Proposed approach to flood management in Masterton

3 months ago

Rivers are the lifeblood of our community.

In fact, the name Wairarapa means ‘glistening waters’. However, sometimes our greatest assets can cause our biggest risk. Masterton District Council and Greater Wellington Regional Council (GWRC) are currently looking at options for managing the current and future flood hazard to Masterton from the Waipoua River. This is being done through the Te Kāuru Floodplain Management Plan.

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As part of the process, draft flood hazard maps have been jointly released by the Councils. The draft flood maps will be finalised following an independent audit, at which point they will replace the current flood hazard maps for Council.

We're proposing a staged approach to managing the flood and erosion risks over the next 30 years. This will allow a better understanding of the current and future risks to Masterton, prior to a large financial investment. We will communicate and engage with the community during each stage of the process.

The flood management responses developed for the urban reach of the Waipoua River include a combination of non-structural measures, increased river channel capacity, and new and upgraded stopbanks. However, these will depend on the information collected during the first stage of the process.

Staged Approach

We are proposing a staged approach, which will first focus on gathering more information and then addressing today’s risk. The first stage includes gathering some more data, undertaking geotechnical investigations on the existing stopbanks and the surrounding environment, considering detailed alignments for potential stopbank improvements, and raising community awareness and preparedness. The later stages will address the future risk of flooding that includes allowing for predicted climate change. A preferred option will be developed with the community and key stakeholders. Following that we will address the areas at immediate risk of a 1% annual chance flood (Oxford Street), then the Akura Road area. It should be acknowledged that all stages following Stage 1 will be dependent on the outcome of the previous stage.

Often this size of flood is referred to as a 1-in-100 year flood, we call it a 1% annual chance flood; it means there is a 1% chance of this sized flood occurring in any given year. It does not mean that there is exactly one of these floods every 100 years. It is also important to remember that several big floods could happen in quick succession.

One of the following stages, in 10-20 years’ time, will look to further review and understand the future flood risk and then a plan to manage that future risk.

The staged approach is to ensure we are not doing unnecessary work and spending money if it is not needed.

The approach of increased channel capacity and stopbanks was agreed as the most viable, supported by non-structural responses such as encouragement of wetland establishment upstream. The increased channel capacity is being considered in order to minimise stopbank heights, but it will have medium-term impact on the look and feel of the river channel and berms. A staged approach has been developed to try to manage the affordability.

Increased Channel Capacity

The majority of vegetation would need to be removed to achieve the greater channel capacity so the river area would change significantly in the medium term until new trees are established. In doing this, it also creates the opportunity to enhance the recreational and amenity values of the river corridor, including improved spaces for walking, running, cycling, and other leisure activities.

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River Corridor Proposal

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Alternative Approaches

A range of approaches have been considered including upstream storage and the removal of at-risk property. Both of these approaches had significantly higher costs (greater than $30 million) and were not considered further. The approach of increased channel capacity and stopbanks was agreed as the most viable.

However, other forms of storage such as wetlands further up in the rural reaches of the Waipoua River are included in the proposed approach to help with the flood management.

What does this mean for you?

This proposed approach aims to provide flood hazard and erosion protection for the Masterton urban community while enhancing the environmental and cultural values of the river. During Stage 1, we will ensure we are doing this in a sustainable and economical way. Stage 2 may require a flood protection investment in the vicinity of $8 million, however, this will be assessed and confirmed during Stage 1. Funding will be loan based as these projects are long term and have a long-term benefit. Repayment of the loan (borrowed money) will be via a rates increase, similar to how ratepayers in the Carterton area are paying for their new Wastewater Treatment Plant.

The cost estimates and timeframes of the plan are indicative only. They will be reassessed at the end of each stage and will be balanced with other council priorities.