Dobbies Garden Centre, Peterborough

Project Background
The developer sought planning permission for the erection of four panel courts with associated infrastructure, lighting, and parking facilities at Dobbies Garden Centre in Peterborough. The development site, located within an established commercial retail park, comprised a mix of existing car parking and greenfield land totalling approximately 0.4 hectares.

The planning consultant, acting on behalf of the applicant, commissioned Air & Flood Consultants to prepare a comprehensive Flood Risk Assessment (FRA) to support the planning application (reference 25/01394/FUL). The assessment was required to demonstrate compliance with the National Planning Policy Framework (NPPF) and satisfy Peterborough City Council's local planning requirements, particularly given the site's identification on Environment Agency surface water flood mapping as having areas of elevated risk.

Site Context and Constraints
The site is situated at grid reference E: 518110; N: 295468 within a predominantly commercial area characterised by retail units, service yards, and extensive car parking. Topographical analysis using LiDAR data indicated relatively uniform ground levels across the site (20-23m AOD), consistent with the surrounding urban landscape and not indicating any notable low-lying areas prone to surface water accumulation.

Geological investigation revealed the site to be underlain by Oxford Clay Formation mudstone with superficial River Terrace Deposits consisting predominantly of sand and gravel. Borehole data from approximately 140 metres east of the site confirmed sandy clay, gravel and sand with groundwater recorded at 2.44m depth. Soilscapes mapping characterised the site as having lime-rich loamy and clayey soils with impeded drainage, suggesting limited infiltration potential would be available for surface water management.

The site was not in proximity to any main rivers, ordinary watercourses, or open drainage channels. Teardrop Lake is located 500m to the east, whilst the Orton Pit Site of Special Scientific Interest (SSSI) lies 370m west of the development boundary. The absence of pollution potential and minimal construction impact meant the proposals were not anticipated to affect these environmentally sensitive receptors.

Technical Assessment Methodology
The Flood Risk Assessment followed established best practice guidance, drawing upon multiple authoritative data sources:
Regulatory Framework:

National Planning Policy Framework (NPPF)
Planning Practice Guidance (PPG)
Peterborough Local Plan 2016 to 2036
Strategic Flood Risk Assessment and Water Cycle Study (2018)
Flood and Water Management Supplementary Planning Document (2019)
Peterborough Flood Risk Management Strategy (2015)

Datasets and Mapping:

Environment Agency Flood Map for Planning
Surface Water Flood Risk Mapping (1 in 30, 1 in 100, and 1 in 1000-year events)
Reservoir Inundation Mapping (wet-day and dry-day scenarios)
LiDAR topographical survey data
British Geological Survey drift and geology maps
Web-based soil mapping
Historic flooding records

The assessment systematically evaluated all potential flood sources in accordance with NPPF requirements: tidal flooding, fluvial flooding, surface water (pluvial) flooding, groundwater flooding, sewer flooding, and flooding from artificial sources such as reservoirs.

Key Findings
Fluvial Flood Risk (Rivers and Sea): The Environment Agency's Flood Map for Planning confirmed the entire site lies within Flood Zone 1, indicating land with less than 0.1% annual probability of flooding from rivers or the sea. No formal flood defences were identified in the vicinity, and the site lay well outside the design flood extent for the 1 in 100-year plus climate change event. Fluvial flood risk was therefore classified as low, and the Sequential and Exception Tests were not required.

Surface Water (Pluvial) Flood Risk: Environment Agency surface water flood mapping revealed portions of the site fell within areas identified as having elevated pluvial flood risk. The mapping scenarios (1 in 30-year, 1 in 100-year, and 1 in 1000-year events) showed that intense or prolonged rainfall events could exceed local drainage capacity, leading to surface water accumulation. The site lies outside any designated Critical Drainage Area, but surface water flood risk was classified as high based on the mapping evidence.
Groundwater Flooding: Neither the DEFRA groundwater flooding scoping report nor Peterborough's Strategic Flood Risk Assessment documents showed records of previous groundwater flooding within the area. Groundwater flood risk was classified as low.

