Flood Risk Assessment and SuDS Strategy — Drainage and Resilience Conditions Discharged, Hackney

Flood Risk Assessment and SuDS Strategy Discharging the Drainage and Flood Resilience Conditions at Downs Road, Hackney
Location: 107 Downs Road, Hackney, London, E5 8DA | Client: Arkidraft Ltd | Services: Flood Risk Assessment; SuDS Strategy; Phase 1 Contamination Risk Assessment

A Site with Nowhere for Water to Go

The site lies within Flood Zone 1, so the risk of flooding from rivers and the sea is low. The governing source is surface water, and the numbers are stark. Environment Agency Surface Water Spatial Planning mapping places 54.6% of the site (255m²) within the 1-in-30-year (3.3% AEP) extent and 83.6% (391m²) within the 1-in-1000-year (0.1% AEP) extent. In the 1-in-100-year event including a climate change allowance, modelled depths across the site are predominantly 150-300mm, with 88m² falling in the 300-600mm band. An overland flow route crosses the site from north-west to south-east.

The existing site makes that worse. It is at present wholly impermeable: the existing building roofs, the single-storey workshop and the yard hardstanding cover the whole of the developed area, and surface water discharges to the combined sewer without any attenuation whatsoever.

We were commissioned by Arkidraft Ltd to prepare a combined Flood Risk Assessment and Sustainable Drainage Strategy in support of an application for approval of details reserved by conditions 3 (sustainable drainage) and 4 (flood resilient and resistant construction) of planning permission 2023/1273, granted on 19 November 2025. The scheme involves the part demolition of a single-storey car repair workshop and redevelopment to provide office (Class E) floorspace together with an additional residential unit. A Phase 1 Preliminary Risk Assessment for land contamination was prepared alongside the flood and drainage work, the site's former use as a vehicle repair workshop being the obvious potential source.

Flood Risk Profile

The site was assessed against all six sources. Tidal flooding is low. Fluvial flooding is low. Surface water (pluvial) flooding is high. Groundwater flooding is low: the Hackney Local Plan records groundwater flood risk across the borough as relatively low, and groundwater vulnerability at the site relates to aquifer protection rather than to flooding. Flooding from sewers is medium. Flooding from artificial sources is low.

Condition 3 — Sustainable Drainage

The strategy takes a site with zero existing attenuation and delivers two things. First, it reduces impermeable area by 87.06m², or 18.6%, converting yard hardstanding to permeable and landscaped surfaces. Second, it introduces sustainable drainage features where none existed, including 45.51m² of permeable paving over a lined 450mm sub-base providing sub-surface attenuation before any residual discharge reaches the combined sewer.

The sub-base is lined rather than open because the strategy is one of attenuation and controlled discharge, not infiltration; the aquifer protection position at the site and the presence of a former vehicle repair use both point away from unrestricted infiltration to ground.

On a site of this character, that combination is the meaningful test. Where the baseline is total impermeability and unrestricted discharge, any properly designed scheme produces betterment — but the betterment has to be quantified against the 1-in-100-year event with a climate change allowance to satisfy the condition, and that is what the strategy sets out.

Condition 4 — Flood Resilient and Resistant Construction

Given modelled depths of 150-300mm across most of the site and an overland flow route crossing it, the resilience specification addresses external and internal measures together. Externally, threshold levels and ground profiling direct overland flows away from the building and toward drainage infrastructure, with sealed thresholds and raised air bricks where practicable. Internally, moisture-resistant construction is specified throughout the ground floor: resilient plaster, low water-absorption insulation, sealed pipework and raised electrical services.

The split matters on a site where water arrives across the ground surface rather than rising from a watercourse. Resistance measures at the threshold handle the depths modelled here; resilient internal construction handles the residual case in which they are exceeded.

Outcome

The submission gave the London Borough of Hackney a quantified drainage position and a resilience specification proportionate to a high surface water risk site, discharging both conditions on a scheme where the existing baseline offered no attenuation and no resilience at all.