Linton Park East Detention Dam Upgrade
Te Whakahoutanga o te Pāpuni i te Papa Rēhia o Linton
At the Old Taupo Road end of Linton Park East there is a ‘flood detention pond’ that helps to control the natural flow of water during heavy rain. The dam was constructed in 1998 and was built to protect properties further downstream from the risk of flooding.
Unlike what most people would consider a dam to be, this dam was built to control water flow rather than to store water for future use. There is a large culvert where the Mangakakahi stream flows which allows floodwater to slowly discharge from the above the dam and make its way down the natural watercourse to eventually reach Lake Rotorua.
About the dam
The existing dam is about 420 metres long, and is shaped like an ‘L’ along the Old Taupo Road and Mount View Road edges of the reserve. If you visit the reserve, it will look like a natural bank about 3 metres above the ground. Where the Mangakakahi Stream flows under the dam you will see an aluminum culvert that allows the water to flow through its natural course.
Rotorua Lakes Council has committed to long-term infrastructure investment to enable more growth, mitigate some existing stormwater issues and to prepare for the effects of climate change. Some of this work includes upgrades to stormwater assets around the city. This includes the Linton Park East dam.
Recently Council carried out investigations into its stormwater network to better understand what improvements can be made. The Linton Park East dam was part of those investigations. The results of these investigations, and analyses have highlighted that the dam in its current state does not meet modern safety requirements and current engineering best practice, and it is no longer fit for purpose.
Why do we need to bring the dam up to standard?
Since the dam was constructed in 1998, there have been significant advances in dam engineering and safety knowledge and practices. A dam safety assessment has highlighted several areas that require improvement:
- Dam embankment and foundation strength and resilient stability require reinforcement;
- Dam embankment seepage controls can be improved;
- Seepage control along the culvert to protect its stability;
- Spillway capacity to allow better controlled discharge;
- freeboard (a factor of safety) needs to be provided;
- The robustness of auxiliary spillway requires to be enhanced;
- The overall system performance monitoring instrumentation needs upgrading; and
- Introduction of new dam safety management systems.
There is a clear need to address these identified areas of improvement to protect people, property, and the environment downstream from the effects of stormwater during severe weather events. Due to the identification of the areas of improvement, the work to bring the dam up to standard will happen under the emergency works provisions of the Resource Management Act (Section 330).
What we need to achieve:
- Bring the dam up to modern safety standards and accepted current industry practise.
- Ensure the upgraded dam continues to manage stormwater flows downstream in a sustainable way.
- No increase of flooding risk to upstream properties.
- Provide as much flood mitigation as possible within the scope of the works to support new housing options.
How will the dam be rebuilt?
The dam has been assessed to have a High Impact classification due to a number of factors. A High classification means the dam has significant safety issues in the case of failure. This also sets the standards for design and ongoing management to keep the risk of dam failure to an acceptably low level. These standards are very high and include being able to safely withstand up to a 10,000-year flood event.
To meet current safety standards, the dam will need be raised and extended within the reserve. The spillway also needs to be replaced and constructed out of concrete to ensure safe operation. Once complete, it will extend past the carpark on Pedlar Street, on the Mount View Road edges of the reserve, and the overall height will be raised by up to 2.6 metres. It’s important to note that the dam is designed to manage stormwater during a 100 year storm event and safely manage flows beyond this. Water will only be ponded in the reserve in large events and will not fill higher than the ground level of surrounding properties.
Access to the reserve during construction
During construction, large vehicles will need to enter and exit Linton Park throughout the day. Council has investigated all possible entry points to reduce impact on local roads and neighboring properties.
Trucks will enter the reserve from Nairn Road. This route uses the least local roads with the fewest directly affected properties. Signage and traffic management will be in place during the construction period. There will be periods when the walkways and cycleways through the reserve will be closed.
Other opportunities for the reserve
While the replacement of the dam is necessary to protect life, property and our environment, it also presents an opportunity to do some enhancements on the reserve for recreational, environmental and better place making amenity.
Council is carrying out work to understand what potential enhancements are possible and what the timeline would be for any improvements. No decisions have been made at this point but we are looking at:
- If existing drainage of the sports fields closer to Edmund Road can be improved.
- What can be done to enhance some existing wetlands within the reserve.
- The existing playground at Deborah Place is up for renewal – there is potential for this to be better integrated into the reserve and include some nature play space, which will be driven by feedback from community engagement.
- Potential to create more connections to the surrounding neighbourhood and existing shared path that follows the Mangakakahi Stream.
Mangakakahi Stream improvement and protection works
Rotorua Lakes Council and Toi Moana Bay of Plenty Regional Council are working together on a programme of works to improve and protect the Mangakakahi Stream between Linton Park and Sunset Road.
There are a number of known issues that need to be addressed to protect the environment, people and properties downstream from Linton Park.
It is expected that any improvement and protection work will begin within the first months of 2022 and that work will include stream bank stabilisation and removal of exotic/pest plants. To maximise these efforts, a planting programme is planned for the stream banks. Establishing appropriate plants on the banks will enhance the natural environment, limit regrowth of exotic/pest plants and help to reduce the ongoing maintenance costs.
Cultural value sites
Rotorua Lakes Council has carried out investigations regarding potential sites of cultural value or wāhi tapu sites. From that research, there are no known wāhi tapu within Linton Park.
No. It’s important to note that the dam is designed to hold stormwater during a 100 year weather event. However, due to the reserve being in a low-lying area, and the height of the dam, the water would not fill higher than the ground level of surrounding properties.
The size and shape of the upgraded dam is contoured to be a rounded mound. It will look similar to the current bank that is there but slightly higher and longer around the reserve. During construction, contractors may need to remove a number of the large trees in the area. From initial investigations the trees look to be exotic (pest) species and Council will consider replanting native plants in the area as future enhancements. The removal of trees may also improve visibility and light for some properties.
The work is scheduled to take place between 2021 and 2022 during the warmer months when conditions are better for construction. Council will be informing the community of the updated construction timeframes once they have been finalised with the contractors.
The earthworks required are substantial, and there will likely be noise and disruptions from large vehicle movements for those that live near the construction area. The amount of disturbance will depend on how close your house is to the work site. Council is working together with the engineers and contractors to find alternative access into the reserve and methods to minimise disturbance to neighbours. If you have any questions or concerns, please contact Rotorua Lakes Council on 07 348 4199.
Council is aware of nearby areas that experience flooding during significant weather events. This work is part of a wider range of works planned over the next few years to mitigate existing flooding issues and provide capacity for additional growth. Upgrading the dam to ensure that it complies with the current regulations means that any risk of it failing will be mitigated. The new dam will provide better protection to the downstream industrial areas.
A 1/100 year flood will occur on average once every 100 years. In the case of a 100 year event, the probability of a storm this size occurring in any one year is 1%. This means it’s possible to have multiple “100 year floods” occurring within a 100 year period because, even if a 100 year flood event was recorded this year, the probability of another event of that size being recorded in the following year (or any other subsequent year) is still 1%.