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Background

RammSanderson Ecology Ltd was commissioned by Midbrook Buildings to prepare a site specific Flood Risk Assessment (FRA) for the proposed redevelopment of flood damaged buildings at the Riverside Garden Centre in Hertford, following the flooding of the River Lea in February 2014.

The site was found to lie within Flood Zone 3 (with a greater than 1% annual probability of flooding) of the River Lea and Bayford Brook. This site had been subject to a successful planning application for the redevelopment of the main retail structure in 2010. As a result of this the new building was constructed to a flood resistant and resilient design. This building was successfully protected during the 2014 flood event.

The Key objective of the project was to assess the risk of flooding at the site and provide details of appropriate mitigation measures.

The Assessment

Modelled flood level data was provided by the Environment Agency for the River Lea and Bayford Brook. A further hydraulic model had been prepared for the previous application for the site which confirmed the accuracy of the Environment Agency model. Topographical data for the site was then compared to the modelled flood levels to determine the depth of flooding expected during various return period events.

Recommendations

Recommendations were made to set finished floor levels of the proposed buildings 600mm above the 1 in 100 year + climate change flood level with specific consideration for the displacement of flood waters from the site to adjacent land with recommendations for flood storage compensation.

A flood evacuation procedure was recommended detailing the requirement to sign up to a flood warning service. This will allow the successful implementation of flood resistant measures (such as shutter boards on doors) to protect the development from future damage.

Site specific recommendations for surface water management were made to prevent an increase in surface water runoff towards the adjacent watercourse. This involved the use of SuDS including permeable paving and a network of swales and infiltration strips to store, balance and control surface water runoff prior to its discharge from the site.

Due to the sites use as a garden centre, a comprehensive rainwater harvesting system was designed to collect runoff from hard surfaces and buildings for use within the site. Excess runoff was then discharged to the River Lea at a controlled rate equivalent to the pre-development brownfield runoff rate less 20% for betterment.