FLOOD1 project

Groundwater flooding in the Pang Valley.

The FLOOD1 project improved our understanding of the role that the unsaturated zone plays in controlling the occurrence and timing of groundwater flooding in Chalk catchments.

This understanding was used to develop an early warning system in an area of Brighton, in southern England, prone to groundwater flooding.

Flood-prone Chalk catchments

FLOOD1, a joint project between the French Geological Survey (BRGM), the University of Brighton and the BGS, was implemented under the INTERREG IIIa initiative of the European Union to develop appropriate early warning systems for groundwater flooding in Chalk catchments.

It was developed following the particularly severe groundwater floods of the winter of 2000–01 and focused on flooding in the Patcham area of Brighton and in the Somme Valley of northern France.

Research sites were set up to the north of Brighton and in the Hallue sub-catchment of the Somme. An additional site was established at East Ilsley in the Pang Valley of the Berkshire Downs.

Improving our understanding of groundwater flood mechanisms in the Chalk

Flood1 drilling site.

As a result of the studies carried out in FLOOD 1, our understanding of the hydraulic behaviour of water flow in the unsaturated zone of the Chalk has developed significantly.

It has long been known that the use of the term 'unsaturated zone' for that part of the Chalk between land surface and the water table is, to some extent, inappropriate due to the high pore water content which cannot be drained due to the small pore throat diameters which are characteristic of chalk.

The monitoring of pore tensions in the unsaturated zone of the Chalk has shown that, following recharge, a critical saturation can be reached that is immediately followed by a rapid rise in groundwater levels.

In times of extreme recharge (as in the winter of 2000–01) this can lead to initiation of flow in high permeability horizons at shallow depths which are normally dry.

Additionally the water table may even intercept the land surface. Both of these instances can give rise to groundwater flooding.

Developing a groundwater flood early warning system

An early warning system was developed for the Patcham area of Brighton that, within the context of the available data, provides a fit-for-purpose methodology for forecasting groundwater flood events in the Chalk and one that is capable of operating within a longer timescale than had previously been possible.

The early warning system takes into account both antecedent groundwater levels and the degree of saturation of the unsaturated zone of the Chalk. The methodology involves a set of nested models based on the research carried out in FLOOD1 and previous understanding of the hydrogeology of the Chalk.