Help shape flood resilience options for Ashley/Rakahuri
Overview
Over the next several years, we’re planning to create new flood resilience measures along the 22km section of the Ashley Rakahuri that runs between the Ōkūkū River and Waikuku Beach.
Our goal is to strengthen flood resilience for Rangiora, Kaiapoi, Waikuku, and surrounding areas.
The first round of consultation has closed. There will be further opportunities to have your say as the project progresses.
Why we're doing this work now
Early flood protection works on the Ashley Rakahuri started in the 1930s. The existing stopbank scheme has protected the district for around 70 years, but over that time, the flood risk has increased.
- Recent events, such as the Kaikōura floods, Auckland Anniversary floods and Cyclone Gabrielle, have demonstrated that extreme rainfall and flooding are happening more often and causing greater damage than in the past. Scientific evidence shows these types of events are becoming more frequent and intense.
- There are now more people, homes, businesses and infrastructure located within the Ashley Rakahuri floodplains. This means the consequences of flooding are higher than they were when the scheme was first built. View our flood modelling maps to see the areas that could be affected during a 1‑in‑500‑year flood event.
- Our modelling shows there is a risk of floods breaking out at different points along the river. We want to ensure our flood protection reflects today’s risks and that they are fit for the future.
Detailed information and flood modelling maps
Flood modelling maps
These maps show the potential extent and depth of flooding if the Ashley Rakahuri were to breach the primary stopbank at the Ōkūkū confluence and near Smarts Road during a 1-in-500 -year flood event.
The Ōkūkū confluence is one of the most likely breakout locations along the primary stopbank, but a flood could break out at any point in the stopbank, as braided rivers are highly dynamic.
A breakout could also occur in a smaller flood event, such as a 1-in-100 year event, as erosion rather than overtopping is the cause of breakouts.
The coloured areas on the map indicate where floodwater could travel across the floodplain and how deep the water could become.
The different colours on the map represent different estimated water depths in metres. See the colour key on the side of the map.
Map 1: Flood modelling upstream of Rangiora
This draft map shows where floodwater could spread and how deep it could be if the Ashley/Rakahuri River broke out during a flood with a 0.2% chance of occurring in any year.
Map 2: Flood modelling across the Ashley/Rakahuri floodplain
This draft map shows where floodwater could spread and how deep it could be across Rangiora, Kaiapoi and surrounding areas during an Ashley/Rakahuri River breakout flood with a 0.2% chance of occurring in any year.
How our flood modelling is developed
We use a combination of the following:
- Historic breakout data
- Updated LiDAR ground levels (remote sensing technology that uses laser lights to determine ground levels)
- Recent hydrology and climate information
- Different flow scenarios, including rare but high-impact events
- Sensitivity testing to understand how water behaves when it breaches stopbanks
Models are peer reviewed internally and will continue to be refined using information from site visits, landowner feedback, and iwi knowledge. The depth ranges shown in the key are estimates based on modelling, not precise measurements at individual properties.
Decision making and next steps
How decisions on flood resilience options will be made
Our next step is to do a comparative analysis of the design options, balancing:
- Protection of life and property
- Feasibility and cost
- Environmental and cultural outcomes
- Equity across communities
- Practicality for farming / land use
Councillors from Canterbury Regional Council (Environment Canterbury) will make the final decision on the preferred option/s after public and iwi input. This will be consulted on within the Long-Term Plan in 2027.
Project timeline
We’re in the early stages of this project, so the feedback and information you provide will help us select one of the options we’re considering.
Indicative timeframes are:
- 2026: Engagement with landowners, iwi, community, and Waimakariri District Council. Options refined into a shortlist.
- 2027: Preferred options selected, resource consent process begins, land compensation / purchase process begins (if required). Project details are updated in Environment Canterbury’s Long-Term Plan.
- 2027–2032: Work begins (depending on Council decisions)
Once an option has been selected, we will complete the work in stages:
- SH1 → the ocean
- Rangiora Railway Bridge → SH1 Bridge
- Ōkūkū confluence → Rangiora Railway Bridge
How this work is funded
This project was socialised in the Long-Term Plan 2024-34 with an indicative eight-year cost of $15 million. This funding will come from a mix of local targeted rates (for directly benefitting communities) and regional general rates, with potential for central government co-investment too.
