Attenuation Design Guide

Attenuation Design Guide

Attenuation design is often undertaken as part of a larger Sustainable Drainage System (SUDS) design. The Attenuation aspect of these designs are typically combined with Soakaways and infiltration systems to reduce the peak discharge from the system during storm events.

Attenuation systems are usually either online or offline. Online being where the water flows through the system at all times, and offline where the water is only diverted to the attenuation system during storm events or when a maximum discharge is exceeded.

Attenuation systems can take the form of above or below ground storage tanks, deliberately oversized drainage pipes, ponds or swales, and even car parks and surfaced areas can be designed to flood in extreme storm conditions and retain the excess rainwater.

Attenuation systems must be designed so that they can be regularly maintained, large offline storage facilities may require permanent maintenance equipment to ensure that they can be utilised at short notice.

The system must be designed to limit the buildup of silt or sediments from the incoming water. Pre-treatment is unlikely to be effective as the system is typically only utilised in high flow conditions. A regular maintenance scheme is likely to be better suited.

Where flow control devices are used, they should include the following features;

• static controls such as vortex flow controls
• self-clearing throttle slide valves can be used for orifices under 150mm
• a penstock or bypass valve should be included to allow the flow to continue when the main flow control is being maintained
• sufficient access for maintenance should be included on both the upstream and downstream sides of the device.The principle of Attenuation Design is to ensure that the system has sufficient capacity to store excess water from a design storm event.

 

Attenuation Design with CivilWeb Spreadsheets

Optimal Attenuation design is achieved with the CivilWeb Attenuation Spreadsheet by following the below procedure;

• Determine the catchment area and characteristics and determine the inflow volume for various storm events. This will require the calculation of the Catchment Area, Runoff Coefficient, Location, Return Period and the Climate Change Factor.
• Next the Attenuation outflow must be specified. This is the maximum rate at which water will be allowed to discharge from the system. This will depend on exactly where the system will discharge and on what restrictions have been placed on this discharge.
• The spreadsheet then calculates the storage volume required.

Finally the attenuation storage area can be designed to suit the volume of water required. This will depend very much on the shape, size and layout of the site and catchment area.