The issue of surface water management is critical at Straitgate Farm if any quarrying is to be permitted. In 2015, Devon County Council warned:
The surface water management is inextricably connected to Flood Risk Management/Airport safeguarding and the need to maintain and recharge watercourses. This issue is so important in terms of the likely significant impacts of the proposal the MPA would wish to ensure that a SWM scheme can be designed to meet all of the requirements identified in advance of the determination of this application.
In 2017, in its Regulation 22 request, Devon County Council asked:
The surface water management strategy should identify location of the infiltration features and how these fit into the site; It should be confirmed how the water will be collected in the catchment for these headwaters to supply the stream heads (bearing in mind airport safeguarding requirements) and the proposed phasing of the site. This should include detailed design regarding their size, details whether infiltration is permitted at the proposed locations and where targeted at areas where infiltration is required to support the spring lines of the existing watercourses (as commented by the Environment Agency), together with details of exceedance pathways. Due to the nature of the flood risk downstream, greenfield performance should be achieved and the site should be designed to the worst case scenario as detailed in the FRA and should ensure that drainage from the site mimics, as closely as possible, the natural hydrograph in perpetuity. The MPA will wish to have it demonstrated that the applicant has engaged with the LLFA, the EA and the Airport to design a scheme that can accord with all of their various requirements.
In answer, Aggregate Industries said:
2.18.2 Figures A2.1, A2.2 and A2.3 in Annex C of the submitted Surface Water Management Plan, provided in Appendix C of the Flood Risk Assessment (FRA) (Amec Foster Wheeler document reference: 33679rr145i4) details the proposed areas of the void base for phases 1, 2 and 3 where runoff will collect as it infiltrates. The figures show the sub-catchment divides, where gradient parallel bunds and contouring will be required to ensure the proportion of runoff draining to different sub-catchments remains the same during the extraction process (i.e. it is targeted to the required sub-catchment).
We have posted before that Aggregate Industries' infiltration plans were unworkable.
Since then, the maximum winter water table contours have been revised, and the problem has become more apparent.
Whilst existing ground elevation contours run parallel to the eastern extraction boundary, the base of any quarry as defined by the newly revised MWWT contours would not.
As shown above – where the infiltration areas overlay the revised MWWT contours – the proposed base of extraction in the areas set aside for infiltration in phases 1 and 2 does not mimic ground elevations to maintain pre-extraction infiltration characteristics. In phase 2, for example, the base of extraction in the infiltration area falls from 140mAOD to less than 135m.
Surface water running off the extraction site would not infiltrate equally along the eastern extraction boundary. In phase 2, run-off would flow to the north-east, directing water away from the Cadhay Wood stream, the stream feeding Cadhay's mediaeval fishponds, and away from the Cadhay spring, the spring supplying drinking water to Grade I listed Cadhay and a whole host of others. In phase 1, water would be directed towards the middle of the infiltration area.
How much water are we talking about? This table is from the Flood Risk Assessment.
Of course, this not only has implications for flooding, springs, stream flows and airport safeguarding, but also for the resource available – freshly cut to 1.06 million tonnes.
