Flood Risk Assessment to extend and refurbish a Grade I and II listed property.
About the project
Ambiental was appointed by the City of York Council to undertake a National Planning Policy Framework (NPPF) compliant Flood Risk Assessment (FRA) for the proposed refurbishment and extension of the York Guildhall.
The development was positioned directly next to the River Ouse and was positioned within a flood Zone 3a, 2, and 1. With the property being in central York , there were already existing flood defences, however with larger flood events and from review of the detailed data sets it became apparent that these existing defences didn’t protect the existing site as best as they could.
The Ambiental FRA report covered the likelihood of flooding for the new development and mitigation measures that should be put in place in the event that a flood should occur. These were discussed with the end client, prior to delivering the FRA.
The proposal then went on to gain planning permission.
A little more about the project
The Guildhall was largely empty after April 2013 when the council moved out, with the building only being used for conferences, seminars, events, and a full council meeting every 3 months or so. With it being a Grade I listed building, it was important any mitigation measures didn’t affect the aesthetic of the building too much.
The council were largely looking to repair and refurbish the building, namely new heating and electrical services, in order to encourage businesses to move into the building and maintain the close links between commerce and governance that’s seen throughout the building’s history.
During the first round of proposals, the Environmental Agency objected to their plan. They wanted to increase the size of the building by 272m². By amending the plans to a 245m² increase, the proposal was considered a “minor development” and was therefore deemed acceptable by the EA.
Ambiental were excited to take part in this historic project, knowing there was a rich history associated with the building. The new proposal for the site would greatly improve the functionality of the building, as well as enhancing the architectural and historical significance of the complex.
Without the proposed extension, the site sits almost entirely within a Flood Zone 3 area. This was not only due to its proximity to the River Ouse, but also its history of flooding. In fact, the Environmental Agency had provided data that showed the site had flooded 3 times within the previous 11 years (September 2012, autumn 2000, and in 1995). The River was determined to be the likely source of the flood water, although the EA didn’t provide further details on the source and extent of the flooding.
Being designated flood zone 3, the site is considered at a high risk of flooding (1 in 100 years). However, the new development was proposed in an area that was positioned within a flood zone 2 and 1 area and was considered less vulnerable under NPPF guidelines (National Planning Policy Framework).
There were defences already in place, consisting of foundations and retaining walls alongside the walkway. The downstream end also ties into an old stone wall that also acts as a flood defence. In addition to this, there is a gate opening in the wall that is often sandbagged by York Council to prevent further flooding.
A topographic survey of the site was provided by BFF Architects and Urban Designers. This showed that the ground levels varied between 10.26mAOD to 11.77mAOD (Meters Above Ordnance Datum).
The site is within the fluvial (river) flood extent for both a 1:100 year event, and a 1:1000 year event. Using the EA data provided, Ambiental concluded that the worst case scenario would cause a flood depth of 1.54m of water in a 1:1000 year flood event and 0.34m in a 1:100 year flood event, even with the defences.
Ambiental worked with the client to make sure that the flood risk remains acceptable and whether new mitigation measures needed to be implemented in order to adhere to NPPF guidelines.
Although the existing site had a history of flooding, even with defences in place, the flood risk assessment demonstrated that the extension had a risk of flooding that could be mitigated. It also included management and mitigation measures in order to make sure the proposed development was as sustainable and compliant to policy.
After the FRA was submitted, along with their other planning documents, both the FULM and LBC planning applications were approved in February 2017, followed by approval from Executive in March 2017.
Images taken from the York Council Website (https://www.york.gov.uk/downloads/file/11032/the_guildhall_complex_site_and_adjacent_contextpdf)