A.J. Gallagher

An early warning system for groundwater flooding in the Chalk

Abstract 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 catchments 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.

Lithostratigraphic controls on recharge to the Chalk aquifer of Southern England

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.

Tracer Investigations of the Nature and Structure of Subsurface Voids in Mildly Karstic Aquifers: an Example from the English Chalk

The Chalk in England is a mildly karstified fractured limestone with high matrix porosity. Chalk landscapes are predominantly a fluviokarst with an extensive dry valley network and with stream sinks and dolines developed in zones close to overlying strata. The nature and extent of subsurface dissolutional voids is poorly understood and field tests using artificial tracers were carried out to investigate their structure. Tracer testing from stream sinks demonstrated rapid groundwater flow (up to 5 km/day) confirming previous Chalk tracer studies. However tracer attenuation was variable with extremely low tracer recoveries at some sites indicating that not all flowpaths fed by stream sinks comprise fully connected conduit systems. A conceptual model of flow along multiple flowpaths comprising a distributary network around stream sinks and a tributary network around springs with variable connectivity between the two is presented. The proposed conceptual model of flowpath structure in the Chalk resembles previous models of the early stages of speleogenesis. It is suggested that groundwater flow in the Chalk and other mildly karstic aquifers occurs within complex small scale dissolutional networks, in which there is a high degree of connectivity between larger conduits, fissures, and a primary fracture network. Advection from karstic channels into smaller voids results in high attenuation of solutes and particulates providing a degree of protection to groundwater outlets that is not seen in more highly karstic aquifers.