Flooding can have a devastating impact on communities. Not only can it cause damage to property and possessions, it can cause injury and death. The Met Office says we should expect an increasing frequency of extreme rain:
…research, published in Nature Communications, found that under a high emissions scenario (RCP 8.5) rainfall events in the UK exceeding 20mm/hr could be four times as frequent by 2080 compared to the 1980s.
Managing flood risk is therefore an important business. Council’s trust consultants to get the figures right.
Last month, Aggregate Industries submitted further information to Devon County Council in relation to its planning application DCC/4399/2024 for Hillhead Quarry near Uffculme, which is in part to deal with the quarry’s ponding problems.
In its revised surface water management plan for the site – where "all surface water will be managed in the former extraction area, with Houndaller Pond only acting as an exceedance route" – Aggregate Industries’ water consultants BCL Hydro – the same crew working on Straitgate – made a big assumption:
The fact that [Houndaller Plantation] pond can sustain long-term and regular abstraction… means that it would be similarly effective as a soakaway.
We questioned this claim in the post AI sense-checks AI... and its soakaway assumptions at Hillhead, and wrote to Devon County Council saying:
There remains, therefore, no cogent evidence – measurements or otherwise – of the soakaway capacity at Houndaller Plantation Pond. It is wrong to assume that Houndaller Pond would be “similarly effective as a soakaway”, and therefore it does not necessarily ‘follow’ that Houndaller Pond could cope with exceedance flows from a storm event.
But there appeared to be other wild assumptions put forward by Aggregate Industries’ water consultants too. As we set out to Devon County Council:
... it appears that the applicant’s revised plans have not allowed sufficient storage space within the extraction area for the design storm event:The applicant claims “the ASV [Attenuation Storage Volume] requirement equates to 3,350 m3 during the design event (1 in 100-yr + 45% allowance for storm intensity due to climate change)” and that “the revised surface water management plan for Houndaller (by incorporating 3,416.60 m3 within the former extraction area) will provide sufficient storage space for the design event.”The applicant claims runoff rates and attenuation requirements were computed using “the assumption that 12.75 hectares [127,500m2] of the catchment area would possess runoff characteristics analogous to that of an impermeable paved surface.” The applicant has not provided any output from those computations to substantiate the conclusions.An ASV of 3,350m3 divided by the impermeable catchment area of 127,500m2, implies the applicant has modelled for a rainfall event of 0.0263m or 26.3mm.This would seem to be an inadequate figure, given the historic records for nearby areas:“June 1946 In Cullompton, 2.35 in [60 mm] of rain fell in 45 minutes and ... in the lower part of the town flooding was 3 ft deep in houses.”“22 October 1960 Flooding occurred in Crediton, following a total of 64 mm of rain on 20 and 21 October (recorded in Exeter)”“21 November 2012 Between 20mm to 39mm (0.8in to 1.5in) fell in 12 hours overnight on already saturated ground.”The University of Exeter Weather Observation Records has the Highest Daily Rainfall at 47.20 mm on 4 September 2024.The UK’s wettest day, in records back to 1891, was 3 October 2020. The average rainfall across the entire UK was calculated at 31.7mm. The Met Office says record-breaking rainfall like that seen on 3 October 2020 could be 10 times more likely by 2100.
Using the FEH22 rainfall depth-duration-frequency model for that area, 26.3mm is less than a 1-hour, 30-year event; by contrast, a 12-hour, 100-year event would exceed 90mm.
A 90mm rainfall event, with the same impermeable catchment area of 127,500m2, would require a much larger storage capacity of 11,475m3...
When it came to Straitgate Farm, consultants Amec Foster Wheeler, writing Aggregate Industries’ Flood Risk Assessment, modelled for a lesser 1 in 100-yr storm event + 10% climate change, and reckoned that across the 25.61ha (256,100m2) site, a total volume of 19,054m3 was required to attenuate runoff. Dividing the latter by the former implies a 74.4mm rain event was modelled for, or 67.6mm excluding the 10% climate change uplift. Assuming Hillhead, 23 miles away, is prone to the same level of storms as Straitgate, then adding back a +45% climate change allowance, means that the surface water management plan for Hillhead needs to accommodate runoff from a 98mm storm event – or in old money, nearly 4 inches of rain.
So why BCL Hydro has only modelled for a not-uncommon 1-inch rain event is anybody’s guess.