Sewer Flooding: The SFRA identified three recorded sewer flood incidents within the PE7 8 postcode area, suggesting localised historical issues but not indicating site-specific risk. Sewer flooding risk was classified as low.

Artificial Sources (Reservoirs): Reservoir breach flood risk mapping for both wet-day and dry-day scenarios confirmed Dobbies Garden Centre lay well outside any potential inundation zones. Risk from artificial sources was classified as low.

Historic Flooding: Review of available records confirmed no historic flooding incidents had been recorded at or adjacent to the site location.

Mitigation Strategy
Given the identified high risk of surface water flooding, the FRA recommended a comprehensive suite of mitigation and resilience measures designed to protect the development throughout its design life whilst avoiding any increase in flood risk beyond the site boundary:
Surface Water Management:

Site Grading and Drainage Design: Ground profiles to be designed to eliminate local depressions and direct surface water runoff positively towards drainage infrastructure. This prevents ponding adjacent to structures and ensures controlled discharge.

Attenuation Systems: Implementation of geocellular attenuation storage beneath permeable or impermeable surfaces, incorporating flow control devices such as Hydro-brake vortex chambers. These systems restrict discharge rates to greenfield runoff rates (typically 5 l/s/ha for the 1 in 100-year plus 40% climate change event), ensuring the development does not increase downstream flood risk.

Construction Resilience Measures: Specification of moisture-resistant building materials including:

- Resilient plaster or moisture-resistant plasterboard on lower wall sections
- Insulation materials with low water absorption characteristics
- Well-sealed pipework with minimal service voids to prevent water ingress
- Raised electrical socket outlets (minimum 1.0m above finished floor level)

These measures ensure that in the event of localised surface water flooding, the buildings can resist water entry and any internal damage can be repaired quickly without requiring fundamental reconstruction.

Policy Compliance and Planning Outcomes
The assessment demonstrated full compliance with the NPPF's flood risk requirements. As the site was located entirely within Flood Zone 1, the Sequential Test (steering development to areas of lowest flood risk) and Exception Test (demonstrating wider sustainability benefits) were not required. The proposals satisfied the NPPF's core principles:

- Future occupants would remain safe throughout the development's lifetime
- The development would not increase flood risk either on-site or elsewhere
- Opportunities to reduce overall flood risk were incorporated through controlled surface water management

The FRA provided Peterborough City Council's planning officers with robust technical evidence to support their determination. By identifying the specific surface water flood risk and proposing proportionate mitigation measures, the report enabled informed decision-making whilst protecting environmental receptors and neighbouring properties.

Professional Standards
The assessment was prepared by Air & Flood Consultants in accordance with Chartered Institution of Water and Environmental Management (CIWEM) standards, demonstrating:

- Systematic evaluation of all relevant flood sources
- Use of current, authoritative datasets
- Transparent methodology with fully referenced technical approaches
- Proportionate and achievable mitigation recommendations
- Clear communication of residual risks

The project exemplifies best practice in site-specific flood risk assessment for smaller commercial developments, balancing technical rigour with practical deliverability and cost-effectiveness.

Lessons and Transferable Insights
This case study demonstrates several principles applicable to similar recreational and commercial developments:

Early Identification: Review of freely available Environment Agency mapping at project inception enables early identification of surface water constraints, informing site layout and drainage strategy from the outset.
Ground Conditions Matter: Understanding soil permeability and geology is fundamental to drainage design. Sites with clay-rich, poorly draining soils require engineered attenuation rather than infiltration-based sustainable drainage systems.

Proportionate Approach: For sites in Flood Zone 1 with manageable surface water risks, appropriate mitigation through conventional engineering measures typically enables development to proceed without extensive study or complex modelling.

Integration with Planning: Successful flood risk assessments align technical findings with planning policy requirements, providing decision-makers with clear, accessible conclusions supported by robust evidence.