H2Ogeo provides hydrogeology and environmental consultancy on matters relating to groundwater and land contamination.
What is Groundwater Flooding?
Groundwater flooding occurs when the water table in permeable rocks rises to enter basements/cellars or comes up above the ground surface. Groundwater flooding is not necessarily linked directly to specific rainfall events and generally remains for a longer duration than other causes of flooding such as surface water or river flooding (possibly lasting for weeks or even months).
This time last year the BBC reported that the British Geological Survey had identified that groundwater levels were so high in some parts of the country that flooding was likely to persist for weeks or even months. The Met Office reported that parts of England had experienced their wettest January since records began more than 100 years ago.
In response to these events in 2013/2014 Local Authorities have been requesting baseline groundwater elevation monitoring throughout winter months prior to granting planning permission for developments. This data is then used in the design and assessment of drainage solutions and foundations. H2Ogeo provides a range of groundwater monitoring services consisting of design and drilling supervision to elevation monitoring and reporting – for more information Contact Us.
Groundwater levels are generally highest in early spring and lowest in early autumn, it is important that the monitoring design and reporting period accounts for these seasonal fluctuations and captures the highest groundwater elevations.
Where are groundwater levels presently?
The Environment Agency provide water situation reports for England on a monthly basis and these are organised by regions and counties. They routinely measure, monitor and report on a range of hydrological parameters to assess the water situation across England. The reports consist of the following information:
- the amount of rain that falls;
- how dry the soils are and how much rain they can soak up;
- the amount of water flowing in rivers;
- the amount of water stored below ground in aquifers and above ground in reservoirs; and
- the outlook for river flow and groundwater over the coming months
The graphic below shows groundwater elevation data for 2013/2014 data versus this year for the end of November and December. It can be seen that groundwater levels in the south east of England are, on the whole, lower this year than they were previously with the exception of the monitoring point in Kent which reports groundwater levels as ‘Notably High’ in contrast with ‘Above Normal’.
Water Situation Reports
The Water Situation Report summary for December 2014 is shown below:
December’s rainfall was below average across England at 87% of the long term average, although regional totals ranged from 64% in the south-west to 125% in the north-west. Soil moisture deficits continued to decrease across most of England during the month and were generally close to or smaller than average by the end of the month. Monthly mean river flows increased at just over half of the indicator sites and were normal or higher for the time of year at the majority of sites. Groundwater levels increased at two-thirds of the updated indicator sites and were normal or higher for the time of year at all sites. Reservoir stocks increased at the majority of sites and stocks were normal or higher for the time of year at all but two sites. Overall reservoir stocks for England were 90% of total capacity at the end of December.
It has been recognised that there needs to better understanding of the risk from groundwater flooding. This has been due both to the occurrence of major flooding events clearly caused by groundwater and the inclusion of groundwater flood risk assessment in European and national legislation. The British Geological Survey identified the need for a dataset that would allow stakeholders to assess the potential for groundwater flooding to occur in an area, this data set is available from the BGS website.