As we develop the design to a preferred option, the cost estimate and funding model will be refined. It is our intent to update the funding in the 2027-37 Long Term Plan.
You can find out more about how we fund river management work on our River Rating District webpage
Detailed project information
What level of flood protection this work may provide
No flood protection system can remove all risk. This project aims to reduce risk to people, property and farmland, not eliminate it. Visit Waimakariri District Council's website for more information about natural hazards in your area
A 1-in-500 year flood (up to 3,900 cumecs - factoring in climate change) has been used as the design basis for these options so far, and the exact level of service (design flood) used in the upgraded stopbanks is to be confirmed through consultation with the community and councils.
Ashley Rakahuri flood history
The last significant breakout near the Ōkūkū confluence was in the 1930s, which was estimated to be around a 1,700 cumec flood. There were a series of breakouts in the 1950s between the Cones Road and railway bridge, which was the last time we had a flood that was above 2,000 cumecs.
In the May 2021 flood, the peak flow was approximately 1,600 cumecs at Rangiora. Flood levels were one metre from the top of the stopbank at locations upstream of Rangiora (near Groyne 8) and some properties were evacuated on a precautionary basis.
Secondary stopbank height
The height of any secondary stopbank above the existing ground level depends on where it is located in relation to terrain features, such as terraces or channels. The height also depends on the expected water depth within the stopbanks, which varies for each alignment option.
The role of gravel extraction
Gravel extraction is an important part of how rivers are managed across Waitaha Canterbury, and more than one million cubic metres of gravel is removed from riverbeds across the region each year. However, there are limits to how much gravel can safely be taken out.
Over the long term, gravel is carried downstream each year and settles in our rivers. Because of this natural process, lowering the riverbed in one place does not provide a permanent solution, as gravel will gradually build up again over time.
Gravel extraction is carefully managed through permissions under the Resource Management Act to ensure we balance gravel supply with negative effects such as undermining flood protection assets, infrastructure, riverbank planting, private land, and important habitats.
To manage these risks:
- Minimum riverbed levels are set in some areas
- Gravel can only be extracted when the riverbed is above those levels, and extraction must stop if those thresholds are reached.
- Riverbed levels are regularly surveyed to make sure extraction remains sustainable.
While gravel extraction can help manage flooding and erosion hazards in some situations, it cannot provide the same level of protection as flood protection infrastructure. Stopbanks are designed to provide a reliable and predictable level of protection during large flood events by helping keep floodwaters within the river corridor.
Our engineers consider a range of tools, including gravel management. In many cases, however, stopbanks are needed to provide the level of protection required for communities.
Insurance considerations
New Zealand’s National Adaptation Framework is clear that there is a difference between sharing information about risk and changes to physical risk on the ground. This recognises that better information helps people, insurers and banks make informed decisions but does not itself change the level of risk landowners face.
For this project, it means discussing options or providing updated information doesn’t alter existing flood behaviour, stopbank performance, or protection levels.
But individual insurers may take different approaches. If you have concerns, we encourage you to speak directly with your insurer.
Find out more on the national adaptation framework webpage
Properties that may be impacted by this work
We have attempted to contact all homeowners whose properties would be impacted by our current flood resilience designs. If you think your property may be affected and would like more information, please email us directly on rivers.delivery@ecan.govt.nz
What options we’re considering
As we follow the Ashley Rakahuri from its confluence with the Ōkūkū River to the coast, the river changes. Flood breakouts can travel in different directions and have different impacts in different places. That's why we're looking at the river in three reaches, each requiring different ways to manage breakout flooding risk.
Reaches:
- Reach 1: Ōkūkū confluence → Rangiora Railway Bridge
- Reach 2: Railway Bridge → SH1 Bridge
- Reach 3: SH1 → the ocean
We’re considering a range of possible solutions that may be combined. Different approaches may be needed for different parts of the river. These include:
- Upgrading or modifying the existing primary stopbank
- One or more secondary stopbanks
- Managed floodways
- Stopbank retreat
- Berm vegetation management
- Wetland creation / room-for-the-river approaches
- Gravel management
Design options
At this early, investigative stage of the project, we have no preferred option/s.
