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The Jubilee River story - Taplow Mill Leat - £1.4m repair cost - Technical Report (Apr 2007)
ENVIRONMENT AGENCY Agenda No. 10
THAMES REGION Report No. T/RFDC/07/16
REGIONAL FLOOD DEFENCE COMMITTEE
MEETING: 12 APRIL 2007
PAPER BY: REGIONAL FLOOD RISK MANAGER
SUBJECT: JUBILEE RIVER: TAPLOW MILL LEAT
Recommendation: The Committee is requested to note the deterioration in the condition of a section of embankment at Taplow Mill Leat and support proposals to undertake urgent remedial works.
1.1 Over recent meetings members have been updated orally on several occasions regarding the condition of a section of embankment at Taplow Mill Leat. This paper summarises the current position and identifies the need for urgent remedial works.
2.1 The Jubilee River forms the main component of the Maidenhead, Windsor and Eton Flood Alleviation Scheme. Taplow Mill Leat is located immediately upstream of the main control structures for the Jubilee River at Taplow Mill, Maidenhead (see location plan, appendix A). The leat includes a 260m long embankment constructed from reinforced earth (plastic geotextile wrapped around granular material) with a central clay core of maximum height 2m. The embankment was constructed in 2001 and replaced an old embankment when the leat was deepened and widened as part of the construction of the Jubilee River.
2.2 Failure of the embankment would create an alternative flow path across Glen Island immediately upstream of Taplow Mill Sluice, effectively creating an alternative, but uncontrolled weir adjacent to Boulters Weir. Exact flow paths would depend on the type and location of any failure combined with river conditions at that time, but clearly the current standard of defence afforded by the Jubilee River would not be achieved.
3.1 Taplow Mill Leat Embankment had only been constructed for 18 months when localised seepage was observed. Seepage control measures were installed in 2002 as part of the Maidenhead, Windsor and Eton Flood Alleviation Scheme including both chemical grouting and the installation of a 30m length of steel sheet piling.
3.2 Following further seepage in 2003 a second phase of chemical grouting was carried out in summer 2003 which again reduced the seepage, but did not eliminate it. Residual seepage is now controlled by a localised drainage system installed during June 2006 funded by FDGIA. A more detailed study was requested via Atkins, a consultant who worked with the Environment Agency to address the remedial works that were necessary to the Jubilee River following the 2003 flood event.
3.3 Members should note that staff continue to monitor the situation. It was through this routine monitoring that we were alerted to the potential problems with the embankment.
Atkins Inspections – August 2006 & January 2007
4.1 Atkins brief was to undertake an initial assessment based on a visual inspection utilising historical data only. The inspection confirmed that the embankment was deformed, poorly compacted, lacked vegetation cover, included damaged sections of geotextile reinforcement and was both narrow and over-steep when compared to the design drawings. Whilst there were no obvious signs of structural failure concerns were raised about the embankments ability to withstand a significant flood event. Atkins recommended a number of actions, including undertaking a full geotechnical investigation and topographic survey to enable design calculation checks to be undertaken under various flood flow conditions.
4.2 Atkins second site visit in January 2007 indicated further deterioration with more pronounced local slumping and voiding evident. This inspection was made easier by the natural winter die-back of the remaining vegetation cover. The geotechnical investigation highlighted that the clay core itself was breaking down with possible deterioration of the chalk layer at depth.
4.3 The surveys have enabled Atkins to undertake a series of calculations on both the importance of the geotextile reinforcement and factor of safety against possible slip failures. Structural stability of the embankment is dependent on the tensile strength of the geotextile, which under limited seepage conditions can be shown to be just adequate. This position is not achieved should seepage occur through the clay core.
4.4 Various slip and breach failures were also tested with a factor of safety of 1.05 obtained for a maximum flood level with seepage through the fissured clay. Failure of the crest material (above the top reinforcement level) has a factor of safety of 0.6 in the same conditions. In either case both factors of safety are significantly below accepted engineering tolerances (typically 1.3 – 1.5) and the Report concludes that the embankment itself is now beginning to show signs of structural distress and may be unable to withstand a significant flood event. Urgent action is recommended to ensure ongoing structural stability and allow the Jubilee River to be used up to its design capacity.
5.1 Staff continue to monitor the embankment and are reporting ongoing minor deterioration in its condition. Whilst monitoring will alert us to any sudden deterioration this is only a short term solution. A full structural solution is proposed that when implemented will address the embankment condition and the seepage prior to next winter.
5.2 In order to ensure the channel is fully available for next winter, only a limited number of options can be considered:
Waterproof river face of the existing embankment.
Rebuild existing embankment.
Install continuous line of steel sheet piles to depth.
5.3 Whilst waterproofing the face will reduce seepage through the embankment it does not address the deeper seepage issues and raises serious landscape concerns. Rebuilding the embankment would again resolve the direct embankment issues but does not address the deeper seepage, nor is it practical given the time constraints. The only viable option to guarantee structural integrity before next winter is to install a continuous line of steel sheet cut-off piles along the line of the existing embankment. Consultation on environmental mitigation is ongoing.
6.1 Committee should note several immediate issues raised by the problems at Taplow Mill:
Doing nothing is not an option. The embankment remains under Agency ownership (the handback process agreed as part of the CPO process has been put on hold whilst a solution is implemented), integrity is now questioned and it is considered unlikely that we can safely operate the Jubilee River to its design capacity.
The embankment will only be repaired to its existing design level and there will be no impact on downstream communities as a result of this work. This guarantee cannot be given should failure of the existing embankment occur.
There may be issues regarding liability for the condition of the embankment. Tony Plytas, Thames Regional Solicitor is investigating whether we are able to take any further action and will be attending the meeting to update the Committee.
The works will be undertaken as permitted development using the Environment Agency’s statutory powers. This will not preclude consultation with the Local Planning Authority, landowners and other key parties.
Environmental mitigation will be fully assessed but may not be implemented until spring 2008 due to growing seasons.
The 2007/08 FDGIA Capital funding allocation was agreed ahead of the Atkins report and it is not clear at this stage whether there is scope for further FDGIA allocation to cover these repairs. A further request for FDGIA funding has been made, should it become available. In the interim this scheme is included in the FDGIA contingency list in the Regional Levy Programme paper, with a current estimated cost of £1.4m.
7.1 Committee is asked to support our proposals to undertake urgent remedial works to the existing embankment at Taplow Mill Leat. Committee will be kept fully updated on our proposals through to implementation.
No specific diversity implications are raised by this paper.
Health and Safety
As well as the loss of flow and the impact on design capacity of the Jubilee River, there is a potential health and safety implication of the failure of the embankment. Regular monitoring of the asset will reduce the risk of any sudden or unexpected failure. This monitoring will increase when higher flows are passed down the channel.
The current estimated cost for the repairs is £1.4m. As explained in paragraph 6.1 above, funding of this essential work has not been fully resolved. Further updates will be provided when available.
Regional Flood Risk Manager
APPENDIX A – SITE LOCATION
Taplow Mill Leat Embankment – site location