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An early warning system has been developed for groundwater flooding and trialled in the Patcham area of Brighton. It provides a fit-for-purpose approach for forecasting groundwater flood events in the Chalk and is capable of operating across longer time scales than had previously been possible. The method involves a set of nested steps or tasks. Initially, the catchment's response to recharge is determined and, using a representative hydrograph, a simple regression model that relates annual groundwater level minima and autumn and winter rainfall to subsequent annual maxima is developed. The regression model is then applied at the end of each summer recession using the observed annual minimum and estimates of winter rainfall to predict the following groundwater level maximum. Based on the results of this prediction a variety of steps may then be appropriate. Where the model predicts potentially high groundwater levels the frequency of groundwater level monitoring observations can be increased. A novel element of the method developed is the monitoring of changes in the matric potential of the unsaturated zone. Specific trigger levels to initiate either the next step of the method or promulgation of warnings of varying severity will be developed through experience of use of the system.

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Newly developed borehole closed circuit television (CCTV) lighting has shown that films of water can develop on open borehole walls at discrete levels within the deep unsaturated zone of the chalk, the levels being associated with marl horizons. The CCTV images, together with jacking tensiometers, have been used to observe unsaturated zone processes in selected boreholes. Work focused on two catchments in southern England, the Pang in Berkshire and the Brighton ‘block’ in Sussex. Deep jacking tensiometers were used to measure the associated pressure head (matric potential). The data suggest that recharge to the water table occurs through the matrix, throughout the year. The data also suggest that marl seams can influence recharge processes significantly, creating horizons with high matric potential. As a result, fracture flow and storage could be more important in chalk sequences where they are abundant and well developed.

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