Your feedback will help select a preferred option by correcting our assumptions and highlighting unintended consequences. This will be balanced with other criteria such as safety, practicality for land use, environmental and cultural outcomes, feasibility and cost.
Reach 1: Ōkūkū confluence to Rangiora Railway Bridge
We’re considering six options to improve flood resilience in Reach 1, between the Ōkūkū River confluence and Rangiora Railway Bridge. These include new secondary stopbanks and upgrades to existing stopbanks. Explore the options below to compare their benefits and limitations.
Option A – Secondary stopbank to the racecourse
This option would see us building a second stopbank further south of the river, starting at the terrace on Mount Thomas Road and connecting back to the primary stopbank just east of the racecourse.
View larger option A map (opens in a new window)
Pros
- This provides two levels of protection for the urban areas of Rangiora and Kaiapoi, as well as the rural areas that are currently within the modelled flow paths for breakouts between the Ōkūkū to Rangiora
Cons:
- This is the longest alignment
- The option crosses a significant number of properties
- This option wouldn’t improve flood protection for properties in between the primary and secondary stopbanks, and would increase flood depths for some of these properties
Option B – Secondary stopbank from Groyne 8 to the airfield
This option would involve building a second stopbank further south of the river, starting around Groyne 8 and connecting back to the primary stopbank just east of the airfield.
View larger option B map (opens in a new window)
Pros
- This would maximise the length of secondary stopbank protection
- This option would provide increased flood protection for more people living on the south of the stopbank
Cons
- This option would have implications for potential land development south of the airfield
- Constriction of breakout flow where the proposed bank first crosses Mount Thomas Road, requiring mitigation for high water velocities
Option C – Secondary stopbank from Groyne 7 to the airfield
This option would see us building a second stopbank south of the river, starting south of Groyne 7 and connecting back to the primary stopbank just east of the airfield.
View larger option C map (opens in a new window)
Pros
- Maximises the length of secondary stopbank protection and improves protection for people south of the proposed stopbank.
- Uses natural terrain features to reduce the earthworks needed during construction.
Cons
- Could affect potential land development south of the airfield.
- Would not provide additional protection for many homes within existing breakout flow paths near the Ōkūkū confluence.
Option D – Shorter secondary stopbanks upstream of Groyne 7
This option would see us build a short secondary stopbank from the confluence of the Ōkūkū River, which returns the breakout flow around Groyne 7.
View larger option D map (opens in a new window)
Pros
This is the shortest option, which minimises impact on people living in the area in relation to impact on land and construction
It focuses on the high-risk section where two rivers meet
It could be a standalone option, or used in conjunction with the other options
Cons
It doesn’t address the risk of the primary stopbank failing further downstream
Its efficacy would rely on extensive primary stopbank upgrades (these may be implemented anyway)
Option E – Secondary stopbank to the west of the airfield
This option would see us constructing a secondary stopbank starting at the low terrace on Mount Thomas Road and joining the primary stopbank along Priors Road, to the west of the airfield.
View larger option E map (opens in a new window)
Pros
- This is a shorter alignment
- It reduces the impact of land use and construction impact on neighbouring properties
Cons
- If the primary stopbank failed, there would be no additional protection between the airfield and Cones Road
- Requires extensive upgrades to primary stopbank from airfield to Cones Road
Option F – Upgrade existing stopbanks in Reach 1
This option considers upgrading the existing stopbanks, keeping the existing alignment the same (likely with a wider footprint), and increasing the height. Engineered erosion protection and strengthening measures such as rock protection and groynes would be required to increase resilience.
View larger option F map (opens in a new window)
Pros
- This option minimises impact to private property as it follows the existing alignment
- Stopbank upgrades are likely to be required in some sections regardless of any alternative option selected
- Minimal impact on proposed developments and existing activities
Cons
- No backup protection if floodwaters overtop or erode the stopbank
- A higher stopbank allows the river to carry more water, which means better protection in most circumstances but potentially greater impacts for nearby properties if the stopbank is damaged during an extreme flood
- Extent of strengthening could be significant as erosion can occur anywhere along the stopbank system
Reaches 2 and 3: Rangiora Railway Bridge to the ocean
We’re considering five options to improve flood resilience in Reaches 2 and 3, between Rangiora Railway Bridge and the ocean. These include upgrading existing stopbanks, moving stopbanks further from the river, creating a managed spillway and modifying stopbanks near the estuary. Explore the options below to compare their benefits and limitations.
