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Latest news from DIG (Drainage Improvement Group)

Please see the following update from Aldbourne’s DIG (Drainage Improvement Group):

Things progress…

On 21st November DIG held the 4th meeting of the “All Agencies” working group. Representatives attended from Thames Water, The Environment Agency, Wiltshire Council , Aldbourne PC and DIG.

Discussion at the meeting covered a number of topics:

1. Groundwater Levels lower than this time last year
The EA presented a graph recording groundwater levels at Water Acre (above the village in the fields along Lottage Road). This shows that current levels are low compared to last winter, but marginally higher than in the previous 2011/12 winter.

2. WS Atkins preparations for their Hydrological study of the village
In late October DIG and the other agencies held a meeting in the village with the WSA team to brief them on the problem and the local geography. Funding is in place from DEFRA, but its future availability is dependent on initial output from the study being presented within the current financial year, which ends on 31st March 2014. This appears to be in hand and WSA expect to put forward a more detailed scope for the project before Christmas

3. Some first steps to slow infiltration of water into the sewers
Following the Connectivity Study carried out in the summer, TW have produced revised maps showing the location of sewers in the lower parts of the village – both public and private. The study had revealed a number of locations where surface run-off water entered the sewage system from private property, but this appears to be a relatively minor cause of sewer flooding, with high groundwater levels the fundamental problem.

There was some discussion as to whether the sewers had adequate capacity, given the scale of building in the village since they were installed in the 1950’s. TW presented graphs illustrating the levels in the sewers during November at two locations – the Square and Southfield – which showed that even on days with heavy rain they were running within capacity. It appears that problems only arise when groundwater is high and infiltrates the sewers.

As an immediate action TW confirmed that replacement manhole covers had been installed in a number of locations in Lottage Road, with seals to prevent surface water flooding down into the sewers at times of heavy rainfall or high groundwater levels. They would still allow the sewers to surcharge upwards, if they reached capacity.

It is still not clear exactly where groundwater infiltration is occurring. TW will be inspecting the sections of the sewers which have been lined, to ascertain if this is effective in preventing it. The WSA study may help to pinpoint areas of concern. DIG will attempt to identify the location of springs with help from the villagers.

The EA now require water companies to produce a detailed Infiltration Reduction Plan before they will consider applications to discharge excess volumes beyond the capacity of the sewers into a watercourse. Essentially these have to show that all reasonable steps to address ground or surface water infiltration are planned or being taken, so that any discharge is a last step, not the first one. TW will be producing one for Aldbourne and this should draw together a lot of these issues.

4. Clearing of the Bourne, culverts and storm water drains in the village
A group of volunteers organised by DIG have cleared most of the weed from the Bourne along South Street (leaving about 20% in place in accordance with the permission given by the EA). Wiltshire Council subsequently arranged its removal to an approved site. Thanks to all those who helped with this.

Wiltshire Council have also arranged for their contractors to clear some rubble left after the culverts under the Square were jetted, apparently on behalf of Hannick Homes, the developers of Barnes Yard.

A couple of culverts along South Street which are partly blocked still need jetting and DIG is taking this up with Wiltshire Council.

Longer term a programme for regular clearance will be developed, guided by the output from the WS Atkins study, to inform how this can be most effective in improving groundwater drainage, and the EA and ARK, to minimise any harm to the natural environment.

5. Help is needed from villagers to locate springs
DIG require information to help with the location of springs in and around the village. If you have any information please contact Jim Oliver, see details below.

For more information about DIG please see this page in the community section or contact Jim directly.

Posted by: RB on 5th December 2013