From the Planning Officer’s report to the West Area Planning Committee, December 2017:
“9.149. During the consultation process, reference has been made to the suggestion within the Factual and Interpretive Ground Investigation Report that the proposed drainage strategy will require the use of lime stabilisation to avoid damage to the paving within the car park expansion from changes to the clay layer below ground and that this needs to be given further consideration as part of any drainage proposals for the site. The concerns raised are that lime treatment is likely to have an impact on the permeability of soils below the car park, and therefore needs to be appropriately considered.
The applicant has confirmed that the surface water drainage strategy has been designed as a tanked system which assumes no infiltration below the attenuation layer, with all storm water discharge from the site via a controlled outfall into Seacourt Stream. An impermeable membrane is included within the construction to prevent water saturating the clay. The underlying clay is of a low permeability whether lime stabilisation is employed or not, and it is envisaged that the attenuation will operate effectively in either scenario.” [emphasis added]
There is no mention in the Application of tanking, nor of an impermeable membrane. We have therefore not known of this till very recently and had no opportunity to comment. While there is little or no detail, the idea that the car park may be separated from the underlying groundwater table, as this implies, raises an extremely serious question. That is, where will the displaced groundwater go? This is a lot of water over such a large area. It is likely that it will cause a significant rise in groundwater levels around this low-lying site. This could cause (new) groundwater flooding within houses (and gardens) nearby. No decision should be taken until the details of what is planned are made clear, appropriate calculations and modelling done, and presented as part of a further revised Flood Risk Assessment.
[By the same token, when it rains, water will be trapped within the tanking , draining only slowly – more pumping needed?]