Image source: Bay of Plenty Regional Council.
Roanna Purcaru, Project Director l Lead Engineer, Storm & Surface Water; and Jamie McNeill, Business Group Leader, Geotechnical, at GHD discuss improving consistency in the assessment, design and construction of our Stopbanks and ancillary structures.
Our country has over 5000 kilometres of stopbanks and many hundreds of ancillary structures, such as flood gates, that form a critical network that helps protect flood-prone communities and infrastructure.
Stopbanks are designed to absorb water pressure, prevent erosion and hold back floodwaters to reduce flood impact. Recently, stopbanks have evolved beyond their traditional role in flood management and are becoming multi-functional community assets. Beyond flood protection, they’re enhancing local biodiversity, promoting urban green spaces, supporting walking and cycling paths, and preserving cultural heritage.
With billions of dollars of flood protection infrastructure across the country, and more than $200 billion of assets protected by them, flood protection infrastructure plays a critical role in our infrastructure network.
Stopbanks are essential for protecting our communities from flooding and, to be effective, they require regular maintenance and upgrades. The local government sector is challenged with carefully balancing investment costs and finance with the need to minimise flood risk to communities and infrastructure.
The Te Uru Kahika report – Before the Deluge 2.0 – Updated Business Case for Co-Investment in Flood Management Infrastructure Following Cyclones Hale and Gabrielle – estimates that $5 billion is required over the next 10 years to upgrade stopbanks. This presents both a significant opportunity to strengthen flood resilience and a major funding challenge.
In response, Central Government and Resources Minister Shane Jones confirmed in May 2024 an initial $200 million in flood resilience infrastructure through the Regional Infrastructure Fund, which focuses on resilience and essential infrastructure. Administered by the Kanoa – Regional Economic Development & Investment Unit, this investment aims to boost flood management capacity across impacted regions.
Dam’s poor cousins
Despite the crucial role flood protection assets play and the significant value of what they protect, there is currently no standardised regulatory framework or guidance for assessing, designing or constructing them.
Stopbanks are specifically excluded from the Building Act (other than stormwater detention dams), Building (Dam Safety Regulations) 2022, and the upcoming revision of the NZ Society on Large Dams (NZSOLD) Dam Safety guidelines. Instead, flood management is handled regionally, with central government playing a key role in recovery.
The lack of clear guidance and regulation has created inconsistencies in how assessments, design and construction are approached and has led to varied methods for stopbank assessment, design and construction.
As consultants, we notice these inconsistencies play out in different ways, from how stopbank and flood gate upgrades are procured to the variability in design outputs from consultants. This lack of standardisation is also a missed opportunity to improve processes and reduce costs in assessment, design and construction. Another critical consideration is how penetrations through stopbanks are managed.
Reducing inconsistencies
GHD’s design of the Rangitaiki River stopbanks, following the 2017 Edgecumbe floods, helped shape the update of the Bay of Plenty Regional Council Stopbank Design and Construction Guidelines published in 2021.
Standardising practices across the region presents benefits such as gaining efficiencies in construction and operation through familiarity with design. It also means communities receive the same level of protection by providing designers with a clear vision and purpose of the assets, by specifying minimum requirements for risk and climate change assessment, construction, maintenance and environmental management.
Engaging with communities is crucial, as it allows local concerns to be addressed within the broader vision of the project.
Our team is currently on a journey with Christchurch City Council to enable standardisation of floodplain management public assets within the residential red zone along the Avon River Corridor, with projects spanning across several decades of planning, design and construction of multi-functional assets.
With a multi-billion-dollar investment requirement over the next decade, there is a need for improved consistency in how stopbanks are assessed, designed and built.
One option would be raising the status of stopbanks to match that of other critical infrastructure, such as roads, bridges, rail and three waters assets.
Developing national best-practice guidance and regulation for stopbank assessment, design and construction would be the logical next step.