In collaboration with the Environment Agency (EA), BGS investigated the causes of groundwater flooding in the urbanised areas of the River Thames floodplain within the city of Oxford. These insights helped the EA develop a flood management strategy for the city.
River Thames flooding in the Oxford area, January 2007. BGS © UKRI.
Oxford: a flood-prone city
Oxford is situated within a narrow valley underlain by alluvial deposits from the River Thames. The city suffers from recurrent floods; for example, in 2007, approximately 200 properties were affected by a 1-in-20-year event. A significant number of these properties were affected by flooding from rising groundwater, which was either the sole cause of flooding or the initial cause prior to inundation by fluvial (river) waters.
Investigating the controls on groundwater flooding
The groundwater and flooding in Oxford project was a collaboration between BGS and the EA that identified the factors that control groundwater flooding in the Oxford area and mapped areas prone to groundwater flooding.
Three dimensional image showing the distribution of superficial deposits in the Oxford area. BGS © UKRI.
The EA was the primary funder of a comprehensive monitoring network that measured the relevant components of the hydrological system. BGS contributed to the design and installation of the monitoring network, quality assured the resulting data and made interpretations based on it.
The conceptual understanding of groundwater flow and surface water interaction benefited from the construction of a detailed 3D geological model that, in turn, formed the basis of a groundwater flow model.
Benefits of the project results
The project informed the assessment being undertaken by the EA of potential measures to reduce the risk of flooding in Oxford. This included the possible impact on natural habitats within the floodplain as a result of these measures.
The project concluded that significant urban groundwater flooding could occur as a result of direct infiltration of rainfall alone. It also showed that high river levels in the days following major rainfall events were very important in controlling groundwater flooding.
The project results provided greater confidence that the main recommendation of the EA’s Flood Risk Management Study — to improve the conveyance of flood waters through the Oxford valley during fluvial flood events — will also reduce the groundwater flood risk.
Ongoing work to understand groundwater flood risk
BGS continues to process and interpret data collected from the Oxford floodplain. This will aid the refinement of proposed flood mitigation measures. It will also improve the understanding of the study area ecology’s dependence on groundwater. A dataset of water levels is growing, which provides a baseline against which to assess the impacts of the flood risk management scheme.
Work is also ongoing to link the groundwater model of the Oxford floodplain sediments and the River Thames model for the area. This would be used to simulate flood conditions and assess the design of flood mitigation schemes.
It is hoped that the Oxford floodplain can become a nationally important field observatory, building on the substantial monitoring network currently in place.
Further reading
Macdonald, D M J, Dixon, A, Newell, A J, and Hallaways, A. 2012. Groundwater flooding within an urbanised flood plain. Journal of Flood Risk Management, Vol. 5, 68–80. DOI: https://doi.org/10.1111/j.1753-318X.2011.01127.x
Macdonald, D M J, Bloomfield, J B, Hughes, A, MacDonald, A, Adams, B, and McKenzie, A. 2008. Improving the understanding of the risk from groundwater flooding in the UK. In FLOODrisk 2008, European Conference on Flood Risk Management, Oxford, UK, 30 September to 2 October 2008. (The Netherlands: CRC Press.)
Macdonald, D M J, Hall, R, Carden, D, Dixon, A, Cheetham, M, Cornick, S, and Clegg, M. 2007. Investigating the interdependencies between surface and groundwater in the Oxford area to help predict the timing and location of groundwater flooding and to optimise flood mitigation measures. In 42nd Defra Flood and Coastal Management Conference, York, UK, 3 to 5 July 2007.
Newell, A J. 2007. Morphology and Quaternary geology of the Thames floodplain around Oxford. British Geological Survey Open Report, OR/08/030. (Nottingham, UK: British Geological Survey.)
Kessler, H, Mathers, S, Lelliott, M, Hughes, A G, and Macdonald, D M J. 2007. Rigorous 3D geological models as the basis for groundwater modelling. In Proceedings of the Annual Meeting of the Geological Society of America, Denver, Colorado, USA, 27 October 2007.
Contact
Please contact BGS Enquiries (enquiries@bgs.ac.uk) for further information.