Brockworth, Gloucestershire

Project: Residential development for 47 homes
Location: Perrybrook, Brockworth, Gloucestershire
Client: Charterhouse Strategic Land
Services: Flood risk assessment and mitigation

The Project

Calibro Consultants was initially appointed to undertake a Flood Risk Assessment (FRA) in support of a detailed planning application to build 47 homes and associated infrastructure on a 2.3 hectare greenfield site north of Perrybrook, in Brockworth, Gloucester.

The developer, Charterhouse Strategic Land, had worked with us before and respects our senior-led and independently audited approach to forestalling problems at an early stage.

Being in Zone 1, the land is appropriate for the proposed development and the flood risk from all sources is low. However, a planning application for a site of this size requires an FRA. Given the particular challenges at the site, this worked included detailed consideration of plot levels, road levels and surface water drainage and how they all work together.

After extensive due diligence and tests on site, including of soakage, Calibro put forward a proposal to collect the surface water runoff from the roofs, roads, pavements and drives of the proposed development by a pipe network and release it through a storage basin in a park on the site, at the current 1 in 2-year greenfield runoff rate. This required a piped connection and outfall to an existing drainage ditch just beyond the site, into which the field currently drains

The Problem

The land, which slopes slightly from west to east, is bordered to the north by the A417 trunk road and to the south-east by the A46. These meet at a large roundabout junction with slip roads – which are drained by a system including a ditch and culvert along the north-east corner of the proposed development site. The ditch and wider drainage network are owned by Highways England (HE) but managed under a 30-year lease by Road Management Services (RMS). There is no other surface water drain to discharge to.

RMS initially refused permission to connect to the HE ditch citing the Department for Transport circular on access which forbids runoff arising from changes of use, and new connections from third party development.

Calibro experts pursued the matter, establishing from planning documents that the ditch had been put in when the bypass was built specifically to prevent “flooding and adverse effects upstream and downstream”, including the proposed development site. Furthermore, senior contacts in HE approached by our Managing Director, Stuart Choak, confirmed not only that the proposed surface water pipe would merely formalise the existing third-party runoff (from the field) but also that the precedent for this had been established elsewhere.

When the application was resubmitted, HE confirmed that the proposals constituted a formalisation of existing runoff into the highway asset and that this would be acceptable subject to existing greenfield runoff rates not being exceeded. However, it recommended conditions to be included with the planning approval as follows:

Hydraulic modelling of its own drainage system, to establish that it was sufficient to handle the runoff from the field site.

A condition survey of the existing highways drainage system under the roundabout

The solution

Stuart wrote a formal complaint to HE, threatening a legal challenge, secure in the knowledge that Calibro’s due diligence and specialist reports showed that HE’s conditions – which would potentially cost the client tens of thousands of pounds – were unnecessary and unjustified, and outside the scope of the planning requirements.

Our meticulous calculations demonstrated beyond argument that our proposals – including the 834m3 water storage basin and restricted outflow – would actually reduce runoff rates discharging into the highway’s ditch, particularly in more extreme weather events, and were therefore an improvement that would benefit HE’s drainage system.

Within a week HE had withdrawn its conditions. The planning application is awaiting local authority approval.

We trust Calibro’s advice completely. Our decisions to promote land and invest in sometimes complex planning strategies often reflects their advice, which reassuringly balances the opportunities and risks, and how these risks can be addressed.

Oliver Taylor
Charterhouse Strategic Land LTD

Related Projects