The Environment Agency’s national Risk of Flooding from Surface Water (RoFSW) mapping identified widespread surface water flood risk across the site, creating significant planning and drainage challenges for the proposed development. Through detailed hydraulic modelling and iterative design support, Aegaea refined the understanding of flood risk across the site, informed the proposed masterplan layout and helped resolve the Lead Local Flood Authority’s holding objection ahead of planning committee approval.

Aegaea developed a site-specific pluvial hydraulic flood model informed by detailed topographic survey data covering both the application site and surrounding catchment area. This allowed local drainage features including ditches, culverts and the adjacent railway embankment to be represented in greater detail than within the national RoFSW dataset.

The flood model was run for the 1 in 30 year, 1 in 100 year, 1 in 100 year plus 45% climate change, and 1 in 1000 year storm events. The flood modelling generated flood extents, flood depths and hazard mapping for each scenario, providing a more accurate understanding of how surface water behaved across the site.

Working iteratively and in collaboration with the masterplanner as well as the wider design team, Aegaea used the modelling outputs to directly inform and advise on the proposed layout. More vulnerable development parcels, including the residential areas and primary school, were positioned on higher ground outside the modelled flood extents, while lower-lying parts of the site were retained for attenuation basins, nature areas and open space capable of conveying and storing exceedance flows. Adhering to the Sequential Approach as set out by the PPG – Flood Risk and Coastal Change. 

This flood-risk-led approach was integrated into the wider drainage strategy, including the use of sub-catchment SuDS basins and acceptance of the water company’s requirement for greenfield Q1 discharge rates to the public sewer network.

Following review of the updated flood modelling and surface water drainage submissions, the Lead Local Flood Authority confirmed that the outstanding issues had been satisfactorily addressed and formally withdrew its holding objection, subject to planning conditions covering detailed SuDS design, exceedance routing, long-term maintenance and construction management.

The planning officer subsequently finalised the committee report recommending approval, and members resolved to grant outline planning permission subject to completion of the Section 106 agreement.

This project demonstrates the value of site-specific pluvial hydraulic flood modelling as a masterplanning and planning strategy tool, rather than solely a Flood Risk Assessment exercise reliant on national government datasets which are not site specific. By refining the published national flood mapping and integrating hydraulic flood modelling into the site layout process from an early stage, Aegaea helped provide a robust technical evidence base capable of supporting sequentially appropriate development and resolving complex drainage and flood risk concerns through proportionate design-stage mitigation.