Another major failing.
Aggregate Industries' plans to quarry Straitgate Farm propose infiltration areas along the eastern boundary to control flooding, maintain groundwater recharge for springs and water supplies, and maintain stream flows – all without becoming permanent bodies of water and an airport safeguarding hazard.
We’ve already shown that these areas can’t work as described. Surface water would not infiltrate equally along the length of the eastern boundary given the gradient of the proposed extraction base.
But with groundwater recorded so close to the surface there are other problems too. To retain surface water run-off, these features are proposed to be 3m deep:
2.18.3 Quarrying operations will be excavating down to a range of between 3m along the eastern boundary and up to 9m at the western most extremity including the overburden.
In addition, bunds along the eastern boundary would be formed:
2.18.3 The north-south bunds shown on Figure A2.1/A2.2/A2.3 are not intended to retain water, their main purpose being visual/noise screening.
It is claimed the bunds would only act to retain surface water during intense rainfall, including:
2.18.4 In the event of an extreme rainfall event filling up void capacity, such that depths exceeded the excavation depth and backed up against the bund. Given a maximum of 1.45m of depth is expected during the design (1 in 100 year + climate change allowance) rainfall event, and the maximum void depth is 3-9m, a significant exceedance event would be required.
However, given the elevated groundwater levels recorded along the eastern boundary, and the newly-submitted maximum winter water table contours – the base of any extraction – it is clear that it would be impossible to dig 3m deep along this boundary without breaching this new MWWT.
The diagram above overlays three plans: the phase 2 infiltration area, the surface contours and the MWWT contours. The numbers indicate the depth from the surface to the MWWT elevation at points where contours from the latter two plans intersect. Clearly, along at least half the length, a depth of 3m is not available for excavation.
The phase 1 infiltration area would have similar problems too.
And what's the impact on flooding? The elevated groundwater levels recorded at three locations along the eastern boundary – PZ2017/02, PZ2017/03 and SG1990/21 – may have informed a new base of extraction, but they have not informed the Flood Risk Assessment which was written in 2016.
Without assessment of this new information, who knows what the flooding implications might be? Obviously not Devon County Council, the Environment Agency, or any councillors unlucky enough to determine this mishmash of chaos.
The issue of how surface water is to be managed has always been recognised as critical if any quarrying were to be permitted at Straitgate Farm. Clearly Aggregate Industries' planning application is still in no fit state to be determined.
