Construction has started on works to reduce flood risk to properties in the upper and mid Heathcote area.

Heathcote flood mitigation works progress

As a result of changes caused by the earthquakes, flood risk has increased along the Heathcote River. The Council's Land Drainage Recovery Programme has investigated the benefits of increasing floodwater storage in the upper Heathcote. The team looked at whether this would help homes along the Heathcote River, which have an increased risk of above-floor flooding as a result of the earthquakes.

Modelling results showed that additional storage areas in the upper catchment would have a positive impact in reducing flood risk above Colombo Street. The first stage of works were completed in June 2016, with two smaller flood detention basins constructed at Eastman Wetlands. These were active and worked well during the rain events in the first half of 2017.

The next stage of works is the construction of four more large storage basins, which when working together will hold back water during flood events and release it after the storm has passed. These storage areas are Wigram East Retention Basin, Cashmere-Worsley Valley, a wetland facility at Curletts Road and further storage in Eastman Wetlands on Sparks Road.  

Construction has also started on the Wigram East Retention Basin, which is being combined with the construction of the Ngai Puna Wai Sports Hub for cost savings on both projects. Detailed design of the Cashmere-Worsley Valley storage project is due to commence in July 2017. The other two basins will be designed in late 2017.

Additional storage in these areas will primarily reduce flood risk in the upper and mid-Heathcote. Due to the tidal influence in the lower reaches other floodplain management options will still be required to reduce flood risk in the lower Heathcote.

Curletts flood storage and stormwater basin

A new stormwater basin will improve water quality in Curletts Stream and the Ōpāwaho/Heathcote River, and reduce flood risk to properties around the lower and mid Ōpāwaho/Heathcote.

The basin at 201 Annex Road is currently being constructed, and hopefully up and running in April next year. Capable of holding up to 130,000m3 of water, it is designed to provide additional flood protection up to a 1-in-50 year rainfall event. It will also treat water within Curletts Stream to remove sediment and contaminants, to improve water quality in the stream and the Opawaho/Heathcote River.

Once complete, the basin will be planted with wetland plants and the surrounding areas planted extensively with native trees and shrubs. The area will be accessible to the public from Mokihi Gardens and Annex Road, providing a lovely new space for walking.

Construction progress

Construction of the basin commenced in April 2018 and has continued throughout winter. During that time approximately 130,000m3 of material has been excavated to create the basin, with approximately 25,000m3 of material still to be removed.

An engineered embankment is currently being constructed to separate the two basins. A channel is also being built that will form the new alignment of Curletts Stream. Much of the basin has had topsoil placed and been grassed to stabilise the site.

The main elements still to be completed are three structures that will allow water to flow between the two basins and a gate to control flood water that will be stored.

Once the basin and new channel are stabilised, the Curletts Stream flows will be transferred to the new channel allowing the existing stream alignment to be removed and the basin to be completed.

Basin excavation progressing in June 2018

First flush basin with new channel, grass establishing

Remaining works

Construction is programmed to be complete by April 2019. At this time, the flood storage element of the project will be operational and provide enhanced protection downstream.

Planting is planned for winter and spring next year with plants currently being grown. Once planting is complete, all water flowing through the basin will be treated as it makes its way through the basin enhancing water quality downstream.