Nailsworth Garden Centre

Project: Nailsworth Garden Centre
Location: Avening Road, Nailsworth
Client: E.A. Chamberlain
Services: Flood Risk, Drainage

The Project

We provided flood risk and drainage services for a proposed farm shop and associated yard in close proximity to a river known as Gatcombe Water in Nailsworth, Gloucestershire.

The Problem

Our client, E.A. Chamberlain, wanted to create a farm shop as an extension to their existing garden centre business. But they were rightly concerned that part of the site was shown to be at risk of flooding from Gatcombe Water, with the flood zone designation representing a potentially significant obstacle to the development of the site. This being the case, it would be necessary to undertake a ‘sequential test’ to demonstrate that there were no other reasonably available sites in Stroud at lower risk of flooding where the development could be located.

There were also potential issues in the mitigation of flood risk: raising the site to make it safe may lead to a loss of floodplain storage. But it was clear from the outset that this loss could not be effectively mitigated within the client’s land holding. It was also evident from our initial review that delivering compliant surface water drainage would be challenging given the limited level difference between the site and the receiving watercourse.

The Conclusion

The interpretation of the flood data presented in our Flood Risk Assessment and the proposed drainage strategy was accepted by the Authority without hesitation – no further information or clarification was requested by the Council’s drainage officer or the Environment Agency, enabling the prompt grant of planning permission on December 1st 2021. Calibro have since been retained to provide detailed design service for the sustainable drainage scheme.

The Solution

Following an initial consultation and site visit, Calibro’s experts carried out a site-specific review of the flood modelling work used to define the flood zones. After determining that the model schematisation was appropriate, we refined the model outputs. This work involved creating a 3D ground model of the site and comparing it with flood model outputs to determine the extent and depths of flooding for key flood events.

This work demonstrated that less than 2m2 of the site was at risk from an extreme 1 in 1,000-year event, and that the vast majority of the site was located in flood zone 1. The work also determined that none of the site would be at risk of flooding during a future 1 in 100 year event, and that raising parts of the site to provide a development platform would not trigger the requirement for compensatory flood storage.

Having addressed the flood risk potential of the site, Calibro’s specialists developed a sustainable drainage strategy, with the aim of keeping water as close to the surface as possible and providing storage that would not be affected by the flood events of Gatcombe Water. The strategy incorporated a system of linear drains positioned around the perimeter of the proposed yard to direct runoff to subtle depressions alongside the existing ornamental pond. This approach avoided the requirement for costly land raising, would require minimal maintenance and make the best use of the existing ornamental pond to provide a final level of water treatment.

Once established with vegetation, the shallow depressions will improve the quality of water running off the site and complement the existing pond, both in terms of aesthetic appeal and ecological value.

Calibro has not only been able to identify and present clear and compelling strategies for large and complex site promotions – ones that don’t just rely on costly infrastructure improvements – but they also take a very proactive and innovative approach in the way they engage with key stakeholders.

Jonathan Steel
Director (Planning)

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