236 West Wycombe Road, Buckinghamshire

Project Background
Archplan Design Ltd, acting on behalf of a homeowner in High Wycombe, Buckinghamshire, commissioned Air & Flood Consultants to prepare a comprehensive Flood Risk Assessment and SuDS Strategy supporting a planning application for a rear extension to an existing terraced dwelling. The proposals involved extending the property at basement and ground floor levels, with first and second floor renovations, alongside demolition of an existing garage and erection of a secure bicycle store.
The site's context presented multiple technical challenges requiring careful assessment and innovative drainage design. Located adjacent to the Chiltern Main Line railway to the north, bounded by established residential development to all other sides, the 540m² plot sat on sloping ground underlain by permeable chalk geology. Most significantly, Environment Agency surface water flood mapping identified the site as being at high risk of pluvial flooding—a classification demanding robust mitigation to satisfy planning requirements.
Site Context and Flood Risk Baseline
The property occupies previously developed land at grid reference E: 484767; N: 194049, within a traditional residential neighbourhood characterised by terraced housing and modest private gardens. LiDAR topographical analysis revealed ground sloping from north-east to south-west, creating natural surface water flow paths during intense rainfall events—a factor contributing to the identified pluvial flood risk.
Geological and Hydrogeological Setting:
British Geological Survey mapping confirmed the site is underlain by New Pit Chalk Formation, a widespread high-permeability bedrock. Borehole data from approximately 520 metres south-east (reference SU89SE68) indicated chalk and gravel substrates with shallow groundwater levels between 0.3m and 0.9m depth. Soilscapes mapping characterised the soils as shallow lime-rich types over chalk or limestone—freely draining profiles offering excellent infiltration potential but also indicating hydraulic connectivity with underlying aquifers.
This geology presented both opportunity and constraint: whilst offering favourable conditions for infiltration-based SuDS, it also indicated medium groundwater flood risk, particularly during prolonged wet periods when water tables rise seasonally.
Existing Drainage and Flood Risk:
The site comprised approximately 335m² of impermeable surfaces (roofs and concrete hardstanding) with all drainage assumed to discharge directly to the combined sewer at the site frontage on West Wycombe Road. With no attenuation or source control measures, the existing arrangement generated significant peak flows during storm events:

1 in 1-year event: 3.44 l/s
1 in 30-year event: 13.49 l/s
1 in 100-year event: 17.74 l/s

These uncontrolled discharges placed unnecessary burden on downstream combined sewerage, contributing to urban flood risk and offering no water quality treatment before entering the public system.
Flood Risk Classification:
Systematic assessment of all flood sources, informed by Environment Agency mapping, Wycombe District SFRA (2014), and Buckinghamshire Local Flood Risk Management Strategy (2024), established the following risk profile:
Flood SourceRisk ClassificationFluvial (rivers and sea)Low – Site in Flood Zone 1; River Wye ~100m south poses no direct riskSurface Water (pluvial)High – EA mapping shows site within high-risk extent for multiple return periodsGroundwaterMedium – Chalk aquifer with shallow water table; site not in mapped emergence zone but elevated seasonal riskSewer floodingLow – No history of sewer flooding at this locationArtificial sourcesLow – Not within reservoir inundation zones
The high pluvial flood risk classification triggered the requirement for detailed assessment and robust mitigation under NPPF policy, even though the development lay within Flood Zone 1 for fluvial/tidal risk.
Technical Assessment Methodology
The integrated approach combined flood risk assessment with hydraulic drainage design, ensuring compliance with multiple regulatory frameworks:
Policy and Standards:

National Planning Policy Framework (NPPF)
Planning Practice Guidance (PPG) on flood risk and SuDS
Non-Statutory Technical Standards for Sustainable Drainage (Defra, 2015)
CIRIA SuDS Manual C753
Wycombe District Local Plan (2019)
Buckinghamshire Local Flood Risk Management Strategy (2024)

Data Sources and Tools:

Environment Agency Flood Map for Planning
EA Surface Water Flood Risk Mapping (1 in 30, 1 in 100, 1 in 1000-year extents)
Wycombe SFRA groundwater flood mapping
FEH22 rainfall data for design storm generation
HR Wallingford StopUP Performance Evaluation Tool for hydraulic modelling
UK SuDS Greenfield Runoff Rate Estimation Tool (IH124 methodology)
LiDAR topographical data
BGS geology and borehole data

Design Criteria:
The drainage system was designed to accommodate:

1 in 100-year storm event plus 40% climate change allowance (upper-end uplift for 2070s)
Discharge limited to greenfield runoff rates where feasible
Zero increase in flood risk to neighbouring properties or downstream systems
Water quality treatment meeting "simple index" approach requirements
Maintenance accessibility for long-term performance

Surface Water Management Strategy
The SuDS strategy prioritised the disposal hierarchy, maximising infiltration to ground whilst providing controlled discharge to the combined sewer as a last resort during exceptional events.
Key Components:
1. Permeable Block Paving (175m²)
Existing concrete hardstanding in the drive and parking areas would be replaced with permeable block paving constructed to SuDS-compliant specification:

