Flood Risk: Mixed Use Development

We were asked to assess the potential significance of flood risk to a mixed use (commercial and residential) development. The proposed redevelopment comprises the extension of the existing building to provide 8 residential units on the first and second floors and commercial space on the ground floor with extension of the existing basement to provide storage.

The site was located in Flood Zone 2 and as such, a Flood Risk Assessment (FRA) was required to aid the planning application for the Site. Our client took advantage of the FloodSmart Plus report, suitable for sites located in Flood Zone 2.

Whilst the end use of the redevelopment was not being changed, the overall area of the building was being increased from its original proportions. Furthermore, the proposed development would result in the increase of overall number of occupants and/or users of the building, thus presenting a more vulnerable end use.

The Site, located in a predominantly residential area, approximately 240m east of the River Irwell, and located within an area designated by the Environment Agency as a flood zone 2, was considered to be under threat of surface water, river, reservoir and groundwater flooding.

Whilst the Site does benefit from flood defences, a residual flood risk remains at the Site. As such, the risk assessment deemed the Site to be at ‘moderate’ risk of fluvial flooding and further work would be required to mitigate the risks. The risks from surface water flooding were however deemed as ‘low’.

In addition to the flood risk presented to the Site, the off-site flood risk was also assessed. The proposed development involves changes to the existing building footprint and hard standing, thus increasing the overall footprint of impermeable area on Site. Given the increase in impermeable area, the volume of runoff on Site is likely to increase, thus requiring additional measures to ensure that this additional volume could be appropriately attenuated without causing further flood risk.

Following a detailed review of available data by GeoSmart’s specialist consultants, moderate risk ratings were identified. However, the report concluded that if recommended mitigation measures were put in place, it is likely that the flood risk to the Site will be reduced to an acceptable level.

These mitigation measures included a minimum level above which residential and sleeping accommodation should be situated. Furthermore, a water entry strategy was recommended for ground floor space to promote flood resilience rather than resistance. These mitigation measures were also deemed appropriate for the mitigation of risks presented by groundwater flooding.

As part of the flood risk assessment a principal escape route was identified, should the site become inundated with water in the event of a barrier breach.

Additional work was also recommended with respect to the additional runoff from the development. A sustainable drainage assessment, such as GeoSmart’s SuDSmart Plus Report or SuDSmart Pro Report, was recommended in order to assess and provide solutions to the increased flood risk from the site to any downstream receptors.

The FloodSmart Plus Report costs £795 plus VAT and offers clarity to make informed decisions as well as available Environment Agency flood risk data.