Article courtesy of British Geological Survey | February 17, 2014 | Shared as educational material
Groundwater levels in some regions of the UK are currently at record highs. This has resulted in ongoing groundwater flooding, particularly in the south and south-west of the UK.
This flooding is located on the Chalk outcrop and also on the floodplain gravels associated with the region’s major rivers.
Although groundwater levels, and the incidences of groundwater flooding, will decrease in the floodplain gravels once the rivers return to more normal flows, due to the nature of groundwater movement in the Chalk aquifer, groundwater flooding may persist in some areas for weeks or months ahead.
Impact of exceptional rainfall
The winter of 2013–14 has seen record rainfall in the UK. December rainfall was 154 per cent of the average for the month across the country. This was followed by the wettest January on record for Southern England.
Infiltration of December rain filled up much of the available storage in the ground (soil, gravels, rocks) in many areas. Much of the subsequent rainfall therefore had nowhere to go except to run off the land, moving quickly to rivers and resulting in widespread fluvial flooding.
Read more in the On the water front blog | January 2014 overview
Groundwater levels in late 2013
Groundwater levels at the end of Autumn 2013 were around the average for that time of the year, slightly lower in some regions.
Groundwater levels that are deep below the ground surface are usually slow to respond to rainfall due to the slow downward movement of water to the water table. However, the amount of rainfall that occurred in December in some regions meant that some groundwater levels did start to rise quickly. Due to the low to average starting point these rises were not initially problematic in the majority of aquifers in the south.
Groundwater flooding in permeable sediments near rivers
There was, however, some groundwater flooding in locations where highly permeable deposits are in contact with major rivers. Here the water table is close to the ground surface throughout the year, but when rivers are in flood, water flows quickly from the river into the floodplain sediments causing the water table to rise away from the area affected by river overbanking, potentially flooding basements and low lying property and infrastructure.
This type of groundwater flooding occurs within days of river flooding but equally flood waters can recede during a similar period of time once river flows return to near normal rates.
Locations within cities such as Oxford, on gravel deposits associated with the River Thames, suffered this type of flooding along with river flooding in January 2014.
West London permeable sediments
Permeable floodplain sediments also occur in West London. These are much more extensive than further upstream in the Thames catchment and the rise in river levels would not normally result in widespread associated groundwater flooding. However, the winter rainfall has seen a period of extended high river levels, unprecedented in recent decades, which has meant a long period of flow of river water into the floodplain sediments. BGS expects that groundwater flooding is being under-reported in this area as the focus is on river flooding. It is possible that the period of flooding may be extended by groundwater flooding which will persist once river flood waters abate.
Groundwater levels in the Chalk of southern England
The high rainfall in the southern part of the UK throughout January and February means that groundwater levels in our bedrock aquifers, such as the Chalk that underlies around 400 000 properties, has continued to rise.
The BGS collects groundwater level data from a series of 32 boreholes across the UK and this shows record groundwater levels in the Chalk of the south and south-west. For example, in the Chilgrove House well, which has been monitored continuously for 179 years, the groundwater level has never been as high.
These record groundwater levels are now starting to result in groundwater flooding. The Environment Agency is reporting that this is affecting properties, roads, the rail network and agricultural land and is also surcharging sewerage systems.
Flooding in Chalk areas could be long lasting
Rainfall recharges the Chalk over a large area, but the areas where groundwater discharges, as springs and to rivers, is limited. This, combined with the relatively slow movement of groundwater in Chalk compared with rivers, means that it will take a long time for groundwater levels to return to normal once the heavy rainfall stops, and therefore continued groundwater flooding.
This can last for weeks and in some cases months, as was seen in the largest groundwater flooding event previously documented, in 2001.
Groundwater flooding elsewhere in the UK
In the northern and eastern Chalk, groundwater levels remain below average. In the Permo-Triassic sandstones, groundwater levels are exceptionally high in the north-west and south-west of England and in other aquifers, such as the Jurassic Limestones, levels are typically above average. However, there has been no widespread flooding associated with these aquifers.