Utility Week

UTILITY Week 4th April 2014

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24 | 4th - 10th April 2014 | UtilitY WEEK Operations & Assets Market view T he Environment Agency estimates that 2.4 million properties in England are at risk of flooding from rivers or the sea, and an additional 2.8 million properties are susceptible to surface water flooding. Fur- thermore, there are a substantial number of properties at risk of flooding from ground- water – shown for the first time on a National Groundwater Flood Risk Map published by ESI, an independent scientific environmental consultancy specialising in water, land and sustainable development. With the recent UK floods affecting large parts of southern England, including the Somerset Levels and the Thames Valley, many utilities customers are not only hav- ing to deal with the devastating effects of groundwater flood damage to their homes and business premises, but the repercus- sions of overwhelmed drainage systems and sewer flooding, too. But how does groundwater sewer flood- ing fit into the wider flooding picture, and what can water companies do to identify customers at risk? ESI's techniques using new data demonstrate how water compa- nies can implement robust sewer infiltration reduction plans to meet the latest regulatory requirements. Groundwater flooding occurs when sub- surface water emerges from the ground at the surface, or into made ground and struc- tures. This could be as a result of persistent rainfall that recharges aquifers until they are full, or may be a result of high river levels or tides driving water through near-surface deposits. Compared with surface water flood- ing, groundwater flooding can last consider- ably longer, with incidents lasting anything from a week to several months, which is why it can prove substantially more costly to land and property owners than any other type of flooding. Groundwater flooding or a shallow water table prevents rainfall infiltration and increases the risk of surface water flooding, which means that many surface floods are actually driven by groundwater conditions. But consideration of surface water in isola- tion, and lack of evidence for groundwater conditions, leads to incorrect analysis of overall causes. Although emergent groundwater tends to be clear and relatively clean compared with muddy fluvial flood waters, it has very real potential to be contaminated by brownfield sites and sewers and, as water companies will know, sewer flooding can cause sig- nificant health, environmental and financial issues. ESI has developed methods that use the new groundwater data in geographic infor- mation systems to allow analysis of the interaction between sewers and groundwater to identify where problems of sewer infiltra- tion will occur. This enables the firm to assist water companies with the development of suitable sewer infiltration reduction plans to reduce the risk of future sewer flooding. Although local authorities and com- mercial organisations are highly reliant on current and accurate flood risk guidance to prevent and predict the costly aermath of groundwater flooding, until now there has been no national-scale authoritative map of groundwater flood risk. The previous milestone achievement in this field was the publication by the British Geological Survey (BGS) in 2007 of a national Susceptibility to Groundwater Flooding data- set at a scale of 1:50,000. This indicates areas where geological conditions could enable groundwater flooding to occur and where groundwater may come close to the ground surface. The BGS clearly states this resource was never intended to be used to calculate the risk of incidence, but in the absence of any other resources, it has been repeatedly relied on for just that purpose. In October 2013, ESI addressed ground- water flooding issues in England and Wales by publishing the first national authorita- tive Groundwater Flood Risk Map. The com- pany's team of specialist hydrogeologists has managed to overcome the constraints of the previous work available, and has developed its model using best practice algorithms and calibrated risk predictions using site-specific evidence of real flooding events from many parts of the country. ESI's National Groundwater Flood Risk Map is already proving popular with a vari- ety of companies in the water and utility sec- tors and the map should see a change in the way that groundwater flooding is considered. The company is working in consultation with dozens of lead local flood authorities and will continue to develop and evolve the map. A move to a risk-based model can only be good news for the hundreds of homes and landowners finally freed from the potential issues that come with being branded a risk in the past, and more in-depth and appropri- ate information can only help with planning projects and infrastructure in the future. Mark Fermor is a hydrogeologist and managing director of ESI Ltd Map to reduce sewer flooding By identifying properties at particular risk of groundwater flooding, the National Groundwater Flood Risk Map helps water companies to develop sewer infiltration reduction plans, says Mark Fermor. Case study: Compton, Berkshire The village of Compton in Berkshire is located in the catchment area of the River Pang and has a long history of inundation. This can lead to overflowing sewers and basement flood- ing, which has led some of the residents to install pumps in their properties as a mitiga- tion measure. ESI's National Groundwater Flood Risk Map can help identify properties in Compton that are at risk so that residents can plan accordingly and water companies can best prepare staff and partners serving the area. The obvious benefit of the map is that it allows the user to get a large-scale understanding of the groundwater flood risks of a region and an indication of the potential flood risks at a given site, at a low cost. This is a powerful screening tool in the early stages of planning for all major projects, including utility connec- tion feasibility. In the case of villages such as Compton, the map assists with clarification of how much risk is faced in different parts of the district, helping those living and working locally to plan ways to avoid exposure to flooding wherever practical.

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