Culverts, weirs, flood and tide gates are essential tools in the flood protection toolbox. They fulfill important functions such as passing water under roads and other infrastructure, controlling the flows within our waterways, and providing protection against storm surges and tidal flows. However, in restricting water flow, sometimes these structures can also restrict fish movement within our waterways.
Around 70 percent of more than 50 native freshwater fish species in New Zealand are threatened or at risk of extinction. Fish need to be able to move upstream and downstream between different river and stream habitats for feeding, finding refuge, or for different stages of their lifecycle, such as reproduction. Without this safe passage, their life cycles can be disrupted.
Flood protection structures in waterways can cause flows that are too strong for fish to swim against or present obstacles they are unable to get through.
Thanks to central government’s one-off shovel-ready climate resilience funding and local investment, we have been able to remediate 14 of these structures, enhancing our biodiversity and mahinga kai (traditional natural food resources and their ecosystems), while also gaining valuable knowledge on fish habitat that will benefit the wider region.
Fish passage the right thing to do
The National Environmental Standards for Freshwater Regulations (NES-F) introduced in 2020 made fish access to habitat upstream a requirement for all new structures built in waterways. Even though NES-F does not require the remediation of existing structures, we were already looking at ways to improve our existing flood protection and drainage scheme structures throughout Waitaha to make them more fish-friendly.
Environment Canterbury Water and Land Committee Chair, Councillor Iaean Cranwell said fish play a critical role in maintaining the health and balance of the precious ecosystems in our waterways.
“Fish are a vital part of healthy and resilient ecosystems. In Aotearoa they sustain people, terrestrial birds, and sea life. They help tell us how healthy our waterways are – with healthy water comes healthy fish,” he said.
“Not to mention, supporting fish lifecycles and habitats is, quite simply, the right thing to do.”
As with any sizeable work programme, funding the works through rates alone would take considerable time. The government’s climate resilience and flood protection funding, announced in 2020, enabled us to scope, plan, and undertake work to remediate our existing instream structures faster to allow for safer passage of fish through these waterways.
An independent consultant identified the highest priority structures from an initial list of 40 that needed assessment and, from there, the project team developed remediation strategies for the sites that would be most positively impacted by remediation, so we could focus funding where it was needed most.
From gates to rocks – the tools of remediation
Many of New Zealand’s native freshwater fish species are diadromous, meaning they must migrate between freshwater and marine habitats to complete their lifecycles.
This includes our breeding population of eels, which leave freshwater to spawn in the warmer waters of the Pacific Ocean; and galaxiids such as īnanga, which cycle between freshwater and the sea.
In recognition of the importance of migration to these species, emphasis was placed on structures that were restricting access upstream.
The techniques we used were based on the site-specific challenges preventing fish passage in each waterway. For some sites, remediation meant replacing an existing narrow culvert – typically a structure channeling water under infrastructure such as roading – with a wider one, as the increase in space creates a decrease in flow.
At some sites, remediation meant replacing the floodgate – designed to control floodwaters within the waterway – with one better suited to the specific needs of the waterway, that does not jam shut, preventing fish access for extended periods.
Sometimes, improving fish passage meant reducing the speed of the current, as fast-moving water can prevent fish from being able to enter a culvert. A good example is the use of rock ramps, where rocks are strategically placed to form a zigzag stairway, which slows water flow and creates small ponds of still water where fish can rest.
Surveying demonstrates fish passage success
Fish habitat survey prior to remediation works was critical to determining the best approach. Waterway sampling for pre and post remediation was achieved using methods such as fyke netting (a cone-shaped netting to capture fish), trapping, and electric fishing (temporarily immobilising fish, allowing them to be counted) and specific methods were selected based on each site’s conditions.
Through this workstream, several staff gained relevant training and even certification in electric fishing, enabling knowledge to stay in-house, which will be valuable for future projects in similar areas.
Additional surveying post-works was conducted to show the impact the changes have had on fish passage. Increases in the number of non-climbing fish species upstream of the structure post-remediation that were not present before work was undertaken demonstrate the fish-friendly remediation works have successfully allowed passage for fish.
Co-investment critical to the future of flood protection
Significant flood protection and community projects like this have been made possible due to the Government’s one-off COVID-19 response shovel-ready funding. However, climate change induced events across the country have highlighted the urgent need to adapt outdated flood protection infrastructure and do more to protect communities.
Currently, regional and unitary councils invest about $200 million each year in flood protection schemes. With ageing structures no longer able to meet the levels of service expected by communities against the challenge of climate change, this is expected to fall short of what’s needed by $150 million per annum.
What is increasingly clear is that a shared investment today means lower overall recovery costs, and better protection for the environment, and for current and future generations.
ENDS
