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Heathcote River flood mitigation update

The July rain event

The Heathcote River reached very high levels following the steady rain on Friday 21 July, combined with a King Tide and an unexpected late storm surge. These factors all combined to see the estuary reach record-breaking levels (flooding areas of Southshore), and in turn, water not draining out of the Heathcote River effectively.

Heavier rainfall in the hill catchments also caused significant runoff into the natural ponding basins around the foot of the Port Hills along Cashmere Stream, resulting in ponding Hoon Hay Valley, Worsleys Valley, Kennedys Bush Quarry reserve and Hendersons basins.

Storage basins in the upper Heathcote filled, and did reduce flood risk further down the river to some extent. But many streets and properties in the low areas along the Heathcote River flooded throughout Saturday. We have recorded 12 properties in these areas that had flooding above the floor.

While street flooding and road closures, and water through a garden or garage can be stressful and an inconvenience, flooding through living areas is extremely distressing, and can see people out of their homes for some time.

We are working to speak one-on-one with all of the people we know of who had floodwater through their homes during the July event. If you had flooding above your floor in this event, and think that the Council does not have your situation recorded, please email us on floodmitigation@ccc.govt.nz

Many houses in flood prone areas along the Heathcote River have been rebuilt since the earthquakes to higher levels as a defence against flooding. We saw many cases in the July event where this prevented flood waters entering the home. However, these properties will often still see water in the section, and under the house.

We are working with the MetService and NIWA to try and obtain better weather forecasts, particularly for storm surges.

Part of the recovery is about community resilience planning, looking at how people in flood risk areas can minimise the impacts for them, their homes and their belongings. This will include advice such as moving valuables from under the house if heavy rain is forecast.

Reducing flood risk - where we're at

The low lying areas of the Heathcote are being prioritised, with $25 million spent on reducing flood risk in these areas in the last 12 months, and $52 million of works in the next 12 months.

But there is no quick fix or silver bullet solution here. Flood mitigation is complex because of the narrow river channel, with roads, infrastructure and private property forming the riverbank in many areas.

The Land Drainage Team have been working hard over the past few years to get a better understanding of the flood risk, and possible mitigation options. They’ve made huge progress recently to get to stage where we can discuss some options next month. Please see details below of the community meetings where you can join these discussions.

There is also a lot already happening:

• Two storage basins have already been constructed in Hendersons Basin in the upper Heathcote area, and four more are planned. Together they will hold a total of 800,000 cubic metres of water and will ease flooding in the upper and mid reaches of the river. When they have all been constructed they are expected to half the number of properties along the river that flood above the floor. The two completed basins worked well over the weekend of the rain event in reducing water levels below where they would otherwise have been.

• A new pump station is being built in Richardson Terrace, due to be finished in April 2018.

• Two new storage basins are being built in the Bells Creek area in Woolston.

• Riverbanks have been stabilised in areas in Beckenham, Opawa and St Martins.

The image below shows the Sparks Road Basin in operation.

Meeting next month - Heathcote flood mitigation options

Several years of investigations into flood risk in the Heathcote are coming together, and next month, we’re going to meet with the community to discuss some of the options we’ve come up with to reduce this risk. Come along and hear what’s being considered to reduce flood risk in your area.

The meetings will be held at the Opawa Community Church, corner of Opawa Road and Aynsley Terrace, Opawa on:

Tuesday 17 October 10am – 12pm

And Wednesday 18 October 6.30pm – 8.30pm

We’ll be back in touch nearer the time with a reminder.

Planting Day and BBQ - Linwood Lower Fields

Please join us for a Community Planting Day and BBQ and play your part in creating the urban forest on the new stormwater basin site, constructed as part of the Bells Creek flood remediation project.

Bring friends, family and a spade. Gumboots are recommended.

The event will be held: Saturday 16 September 2017, 10am–12noon
At: Former Linwood Lower Fields site, 521 Ferry Road.

Please meet in the carpark.