Flood Risk Assessment and Sequential Test for a Redevelopment Scheme in Elmbridge - Case Study

Flood Risk Assessment and Sequential Test for a Redevelopment Scheme in Elmbridge – Case Study

In Case Studies by Ambiental Environmental Assessment

Flood Risk Assessment and Sequential Test for a Redevelopment Scheme in Elmbridge - Case Study

About the project

Ambiental were approached to provide support with Flood Risk and Sequential Test works for a small redevelopment scheme in Elmbridge. The development consisted of the demolition of a garage and construction of a new, end of terrace, residential dwelling.

Elmbridge is a Borough with a significant flood context. The River Thames form Elmbridge’s Northern boundary, with the Rivers Wey, Mole, and Rythe running through it. There are also areas of the Borough at risk from surface water flooding.

The Elmbridge Strategic Flood Risk Assessment (SFRA) suggests that over 12,500 properties are at risk of flooding

The client had received a favourable pre-app response. However, it did identify a number of flood risk related issues associated with the site being located within an area of Flood Zone 2 and in close proximity to Flood Zone 3.

Flood Risk Assessment and Sequential Test for a Redevelopment Scheme in Elmbridge - Case Study

The Problem 

The development itself was located on the end of a terrace. By nature, this has a number of constraints associated with it relating to the aesthetics of the proposed new dwelling and making sure that appropriate flood mitigation measures were incorporated into the design.

Elmbridge Core Policy CS26 (Flooding) seeks to reduce the overall and local risk of flooding in the Borough. It requires that a new development is located, designed, and laid out to ensure that its safe. This means that the risk from flooding is minimised, whilst not increasing the risk of flooding elsewhere and that residual risks are safely managed.

Generally, national guidance advises that minor developments are unlikely to raise significant flood risk unless specific localised issues arise. However, Elmbridge Strategic Flood Risk Assessment (SFRA) published May 2015 and the Elmbridge Supplementary Planning Document (SPD) published in 2016 advises that in parts of the Borough there is potential for minor developments to have a cumulative impact on flood risk in the local area as a result of impact on flood storage capacity and flood flows. Therefore, it is appropriate to take a precautionary approach to all development within areas identified as being at risk of flooding.

The new extension increased the footprint by approximately 25m2 compared to the existing site. Although it was only a small increase, compensation was still required in order to reduce the flood risk to the surrounding area. Initially, under-floor voids were requested in order to compensate for this.

However, sole reliance on the use of under-floor voids to address the loss of floodplain storage capacity is generally not acceptable on new developments. On smaller scale developments, using a void may be suitable where it is not be possible to achieve all the direct compensation required or where it can be difficult to achieve full compensation. However, it will also typically require a legal agreement or planning condition for a maintenance plan to satisfy that they remain open and useable for the lifetime of the development.

The Solution

Initially, Ambiental conducted a flood risk review of the site and provided a short technical note which detailed the flood levels that needed to be designed to, the required mitigation measures (including finished floor levels), and other recommendations which were all used in the design of the building to increase the chances of planning approval.

Compensation calculations were undertaken and ultimately, due to the predicted flood levels, it was decided that non-habitable rooms (garage, storage, utility room etc.) should be located on the ground floor and designed to be flood resilient.

Once the design had been finalised, the analysis, undertaken as part of the initial review, was used to inform the Flood Risk Assessment and Sequential Test which was submitted as part of the planning application.

The Outcome

Our design plans and mitigation measures were accepted by the client and they incorporated them into their planning application.

Steven Brown Ambiental Associate

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