Sustainable Drainage – Does Size Matter?

Written By: Sustainable Drainage Engineer, Cambridge City Council

In the case of SuDS is bigger always better?

I have heard all sorts of arguments for why a single pond is the only solution to a site’s surface water drainage scheme. “Maintenance will be easier”, “there will be more biodiversity with a bigger pond”,  “bigger ponds have more storage and therefore the risk is lower the bigger they get” and I kid you not “big ponds are prettier”.

SuDS is not the new terminology for balancing ponds, it is the philosophy of having a system of features to manage surface water. Dealing with the water where it falls (source control) is an essential element of the ‘system’. Small distributed features can not only benefit the development in controlling flood risk, but can also significantly add to the quality of the environment. But flow controls can’t handle flows that low I hear you say. Well this is where ‘engineering and not specifying’ comes into play, make the water work for you, filters, gradients and vegetation can all control the flow of water where mechanical flow controls don’t perform well.

So will there be more biodiversity in a larger single feature rather than smaller distributed features? A study is 2001 (Beat Oertli, Dominique Auderset Joye, Emmanuel Castella, Raphaelle Juge, Diana Cambin, Jean-Bernard Lachavanne., The relationship between pond area and biodiversity, University of Geneva, 2001) showed that a set of ponds of small size has more species and has a higher conservation value than a single large pond of the same total area. However, there were some species that were only found in the larger ponds. So if it is conservation value you are after, smaller is better.

What about maintenance is bigger better? From my experience of maintaining water features in Cambridge, bigger is definitely more difficult which means more specialist equipment, which inevitably means higher cost. Our approach to maintenance is small and often, smaller features that are easy to be maintained by a man in a van, which we believe will lead to lower whole life costs.

Large ponds do have their place as part of system, but there needs to be those upstream features to reduce the risk, to improve the water quality prior to it entering the pond and if carefully designed, to promote a range of habitats to enable high quality biodiversity. This should also lead to a high quality landscape that will be of benefit to the both the developer in being able to sell properties at a higher premium and also to the residents that will eventually live there.

Read about Cambridge City Council’s SUDS Infrastructure at the Clay Farm Housing Development

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