WET News

24 WET NEWS OCTOBER 2016 Climate change continually throws new challenges for flood risk managers and resilience planners – on managing surface and fluvial flood risk better. ESI's Mike Streetly explains. The challenge: Slowing the flow INSIGHT Flood risk management • T raditional flood risk man- agement approaches have been centred on structur- al flood defences such as flood- walls, which aim to keep flood- water away from vulnerable areas. Structural defences are restricted by high capital, main- tenance and upgrade costs, and cannot be raised indefinitely in response to increasing risk Storm Desmond demonstrat- ed to the people of Carlisle in December last year that what was built in response to the 2005 floods was not enough, and that the "new normal" in flood events just got a lot higher. The government has prom- ised £3bn in flood defences from 2015 to 2021, including a £700M boost announced in the last budget. So it seems odd that in the context of such a sig- nificant investment number that the publication of the latest National Flood Resilience Re- view has focused back on a £12M investment figure for tem- porary flood defences around 500 critical infrastructure sites. This is fine, but relies on human organisation and rapid response in oŠen difficult circumstances, especially if that infrastructure grinds to a While the government has committed to investing billions into flood defences in reaction to ever more catastrophic flood events, there is a case for adopting more holistic, natural approaches. A solution: Natural flood risk management halt and it cannot be deployed quickly enough. Flood management is more than just about temporary barriers or even building even higher permanent (and oŠen unsightly) barriers in our towns and villages close to offending rivers. S lowing the flow is what it is all about. Natural flood risk management (NFM) is an approach that deploys a range of natural measures across a whole river catchment, rather than focusing on specific defence locations. Designed to enhance, restore or mimic natural processes, a key component is allowing identified areas to flood in order to decrease the flood risk elsewhere. Catchments are a whole system; rivers are highly interlinked and flooding in lowlands may be a result of local conditions and activities upstream affecting surface run- off. As the geography, biology and water cycle of every catchment is unique, NFM plans are tailored to specific areas using expert and local knowledge. While fluvial flood risk maps and assessment are easily available and understood, NFM risk management areas do not necessarily overlap with areas at risk of flooding; measures oŠen work most effectively by holding back water or keeping soil on the land in other areas of the catchment. In particular, the manage- ment of upland areas can ame- liorate flooding by slowing run- off at its source, through peatland restoration, blocking upslope drainage channels, and other measures. Regional opportunity maps have been developed to identify where there is potential for wetland creation, tree planting or both to mitigate flood risk or improve water quality. It also contrib- utes to AMP WFD objectives around water resource manage- ment and maintaining river flows during dry weather. Our project work with water compa- nies has demonstrated how a more sustainable catchment ap- proach has helped conserve and realise resources under stress. Reducing the likelihood or severity of flooding by gradual- ly lowering or spreading out the flood peak as it passes along a river is known as attenuation. This approach expands the drainage network of a river dur- ing periods of high flow and purposefully floods some areas while reducing the flow of water to areas more vulnerable to flood damage, as well as trap- ping sediment. It is important to control overland flows in the upper catchment by placing appropri- ately designed measures as close to the source of surface run-off as possible. Examples of these include: Better Soil Management Intensive farming practices, including replacing grazed pasture with arable cultivation can leave soils bare and compacted, with a degraded soil structure that lowers absorbency, causing more rainfall to run-off across the surface of the land. Practices that keep soil on the land are not only beneficial for farming, but are more cost effective than managing excess sediment build up in river channels. Slowing Rural Surface Water Runoff When runoff occurs, it can be controlled at source by a range of attenuation features such as storage ponds, drain and ditch barriers, rough-grassland or vegetated buffer strips and soil banks. Storage ponds, drain and ditch barriers, need regular maintenance to manage sedi- ment build-up, but can reduce diffuse pollution as well as miti- gating flood risk. Such meas- ures are effective in reducing flood risk for farms and small communities where structural flood defences are neither feasi- ble nor cost-effective. Making Floodplains work Floodplains slow and store flood water by allowing it to spread outside the river chan- nel. Lowering embankments and deliberately flooding strate- gic locations takes local pres- sure on downstream flow and can benefit water quality. Farm- ers can diversify cropping through hay meadow planting that thrives well in these varia- ble conditions. River Restoration Many UK lowland floodplains, particularly in urban areas, have been disconnected from the river channel by a combina- tion of land drainage, channel straightening, or embankments. This increases the risk of passing flooding on to critical areas downstream. Measures include re-meandering straight- ened channels to decrease the river's gradient, increase its length, and slow the flow of water downstream. It can also involve removing artificial bar- riers to flow including culverts and weirs. Riparian Planting Woodland planting reduces flood peaks for individual events by enhancing infiltra- tion of water into the soil. Tar- geted planting along water- courses, and subsequent woody debris in channels upstream of vulnerable areas, can slow the flow of floodwater. It can also reduce soil erosion along river banks and improves biodiversity. SuDS SuDS aim to manage surface water run-off in a way that mimics the natural water cycle. Features that slow and store water, like vegetated roofs and retention ponds, or increase infiltration like permeable paving and rain gardens can be imaginatively applied within existing and new developments. Requirements for new devel- opments to adopt SuDS were set out in the Flood and Water Management Act 2010. While SuDS have yet to be mandated for large developments, the House of Lords is pursuing a re- Rivers cause sudden, short term and the most obvious impacts to human life

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