Slowing the flow – a holistic approach to flood protection
Current planning regulations require the attenuation of flows from new development, irrespective of the location of the site within the catchment. Motion Director Richard Bettridge argues for three improvements to flood risk management strategies in relation to new schemes.
Sites near the seaside have needed measures to attenuate surface runoff, when it may be better to release excess flows to sea quickly, clearing the way for upstream flows. Sites at the head of a catchment are allowed to discharge attenuated flows under extreme conditions, exacerbating flooding downstream.
Schemes which operate on a sliding scale should be considered. Methods to hold the runoff at the head of the catchment could be employed. Attenuation could be used mid-catchment, with free discharge at the bottom of the catchment.
The level of flood protection is often designed to operate under particular rainfall or flood events. More needs to be done to revise the criteria to take account of successive extreme events. Although in theory these events are rare, in practice more are being experienced. The recent floods in the north of England and Somerset appear to arise from a number of successive serious rainfall events. These expose the inadequacy of flood defence measures, which may have been designed on the traditional basis.
Finally, the needs of water supply need to be matched to those of flood protection by the same means: impounding and storing excess water in upstream areas. At first glance, the answer to both challenges is identical.
The solution is not a simple one. In order to address all the issues, a more holistic approach may be required. The following three measures could help channel the course of flood protection strategies for the better:
– Relate sustainable drainage solutions to the behaviour of the catchment
– Consider the impact of successive extreme events
– Link the needs of water supply and flood storage
This article is based on a letter published in Environment magazine, March 2016.