Don’t laugh, but Aggregate Industries' plan for Straitgate Farm – the one it’s been working on for the last decade – is reliant on someone else's small, easily-blocked, 300mm culvert.
A large portion of surface water run-off from any quarry at Straitgate Farm would be discharged to the Cadhay Bog watercourse, via the above third party culvert that runs underneath a neighbouring field.
This issue has been around some time. In 2013, Aggregate Industries' consultants concluded:
There is extensive evidence of natural flooding across the eastern extent of Straitgate Farm. The capacities of roadside ditches and streams have not been adequate to deal with recent levels of rainfall in the area.
In 2015, for Aggregate Industries' previous application, the Environment Agency recognised in this Devon County Council Regulation 22 document that the existing culvert would need to be improved "to reduce the risk of flooding, which has been seen to have a significantly adverse impact on surrounding settlements." A meeting was held:
It was agreed by DCC and the EA that the previous comments regarding de‐culverting off‐site watercourses which had been made on biodiversity grounds would not be pursued as there was a possibility that this could increase downstream flooding.
In any case, Aggregate Industries claimed the issue wasn't anything to do with them, because they wouldn't increase the amount of surface water running off the site:
3.12 Improvements to the existing drainage should focus on increasing roadside ditch capacity and ensuring that culvert inlets and outlets are suitably maintained to avoid blockages from vegetation or siltation. 3.13 There is a need for work on the existing drainage beneath Birdcage Lane to improve conveyance and prevent episodes of flooding along the road, this should however be the statutory responsibility of the local highways department if it can be demonstrated by AI that watercourse peak flows are not increased by extraction activities at Straitgate Farm.
And no increase in surface water run-off is what Aggregate Industries is still attempting to claim:
2.13.7 Two discharge points are proposed: One serving the ancillary area and upper portion of the access road north of the Cadhay Bog Stream which will discharge directly to the watercourse at ‘Point A’ on Figure 2.6 below; and The remaining section of the access road south of the Cadhay Bog Stream will discharge to the roadside ditch in the far south-east of the site, which drains northwards to join the Cadhay Bog stream, as shown on Figure 2.6 as ‘Point B’. 2.13.8 This arrangement ensures that the runoff from these developed areas (ancillary area/access road) is managed within the Cadhay Bog sub-catchment, and the final rate and volume of flow discharged to the Cadhay Bog stream replicates the natural rate and volume.
But that's just wishful thinking. In the real world, with extreme weather events becoming more unpredictable, and with a quarry upslope removing the majority of the unsaturated layer, who knows what would happen? What allowance for improving this culvert has been made in this application? None. And it doesn’t matter how many discharge points there are, both Point A and Point B would feed into the one 300mm culvert:
Given that it was recognised this culvert struggled to take the existing run-off from the site, it was surprising that the introduction of a quarry in its upslope catchment, together with access road, 50mx50m loading/stocking/lorry parking area, wheel wash, weighbridge, tip off bay, and lagoon (variously described as 1,286m2 to less than 600m2, or 20mx20m) had not prompted a little more thought.
But with the recent submission of revised maximum winter water table contours – the proposed base of extraction – matters have taken a new twist.
We have already posted that the revised base of extraction does not mimic existing ground elevation contours to maintain pre-extraction run-off characteristics.
Overlaying the revised MWWT contours over the Extraction and Ancillary Development plan shows that the base of extraction would direct more surface water to the Cadhay Bog water course than existing ground levels. More surface water would flow towards the loading area, down the swale adjoining the access road, into the lagoon, and towards the ditch on Birdcage Lane. And all of that would then need to pass through the one 300mm culvert.
What could possibly go wrong?
The Flood Risk Assessment has made no allowance for increased run-off from the revised extraction contours. The swale capacity makes no allowance for this. The lagoon capacity makes no allowance for this. The ditch capacity along Birdcage Lane makes no allowance for this. The 300mm culvert makes no allowance for this. Clearly, it's another major failing.
And how much water can already flow down the Cadhay Bog watercourse on a bad day?
