Flood Risk Assessment and SuDS Strategy — Basement Extension, High Wycombe, Buckinghamshire

Flood Risk Assessment and SuDS Strategy — Rear Extension with Basement, High Wycombe, Buckinghamshire
Location: West Wycombe Road, High Wycombe, Buckinghamshire | Client: Archplan Design Ltd | Services: Flood Risk Assessment, SuDS Strategy

Why a Rear Extension Needed a Hydraulic Model
On paper, a rear extension with basement to a terraced property in High Wycombe sits well within the category of minor residential development. In practice, the flood risk and drainage picture at this site was considerably more involved than the project type might suggest. EA surface water mapping placed the site within the high-risk pluvial flood extent across multiple return periods, driven by a north-east to south-west ground slope that concentrates overland flow across the plot. The existing impermeable surfaces — 335m² of roofs and hardstanding in total — discharged entirely to the combined sewer with no attenuation, no source control, and no water quality treatment. And beneath the site, a chalk aquifer introduced medium groundwater risk to the proposed basement excavation.

We were commissioned by Archplan Design Ltd to prepare an integrated Flood Risk Assessment and SuDS Strategy for the site. The brief was clear: address the high pluvial classification, manage the groundwater risk to the basement, achieve measurable betterment over the existing drainage situation, and demonstrate compliance with NPPF, Defra Non-Statutory Technical Standards for SuDS, and Wycombe District Local Plan policy.

The Existing Drainage Baseline — and Why It Mattered
Before designing any new drainage measures, characterising the existing drainage performance was essential. With no attenuation infrastructure in place, the existing impermeable surfaces generated peak discharge rates of 3.44 l/s in a 1-in-1-year storm event, rising to 17.74 l/s in the 1-in-100-year event. Every litre went directly to the combined sewer at the site frontage. That is the kind of baseline that gives combined sewer networks the problems they have in heavily urbanised catchments — uncontrolled peak flows with no reduction, no water quality improvement, and no resilience against more extreme rainfall.

That existing situation also created an opportunity. Where the pre-development performance is this poor, a well-designed SuDS scheme has real scope to deliver meaningful betterment rather than merely meeting minimum requirements — and that shaped the ambition of the drainage design from the outset.

Geology as an Enabler
The site is underlain by New Pit Chalk Formation — a high-permeability bedrock that makes infiltration-based drainage genuinely viable rather than simply aspirational. Borehole records from approximately 520 metres south-east of the site confirmed chalk and gravel with a seasonal water table ranging between 0.3m and 0.9m depth. Soilscapes mapping characterised the soils as shallow, lime-rich types over chalk with excellent drainage characteristics.

Under the SuDS hierarchy, infiltration to ground is the preferred disposal route wherever it is achievable, and at this site the geology actively supports it. The design needed to account for seasonal groundwater table rises and incorporate a 40% climate change uplift on rainfall inputs — both of which were modelled explicitly — but the underlying chalk provided the drainage capacity to make a fully infiltration-based solution deliverable.

The SuDS Scheme
Hydraulic modelling was carried out using HR Wallingford StopUP software with FEH22 rainfall data and IH124 greenfield runoff methodology, ensuring the design was based on current best-practice hydrological inputs.

The scheme comprised three complementary components. Permeable block paving replaced 175m² of existing concrete hardstanding across the driveway and parking areas. The specification — permeable concrete blocks over a 200–300mm open-graded Type 3 sub-base with geotextile separation — provides immediate source control at the point of rainfall, with approximately 13m³ of sub-base void storage at 30% porosity and continuous infiltration to the underlying chalk. This alone addresses the largest single area of existing impermeable surface on the site.

Two soakaways totalling 6m³ capacity receive roof drainage from 155m² of tiled roof via downpipes and underground pipework. Both units are sized in accordance with BRE Digest 365 principles, positioned a minimum of 5m from the building, with bases set at least 1m above the seasonal high groundwater level to maintain year-round infiltration efficiency. Inspection access and rodding eyes are incorporated to allow routine maintenance without specialist equipment.

A vortex flow control device limits any emergency overflow discharge to 0.09 l/s — deliberately specified below even the calculated greenfield runoff rate for the 1-in-100-year event — with connection to the existing combined sewer engaged only when soakaway capacity is exceeded in an extreme scenario. A two-stage water quality treatment train was incorporated for both roof and permeable paving drainage, meeting the treatment levels recommended in CIRIA SuDS Manual C753 for residential surface types.

The Results
The hydraulic modelling outputs were notable. Across all modelled storm durations and return periods — including the 1-in-100-year event with the full 40% climate change uplift — post-development peak discharge to the combined sewer was zero. The combined 19m³ of storage provided by the permeable paving sub-base and soakaways absorbed all rainfall within the design envelope through infiltration alone. No exceedance was recorded in any modelled scenario.

Against a pre-development peak discharge of 17.74 l/s, this represents a 100% reduction in peak runoff rate. The post-development performance is not merely below greenfield rate — it eliminates surface water discharge to the combined sewer entirely under all design conditions. That substantially exceeds minimum policy requirements under the Defra Non-Statutory Technical Standards for SuDS (2015) and delivers the kind of drainage betterment that meaningfully benefits the downstream combined sewer catchment.

Basement Waterproofing and Groundwater Risk
The chalk aquifer that makes infiltration drainage viable at this site also introduces medium groundwater risk to the proposed basement. A dedicated waterproofing strategy was therefore developed alongside the SuDS scheme, specifying structural protection to BS 8102:2009 using a combined approach: Type A barrier protection, Type B structurally integral waterproofing, and Type C drained cavity protection. This three-type combination provides defence in depth, ensuring that no single failure pathway can result in water ingress to the basement.

Standpipe groundwater monitoring was recommended during the construction phase — particularly during excavation — with temporary wellpoint dewatering on standby should groundwater be encountered at a higher level than anticipated from borehole records. A permanent perimeter land drain system connecting to a sump pump with battery backup and high-level alarm completes the below-ground protection package for the operational building.

Outcome
The integrated FRA and SuDS Strategy demonstrated compliance with NPPF flood risk policy, Defra Non-Statutory Technical Standards for SuDS, and Wycombe District Local Plan requirements across all relevant criteria. Buckinghamshire Council received a submission that went beyond demonstrating that the high pluvial risk site could be developed safely — it demonstrated that doing so would deliver a 100% improvement in drainage performance compared to the pre-development baseline, with measurable benefits to the wider combined sewer network. For a rear extension on a 540m² residential plot, that is an outcome worth documenting.