Although often occurring less frequently than fluvial (river) flooding, the effects of groundwater flooding can be as devastating, if not more damaging. Groundwater flooding is caused by water emerging from underground, for example, from areas underlain by chalk and aquifers in contact with the sea. Groundwater flooding can cause flooding of basements, ground floors, farmland and roads and can leave areas submerged for months.
The EU Floods Directive requires Member States to carry out a preliminary assessment by 2011 to identify areas at risk of flooding. Member States must draw up flood risk maps by 2013 and establish flood risk management plans by 2015. In the UK, data collection and collation on groundwater flooding has been locally focused and conducted gradually, area by area. This research summarises possible data and techniques that could be used to produce groundwater flood maps and flood management plans by examining existing data on groundwater flooding in the UK.
Before Spring 2006, there was no single organisation responsible for monitoring groundwater flooding in the UK. However, some valuable data were collected during the groundwater flood events of 2000/2001 and 2002/2003. The lack of consistent records of groundwater flooding indicates a need for a database that captures information on flooding from all sources to enable a joined-up approach to flood risk management.
Records of groundwater flooding are a source for identifying susceptible areas. However, under-reporting and uncertainty make it difficult to assess the likelihood, as well as the possible depth, of flooding. The study suggests that a more comprehensive approach would be to apply our understanding of flooding mechanisms in areas where flood risks exist, such as chalk valleys.
Hazard maps developed for these areas could assess the frequency of drivers such as rainfall or prior groundwater levels. They could also assess surrogates (or measurements that can represent flooding levels) such as spring discharge rates or flows from rivers draining aquifers. The combination of drivers and surrogates could also be examined.
Alongside hazard maps, risk maps should indicate potential adverse consequences on population, economic activity and vulnerable buildings.
To further meet the requirements of the EU Flood Directive, the study recommends a UK information service to assist in the preparation for potential groundwater flooding. This includes making data and maps available to the public, helping develop community action plans and expanding the existing fluvial flood warning service to communities at risk from groundwater flooding.