Option F – Upgrade existing stopbanks in Reaches 2 and 3
As with Reach 1, this option proposes raising and strengthening existing stopbanks where required to increase the level of flood protection provided. A geotechnical engineering consultant is investigating how and where this is possible.
View larger option F for reach 2 and 3 map (opens in a new window)
Pros
- Minimises impacts on private land
Cons
- Doesn't address flow restrictions at the SH1 bridge, so a spillway may still be required.
- May not be feasible to upgrade all sections of stopbank.
Option G1 – Stopbank retreat on council-owned land
This option proposes building a new stopbank on Environment Canterbury-owned land on the north side of the river and, once established, lowering or removing sections of the existing stopbank.
For both Option G1 and Option G2, new flood protection vegetation would be planted and allowed to establish before the existing stopbank is lowered or removed.
View larger option G for reach 2 and 3 map (opens in a new window)
Pros
- Increases flood-carrying capacity in this section.
- Reduces constriction created by the existing stopbank alignment.
- Primarily uses Environment Canterbury-owned land.
Cons
- Will take time for new flood protection vegetation to establish.
- Addresses capacity issues over a relatively short section of the scheme (1.2 km).
Option G2 – Extended stopbank retreat
This option builds on Option G1 by extending the proposed stopbank alignment along the edge of an old river terrace through privately owned land. For both Option G1 and Option G2, new flood protection vegetation would be planted and allowed to establish before the existing stopbank is lowered or removed. This staged approach helps maintain flood resilience during the transition.
Note: We will publish the map/s for Option G2 following further discussion with relevant landowners.
Pros
- Increases flood-carrying capacity in this section of the river.
- Reduces constriction created by the existing stopbank alignment.
- Creates a wider flood corridor, allowing floodwaters to move more freely during large flood events.
- Provides greater room for the river to operate naturally.
- Supports long-term flood resilience by moving flood protection further from the river.
- Enables establishment of new flood protection vegetation before the existing stopbank is lowered or removed.
- May provide environmental benefits through expanded riparian vegetation and habitat creation.
Cons
- Requires use of privately owned land and may affect existing land uses.
- May require land acquisition, easements, or other property agreements.
- Could impact farming operations, property access, drainage, and infrastructure.
- Higher implementation costs than shorter retreat options.
- Longer planning and delivery timeframes due to landowner engagement and property requirements.
- Requires ongoing management and maintenance of new flood protection vegetation.
- May result in changes to the landscape and how surrounding land is used.
Option H – Managed spillway upstream of the SH1 bridge
The State Highway 1 bridge limits how much water can safely pass through this section of the river during flood events.
If flood capacity is increased, a spillway could be used to allow water to overflow from the river in a controlled manner to the north and into either Saltwater Creek or across the highway and into the estuary.
Note that the shaded area is indicative only and does not represent exact flows.
View larger option H for reach 2 and 3 map (opens in a new window)
Pros
- Reduces flood pressure on the SH1 bridge.
- Improves flood protection for Waikuku.
- Allows overflows to occur in a controlled and predictable way.
- Guidebanks can help direct flood flows along a defined path.
Cons
- Will result in flooding of properties within the floodway when activated (likely in excess of 3,000 cumecs, almost twice the recorded peak flow in May 2021).
- Some properties in this area are at risk of flooding under existing conditions.
Option I – Modify stopbanks near the estuary
This option considers modifying the existing stopbank on the north side of the river downstream of SH1.
This might include lowering or removing sections of the existing north-side stopbank to relieve pressure on the south-side stopbank, and to lower flood levels near Waikuku Beach.
Note that the shaded area is indicative only.
View larger option H for reach 2 and 3 map (opens in a new window)
Pros
- May reduce flood levels near Waikuku Beach.
- Allows for the future effects of sea level rise by retreating flood protection from land that may be inundated.
- Creates opportunities for large-scale estuary restoration.
Cons
- May affect private land and farming activities.
What happens next
Feedback is now closed. Thanks for telling us what you think. We will summarise feedback themes and share how it shaped decisions on this page later in 2026.
Areas
- Selwyn District
Interests
- Flood protection and river management
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