Flood Risk Assessment for Commercial Development: Planning Approved

Project Overview

Sigma Architects instructed FPS Environmental on behalf of their client to help assess flood risk for a development in Ilkeston, Derbyshire. As part of our wider work supporting architects with Flood Risk Assessments for planning, we were appointed to review the site and confirm its suitability for development. The development proposal consisted of the addition of new first floor to an existing single storey office building (B2 – General industrial). The site falls within Flood Zone 2 which meant the proposals required a site-specific Flood Risk Assessment in order to support the planning application. FPS Environmental demonstrated the site’s suitability for the proposed development confirming its suitability.

Flood Risk Evaluation

A desktop study was conducted to review the flood risk posed to the existing building. To do this LiDAR data was assessed to understand topography and local hydrology was reviewed to offer a baseline for the assessment. Using a variety of sources and data it was identified that:

The site is located within an area of recorded historic flooding, according to Environment Agency sources.

• The site is located within an area of recorded historic flooding, according to Environment Agency sources.
• The watercourses in the vicinity of the site are not tidally influenced.
• According to Environment Agency data available at the time, surface water was shown to not affect the site in either a high risk (annual chance of flooding of greater than 3.3%) or medium risk (annual chance of flooding of between 1% and 3.3%) scenario but was shown to be affected in a low risk scenario (annual chance of flooding of between 0.1% and 1%). Areas adjacent to the site showed a medium risk of surface water flooding.
• The site is located within an area at risk of flooding in the event of reservoir failure.
• Bedrock and superficial geologies are present at the site, with the site bedrock defined as a moderately productive aquifer.

Modelled Flood Levels

Upon request the Environment Agency provided the outputs of the Erewash and Tribs (AECOM, 2023) Nut Brook model. Floodplain heights for various return periods were provided and included within the FRA. Following advice on projected increases in peak river flows that may arise due to climate change, these were reviewed alongside modelling outputs whereby appropriate climate change allowances could be considered. These outputs subsequently allowed for the finished flood level of the building to be assessed.

Development Viability & Design Principles

Following the assessment of flood risk an analysis was undertaken to confirm if the development proposals would be acceptable given the outcomes considering multiple flood risk sources. Applying Annex 3 of the NPPF it was determined that the site should be classified as ‘Less Vulnerable’. Subsequently this meant that as the site is located in Flood Zone 2 and ‘Less Vulnerable’ that an Exception Test would not be required.
It was recommended that the development proposals:

• Considered moving vulnerable uses to upper floors.
• Applied an appropriate Finished Floor Level relative to flood risk.
• Diverted water away from buildings but safely manage it within the site so as not to impact flood risk on neighbouring buildings/ infrastructure.
• Considered the implementation of flood resistance and/or flood resilience measures.

Planning Success

Erewash Borough Council reviewed the planning application and approved the development. The application did not receive any objection from the Environment Agency, and it did not have any planning conditions relating to flood risk or drainage imposed. Erewash Borough Council concluded in their decision notice that “the proposed development is for the addition of a second storey to the existing office building which is considered to make it safer from a flood risk perspective and is therefore acceptable”.

Need for a Site-Specific Flood Risk Assessment

Continual updates to local and national planning policies mean that flood risk management requirements for developments are becoming stricter and are starting to apply on a greater scale to smaller sites than previously set out in legislation. It is more important than ever to appropriately consider flood risk for your development. Not preparing a Flood Risk Assessment or considering drainage from the start of proposals can be costly and cause delays to your project.

FPS Environmental is here to help, contact us if you have questions about planning requirements for flood risk and/or drainage and we will be happy to examine the requirements for your development.