - keeps water on-site

SuDS (Sustainable Drainage Systems) favour soakaways and permeable surfaces. Check with your Local Planning Authority for requirements.

Runoff Coefficients (Cv)

Surface	Cv
Flat roof (GRP / EPDM)	0.95
Concrete / tarmac	0.90
Pitched roof (tile/slate)	0.75
Block paving	0.65
Gravel / hardcore	0.35
Lawn / grass	0.25
Meadow / wildflower	0.10

The effective area (Ae) is the sum of each catchment area multiplied by its Cv. Only this fraction of rainfall becomes runoff.

Soakaway Soil Guide

Soil	f (m/s)
Gravel / coarse sand	1×10^-3
Medium sand	5×10^-4
Fine sand / silt	1×10^-4
Silty clay	1×10^-5
Clay	Not suitable

BRE 365 requires soakaways to empty within 24 hours. Confirm soil type with a percolation test on-site.

Design Storm Notes

Residential extensions typically need 1 in 30 year return period as a minimum.
New developments may require 1 in 100 year plus a 30-40% climate change allowance - increase rainfall manually to account for this.
Rainfall values shown are approximate UK figures. Use site-specific data from FEH or approved software for formal submissions.
The critical storm duration is the one that produces the largest storage volume - try multiple durations.

Rainwater Attenuation & Infiltration Calculator