Assessing the risk of groundwater-induced sewer flooding to inform water and sewerage company investment planning

During the last two winters, and that of 2000-01, Chalk catchments of southern England experienced severe groundwater flooding. Rising groundwater tables caused rivers to flow high up-catchment in normally dry valleys, flooding homes and businesses in these locations and further downstream. Groundwater ingress into the sewer network led to restricted toilet use and the overflow of diluted, but untreated sewage to road surfaces, rivers and water courses. Increased sewer flows reaching sewage treatment works caused overspills and the contamination of water flowing into rivers. The water and sewerage company Thames Water Utilities Ltd (TWUL) estimate that they spent in the region of £19m responding to the extreme wet weather of 2013-14 and used a fleet of over 100 tankers. However, the magnitude of the event was so large that these efforts could not stop the discharge of sewage to the environment. A particular challenge in managing groundwater flooding in Chalk catchments is that it can last for weeks to months as they are slow to drain after rainfall stops.

In response to these groundwater flooding events TWUL are working to reduce the infiltration of groundwater from the Chalk into sewers, and to understand the risk of groundwater flooding to the network. This involves the development of "Infiltration Reduction Plans", which will be submitted to the Environment Agency, and strategic planning of investment in their infrastructure. This project brings together researchers from the British Geological Survey and Imperial College London, to work in partnership with TWUL infrastructure managers and technical specialists to support this investment planning. The overarching aim of the work is to quantify the scale of the risk of groundwater-induced flooding to the sewer network in a Chalk catchment and to translate this knowledge into options for investment planning. The project builds on recent NERC funded research involving the partners that has developed a series of highly relevant datasets, tools and models. Specifically, we will: (i) quantify the risk of groundwater flooding under current climate using an ensemble of weather sequences generated using the state-of-the art weather generator, GlimClim; (ii) assess changes in the likelihood of flooding under future climate using new probabilistic climate projections; (iii) investigate the propagation, in space and time, of the interaction of groundwater flooding with the sewer network, and where investment should be targeted first to minimise sewer overloading; and (iv) translate this improved understanding into TWUL's investment planning, for example, as part of Ofwat's Asset Management Planning 5-year cycle. The project will use the River Lambourn, Berkshire, which has been impacted by these issues, as a case-study catchment.