Surface layer: Permeable concrete blocks with 3-6mm joints filled with 2-6mm aggregate
Bedding layer: 50mm sharp sand
Sub-base: 200-300mm open-graded stone (typically Type 3 sub-base)
Geotextile membrane: Separating sub-base from underlying chalk, preventing fines migration whilst maintaining infiltration capacity

This component provides immediate source control, intercepting rainfall at the point it lands and facilitating infiltration through underlying strata. The sub-base void space (typically 30% porosity) provides temporary storage during intense events, with infiltration to the chalk occurring continuously.
2. Soakaway System (Two Units, 6m³ Total Volume)
Roof drainage from 155m² of tiled roof area would be directed via traditional downpipes and underground pipework to two discrete soakaways positioned in garden areas:

Construction: Precast concrete ring chambers or wrapped geotextile-lined excavations filled with clean angular aggregate
Sizing: Based on BRE Digest 365 principles and confirmed through hydraulic modelling
Location: Minimum 5m from buildings, positioned to avoid undermining foundations or creating differential settlement risk
Depth: Bases positioned minimum 1m above likely seasonal high groundwater level to maintain infiltration efficiency

Each soakaway incorporates inspection access for monitoring and maintenance, with rodding eyes allowing jet-washing if infiltration rates decline over time.
3. Flow Control and Emergency Overflow
Recognising that extreme events exceeding design capacity could occur, particularly with future climate uncertainty, the system incorporates:

Vortex flow control devices (Hydro-Brake or similar) limiting discharge to 0.09 l/s—below calculated greenfield rate of 0.04 l/s for the 1 in 100-year event
Emergency overflow connection to existing combined sewer, only engaged when soakaway capacity exceeded
High-level warnings (e.g., visual indicators at inspection chambers) alerting occupants to system stress

4. Water Quality Treatment
Pollutant removal follows a treatment train approach aligned with CIRIA SuDS Manual guidance on the "simple index" method:
Surface TypeTreatment StagesMechanismsRoof (155m²)1. Green roof/water butt interception2. Trapped gullies and silt traps3. Infiltration through soakaway mediaSedimentation, filtration, biodegradationPermeable paving (175m²)1. Surface filtration through block joints2. Sub-base storage and filtration3. Infiltration to chalkPhysical straining, adsorption, biodegradation
This achieves the two levels of treatment recommended for residential roof and pedestrian areas, ensuring protection of groundwater quality before infiltration. Regular maintenance (detailed in Appendix E) prevents pollutant accumulation and maintains treatment efficiency.
Hydraulic Performance and Flood Risk Reduction
Detailed hydraulic modelling using the HR Wallingford StopUP tool quantified system performance across multiple return periods and storm durations (60-minute and 1440-minute events). Results demonstrated exceptional flood risk reduction:
Pre-Development Peak Runoff Rates:
Return PeriodStorm DurationPeak Flow (l/s)1 in 1-year60 min3.441 in 30-year60 min13.491 in 100-year60 min17.74
Post-Development Peak Runoff Rates (including 40% climate change):
Return PeriodStorm DurationPeak Flow (l/s)Betterment (%)1 in 1-year +CC1440 min0.00100%1 in 30-year +CC1440 min0.00100%1 in 100-year +CC1440 min0.00100%
For all modelled scenarios up to and including the 1 in 100-year event plus 40% climate change, the SuDS system achieved zero discharge to the combined sewer. All rainfall was successfully managed through infiltration via permeable paving and soakaways, with no flooding from the drainage system recorded in any modelled event.
This represents a 100% reduction in peak runoff rates compared to existing conditions and achieves discharge rates below greenfield baseline (calculated at 0.04 l/s for 1 in 100-year event using IH124 methodology on the 0.054 ha site).
Volumetric Storage and Climate Resilience:
The combined storage volume provided by permeable paving sub-base (~13m³ at 30% voids) and soakaways (6m³) totals approximately 19m³. Even accounting for:

Partial saturation of sub-base during antecedent rainfall
Reduced infiltration rates during winter when water tables elevated
40% increase in rainfall intensity under climate change projections

...the system maintains zero exceedance, demonstrating substantial resilience and over-design margin for uncertainty.
Groundwater Flood Risk Mitigation
Whilst the drainage strategy focused primarily on managing pluvial flood risk, the medium groundwater risk required specific consideration to protect the proposed basement extension.
The Wycombe SFRA groundwater mapping placed the site within an area of medium groundwater flood risk, defined as locations where water tables may periodically rise close to the surface during extended wet periods. The underlying chalk aquifer responds slowly to recharge, meaning groundwater levels can remain elevated for months following sustained rainfall.
Basement Construction Mitigation:
Although not the primary focus of the drainage strategy, the FRA recommended the following measures for the proposed basement:

