Groundwater and Flooding in Oxford
British Geological Survey and Environment Agency
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. A small project funded by the British Geological Survey was already ongoing in south Oxford studying groundwater flooding. In 2005 a larger jointly-funded collaborative project was initiated between the Environment Agency and the British Geological Survey to address the groundwater issues and their impact on flooding across the whole of the Environment Agency study area.
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.
Bringing together the existing data: Work to understand the relationship between surface water, groundwater and ecology in the study area, has been underway for more than 25 years. The principal focus of this is the Oxford Meadows SAC. The data are being collated and archived by BGS for collaborative use.
Setting up of a monitoring network: The existing coverage of groundwater and surface water level monitoring points was expanded. Over 150 groundwater sites and some 50 surface water sites are now routinely monitored across the area, a large number using digital water level recorders on a 15-minute interval. In addition, data from routine monitoring of river flow and rainfall is also available.
Development of a conceptual model of groundwater system: The available data, including water levels obtained through the monitoring network, is helping to better understand key aspects of the groundwater and surface water system. This includes valuable data collected during the July 2007 flooding. The development of the conceptual understanding has been aided by the creation of a 3D geological model, which is based on the large amount of data for the area in BGS's borehole geology database.
Development of a groundwater model: The processes identified through the development of the conceptual model have been incorporated into a groundwater model of the system. This will be used to assess implications of potential mitigation measures. It is hoped that this can be linked with the Environment Agency's Thames River model for the Oxford area.
Macdonald DMJ et al., 2007. Groundwater and flooding in Oxford (PDF 1 MB). Defra 42nd Flood and Coastal Management Conference, York 2007
British Geological Survey project leader: David Macdonald
Project leader for Environment Agency's Oxford Flood Risk Management Study: Geoff Bell
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