Abbotts Site and Mill Pond Site, Dartford

EAS prepared the flood risk assessment and drainage strategy for two residential led mixed-use developments at the Mill Pond and Abbotts Sites in Dartford, Kent. These sites comprised former industrial uses and had contamination and flood risk constraints which needed to be considered with respect to the drainage and floodplain compensation.

The local flood mechanisms at the Abbotts site was identified to be from the fluvial River Darent and a breach on the Dartford Creek. The EA’s Dartford Creek hydraulic model was analysed to determine the worst case tidal breach model. EA modelled flood levels were used to estimate a 1 in 100 year (+35%CC) fluvial level through a stage-discharge graph. It was agreed with the EA that all finished floor levels across the site were set above the fluvial level. All sleeping areas were set above the tidal breach level.

A floodplain compensation study demonstrated that as fluvial flows would be directed towards the new semi-basement car park within the Abbotts site, this would provide suitable floodplain storage and no further lowering of levels within the site was necessary.

Green roofs were used on the courtyard areas to increase biodiversity and reduce runoff as part of a SuDS drainage strategy. In addition, amenity ponds/wetlands were used which were partially fed by the runoff from the roofs. Due to the potential for contamination across both the Mill Pond and Abbots Site, no deep excavations could be used for tanks and the like. Instead, runoff was discharged unattenuated via sewers to the River Darent. However, the inclusion of green roofs, wetlands and silt/hydrocarbon traps within the developments improved the biodiversity, filtered the surface water and resulted in a reduction of runoff discharging to the river.