Attenuation

Attenuation systems store rainwater from roofs and paved areas underground and release it slowly, so a site doesn't flood the sewer or watercourse it drains to. A typical system has three parts: a silt trap on the way in, crates or a tank to store the water, and a flow control on the way out.

Read the buying guide and questions
  • Crates: AquaVoid 20 tonne, StormMaster 40 tonne and Hydrocell 62 tonne
  • Membranes: non-woven geotextile, 300 and 500 micron DPM and lap tape
  • Top hat seals for 110mm and 160mm sewer pipe and 150mm and 225mm twinwall
  • Silt traps: Turtle SiltBlok, Silt Guard and Gulliblok, Oaklands Silt Guardian and JKH GRC silt trap inlets
  • Flow controls: Vortiflo and bespoke vortex units, Oriflo orifice chambers

Shop by type

How an attenuation tank works

An attenuation tank is an underground store for storm water. In heavy rain, water from roofs and paved areas runs in faster than it is let out. A flow control on the outlet releases it at a rate the sewer or watercourse downstream can take. The tank fills during the storm and drains down slowly once it has passed.

The water leaves through the flow control chamber, by a gravity pipe where levels allow, or through a pumping chamber where they don't. Crate tanks need a separate flow control chamber; large cylindrical tanks often have one built in.

Holding the water back lowers flood risk downstream, takes load off the sewers, reduces erosion where the water comes out, and helps a site meet its sustainable drainage (SuDS) requirements.

Types of attenuation storage

  • Crate tanks: plastic crates wrapped in an impermeable membrane, built to suit the space and depth on site. The usual choice, from driveways to car parks. See attenuation crates.
  • Large tanks: cylindrical tanks in GRP, polyethylene or steel. Pipes bring water from a wide area to one tank, which suits larger commercial sites. See storm attenuation tanks.
  • Oversized pipes: large-diameter pipe that stores the water within the pipe run. See storm attenuation pipes.

For crates, check each crate's storage volume, load rating and cover depth on the maker's data sheet against the traffic above.

Sizing

The tank volume is worked out in the drainage design from:

  • the roof and paved area draining to it
  • local rainfall
  • the design storm, given as a return period such as 1 in 30 years
  • the discharge rate the approval allows
  • the soil's infiltration rate, where some water can soak away
  • the depth available for the drainage

Questions about attenuation

What is the difference between attenuation and a soakaway?

A soakaway lets water soak into the ground. Attenuation stores it in a sealed tank and lets it out slowly through a flow control. Use attenuation where the ground won't drain or the approval limits the discharge rate.

What size attenuation tank do I need?

Your drainage design sets the storage volume and the discharge rate. Send your drawing for a free drawing take-off and we will list the crates, membrane, silt trap and flow control.

Why fit a silt trap before the tank?

Silt is what blocks crates and flow controls. A silt trap catches it where it can be cleaned out, instead of inside a buried tank.

What discharge rate will my site be allowed?

In England, it is set as part of the drainage approval. New developments are usually held to the rate the site drained at before it was built on. On brownfield sites the rate is agreed with the council. Confirm the figure early, as it sets the size of both the tank and the flow control. Wales, Scotland and Northern Ireland have their own rules.

Do I need SuDS approval?

Most new developments need their drainage strategy approved. In England, the council approves it, with advice from the lead local flood authority, and will want to see how the system will be maintained, so allow access to the silt trap and flow control. Agree it early, before the site layout is fixed. Wales, Scotland and Northern Ireland have their own rules: in Wales, most new developments need approval from a SuDS Approving Body before work starts.

Pricing a whole job? Send your drawings for a free take-off, ask for a trade quote or call 0121 351 3230.