Avonmouth to Aust flood defences, UK

The Environment Agency, South Gloucestershire Council and Bristol City Council have used NEC to improve 17 km of flood defences between Avonmouth and Aust on the east bank of the Severn Estuary, England. The client partnership let the complex £100 million design and build project to a joint venture of Bam Nuttall and Mott MacDonald under an NEC3 Engineering and Construction Contract (ECC) Option C (target contract with activity schedule) in April 2019. Despite the Covid-19 pandemic, the work was completed on time and below the target cost in June 2025. 

Known as the Avonmouth Severnside Enterprise Area (ASEA) Ecology Mitigation and Flood Defence Project, the multiple-worksite scheme has reduced flood risk to 2,500 homes and businesses arising from climate change and asset deterioration, created 80 ha of new wetland habitat, enhanced local infrastructure and helped to unlock 12,000 new jobs and £3.9 billion of local economic growth. 

The defences include 5.9 km of raised earth embankments, 2.5 km of steel sheet-piled walls, 2.3 km of precast reinforced concrete walls and 0.6 km of in-situ reinforced concrete walls. Other works included strengthening and increasing the height of eight existing watercourse outfalls, installing five new flood gates, building 188 m of glass panel flood walls and planting 2 ha of native trees. Two new wetland areas were also created as required as habitat compensation to allow economic development to continue without adversely affecting the Severn estuary. 

The scheme won the Showcase, Resilience and People’s Choice categories in the 2024 Institution of Civil Engineers South West Civil Engineering Awards. Arcadis was the NEC project manager and WSP Binnies was the NEC site supervisor.

Flexibility and collaborative ethos

Colin Taylor, senior flood and coastal risk management advisor at the Environment Agency, says NEC3 ECC Option C was selected by the client team for its flexibility and collaborative ethos. ‘It allowed the client partnership and delivery team to share risk and reward fairly, while maintaining strong budgetary control and transparency.’

He says all parties embraced the NEC requirement to ‘act in a spirit of mutual trust and co-operation’, which enabled a collaborative, multi-agency delivery model. ‘Co-locating workspaces for up to 30 partners also fostered a one-team culture and rapid decision making across the project team.’

Taylor says the contractor actively encouraged the client team to contribute throughout the design development process. ‘A design philosophy statement was used to capture the principles and document key client requirements, and weekly calls were held between the client and delivery teams, providing a regular forum for collaboration. This positive relationship helped to overcome the various challenges faced during the construction works.’

Collaborative leadership approach

Juraj Popovic, commercial manager with Mott MacDonald, says the client provided clear strategic direction as well as unified oversight and decision making throughout the project lifecycle. ‘This collaborative leadership approach was instrumental in engaging stakeholders, driving successful delivery and ensuring the project achieved its objectives.’ 

He says a key benefit of the contract was the immediate discussion of emerging issues via the NEC early warning process, with regular weekly meetings to agree actions to mitigate and/or minimise impacts on the defined cost and programme. ‘Any resulting compensation events were handled transparently and fairly, and the final forecast remained within the agreed budget.’

Popovic says the ECC Option C pain/gain share mechanism also incentivised innovation and value engineering. ‘An example was construction of a 200 m long trial embankment to measure ground response, which cut fill by 5,000m³ and saved £200,000 and 100,000 kg of carbon dioxide emissions. By proposing value engineering items we were able to reduce the cost by £3.3 million, benefiting both the client and contractor.’

He says the NEC requirement for a robust and regularly revised project programme combined with weekly production calls and collaborative planning sessions ensured programme control and agility. ‘Agile planning and wellbeing initiatives also ensured continuity during the Covid-19 pandemic, as most of the design was delivered while team members worked from home.’

Popovic adds that lessons learned from the contract were captured through workshops and shared across the Environment Agency and local authorities to inform future projects. ‘These lessons are now influencing best practice in flood defence design and being implemented on other similar infrastructure projects across the UK.’

Benefits of using NEC

• NEC provided a clear, robust and fair framework of collaborative processes, enabling all parties to work together to solve issues and work in the required ‘spirit of mutual trust and co-operation’.
• NEC early warnings ensured many risks were mitigated or avoided, and the compensation event process allowed change to be agreed as the job progressed.
• NEC ECC Option C provided full transparency of cost and programme throughout the works and incentivised all parties to work together.
• The project was completed on schedule and below the target cost, so both parties benefited from the NEC pain/gain share mechanism.