Flood Risk Assessment and SuDS Design
About the project
Ambiental were approached to create a Flood Risk Assessment and Drainage Strategy to support the development of the Hollings Building, more affectionately known as the “Toast Rack” and supporting buildings in Manchester.
This iconic building was built in 1960 and was given Grade II listed building status in 1998 by English Heritage for its distinctive and memorable design. Having been a part of the Manchester Metropolitan University for much of its life, it was closed in 2013 and later sold in 2014 to developers Generation Ltd.
The development involved changing the use of the “Toast Rack”, and the three other buildings within the site; a clothing block, admin block, and the “Drum” (or the ‘Fried Egg’). The plan was to refurbish the current building and extend the site to include an 11-storey residential building (named the Gateway Building), a further 7,000 square feet of retail spaces, and a 20,000 square foot gym and leisure area. Car parking, access, and landscaping works were also included in the planning.
In order to fully assess the flood risk of the site, Ambiental liaised with the Environmental Agency to collect previous flood data and LiDAR data to fully assess the likelihood of flooding in different scenarios.
The site is located in Flood Zone 1 and under the NPPF, it is considered a mixed vulnerability of “More and Less Vulnerable”. The site is also located within the Critical Drainage Area of Levenshulme and Fallowfield as identified by the Manchester City Strategic Flood Risk Assessment in 2011. Given the size of the development it was deemed to require an Environment Impact Assessment (EIA). Ambiental supported in providing the Environmental Statements (ES) for Water, Flood Risk Assessment, and Surface Water Drainage.
As a result of this, the development needed to demonstrate that it can be achieved in a sustainable manner, with an overall reduction of flood risk to the site and surrounding area.
With the new development, the percentage of hardstanding/impermeable surfacing would be increased. In fact, the proposed impermeable surfacing was increased up to approximately 80%.
Such increases can cause greater need for surface water drainage and implementation of Sustainable Urban Drainage Systems (SuDS) as the water would become impeded (pool) if these were not to be considered. By not improving the situation and complying with policy there is a greater risk of flooding to the site and elsewhere.
Ambiental contacted the local council and the EA to look for past flood data and found that no flood data was available for the site or within the immediate vicinity of the area. Given the EA flood map for planning demonstrated the site to be Flood Zone 1 – the risk of flooding from rivers and sea was confirmed to be low.
As the site is positioned within a critical drainage area, there was known to be a risk of surface water flooding even without historic data to look at. As such, the Environmental Statement of the EIA, included a Flood Risk Assessment (FRA) and Surface Water Drainage Strategy, was able to suggest a series of mitigation measures in order to make sure the development was as sustainable as possible in the event of flooding.
This risk of surface water flooding meant that SuDS drainage should be implemented across the site in order to mitigate any risk as well as make the site more sustainable.
Due to the nature of the soil at the site, infiltrating devices were dismissed. However, attenuation and interception devices were suggested as ways to manage the superficial water runoff.
You can find out more about the different types of SuDS devices and techniques in our article here: A SuDS Definition – Everything you need to know about Drainage Design and Strategy
Following the guidelines contained within the NPPF, the proposed development was suitable assuming appropriate mitigation (including adequate warning procedures) can be maintained for the lifetime of the development. The application was approved for planning.