Flood Risk Assessment and SuDS Strategy — Extensions and Garden Room on a High Surface Water Risk Site, Edgware
We prepared a Flood Risk Assessment and SuDS Strategy for single storey rear and side extensions, a first floor extension and a detached garden room at 100 Whitchurch Gardens, Edgware, HA8 6PB — a Flood Zone 1 site in the London Borough of Harrow where surface water risk is high — on behalf of Alexander Awramenko of AS Made Studio. Tidal, fluvial, groundwater, sewer and artificial-source risks are all low; the high surface water rating follows from the low-permeability London Clay, on which rainfall that is not intercepted becomes runoff almost immediately. The proposals increase impermeable area from approximately 176m² to 302m². The strategy answers that 126m² increase with 179m² of permeable paving over a 300mm sub-base at 25% porosity taking the whole roof catchment, a silt trap upstream of the sewer connection, and 180m² of retained soft landscape.
Flood Risk Assessment and SuDS Strategy for Extensions and a Garden Room on a High Surface Water Risk Site, Edgware
Location: 100 Whitchurch Gardens, Edgware, London Borough of Harrow, HA8 6PB | Client: Alexander Awramenko, AS Made Studio | Services: Flood Risk Assessment; SuDS Strategy
An Increase in Impermeable Area on a High Surface Water Risk Site
This is the combination that drainage strategies exist to resolve. The proposals at 100 Whitchurch Gardens comprise single storey rear and side extensions, a first floor extension and a detached garden room. Taken together they nearly double the built and hard-surfaced footprint of the plot, and they do so on a site where Environment Agency surface water mapping already indicates a high risk. Without a designed drainage response the development would demonstrably increase runoff into an area that floods.
The starting point for the strategy is therefore an honest statement of the increase: impermeable area rises from approximately 176m² to 302m², an addition of 126m². Everything the strategy then does is measured against that figure rather than against a characterisation of the scheme as broadly neutral.
We were appointed by Alexander Awramenko of AS Made Studio to prepare the Flood Risk Assessment and the SuDS Strategy in support of the planning application to the London Borough of Harrow.
Site Characteristics and Flood Risk Profile
The site occupies a suburban residential plot in Edgware, within Harrow's Canons ward, bounded by dwellings and gardens. Ground levels across the plot are broadly uniform.
We drew on the Environment Agency Flood Map for Planning, the Risk of Flooding from Surface Water mapping, BGS geological mapping, LiDAR topographic data, sewer flooding records for the development address and the borough Strategic Flood Risk Assessment. The site lies within Flood Zone 1.
Fluvial and tidal risk are low. The site sits outside the 1-in-100-year plus climate change extent and there is no watercourse in the immediate vicinity. Groundwater risk is low, consistent with the low-permeability London Clay that underlies much of this part of north-west London. Sewer flooding risk is low, with no incidents recorded at the development address. Risk from artificial sources is low.
Surface water risk is high, and it is the governing source. The London Clay geology is the reason: it is effectively impermeable, so rainfall that is not intercepted or attenuated becomes runoff almost immediately, and the surrounding urban fabric offers few natural flow paths to take it away.
Surface Water Management Strategy
The strategy is built around a single high-capacity feature sized to take the whole roof catchment, rather than a scatter of small devices each treating a fraction of the plot.
Permeable paving: 179m² over a 300mm sub-base at 25% porosity, receiving the whole roof catchment. The hydraulic model conservatively adopts 170m², which understates the storage actually provided.
Silt trap or catchpit upstream of the connection to the public sewer, protecting it from sediment. It is a small detail, but it is the one that determines whether the system still works in ten years' time.
Retained soft landscape: 180m², 27% of the plot, preserved as permeable surface.
Directing the roof catchment into the permeable paving sub-base is what makes the arithmetic work. It converts the largest single source of runoff on the plot into a volume that is stored and released slowly, rather than discharged directly to the sewer at the rate it arrives.
The network is designed to the 1-in-100-year storm event with a 40% allowance for climate change.
Outcome
Subject to the measures set out in the strategy, the proposed development is acceptable in surface water drainage terms and surface water flood risk to the surrounding area is not increased. The commission shows that a substantial increase in impermeable area is not by itself a barrier on a high surface water risk site, provided the drainage design is sized against the increase and the roof catchment is put to work.