FP McCann’s Hybrid SuDS Solution Specified At Snettisham Housing Development

Sep 25

2 min read

FP McCann has successfully delivered a bespoke hybrid Sustainable Drainage System (SuDS) for Bennett Homes' The Spires housing development in Snettisham, Norfolk, combining innovative design, rapid response and efficient installation to overcome challenging ground conditions.

Large white concrete foundation slab at a sunny construction site, with excavators, trenches, and houses in the background.

The development required a drainage solution that would satisfy both planning authority requirements and local flood authority regulations while providing effective long-term stormwater attenuation for the new housing scheme.

However, during excavation works, previously unidentified groundwater was encountered which had not been identified during the original site investigations. This unexpected discovery meant the original drainage strategy was no longer suitable and required a rapid redesign to prevent delays to the development programme.

Construction site with a large concrete slab in a deep excavation, surrounded by scaffolding and excavators under a clear blue sky.

The challenge for both Bennett Homes and consulting engineers, Canham Consulting, was to find a robust solution that could operate effectively within groundwater conditions while maintaining project timelines and regulatory compliance.

Working closely with Canham Consulting, FP McCann's Specification and Technical Team quickly assessed the revised site conditions and developed an alternative drainage strategy within approximately two weeks. The solution centred around a 523m³ precast concrete StormTank, specifically designed to perform in groundwater conditions while delivering guaranteed watertight performance. Installed partially within the groundwater table, the StormTank forms the foundation of a unique hybrid SuDS approach.

Construction site with a large white concrete slab, excavators, and dirt piles under a clear blue sky.

The system integrates underground and above-ground attenuation through the combination of the precast concrete StormTank and a landscaped attenuation basin positioned directly above it. During normal rainfall events, stormwater is stored within the underground tank. During more extreme weather conditions, excess water rises into the attenuation basin above, providing additional storage capacity before safely draining away. Once conditions return to normal, water gradually drains back through the tank.

Leveraging FP McCann's Design for Manufacture and Assembly (DfMA) approach and Modern Methods of Construction (MMC), the precast modular solution was manufactured and supplied rapidly, enabling installation to be completed in just two weeks with minimal disruption to the wider construction programme.

Key benefits of the StormTank system include:

100-year design life
Installation depths of up to 6 metres
No footprint size limitations
Suitable for loading requirements from pedestrian areas through to LM1 highway loading
Guaranteed watertight performance
Proven suitability for groundwater applications

The Result

The revised drainage solution enabled the project to continue ahead of programme despite the unforeseen groundwater challenge, avoiding significant delays and providing a compliant, sustainable and future-proof drainage system.

Concrete drainage channel with raised side walls and open slots along a sunlit construction site, no people visible

The innovative hybrid design not only delivers effective flood management but also creates a valuable community amenity. The landscaped attenuation basin provides an attractive green space that can be planted with wildflowers and enjoyed by residents during dry weather, delivering both environmental and social benefits.

Commenting on the project, Rhys Buckle, Canham Consulting, said: "We selected FP McCann's StormTank solution due to its guaranteed watertight performance, proven suitability for installation in groundwater conditions, and the long-term reliability of its precast concrete systems. Throughout the project, FP McCann's technical team provided exceptional support, working collaboratively with us and delivering design updates ahead of programme."

The Snettisham scheme demonstrates FP McCann's ability to combine technical expertise, collaborative design support and modern off-site manufacturing techniques to deliver adaptable and sustainable drainage solutions, even in the most challenging ground conditions. www.fpmccann.co.uk