Flood Risk Assessment and SuDS Strategy, Harrow Weald
Four new dwellings on 0.064 ha of former garden land in Harrow Weald lie in Flood Zone 1 with every flood source assessed as low, but within Critical Drainage Area Harrow_08, where a flood risk assessment incorporating a drainage strategy and greenfield runoff rates were required. The Air & Flood Consultants FRA and SuDS Strategy showed that the permeable paving and water butts first proposed could not meet greenfield rates on the London Clay, and added a lined 26.8 m³ geocellular crate beneath the parking with a 20 mm flow control, holding discharge to 0.25, 0.44 and 0.51 l/s in the 1, 30 and 100-year events.
The site is 643 m² of former garden land in Harrow Weald, in the London Borough of Harrow. The proposal is four three-bedroom dwellings in two semi-detached pairs, each of two storeys with rooms in the roof, with four parking spaces, permeable hardstanding and soft landscaping. The site lies in Flood Zone 1 on the Environment Agency's Flood Map for Planning, and fluvial, tidal, surface water, groundwater, sewer and artificial-source risks were all assessed as low. The site does, however, fall within Critical Drainage Area Harrow_08, identified in the Harrow Surface Water Management Plan (2012) and mapped in the West London Strategic Flood Risk Assessment Level 1 (April 2019). On that basis the council's pre-application advice required the planning application to include a Flood Risk Assessment incorporating a drainage strategy.
The drainage design was tested against Policy CN4 (Sustainable Drainage) of the Harrow Local Plan 2021-2041, adopted on 24 March 2026, and London Plan Policy SI 13, which require greenfield runoff rates and the London Plan drainage hierarchy. The former garden land was treated as greenfield. Greenfield rates were calculated by the IH124 method at 0.25, 0.66 and 0.93 l/s for the 1, 30 and 100-year events. BGS mapping and the nearest borehole record show London Clay at surface, and the proprietary Air & Flood Consultants infiltration potential model indicates a permeability below 1×10⁻⁶ m/s. The only locations available for storage are also within 5 m of the dwellings, inside the soakaway offset in BRE Digest 365. Infiltration was therefore ruled out, and the paving and storage are fully lined.
The permeable paving and water butts shown on the application drawings were modelled first, in the HR Wallingford StopUP SuDS Tool with FEH22 rainfall. With a 50 mm outlet the paving discharged 2.86 l/s in the 100-year event, three times the greenfield rate. Restricting the outlet to 20 mm brought the peaks down but overflowed the 9.7 m³ paving sub-base by 11.6 m³ in the 100-year event with a 40% climate change allowance. The additional storage is provided as a lined geocellular crate, 47.0 m² by 600 mm deep at 95% voids (26.8 m³), confined to the parking bays and clear of an existing tree. Sizing runs showed about 46 m² to be the smallest crate that holds the design storm at that depth. A smaller, deeper crate was rejected because it raised the 1-year discharge above the greenfield rate.
With the crate and a 20 mm flow control, modelled discharge to the 175 mm Thames Water surface water sewer in the adjoining road is 0.25, 0.44 and 0.51 l/s in the 1, 30 and 100-year events, and 0.63 l/s in the 100-year event with a 40% climate change allowance. No flooding from the drainage system is predicted in any modelled storm. Without SuDS, the same roofs and paving would discharge 42.73 l/s in that event. Air & Flood Consultants issued the FRA and SuDS Strategy to the client in October 2026 for submission with the planning application, with the drainage layout drawing, the London Sustainable Drainage Proforma, the StopUP model outputs and a maintenance plan for the paving, crate and flow control.
More on these services:
Flood risk assessments: https://www.airandfloodconsultants.co.uk/floodrisk
SuDS strategies: https://www.airandfloodconsultants.co.uk/suds
Flood risk assessments in London: https://www.airandfloodconsultants.co.uk/flood-risk-assessment-london