Groundwater Flooding in the Chalk of England

During the winters of 2001 and 2003 extensive long duration flooding occurred in the valleys of the Chalk downlands in England. Chalk streams of the downlands are characterized by seasonally driven ephemeral “bourne” behavior and flooding often occurred further up-stream than typical in these dry valleys. Detailed studies in the Rivers Pang and Lambourn have included analysis of flow records and borehole hydrographs which have revealed that the groundwater flooding mechanisms are intimately related with antecedent rainfall and high groundwater conditions, leading to the development of sustained groundwater mounding on the stream interfluves. Data concerning the location of unusual spring flow and ponding water has been gathered from aerial photography and surveys by regulatory authorities, but much valuable information was obtained from a questionnaire sent to farmers in the area. The location of flooding is influenced by the topography and local geology. Two mechanisms are prevalent in different parts of the catchments. In the downstream areas, flow is generated by the groundwater level reaching the ground surface in an expected manner, but in other areas the presence of hard grounds and high transmissivity horizons causes spring generation on the sides of normally dry valleys.