This page shows the most recent entries of our blog, On the water front — groundwater matters at BGS, which relate to groundwater levels and the potential for groundwater flooding and groundwater drought.
January saw a continuation of the exceptionally wet weather that began in December 2013. For example, much of central and southern England received over twice the average rainfall for January.
The persistent rainfall has led to significant recharge across the southern Chalk outcrop, with new January maxima established for six Chalk boreholes. For example, Chilgrove House well has overflowed. We believe this well has the longest continuous observation of water levels in the UK - the record starts in 1836 - and such artesian conditions have only happened 6 times before. Groundwater emergence in bournes has also been observed, e.g. the South Winterbourne (Dorset), Aldbourne (Berkshire), Lavant (Hampshire) and Nailbourne (Kent).
Levels are now increasing in the northern and eastern Chalk, which received less rainfall in December, but these are still below average.
Above average levels have been observed in the Permo-Triassic sandstones of the Midlands and north Wales. In the north-west and south-west these have been exceptionally high, with record monthly levels recorded at Newbridge, Skirwith and Bussels.
In other aquifers, levels were typically above average, with Ampney Crucis (Jurassic limestone) registering exceptional levels.
With rising water levels in the southern Chalk, concerns over sewer surcharging and groundwater flooding heightened through January; flood alerts were widespread across the southern Chalk (from Dorset to Kent, but predominantly in the west), and were also issued for the Lower Greensand in south-west Surrey. The high water levels on the interfluves will gradually feed into the lower parts of catchments over a period of weeks to months, and it is likely groundwater flooding will persist well into the spring. In some of the major river valleys (e.g. the Thames Valley), elevated groundwater levels in superficial sands and gravels are exacerbating current flooding and the risk of future flooding.
For more information, see the hydrological summary for January 2014 [PDF].
David Macdonald appeared in last night's Channel 4 documentary, "The Year that Britain Flooded". As well as impressive footage of flood events, the programme explained some of the meteorological and hydrogeological processes which contributed to the flooding.
David used a water level dipper to measure the depth to water in one of our boreholes. The water level was close to the top of the borehole casing (metal tube), which sticks up above the ground level.
This borehole penetrates a confined aquifer: in this area, the Upper Greensand Formation is confined by overlying low permeability clayey chalk (chalk marl). The water level is frequently above ground level, so the borehole was designed with the extra casing above ground level to allow us to measure the water level accurately over a greater range. At times, including earlier this year, the water level rises even higher and water naturally flows out of the casing when the flange sealing the top is removed.
We expect to see groundwater levels rising in December, as this is part of the usual recharge season when soils are wet and rainwater can infiltrate into aquifers. However, the final month of 2012 was still exceptional - as the map shows, groundwater levels in over three quarters of our index wells exceeded their December average.
Record levels for December were seen in 7 index wells, e.g. Skirwith in the Sherwood Sandstone and Wetwang in the Chalk. The second highest levels on record were seen at a number of sites including the long record boreholes Chilgrove House (1836) and Compton (1894) in the South Downs. Further extensive groundwater flooding is probable given the high levels in the Chalk.
Groundwater flooding has been occurring in some Chalk areas, and groundwater discharge has been contributing to high flows in some streams and rivers for which the Environment Agency have issued flood alerts.
For more information, see the hydrological summary for December 2012 [PDF].
It's been more of a wet Christmas than a white one! The BBC have been interviewing BGS hydrogeologists to find out what impact recent rainfall has had on groundwater levels and the possibility of flooding.
We have a few boreholes on site that we use to monitor water levels. They are also useful for testing equipment like pumps and downhole logging probes before we take them into the field.
Groundwater levels in many areas have continued to rise during November. High levels this early in the winter mean there is a strong possibility of localised groundwater flooding in the coming months. Groundwater levels in areas of the western Chalk (e.g. South Winterborne in Dorset, Tilshead and the River Og in Wiltshire, King's Somborne and Bishop's Sutton in Hampshire and the Lamborne in Berkshire) have already risen to the Environment Agency's flood alert trigger levels.
Levels in the Chalk outcrops are generally above average for the time of year, rising by about 10 m at Chilgrove, Compton and Tilshead. Previous monthly maxima have been exceeded at Ashton Farm and West Woodyates during November.
For more information, see the hydrological summary for November 2012 [PDF].
We usually expect water levels to be around their lowest in early October, but this year has been very different, and aquifers have already been well topped up by the exceptional spring and summer rainfall. The water resources situation is therefore healthier than usual for this time of year. However, going into the late autumn and winter (the time of year when groundwater recharge normally occurs) with groundwater levels that are already anomalously high means an increased likelihood of groundwater flooding in susceptible areas.
Groundwater levels are particularly high in the Chalk of the south of England were groundwater flood warnings were already issued earlier in the autumn. For more information, see the hydrological summary for October 2012 [PDF].
Hydrogeologists got just as excited about the big national events of this year - the London Olympics, the Jubilee - as everyone else. But we were even more excited by the unusual pattern of rainfall which has led to some very interesting responses in groundwater levels.
Late autumn is usually when UK groundwater levels start to rise again. Plant growth slows at this time of year, and as the plants are taking up less water, soils 'wet up' so that rain can once again infiltrate down to replenish aquifers.
Across much of the UK the pattern has been different this year. Rainfall was lower than usual last winter, so groundwater levels did not rise as expected. Droughts were predicted and eventually declared in many areas. Exceptional rainfall between April and July then ended the drought, causing remarkable rises in groundwater levels. Groundwater hydrographs were practically inverted across the winter-spring-summer 2012 period, as illustrated by the example in the image.
An incisive overview of the hydrological events of this period has been published by CEH: "An overview of the 2010-12 drought and its dramatic termination".
We should now begin to see 'normal' autumn/winter recharge on top of the 'unusual' spring/summer recharge. This puts us in a good position in terms of water resources, but some areas will be more prone to groundwater flooding than at this point in most years.
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Contact Andrew McKenzie for further information