Structural Waterproofing: BS 8102:2009-compliant tanking incorporating Type A (barrier), Type B (structurally integral), and Type C (drained cavity) protection systems
Groundwater Monitoring: Standpipe installations during excavation to confirm water table position and inform construction methodology
Dewatering Provisions: Temporary wellpoint systems if groundwater encountered during construction
Permanent Drainage: Perimeter land drains around basement walls connecting to sump pump with battery backup and high-level alarm

These measures ensure the basement remains dry and structurally sound even during seasonal groundwater table fluctuations, whilst the surface SuDS strategy simultaneously reduces surface water ingress risk.
Policy Compliance and Planning Outcomes
The integrated FRA and SuDS Strategy provided Buckinghamshire Council with comprehensive evidence demonstrating compliance with multiple policy requirements:
National Planning Policy Framework (NPPF):

✓ Development will be safe for its lifetime without increasing flood risk elsewhere (Para 167)
✓ SuDS incorporated unless demonstrated to be inappropriate (Para 169)
✓ Opportunities taken to reduce overall flood risk (Para 169)

Non-Statutory Technical Standards for SuDS (Defra 2015):

✓ Peak runoff rate limited to greenfield rate (S2)
✓ Runoff volume controlled for 1 in 100-year event including climate change (S3)
✓ Flood risk managed for events exceeding design standard (S4)
✓ Appropriate treatment levels for water quality protection (S6)

Local Policy (Wycombe District Local Plan 2019):

✓ Drainage hierarchy followed (infiltration prioritised over sewer discharge)
✓ Surface water separated from foul drainage
✓ Betterment achieved over existing situation

The assessment's transparency—providing full hydraulic calculations, design drawings, and maintenance schedules in technical appendices—gave planning officers confidence in the proposals' technical robustness. By demonstrating a 100% reduction in peak flows and zero increase in downstream flood risk, the strategy exceeded minimum policy requirements whilst addressing the site's high pluvial flood risk classification.
Maintenance and Long-Term Performance
Sustainable drainage systems require ongoing maintenance to preserve design performance. The strategy includes a comprehensive Operation and Maintenance Plan (Appendix E) detailing responsibilities and frequencies:
Permeable Paving Maintenance:

Annual vacuum sweeping after autumn leaf fall
Weed control as required (mechanical removal or targeted glyphosate application)
Monitoring for surface clogging or reduced infiltration (quarterly for first six months, then annually)
Rehabilitation sweeping every 10-15 years if infiltration performance declines significantly

Soakaway Maintenance:

Annual inspection of silt traps, inspection chambers, and manhole rings
Gutter and downpipe cleaning annually
Sediment removal from pre-treatment components as required based on inspections
Root trimming if vegetation encroachment observed
Full reconstruction only if catastrophic failure occurs (expected lifespan >25 years with routine maintenance)

Responsibility:
As a single private dwelling, maintenance responsibility rests with the homeowner. The Maintenance Plan provides clear, accessible guidance enabling non-specialist householders to understand their obligations. For more complex tasks (e.g., soakaway reconstruction), professional drainage contractors should be engaged.
Lessons and Transferable Insights
This case study demonstrates several principles applicable to small-scale residential development in areas of elevated pluvial flood risk:

Infiltration Viability: Even sites with medium groundwater risk can successfully employ infiltration-based SuDS where permeable geology exists and appropriate separation distances maintained between drainage features and seasonal water table.
Retrofit Potential: Existing impermeable surfaces represent significant opportunities for betterment. Converting 175m² of concrete hardstanding to permeable paving achieved dramatic flood risk reduction at modest cost.
Climate Change Resilience: Designing to 1 in 100-year plus 40% climate change standard with generous storage margins ensures long-term performance despite future uncertainty. The 100% reduction in discharge provides substantial headroom for potential underestimation in climate projections.
Integrated Assessment: Addressing flood risk holistically—combining FRA with SuDS design—ensures coherent solutions. Groundwater risk informed basement construction methodology whilst chalk geology enabled ambitious infiltration targets.
Hydraulic Modelling Value: The HR Wallingford StopUP tool provided quantitative evidence of performance, moving beyond qualitative assertions. Demonstrating zero discharge across all modelled scenarios gave planning officers confidence in technical robustness.
Treatment Train Efficiency: Modest residential development generates low pollutant loads. Simple, passive treatment measures (trapped gullies, permeable media filtration) achieve appropriate protection without complex engineered systems.
Maintenance Accessibility: Prescriptive, frequency-based maintenance schedules aligned with CIRIA guidance enable non-specialist property owners to maintain system performance without requiring technical expertise or specialist equipment for routine tasks.
High-Risk Site Acceptability: Even developments in areas mapped as high pluvial flood risk can proceed where robust mitigation achieves betterment. The key is demonstrating the proposed situation represents improvement over existing conditions—here, a 100% reduction in peak flows and elimination of direct runoff to sewers.