EAS were instructed to provide transport advice and to prepare a SuDS Flood Risk Assessment Report to support a planning application for 3no. residential dwellings at Newlands Avenue, Radlett.
The proposals include for the demolition of an existing dwelling and outhouses and provision of improved access to serve the development. In terms of flood risk, the site was not located in any areas subject to fluvial, surface water, groundwater or other sources of flooding. The challenge on this site was to identify a suitable surface water drainage strategy.
The existing property did not benefit from a positive surface water drainage system, waters simply shed to ground. Infiltration tests to BRE 365 Standard were undertaken to ascertain if soakaways would be viable, however these were unsuccessful. As no watercourses or surface water sewers were present in the vicinity of the proposed development, outfall to an adopted foul water sewer in Newlands Avenue was explored. The Water Authority would not grant permission to outfall to this sewer, stating that other methods of surface water disposal should be explored.
Due to site constraints, the client had resisted undertaking borehole infiltration testing, however it had become apparent that unless testing was undertaken, it would not be possible to provide the Local Planning Authority (LPA) and Lead Local Flood Authority (LLFA) with a viable surface water drainage strategy for the site. It was concluded by the Client and Project Manager that borehole testing undertaken at a neighbouring site could be put forward as evidence that deep borehole soakaways are viable. EAS advised that a suitably worded Planning Condition could be suggested to demonstrate to the LPA and LLFA that the Applicant would undertake site specific testing prior to commencement of the development should Planning be granted.
EAS provided input to the Master-planning, ensuring the Design Team were informed on matters relating to site access as well as space for SuDS Features and easements. A new site access within an existing change in level has therefore been designed to facilitate access into the site. Root Protection Zones were a major constraint as well as the site levels and gradients. A small catchment of the site would require waters to be pumped from a low-point to the Attenuation Basin, therefore a private package surface water pumping station would be utilised and necessitated careful consideration of the infrastructure to serve it and fitting this into a confined space at the entrance of the site.
A surface water drainage strategy was developed by EAS, utilising SuDS Features such as lined Permeable Paving, Raingarden Planters, Swales and an Attenuation Basin. The 4 Pillars of SuDS (Biodiversity, Amenity, Water Quality and Water Quantity) were therefore satisfied. In line with National Planning Policy Guidance and LLFA Requirements, the surface water drainage system was designed to manage all storms up to and including the 1 in 100yr + Climate Change Event and provides suitable treatment stages.
Client:
ACRE
EAS Input:
Pre-application Advice, Transport Statement, Flood Risk Assessment, SuDS Drainage Strategy, Planning Negotiations
Location:
Newlands Avenue, Radlett
