Whangamatā Stormwater Flood Modelling Project
- Project typeStormwater infrastructure
- Project value$9.2 million
- Completion Date30 June 2030
Whangamatā Stormwater
The township of Whangamatā has been facing stormwater issues for some time. To ensure a coordinated approach to remedial work, our Council is undertaking a stormwater modelling exercise to understand the issues and where work needs to be carried out.
Whangamatā Stormwater Flood Model
We can’t control the weather, but our Council has developed a flood model and map for stormwater flooding to understand the risks, allow our communities to be prepared and become more resilient to flooding.
We are using the information gained from the stormwater flood model maps for our stormwater infrastructure and emergency planning.
Climate change is increasing the number and intensity of storm event and the modelling undertaken helps us incorporate these changes.
What is the result of the stormwater flood modelling in Whangamatā?
During storm events, stormwater runoff naturally flows overland to the lowest point. Typically, in Whangamatā, we experience stormwater ponding at low points for short periods of time, with the stormwater infiltrating into our sandy soils quite rapidly.
However, during large storm events, our sandy soils and stormwater network will reach capacity, or there are blockages, which can result in flooding.
The majority of flooding in Whangamatā can be described as ponding. Ponding is where water can become trapped and collect during heavy rain.
The lack of velocity in the ponding flood waters significantly reduces the risk to people and damage to property.
It is important to understand the Whangamatā Stormwater Flood Map shows the extent of an extreme storm event, of which our district has not experienced before.
Access the Flood Map here
You can find easy step-by-step instructions on how to best use this map here(PDF, 462KB).
Capital Expenditure
The 2024-34 Long Term Plan (LTP) has been finalised and approved with $9.171M of capital funding granted to address stormwater issues in Whangamatā. Details of Stormwater funding can be found on page 69 of the LTP.
Our Council has been working with the Whangamatā Stakeholder Community Engagement Team (Community representatives and Community Board and Council members) regarding these stormwater issues. Council has also met with local iwi.
Council developed a Stormwater Project Master Plan, which was used to prioritise projects for the $2.135M of capital expenditure available in 2024/25. Using the Stormwater Flood Model, nineteen improvement options were generated. The Stakeholders Engagement team had an input into assessing each option using multiple criteria including;
- Achieving the target level of service
- Proposed location in stormwater network
- Practicality of design
- Relative capital cost
- Level of impact on flooding
- Adaption to climate change
Council have selected the 2024/25 projects which involve upgrade works to two stormwater outfalls to the Harbour. These projects have been chosen as we need to undertake upgrade works from the downstream end of the network first. If we attempted to fix issues in the centre of town first, it would increase flooding downstream and not solve any flooding issues.
This work involves upgrading the existing stormwater outlets on the northern side of Hetherington Road and southern side of Harbour View Road. These works will improve stormwater drainage within the northern side of town.
Williamson Park Pond
As part of addressing the stormwater issues in Whangamatā, our Council has undertaken works at Williamson Park Pond. The purpose of this project is to drain the upstream network stormwater flows out to sea quickly and without excessive ponding.
Civil works have been completed on the Dry Pond. Observing the Williamson Pond during rainfall events indicates the system is working as designed – there is no excessive ponding and stormwater is discharging out to sea quickly.
We are currently monitoring stormwater quality in the pond. If the water quality analysis shows that we need to improve the water quality prior to discharging into the sea, then the dry pond will be converted into a wetland. The water will then flow from the inlet pipes and through the wetland to the weir before heading out to sea. The plants in the wetland will provide natural cleaning of the water.