Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | APRIL 2014 | 9 Comment I n recent years and specifically over the last few months, flooding where we live and work will have affected many of us. The impact of flooding on people's health and well- being is devastating. We design drainage in urban areas to accommodate different size rainfall events. Typically, sewers can accommodate rain with a 1-in-30 chance of occurring in any given year. Highway drainage is o-en to a lower standard, 1-in-5 or 1-in-10 chance of occurring in any given year. Sometimes we add an allowance for climate change. For new development, we should be designing to a higher standard to keep water out of properties. Planning We have emergency and contingency planning / procedures to cope with the consequences of extreme events. But what about rainfall that is bigger than the drainage design, which may not be called extreme? Urban flooding - creating spaces in places Why we should design for exceedance How should we manage the consequences of rainfall that exceeds the capacity of our drainage systems? Should we ignore it, let it happen and deal with the consequences or should we decide to act? It is unsustainable and unaffordable to build ever bigger drainage for events that have a low probability of occurring. Building sustainable drainage systems (SuDS) or a grey-green infrastructure on new development, or retrofitting form part of the solution, but will not completely solve our current challenge of managing rainfall that, in the past, we did not design for. Design What we can and need to do is to design for exceedance. This approach helps to manage the excess flow that may occasionally occur on the surface within the existing urban environment to reduce its impact on homes and vulnerable assets. It is about making the most of shared surfaces and spaces so they have more than one function or use. For example, we can use roads as flood pathways to channel and convey exceedance by strategically increasing the height of kerbs, or profiling the carriageway to direct flow away from vulnerable locations. Surface changes O-en small changes to the surface create the greatest impact such as changing the slope of a footpath, building or removing a drop kerb. We can create multi-functional spaces to store water, for example in car parks or open green space. We can build floodwalls around properties and install property protection measures. We may only use these measures infrequently, but they create resilience within an area. Many can and should be designed to fit aesthetically within the urban area. Exceedence We have a big opportunity to manage rainfall and surface water flooding in our urban areas by designing for exceedance more. To do this, we have to make the most of the opportuni- ties we have. Firstly, we need to ensure that when building new developments we check where exceedance may occur, and where it will go. Managing exceedance is a requirement, referred to in the National Planning Policy Framework for England, Sewers for Adoption (7th edition) and the dra- national standards for sustainable drainage. However, despite the requirements, assessing and designing for exceedance is not consistently applied. There have been missed opportunities to make new developments more resilient. We need to make best use of drainage models for new development and take the small extra step to include surface topography in the model. ChriS Digman SenIoR PRIncIPAL engIneeR mWh

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