British Geological Survey

In February 2002, the Environment Agency commenced a study to identify sustainable methods to reduce the flood risk in Oxford. Over 3,600 properties are within the floodplain of the Rivers Thames and Cherwell with a 1 % (1 in 100) annual flood exceedance probability. During the early stages of the study, it was recognised that out-of-bank flows from the rivers was the most significant factor but that groundwater flooding could not be ignored. This paper addresses the output of a collaborative project that was set up between the Environment Agency and the British Geological Survey to address the groundwater issues and their impact on flooding. Groundwater level in the shallow gravel aquifer that underlies Oxford responds rapidly to both fluctuations in river level and rainfall. During periods of high rainfall and river level, it is often the emergence of groundwater that is the first sign of flooding for many of the residents. In addition, in recent times, a significant number of properties have suffered flooding from groundwater; fluvial flow was not a factor. Where the latter was significant, the preceding and subsequent high groundwater levels often prolonged the period of flooding. The aims of the joint project are to: • better understand the groundwater system and its interaction with surface waters; • identify the controls on groundwater flooding • improve the prediction of the timing and duration of groundwater flooding; and • provide tools to examine the potential flood risk management options. The tools will also help to predict the impacts of the options on groundwater within the many ecologically sensitive sites in the floodplain.