the role of groundwater in flooding events

Project Rationale

Project Diary


FLOOD 1: The role of groundwater in flooding.

Floods in Chalk catchments were, until recently, considered to result primarily from surface runoff. All existing flood prediction tools are based on this concept. These tools are normally used over a time scale of up to 48 hours. However, some recent flood events, including those in the Somme and at Brighton, have shown that groundwater played the principal role and time periods from weeks to months are involved.

During 2000-2001, large floods occurred in the northern part of France, Belgium and the southern part of United Kingdom, especially in valleys that are developed on Chalk outcrops. In many of these regions, a sudden rise of the groundwater level was recorded just before the floods. Another common characteristic was the long duration of the flood events which lasted weeks or months.

The key to understanding this flooding process is believed to lie in the unsaturated zone (the zone located between the water table and the ground surface), which is not currently part of any flood monitoring system in the World. Indeed all available observations and measurements suggest that the unsaturated zone plays a fundamental role in the generation of those groundwater floods, notably by storing water during periods of high rainfall and discharging this stored water after reaching a threshold of water content.

Preliminary modelling and chemical investigations of different types of water (springs, rivers, groundwater) carried out by BRGM in the Somme river catchment showed that groundwater was a major contributor to the triggering and the persistence of these floods.

Chalk has two principal kinds of porosity, the matrix (material) and fracture. Experimental measurements in the field (down to 3 m depth) or in the laboratory on chalk blocks, suggest that the water may first flow through the matrix at a low velocity. Then, when the water content of the unsaturated zone reaches a certain threshold, it is thought to flow through the fractures, with an increased velocity. This kind of dual behaviour, suggested by both theoretical and experimental work, could explain the triggering of groundwater flooding. If this threshold can be identified for different kinds of chalk, improved monitoring networks and better flood forecasting will result.

There is little information and reliable data available on the deep (below 3 m) unsaturated zone for three reasons: (i) the available technology for monitoring water content in the deep unsaturated zone is relatively complex and expensive. (ii) It requires an infrastructure that disturbs the studied environment and it is, therefore, difficult to rely on the accuracy of the observed data. (iii) It has had little economic interest until very recently (no resource potential for agricultural and water management purposes).

Better predictions of flood magnitude and duration will enable stakeholders to identify vulnerable elements within a community and more accurately assess risk. This new and different approach will also minimise false alarms since it will take account of the state of the groundwater system and the severity of the rainfall event. Authorities responsible for flood warning and relief will be able to take more appropriate and timely decisions. This project will be the first attempt anywhere in the world to instrument and monitor the unsaturated zone of an aquifer and the causes of groundwater induced floods.

The main objectives of the proposed project therefore are :
i. to understand the hydraulic behaviour of water flow in the unsaturated zone which leads to triggering of groundwater flood events.
ii. to develop unsaturated zone monitoring techniques, including non-intrusive ones such as Magnetic Resonance Sounding (MRS), to reduce cost and environmental impact, and to improve areal representation of the data.
iii. to produce more appropriate methodologies and tools for forecasting groundwater flood events capable of operating within a much longer timescale than is currently possible (i.e. days and weeks rather than hours).

Actions envisaged

The project is expected to be carried out over a 2 + 2 years basis (total of 4 years). To achieve the objectives set out above the following actions will be undertaken:

1. Develop a technical programme

i. A bibliographical study, preliminary analysis, and detailed water balance of the studied chalk aquifer system (including the unsaturated zone).
ii. Design and build experimental sites dedicated to the study of groundwater induced floods in the Brighton and Somme catchments, to monitor both the saturated and unsaturated zones (groundwater level, pore-water pressure, water content) at varying depths and times. Collect and analyse data for stream flow, flood profile in space and time, and climate (rainfall, evapotranspiration) in and around the project areas.
iii. Investigate the physics of groundwater flow in the different types of chalk and the different geological settings in the Brighton and Somme catchments.
iv. Develop a methodology for indirect and non-destructive measurement of water content in the unsaturated zone e.g. by refining Magnetic Resonance Sounding (MRS).
v. Improve the existing flood risk forecast models for the Brighton and Somme catchments by introducing a representation of the mechanisms operating in the unsaturated zone.

2. Prepare publicity, dissemination and communications

i. Appointment of a technical support person to build a project database and bilingual website. This site will be managed for the first year by a website database/GIS specialist and thereafter by the Research Officer(s) appointed to the project.
ii. Link project website to European Environmental Network
iii. Create project announcements and leaflets and organising project initiation team meetings and steering group meetings with the Environment Agency, local and regional government officers and industrial partners.

3. Establish an Advisory Group

The project will be supervised by an advisory group made up of engineers, scientists, local authorities and end-users from France and the U.K. Its composition will be established within 30 days after the issuing of the Grant Offer letters.This committee will act independently to assess all aspects of the work and ensure the project produces outcomes beneficial to the region. Advisory Group members have already reviewed and contributed to the application.

Target group

Local and regional governments, their agencies including the Environment Agency and the Highways Agency and water companies with responsibility for flood control and flood relief, populations affected by flooding events and insurance companies will all benefit from this project . This project will:
- improve flood risk forecasting, limit damage to properties and reduce the cost to society,
- put the INTERREG region on the global research map as the first attempt to instrument, monitor and understand groundwater behaviour in the unsaturated zone.

Cross-border nature of the project

The chalk geology shared by both countries has been subject to major and regular flooding events affecting communities and communications in many parts of this INTERREG region. Despite the reviews carried out in France and U.K., the causes of flooding events, particularly their duration, are still not well understood at this stage. Hypotheses have been made, but not verified.
BGS and BRGM are leading authorities on chalk groundwater with complementary skills. The University of Brighton has specialist expertise on all aspects of the Chalk of England and France. Cross-border sharing of this expertise, exchange of skills, software, techniques and personnel and the creation of a cross-border team to carry-out the investigations provides a better chance of gaining a sound understanding of these events which could not be achieved by one of the institutions alone.

Impact on wider cross-border development

The Chalk is the major source of potable water in both northern France and southern England (Brighton and Hove City obtaining >90% of its drinking water from the Chalk). The understanding of the behaviour of groundwater in the Chalk will have wider implications for the whole of the Anglo-Paris Basin in terms of water resource development, control of pollution, sustainability of river eco-systems as well as flooding. In addition, many other areas of Europe such as Denmark, Germany and Poland have large chalk or carbonaceous aquifer systems: these will also benefit from the results obtained in this project.

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