Stormwater Backflow Prevention Programme

The Brief

Following significant flooding in Westport, Buller District Council engaged us to support improvements to the town’s stormwater network. Our role was to help reduce the risk of river backflow into the stormwater system and improve network resilience during heavy rainfall and flood events. Challenges Identified

Given the age and materials of the former buildings, there was a risk of contamination from lead-based paints and fragments of asbestos-containing material on site. Assessing and managing these risks safely was a critical early step. 

What We Did

We designed structures for WaStop® backflow devices at four locations in the town. In combination with other works – outfall remediation and pipe upgrades – the use of these valves would prevent river backflow into stormwater reticulation and enable Westport’s underground stormwater network to become more resilient during heavy rain. The projects include: 

• Brougham Street Brick Arch Culvert – 900 mm I.D. outfall design, access manhole, bespoke structural shear wall, and construction supervision (2023) 
• Henley Street Stormwater Manhole – 2050 mm diameter 3m deep stormwater manhole and outfall with manual penstock design, allowing for future Wastop® installation (2023) 
• Cobden Street Emergency Stormwater Outfall – including 21 m of 1200 mm diameter replacement of the existing 450 mm dia. pipe and new manhole design, mechanically stabilised earth (MSE) retaining wall and rock protection, and construction supervision (2025) 
• Hughes Place Stormwater Outfall – including rock armouring and 130 m of 1050 mm dia. replacement of existing 450 mm dia. pipe and new manhole design (2025) 

We provided multi-disciplinary services, with deliverables including the detailed design package comprising a geotechnical engineering design report, civil and geotechnical design drawings, specifications and schedule of quantities. 

Innovations included a proprietary headwall design for Cobden Street – an MSE wall embedded in the riverbank and raked back at a 50-degree angle, with the backfilled slope rock riprap protected. At Brougham Street, a bespoke shear wall design enabled the 900 mm diameter outfall and new manhole to be connected to the existing brick arch culvert. 

Our civil engineering lead was regularly in Westport during the design and construction of the projects. Frequently on-site to assist with construction monitoring, they were readily available to provide constructability advice to Buller District Council representatives and the contractor for safe and efficient construction.