The issue of surface water management is critical at Straitgate Farm if any quarrying is to be permitted.
Surface run-off must not increase flooding downstream, it must maintain stream flows and groundwater recharge, and must not create permanent bodies of water that might attract birds and cause a birdstrike issue for planes landing at Exeter Airport. DCC warned in 2015:
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. 17
And again in 2017:
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. 2.18
Over the years countless consultants' reports have been written on the subject; they are not always consistent. Take the Updated Flood Risk Assessment for example:
Flows from the spring line adjacent to the Site may become more responsive to rainfall events as the removal of part of the unsaturated zone will decrease the time taken for water to recharge the aquifer. 3.5.4
Whilst this response a few months later from the same group of consultants said the opposite:
This increase in the proportion of rainfall passing to groundwater recharge will act to slightly flatten the hydrographs of the watercourses during storm events... reducing downstream impact. 2.11.3
It’s bad news when even the consultants are confused.
AI’s current answer to surface water management at Straitgate is to have infiltration areas along the eastern boundary of the proposed extraction area, shown in blue on the maps below:
Quarrying operations will be excavating down to... 3m along the eastern boundary... 2.18.3
The proposed infiltration areas at the Straitgate Quarry [sic] have been designed to meet national guidance on design events (1 in 100 year with climate change). 2.18.5
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. 6.1.9
But from recent posts – in particular AI has ‘forgotten’ one 1990 borehole – that puts groundwater 2.8M ABOVE MWWT – you can see that, as things stand, this can't work.
Surface water would obviously not soak away in these infiltration areas in the way that AI and Amec hope, not if groundwater is as close to the surface along this boundary as these piezometers have recorded: PZ2017/02: 1.72m, PZ2017/03: 0.43m, SG1990/021: 1.26m.
So unless the intention is to flood downstream communities, and have long stretches of ponding along the eastern boundary (meaning a run-in with Exeter Airport), AI’s flooding consultants will need to go back the drawing board.
