Devon County Council's Flood Risk Management Team – in its role as Lead Local Flood Authority – recently objected to Aggregate Industries' plans to quarry Straitgate Farm, and requested cross sections to show how the infiltration areas to prevent downstream flooding could be accommodated within the areas indicated:
The applicant should provide cross sections of all the proposed basins.
However, Aggregate Industries – in its "final response", submitted yesterday – has refused to supply cross sections. The company said:
The void created by mineral extraction acts as the infiltration basin so there is not a single cross section as this will change as extraction progresses and Figures A2.1, A2.2 and A2.3 dated July 2017 to cover all phases have been submitted.
This is one of those figures – A2.2 – with the newly revised MWWT superimposed:
It clearly shows that Aggregate Industries' infiltration plans can’t work as described. They could not be dug to the described 3m depth without breaching the maximum water table – the MWWT. Furthermore, the newly revised MWWT contours show these infiltration areas would not be level, and run-off would not infiltrate evenly along their lengths.
Furthermore, the infiltration basin capacities have been calculated on the basis of the base along the working face of the bund being level. As it would be sloping, the infiltration capacity would obviously be reduced. The following is taken from the out-of-date Flood Risk Assessment:
As the infiltration basin would be sloping along the eastern boundary of the excavation areas, then water would not collect evenly along the entire length of L – "the length of the working face of the bund" in the above diagram. The capacity would be reduced. The following figures in the FRA are therefore incorrect:
For the expected 4% slope of the infiltration area, a total surface area of 1.87 ha would be required to provide the 11,980 m3 of storage based on the equations above. Water would back up against the working face to a depth of 1.21m (hw) and would form a pool of width 30.36 m (d) along the working face of length 650m (L) (representing the eastern boundary of the site).
A best estimate runoff scenario has been calculated for the entire proposed excavation area (25.13 Ha) for the 1% AEP plus Climate Change extreme rainfall event, where 84 % winter rainfall runoff occurs. All runoff can be infiltrated via the provision of the storage volumes specified in the void base for each phase to allow for infiltration into the BSPB. The volumes are: Phase 1: 6,747m3; Phase 2: 7,761m3; and Phase 3: 4,546m3. Bunds will need to be constructed at the downslope end of any infiltration basins to prevent uncontrolled discharge to local watercourses. A total of 364m3 of storage to facilitate infiltration should be provide for the access road/hardstanding.
Plainly, Aggregate Industries knows its infiltration plans cannot work as described. Otherwise, why not supply some simple cross-sections to allay concerns?
Why won’t Aggregate Industries come clean? Because, to produce infiltration trenches that could function as described without breaching the MWWT would lose yet another large chunk of resource – to the east of the red line shown below, as a minimum. The red line indicates where a 4m trench – backfilled with 1m of material to produce a 3m face, and following existing contours in order that infiltration areas would mimic existing drainage – could be dug in phase 2 of the operations, that would be level and would not breach the MWWT.
How has it got to the stage that something so critical to this proposal – the flood risk to downstream communities – is treated so flippantly by Aggregate Industries? Flooding has been one of the major issues right from the off – and yet we have an out of date FRA, an FRA that has not been informed by the recently recorded elevated groundwater levels, an FRA that contains flood mitigation plans that have NO prospect of working, and a company not prepared to show how they could work.
