Wai i te reporepo, wai i te wao: what nature tells us about how to understand and prepare for flooding

The Nature-based Solutions study will model nature-based methods to find out how they may lessen the effects of flooding and enhance biodiversity. The results will benefit the entire region.

This project aligns with our draft Strategic Climate Action Plan, the Hill Country Erosion programme and work being done by our Natural Hazards team. 

 

Te Hakapupu as depicted by Dall-e

The Otago Regional Council has been partnering with Kāti Huirapa Rūnaka ki Puketeraki to enhance the mauri and health of the Te Hakapupu / Pleasant River system in East Otago.

Together, we are on a journey to create a catchment restoration action plan to inspire us to sustain the awa. The plan aims to enhance the wider ecosystem, improve water quality by reducing the amount of sediment and nutrient input to the rivers and estuary, and use the best of Kāi Tahu mātauraka (knowledge) and modern science to sustain our efforts.

 

A new study

Natives planted to prevent erosion
and restore biodiversity

As a regional council we are always looking for new and better ways to protect people, livelihoods and infrastructure. The Toitū Te Hakapupu Restoration project has built relationships and developed a knowledge base which make this catchment ideal for a pilot study modelling the use of nature-based solutions.

The year-long Nature-based Solutions Study, is fully funded by the Ministry for the Environment’s (MfE) Essential Freshwater Fund and will use work already done in the catchment to explore the potential of Nature-based Solutions or ‘green Infrastructure’ like wetlands and planting to help manage flooding, clean our water and support biodiversity in the region. We have engineering, water, planning and climate change experts helping us out and we’ll be combining local knowledge with international best practice to come up with ideas that could work.

Our partners have existing relationships in this area, so it makes sense to use this location as a pilot and get to work on a Feasibility Study which will be completed this year, to help us understand what might work and where.

An Engagement study, looking at perceived barriers and people’s willingness to adopt these ideas, will be finished in 2025. The results from both of these studies will help inform future decision making.

What are Nature-based Solutions?

Known globally as Nature-based Solutions (NbS) these are practical ways to use nature's own processes to solve challenges and include things like wetlands to soak up floodwater and filter pollutants from stormwater run-off, and native planting to provide habitat for threatened species. These measures could be a win-win for the environment, industry and the local community, helping us play a part in reaching Aotearoa New Zealand’s environmental goals, while reducing the impacts of flooding and climate change on people and infrastructure.

Rather than engineering or built structures, natural solutions rely on green infrastructure - such as wetlands - to soak up floodwater and filter pollutants from stormwater runoff and native planting to prevent erosion, improve air quality and increase biodiversity.

  • Once we have got a list of ideas, we will test these with mana whenua, industry and land user groups, like forestry, farming and lifestyle block owners in Te Hakapupu / Pleasant River before we finish the Feasibility Study.
  • Once this report is complete, we’re planning to test some of the ideas more widely, with land users around Otago.
  • At the end of the project, we’ll have two documents – a Feasibility Study focusing on possible designs for the Te Hakapupu catchment, as well as an Engagement Study about the opportunities and roadblocks to adopting nature-based solutions in the wider Otago catchments.
  • From there we’ll be better placed to make a case around if and how Nature-based solutions could be rolled out in this region in future.

Feasibility Study

The feasibility study is investigating the opportunities, barriers, and practical viability of nature-based solutions within the Te Hakapupu | Pleasant River Catchment that could contribute to flood mitigation.

As climate change intensifies and flood risks rise, the search for sustainable solutions to protect people, infrastructure and biodiversity becomes more important.  Nature-based solutions (NbS) are practical, cost-effective ways to use nature's own processes to solve challenges - think wetlands to soak up floodwater and filter pollutants from stormwater run-off, and native planting providing habitat for threatened species.  

To evaluate how NbS may be applied to reduce flooding hazards within the Hakapupu catchment, a hydrology assessment and hydraulic flood modelling has been conducted. This modelling characterises how the current catchment will likely respond during an extreme rainfall event, in this case a one-in-100-year rainfall event (100-year annual return interval) with the added effect of climate change. The model simulates how rain, captured in the steeper inland areas of the upper catchment, flows through the main channels of Pleasant River, Trotters Creek, and Watkin Creek into the flatter areas of the lower catchment, causing flooding of farms, homes, and roads.

Based on historic rainfall data, the one-in-100-year rainfall event simulated in the flood model is projected to receive rainfall depths of 176mm to 190mm across the catchment. To put this in perspective, the flooding that occurred across North Otago in July 2022 was caused by between 110mm to 150mm of rainfall.

Flood modelling

As part of our Nature-based Solutions study, we are carrying out a Feasibility Study investigating the practical viability of Nature-based Solutions within the Te Hakapupu | Pleasant River Catchment and whether they could contribute to flood mitigation.

A hydrology assessment and hydraulic flood modelling has been conducted, looking at how the current catchment will likely respond during an extreme rainfall event, in this case a one-in-100-year rainfall event with the added effect of climate change.

Next step: creating modelling looking at the same event, if Nature-based Solutions were in place.

Predicted peak flow speed during 100-year ARI event

 

The depth model above, in blue, illustrates the simulated depth of flood waters during an extreme rainfall event (one in 100 years), with flood depths ranging from 0 and 5m, to show where surface flooding is expected to occur.

Predicted peak flood depth during 100-year ARI event

 

The velocity model illustrates the flow speed of flood waters during a 1 in 100-year flood event, with flow speeds between 0 and5.6m/s, to show where flood water is pooling and where it is swiftly moving.

Frequently Asked Questions

Where can I find out more information?

To learn more about the project email the project manager Melanie White.

Or follow the links below for more information about:

What the Ministry for the Environment is doing, and case studies of nature-based solutions from around the country, visit: Working With Nature

Previous ORC projects in Te Hakapupu, like the Toitū Te Hakapupu Restoration Project