The immediate impact of the exceptional and prolonged rainfall that we have experienced, over much of Britain in late December 2013 and early January 2014, was high river flows and flood alerts as water, unable to infiltrate into saturated ground, ran off into rivers.
As the rain eased most rivers returned quite quickly to more normal flows — but in some areas the effect of the wet weather may still linger as groundwater flooding.
Groundwater flooding occurs when high rainfall raises water levels in aquifers well above the normal range, to the point where water flows out onto the land surface or into cellars and basements, often causing sewers to back up
Because groundwater flows fairly slowly through the soil and rocks groundwater can continue to rise for weeks, or sometimes months, after a rainfall event.
Aquifers were generally at or near normal levels before the storms, which means there is some capacity to absorb rainfall without flooding, but inevitably there will be some localities where problems may occur, and since the rainfall has ended, groundwater flooding alerts have been issued in several catchments in Southern England.
We have seen the water rise by more than 30 metres in some observation wells in the Chalk of Hampshire.
Recent groundwater flooding occurred in the floodplain of the River Thames in Oxford. The BGS has an ongoing project here investigating the mechanisms and risk of groundwater flooding in shallow floodplain sediments as part of the research being undertaken in the BGS Oxford Field Observatory.
Water levels, both river and groundwater, are being monitored and this monitoring network is provided very useful data from the recent flood event.
Project leader David Macdonald, whose own house in Oxford was surrounded by flood waters, kept an eye on the situation from the ground. Flooding from the Thames in south Oxford reached levels higher level than in 2007.
Groundwater flood levels have also been higher than in 2007 and are the cause of flooding in some low-lying properties that are not being directly inundated by river flood waters. The permeable sediments beneath provide a subsurface route for flood waters to bypass high ground that would otherwise protect these low-lying areas.
Groundwater can flow into sewers and storm drains when the water table is high which contributes to the drainage problems that are occurring during the floods, such as getting rid of household waste water. The surcharging of sewers is also proving to be a big problem.
Work is being undertaken by the BGS to link the complex river flood model of Oxford with a groundwater model to improve the assessment of the risk of groundwater flooding. Data being collected at the moment will help to check the results of the linked models.
Oxford is one of several areas being monitored by BGS scientists where groundwater flooding is known to be a problem. We will be going into the field to collect data that can be used to refine our maps of groundwater flood-prone areas and help improve our ability to forecast and model these events.
Keep in touch with the latest flood situation in your area via local alerts provided by the relevant environment regulators, for example, in England it is the Environment Agency.
If you have any general questions on groundwater flooding please see our Frequently Asked Questions.
For more information contact David Macdonald