Groundwater flooding has moved up the policy‐makers' agenda as a result of the United Kingdom experiencing extensive groundwater flooding in winter 2000/2001. However, there is a lack of appropriate methods and data to support groundwater flood risk assessment. The implications for flood risk assessment of groundwater flooding are outlined using a study of the Chalk aquifer underlying the Pang and Lambourn catchments in Berkshire, UK. Groundwater flooding in the Chalk results from the water table reaching the land surface and producing long‐duration surface flows (weeks to months), causing significant disruption to transport infrastructure and households. By analyzing existing data with a farmers' survey, it was found that groundwater flooding consists of a combination of intermittent stream discharge and anomalous springflow. This work shows that there is a significant challenge involved in drawing together data and understanding of groundwater flooding, which includes vital local knowledge, reasonable risk assessment procedures and deterministic modelling.

[1] A new approach to regionalization of conceptual rainfall-runoff models is presented on the basis of ensemble modeling and model averaging. It is argued that in principle, this approach represents an improvement on the established procedure of regressing parameter values against numeric catchment descriptors. Using daily data from 127 catchments in the United Kingdom, alternative schemes for defining prior and posterior likelihoods of candidate models are tested in terms of accuracy of ungauged catchment predictions. A probability distributed model structure is used, and alternative parameter sets are identified using data from each of a number of gauged catchments. Using the models of the 10 gauged catchments most similar to the ungauged catchment provides generally the best results and performs significantly better than the regression method, especially for predicting low flows. The ensemble of candidate models provides an indication of uncertainty in ungauged catchment predictions, although this is not a robust estimate of possible flow ranges, and frequently fails to encompass flow peaks. Options for developing the new method to resolve these problems are discussed.

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Using synthetic urban catchments, represented by a distributed nonlinear rainfall-runoff model, the effects of storm velocity on the runoff hydrograph are quantified for different catchment areas and response parameters. Results are considered in comparison with equivalent stationary storms and the implications for current design practice are examined. Yen, B.C., and V. T. Chow, A laboratory study of surface runoff due to moving rainstorms, Water Resour. Res., 5(5), 989-1006, 1969.

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