What is storm water attenuation?

Storm water attenuation is the temporary storage of rainwater during heavy storms to prevent flooding downstream. When a site is developed (roofs, driveways, patios), rainwater runs off much faster than from the original greenfield. Attenuation tanks or crates store the excess water and release it slowly at a controlled rate (the greenfield runoff rate), preventing overwhelm of the drainage system or downstream flooding.

What rainfall intensity should I use?

For UK planning applications, a 1:100 year storm event with 40% climate change allowance is typically required. A commonly used design rainfall intensity is 50mm/hr, though this varies by location. Your local authority drainage engineer or SuDS Approving Body (SAB) will specify the exact design storm return period and climate change allowance required for your planning condition.

What is the greenfield runoff rate?

The greenfield runoff rate is the rate at which water would naturally drain from a site before development. It is typically expressed in litres per second (l/s) and is determined by soil type, topography, and catchment area. For planning applications, the local authority will specify the maximum discharge rate, often 2-5 l/s per hectare. For small domestic developments, a flat rate of 2-5 l/s may be permitted.

What are attenuation crates?

Attenuation crates (also called soakaway crates, geocellular storage, or stormwater crates) are modular plastic units that are assembled underground to create a void space for storing water. They have a void ratio of approximately 95%, meaning 95% of the crate volume is available for water storage. Common brands include Polypipe, Wavin, and Graf. They are wrapped in geotextile membrane and buried in an excavation.

Do I need planning permission for attenuation?

Attenuation systems themselves do not usually require separate planning permission, but they are typically a condition of planning approval for new developments. Under the National Planning Policy Framework (NPPF) and local planning policies, developments must demonstrate that surface water runoff will not increase flood risk downstream. A surface water drainage strategy is usually required as part of the planning application.

Storm Water Attenuation Calculator — How Much Storage Do I Need?

Calculate storm water attenuation storage volume and crate quantities to manage surface water runoff. Based on CIRIA SuDS Manual (C753) and Part H Building Regulations.

Impermeable Area

Total hard-surfaced area (roofs, drives, patios)

Rainfall Intensity

50mm/hr typical for 1:100 year storm + climate change

Greenfield Runoff Rate (allowable discharge)

Set by local authority, typically 2-5 l/s

Storm Duration

Design storm duration (typically 60 min)

Attenuation Crate Type

Standard crates are most common for domestic projects

Price Per Crate (optional)

Price per attenuation crate for cost estimate

How We Calculate This

This calculator determines the volume of storm water attenuation storage needed and the number of crates required, using a simple inflow-outflow method.

The formulas

Peak inflow rate (l/s) = (Rainfall intensity × Impermeable area) / 3600

Storage volume (litres) = (Peak inflow - Allowable discharge) × Storm duration (seconds)

Number of crates = ceil(Storage volume / (Crate capacity × Void ratio))

How it works

The peak inflow rate is calculated from the design rainfall intensity falling on the impermeable (hard-surfaced) area. The allowable discharge rate is subtracted to determine the net excess flow that must be stored. This excess, multiplied by the storm duration, gives the required storage volume.

The storage volume is then divided by the effective capacity of each crate (nominal capacity × 0.95 void ratio) to determine how many crates are needed.

Important notes

This is a simplified calculation suitable for initial sizing. For planning applications, a full drainage strategy using software such as MicroDrainage or WinDes is typically required, modelling multiple storm events and return periods. The design should be carried out or verified by a drainage engineer.

Standards

Storm water management should comply with Part H of the Building Regulations, the CIRIA SuDS Manual (C753), and the Non-Statutory Technical Standards for Sustainable Drainage Systems (Defra, 2015). Attenuation crates should comply with BBA certification or equivalent.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.