About the project

Ambiental were approached by St Paul’s School in London to undertake a NPPF compliant Flood Risk Assessment (FRA) and Environmental Impact Assessment Chapter for Flood Risk and Surface Water Drainage (Water). The development was for a minor material amendment of an existing planning permission for the ongoing redevelopment of the building. The existing planning was granted planning permission in 2009.

St Paul’s School is an independent school offering education to boys aged 7 to 18 years.  It’s located on a 43-acre site in Barnes, London right on the River Thames. It has a rich and diverse history and at its inception in the 16^th century, it was the largest school in England. Since then, the school has moved 4 times before settling in its current location.

A little more about the project

The existing planning was approved for the demolition of most of the existing buildings on site and the refurbishment of the sports hall, with the construction of a 13,159m² educational facility including classrooms, a dining hall, kitchen, library, chapel, and further residential accommodation.

Changes to this development meant a new Flood Risk Assessment was required as part of the Environmental Statement – Water to accompany the full EIA. The new development was to add on to the existing plan the reconfiguration of the car parking facilities and the construction of an additional residential area with a 1,084m² footprint.

Flood Map for Planning showing the Flood Zones of the site

The Problem 

The site was located across Flood Zones 1, 2, and 3, although it was mostly in Flood Zone 3. The new development was ‘More Vulnerable’ under the NPPF policy due to the residential areas and its use as an educational facility. In accordance with the National Planning Policy Framework an exception test was required, which meant the development had to provide wider sustainability benefits to the community that outweigh the flood risk and the FRA had to demonstrate the development won’t increase flood risk elsewhere and where possible, reduce the flood risk overall.

According to the EA, the potential flooding sources were from tidal (River Thames), surface water, groundwater, and sewers. The EA Flood Map showed that the site had a greater than 0.5% annual chance of flooding from the River Thames, however, the defences in place provided benefit.

In fact, the existing flood defences were upon analysis designed to provide a defence up to a 1:1000 year flood event.

It is important to note that tidal flooding is generally caused by low pressure weather systems creating storm-surges (or storm tides), chiefly via high speed winds. These winds (and to a certain extent, the low pressure) create a ‘bulge’ of water which, if it coincides with high tide, can generate very high, stormy, water levels. However, because this mechanism is well understood, it is very likely that an early warning will be issued before such an event occurs. As such, it is very unlikely that the site would be subject to tidal flooding without several hours of early warning.

Although the mechanisms of tidal flooding are well understood, and the site benefited from defences, there was also the risk of flooding from the failure of the defences (breach) the residual risk. The “what if?” scenario.

As the project evolved through planning so too did the Environment Agency models for the River Thames in these locations. The greatest change being the need to consider the residual risk more so than ever. In addition, the client further included boarding school facilities and the level of the sleeping accommodation became ever more so important in light of this new information.

The complexity of this new data and how it affected the sleeping accommodation level of the new building was further exacerbated as some of the new buildings would connect to existing buildings. With these new building the finished floor levels would have to differ compared to those that were existing. Our team of Flood Risk Consultants worked extensively with the client, the planning consultant, the architect, and others in the planning team to contribute in finding the most effective solution with minimal impact to the proposed designs and connection to the existing buildings.

We had to adapt quickly considering this new data and how it would be incorporated into the design of the proposed development. The new data as mentioned did complicate the design however our Flood Risk Consultants arranged multiple meetings with the EA and Local Planning Authority in London to work through their concerns and agree appropriate mitigation measures that supported St Pauls School, the client, and the Environment Agency.

The Solution

The new development post completion of the meetings and review of all the important data, it was found to have negligible environmental impact on the surrounding surface water environment in both the short (construction) and long term (operation) so long as suitable mitigation measures were put in place.

The mitigation measures put in place on the original plan were still compliant to policy and were extended to the new buildings, so no further additional flood mitigation measures had to be recommended.

The Outcome

The development was considered suitable assuming appropriate, maintainable mitigation was put in place, including adequate warning procedures and an evacuation plan.

The development went on to get planning approval and building is already underway.