Extreme Rainfall Impacts in Fractured Permeable Catchments

Serious groundwater flooding has occurred on Chalk catchments in the UK and North West Europe in the last decade causing substantial amounts of disruption and economic damage. Consequently, there is a need to quantify how water tables in fractured permeable catchments respond to different types of rainfall and the implications for groundwater flooding. Logged measurements from a Chalk catchment (River Pang, Berkshire, UK) of rainfall (tipping bucket), actual evaporation (eddy flux correlation), soil water content (profile probes and neutron probes), near surface matric potential (tensiometers and equitensiometers), deep (>10m) matric potential (deep jacking tensiometers) and water table elevation have been used to provide an integrated dataset of recharge processes. Conventional treatment of recharge in Chalk aquifers considers a fixed bypass component of rainfall, normally 15%, to account for the role of the fractures. However, interpretation of the field data suggest three modes of recharge: under low rainfall intensities recharge is slow (lags of > 100 days) and through the matrix; under moderate intensities recharge is via the matrix and partially saturated fractures (lags of 10s of days) which, if sustained, can lead to flooding (as in 2000/2001); under high intensities fractures can transmit rainfall preferentially leading to a large, rapid (<1 day) water table response even at tens of metres depth. By examining rainfall-water table response patterns, a combined intensity-duration characteristic of rainfall events is proposed which can be used to predict when preferential recharge is likely to occur. Detailed understanding of the processes governing the various responses has been provided using a physically based, dual permeability model of the Chalk, which couples unsaturated and saturated zones and conditioned on the field observations. This has demonstrated the importance of the soil and weathered chalk layers for controlling matrix and fracture flow in response to different types of rainfall infiltration.