Attenuation is the process of temporarily storing surface water during a storm event and releasing it gradually at a controlled rate. It is a fundamental principle of sustainable drainage and is required on virtually all new development to prevent increased flood risk downstream.

The need for attenuation arises because development typically replaces permeable surfaces (soil, grass, vegetation) with impermeable surfaces (roofs, roads, car parks). Without mitigation, this increases both the volume and the peak rate of runoff, potentially overwhelming receiving watercourses, sewers, or downstream properties.

Attenuation storage is designed to hold back the difference between the inflow (from the developed site) and the permitted outflow (typically limited to the greenfield runoff rate). The volume of storage required depends on:

The size of the impermeable area being drained
The permitted discharge rate (set by the LLFA or sewerage undertaker)
The design storm duration and return period (usually up to the 1% annual exceedance probability event)
Climate change allowances applied to future rainfall

Attenuation can be provided by a range of SuDS features:

Above-ground: Detention basins, ponds, swales, and landscaped depressions that fill during storms
Below-ground: Geocellular storage crates, oversized pipes, and below-ground tanks

The controlled release is managed by a flow-control device (typically a hydrobrake or orifice plate) at the outlet. Aegaea sizes attenuation systems using industry-standard software as part of our drainage design service.