ATTENUATION: The myth of splitting storage volume 1 for 1

January 24, 2024

With the requirement of SuDS on almost every project, developers often approach the flood risk and drainage design with a question:

How big does the attenuation tank need to be?

Often, Developers will try to keep costs down by using a high-level estimate for site attenuation for planning submissions without doing a Detailed Drainage Design.

Once on site however and the actual site constraints are evident (e.g. high voltage cables, steep topography), it is often assumed this previously estimated attenuation volume can be split between different locations and SuDS features (for example in the form of blueroof, permeable paving, geo-cellular storage tanks etc) without compromise.

This may work for minor planning applications and smaller projects, however due to some of the factors described below, this approach is not failsafe and can often lead to problems at the detailed design stage.

  • Steep sites and permeable paving. When working on developments with steep topography, the effectiveness of permeable paving might be reduced due to water flowing downstream too quickly and not using all the available space within the underlying pavement sub-base. To avoid this, it is necessary to create a flat sub-base formation level and/or use intermediate flow controls. However, when using intermediate flow controls, an additional delay is brought into the system resulting in the overall discharge capacity of the system slowing down, thus reducing the overall efficiency of the system as well as increasing blockage risk.
  • Blueroofs. When the constraints are at ground level, an alternative attenuation of water could be achieved in the form of a blueroof. These are installed with their own flow control devices to maximise the use of storage spaces but similarly to the case above, will likely require a larger overall site attenuation volume due to the use of intermediate flow controls.
  • Half drain time capacity. When the attenuation tank is large and the imposed maximum flowrate is small, the drain down time of the structure is often greater than 24 hours. It is a standard planning requirement; the tank needs to half its volume within 24 hours of the critical attenuation level. This is often a challenge, and a general accepted compromise by approving authorities is to increase the tank size so it can attenuate a consecutive 10 return period storm after the critical rainfall event.

Preparing a Detailed Drainage Design from the onset will take into account these and other factors, using accurate hydraulic modelling to design a drainage strategy that works based on the independent constraints of the site.  This will save a developer time and money in the long run, by getting the drainage strategy right first time.

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