Summary
- The Karehana Bay area in Plimmerton is one of the most flood prone catchments in the Wellington region.
- After severe flooding in late 2020 affecting 50 homes, Porirua City Council and Wellington Water (WWL) set out to mitigate flood risks, with a budget of $17.9m.
- Awa Environmental drove modelling for the catchment-wide optioneering, in partnership Stantec, who led the project overall and delivered engineering design.
- We rapidly assessed a wide range of stormwater solutions in an iterative process that involved dozens of model runs, testing numerous options and configurations until optimal solutions were identified and developed.
- The high-quality information provided by Awa Environmental and Stantec about the potential cost, risks, and benefits of the preferred solutions led to a broader consideration of flood resilience across the region and a decision not to proceed with these works.
Challenges
Fiscal constraints and competing demands
- After the 2020 flooding, WWL and Porirua City Council sought to reduce future flood impacts on residents, property, and infrastructure.
- To justify the investment they needed detailed, high-quality information on the costs, risks, and benefits of potential solutions.
- An early estimate set the project’s capex budget at $17.9m.
- However, the Council had already invested heavily in wastewater and drinking water infrastructure, so budget pressure was high.
- Any stormwater expenditure had to compete with other priorities across the district.
Catchment-wide design challenges
- Catchment-wide optioneering required an understanding of how individual design elements performed independently and as part of the wider system.
- For example, moving more water through the network in one location may worsen flooding elsewhere, while redirecting flows could undermine other design elements.
- The challenge was compounded by all stormwater flows in the catchment converging at a single constrained outlet to the coast, heavily affected by tides.
- High-quality modelling was critical to inform these interdependencies and risks.
Solutions
- WWL engaged Awa Environmental to lead the modelling for the optioneering phase, working closely with Stantec, who led engineering design and the project overall.
- Awa Environmental had delivered several flood models across the Wellington region, including one for the Karehana Bay catchment, built before the 2020 floods.
- We first validated the accuracy and reliability of the model by gaining an understanding of the latest flooding and checking the model emulated what happened, making updates to the model as required.
- This provided the project team with a reliable tool and an in-depth understanding of localised flood risks and the performance of the existing stormwater system under extreme conditions.
Complex modelling, iterative design
- Awa Environmental and Stantec then assessed a wide range of stormwater options starting with comprehensive full-system concepts.
- This process was highly iterative and fast-paced, so modelling and design progressed in parallel.
- Typically, Stantec and WWL proposed options and approaches for Awa Environmental to test.
- Based on model outputs, Stantec engineers would refine designs, which were re-tested and so on until optimal solutions were developed.
- Awa Environmental also suggested options and approaches, based on our familiarity with the catchment and stormwater network.
- The modelling was highly complex: each design element had to be considered in combination with others, and then entire system designs were assessed – together with the impacts of climate change.
Enabling rapidly repeatable benefits measurement
- To support decision-making, Awa Environmental also worked with WWL, Stantec, and others to establish a rapidly repeatable process for measuring the benefits of potential solutions.
- Model results were used to quantify reductions in flooding of buildings and corresponding cost savings.
- This approach allowed benefits to be consistently measured across design iterations and fed into the cost-benefit analyses at key milestones.
Scaled-back options under budget pressure
- The preferred concept design – featuring a pump station, stream upgrades, hydraulic improvements, pipe diversions, and inlet protection – was approved for detailed design.
- However, subsequent work revealed it would exceed the budget, so the team was asked to develop scaled-back options.
- Further modelling and design identified the preferred scale of individual options alongside the best-value combination of them that balanced project costs and benefits.
- Yet even these designs were unlikely to deliver sufficient performance or value to justify the associated costs and risks compared to other potential projects.
- Ultimately, this enabled WWL and Porirua City Council to make the difficult but informed decision not to proceed with the works.
Benefits
- Awa Environmental’s stormwater modelling capability and collaborative, delivery-focused approach enabled us to work productively and efficiently with Stantec, leveraging respective strengths.
- Our team’s knowledge of local hydrology, geology, and stormwater system challenges added value to the partnership and aided design decisions.
- Overall, the project assist WWL to:
- Understand the localised flood risk for the catchment
- Explore and refine catchment-wide options from initial concept to detailed design
- Quantify the costs, risks, and benefits of dozens of options combinations
- Develop optimal preferred designs.
- Most importantly, the work equipped WWL and Porirua City Council with the evidence needed to weigh this project against broader regional flood resilience goals.
- In the end, decision-makers chose to direct resources to other projects with greater overall benefit
