Thames Water drainage Scheme and the Trees in Blundells Copse

 

Report of meeting in Blundells Copse and nearby Bran Close Green, Mon 14 Jan. 2008

 

Present: James Hern: Halcrow (hired by Thames Water for this project) Steve Radford (RBC, i/c care and maintenance of RBC trees) ,Sarah Hanson (RBC Planning Dept. i/c TPO etc )

Philip Allen, Ron and Sheila Smith, Stuart Faulkener, Liz Ellis, Jenny Cottee  (members of Tilehurst Globe)

Background: The meeting had been arranged following the Globe meeting to discuss the flood alleviation scheme in December. The purpose was to find the location of the pipes etc that would be least damaging to public amenities (Blundell's Copse etc). Whilst Thames Water have permitted development powers to undertake any scheme they wish underground, only needing planning permission for above-ground work, they wish to work with RBC and the local community to find a mutually agreed scheme.

Design Note: All levels, distances and measurements are subject to change as the design is still progressing.  The finalized design is expected to be completed in February.

Underground Storage tank. The cylindrical vertical   tank (approximately 10.5m diameter, 15m, depth) will be buried underground with a small concrete top (which will be flush with ground level) for maintenance work on the pumps (approx 4m x4m) visible at about ground level. There will be an access track leading to the tank   made of plastic mesh to protect the grass. To minimize the visual impact of the tank, Halcrow propose to locate it towards the edge of the green area nearest Thicket Road. The concrete cap will thus be shielded   in some degree from sight and will not be on the main path to the steps. Subsidence risks are being taken into consideration. The tank will be constructed by progressively lowering sections of pipe.

Pump Kiosk .The kiosk (approximately 2.4m high, 1.2m deep, 3.4m wide) will house the pump in a secure steel container (dark green). It will have a door (s) for access to service the pump etc. Halcrow believe the best place for the kiosk is close to the fence near the tank. They are uncertain whether the kiosk will be best placed on the copse side of the fence- possibly moving the fence, or abutting the fence, but on the green side. There will need to be a padlocked   gate if the kiosk is inside the wood. The advantage of being inside is that would be less visually obtrusive. It might be possible to have it at a slightly lower level. The design of the door (s) and gate would need attention.

Pipes The 0.45m inlet pipe leading to the storage tank from Ogmore Close will be installed using a microtunnelling machine. The exit pipe will be in the region of 0.1m diameter and will be laid from the tank to the Bran Close, under the grass at a depth of between 1-2m.

Trees in Blundell’s Copse The most delicate decisions regarding the pipes concern the path of the entrance pipe. The pipe needs to be laid underground   from the garden of 21 Ogmore Close, across part of the Copse including the accessible path, into the green to join the tank. The vast majority of tree roots will be in the top 1-1.5m of soil. There are significant oak trees in the area near the proposed path. In discussing the route the group noted that oak trees have tap roots. Deeper than 1m the tap   root is likely to be subdivided into many smaller roots. The root ball extends to the area likely to be larger than the canopy of the tree or ‘drip line’. The Root Protection Area (RPA)[1] is the significant area that should be avoided at the depth that contains roots. The proposed path of the feeder pipe is shown running between trees, at least 3m from all mature trees, but within the canopy of several trees. The depth of the pipe underground   will be approximately 1.5-2m at the Ogmore Close entrance to the Copse, rising to 2.5m under the path, and 5m at the top of the Copse- i.e. the entrance to the green. In the proximity to the two oak trees the depth of the pipe will be 3.5m and 5.5m respectively.  These depths are approximate at present and the direction and depths will be finalized on completion of the design work. 

Problems that might occur when the boring is undertaken. If the drilling hits an obstacle it may have to stop. Possibly be left in place and a new route taken. If subsidence to the path occurs   remediation work would be undertaken immediately. The access to the copse via bran Close will be maintained all the time whilst work is in progress. At present, it is expected that access to the site by lorries, diggers etc will be via Corwen Rd only.

Community benefits the scheme will be finalized by mid February so proposals for improvements etc need to be sorted out by then, with proposals made by say end of January.

 

Record prepared by J Cottee

 

 

[1] This area is defined as 'an area surrounding the tress that contains sufficient rooting volume to ensure the survival of the tree'.  It is calculated by taking the diameter of the trunk in mm (at 1.5m above ground level) X 12, then divide that by 1000 to give you the radius of the area in metres.  The RPA often extend further than the drip line.

 

 

 

 


[1] This area is defined as 'an area surrounding the tress that contains sufficient rooting volume to ensure the survival of the tree'.  It is calculated by taking the diameter of the trunk in mm (at 1.5m above ground level) X 12, then divide that by 1000 to give you the radius of the area in metres.  The RPA often extend further than the drip line.