SuDS Strategy — Lorry Park Change of Use in Flood Zone 1, Chippenham

"SuDS Strategy — Lorry Park Change of Use on Cornbrash Limestone, Chippenham
Location: Chippenham Pit Stop, Chippenham, SN15 5LH, Wiltshire Council | Client: Pearce Planning | Services: SuDS Strategy

Formalising a Lorry Park Just Off M4 Junction 17

Chippenham Pit Stop has operated as informal lorry parking for decades — greenfield land stripped to grade 1 aggregate in the 1980s and used, on anecdotal evidence, by HGVs ever since. The planning application to convert it formally to a 20-bay lorry park needed a SuDS Strategy that could satisfy Wiltshire Council, the NPPF and the SuDS Manual without losing sight of the fact that the site already drains freely into limestone.

We were appointed by Pearce Planning to prepare the SuDS Strategy for the change of use at Chippenham Pit Stop, SN15 5LH — half a kilometre east of M4 Junction 17.

Site Context and Ground Conditions

The site covers approximately 3,550m² of broadly flat hardstanding bounded by the B4122 to the north, surrounding agricultural land, and the existing pit stop building and forecourt to the south. Topographical survey confirmed a gentle south-easterly fall consistent with the wider landscape, with no notable low-lying areas prone to surface water accumulation.

The underlying geology is Cornbrash Formation limestone bedrock. A nearby BGS borehole record (reference ST97NW10, approximately 220 metres north) recorded clay, limestone and silty clay with rock fragments — close to the mapped mudstone boundary but consistent with the dominant limestone profile. Soilscapes mapping characterises the site as freely draining shallow lime-rich soils over limestone, which made infiltration the obvious primary discharge route.

The nearest watercourse is an unnamed tributary of the River Avon flowing east approximately 100m to the south, joining the Avon near Christian Malford. There are no public surface water sewers in the immediate vicinity. The existing site discharges to the tributary via a private piped system supplemented by a 360m³ cellular soakaway.

Flood Risk Assessment — Site in Flood Zone 1

Fluvial risk was low — the EA Flood Map for Planning places the site within Flood Zone 1, outside the modelled 1-in-100-year plus climate change flood extent. Tidal risk was negligible. Pluvial risk was low — EA surface water mapping for the 1-in-30, 1-in-100 and 1-in-1000-year events showed the site lies outside any identified areas of elevated surface water flood risk, with localised risks on adjacent roads intercepted by existing gullies before reaching the plot. Sequential and Exception Tests were not required.

SuDS Strategy — Treatment Train Into Limestone

The strategy applies the SuDS Manual (CIRIA C753) management hierarchy as far up as feasible. Infiltration was the most appropriate discharge route given the underlying Cornbrash Limestone — there are no surface water sewers nearby, and discharging to the Avon tributary was avoided in favour of source control.

The proposed treatment train manages runoff from approximately 2,960m² of new impermeable hardstanding through three sequential stages: a grassed filter strip providing primary filtration and removing suspended solids; a bypass oil separator intercepting hydrocarbons and any residual contamination from HGV operations; and a 120m³ cellular soakaway disposing of treated flows by infiltration into the limestone bedrock.

Vegetated SuDS — swales, rain gardens, bioretention systems, detention basins, ponds — were considered and ruled out given the site's tight layout and limited soft landscaping area. Grassed filter strips provide the equivalent source-control function within the constraints. Catchpits and silt traps within inspection chambers and gullies protect the soakaway from sedimentation through the system's design life.

Hydraulic Performance and Climate Change Allowance

The system was designed to the 1-in-100-year storm event with a 40% allowance for climate change, in line with current DEFRA guidance. Runoff calculations used the HR Wallingford SuDS performance evaluation tool (StopUP) and FEH22 rainfall data; greenfield rates were derived using the IH124 methodology via UKSUDS.

Pre- and post-development peak runoff rates demonstrate substantial betterment: 33.37 l/s reduced to 4.69 l/s for the 1-in-100-year event — an 85% reduction — and 25.38 l/s reduced to 3.49 l/s for the 1-in-30-year event. The 1-in-100-year plus 40% climate change peak flow is 6.37 l/s. The greenfield baseline (QBar) is 1.9 l/s.

Water quality is treated to three levels per the SuDS Manual — one for the filter strip, two for the bypass separator, three for the soakaway. The first 5mm of rainfall, carrying the highest pollutant concentration, is fully retained and treated on-site. Exceedance flows in events beyond the design storm are safely retained within the site boundary.

Outcome

The SuDS Strategy demonstrated full NPPF and Wiltshire Core Strategy compliance, confirming that the change of use to a lorry park can proceed without increasing flood risk on or off-site. Discharge is by infiltration only — no flows enter the sewer network or the nearby Avon tributary. The 85% betterment in peak runoff, combined with three-level water quality treatment, provides Wiltshire Council with the technical evidence needed to support the planning application. A maintenance schedule covering the soakaway, bypass separator and filter strips is in place for the lifetime of the development."