BGS on Channel 4 flooding documentary

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.

An image from inside the borehole casing showing David lowering the water level dipper down into the hole.

December 2012 overview

Groundwater levels - December 2012

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 (since 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].

BGS hydrogeologists meet BBC's David Shukman

A screenshot from the BBC website.
Yes, they've got our name wrong!  Survey, not Society!

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.

Andrew McKenzie appeared in the lead item on the 6 O'Clock and 10 O'Clock news last week - you can view the clip on the BBC website.  That's our Wallingford office in the background.  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.

Andrew was also interviewed on BBC Radio Berkshire's breakfast show on Monday.

November 2012 overview

Groundwater levels - November 2012

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].

October 2012 overview

Groundwater levels - October 2012.

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].

2012 so far: an exceptional year

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.

What we expect

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.

The unexpected

The dashed black line shows the monthly average water level for this borehole. The red ellipse highlights the winter of 2011/12 when groundwater levels did not recover as usual.  The blue ellipse shows how groundwater levels rose significantly in the summer of 2012, a time when we would expect to see levels fall.

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".

Whatever next?

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.