GROUNDWORKS 63 SPACE FOR SUDS
Howard Gray of GreenBlue Urban highlights the need for early education and planning for green and blue infrastructure from the outset of projects, to help mitigate the eff ects of fl ooding
E
arly in the development process comes the allocation of land for buildings, paving, open space, and drainage requirements – usually this allocation is re ected in the value of the land. The land is a diminishing resource, and to make profi ts, developers must allocate land in the most effective way. For generations, the percentage of land to open space has been the same – property densities as high as reasonably possible to make developments viable. However, the SuDS requirement has thrown a new challenge into the pot: allowing enough room to treat and store surface water on site rather than throw it into the pipework and away into the drainage network means that the tried and tested allocation ratio must change. This is not simple to answer, except to say, by enough. Calculations must be
made to ensure that the total rainfall volume for the design storm event can be retained onsite for the design period. This period would usually be specifi ed by the Lead Local Flood Authority (LLFA) as being the time required to allow a storm event to pass through the drainage network so that it does not become inundated by stormwater. But this could be as much as 24 hours, so the volume of storage is not insignifi cant. We must also consider the health and safety aspect of the retention feature; both for general use and for maintenance. It is not usually acceptable to create a deep storage pond, unless no other option is available, as this poses a risk for users and maintainers of the space. Most commonly, a shallow basin is preferred, if possible dry through much of the year, and only full in exceptional rain events.
THE USE OF MULTIPURPOSE SUDS FEATURES, SUCH AS TREE PIT SYSTEMS, IS AN ALTERNATIVE WAY TO MINIMISE LAND TAKE
BUT, HOW MUCH LAND MUST BE SET ASIDE FOR THIS?
A rule of thumb is that there should be a maximum ratio of 10:1 between imperme- able to the base of water storage areas close to buildings and that the maximum depth of stored water should be no more than 300 mm. However, this is a guide only and based on open SuDS features such as attenuation basins or swales.
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