There is good news – headwater wetland earthworks, basins and reconstructed stream beds are part of the present and future of the Ōpāwaho Heathcote River. It is slow work, but several years and millions of dollars worth of earthworks, planting and infrastructure will return parts of the Hoon Hay floodplain to wetlands, the Cashmere Stream to its original streambed and, hopefully, reduce flooding and sedimentation in the Ōpāwaho Heathcote River.

The most obvious activity in the headwaters at the moment is the springing up of several residential sub-divisions on land that used to be farmland and before that, wetland.  The developers of these sub-divisions are required to treat and retain the stormwater run-off that they produce, infiltrating it into groundwater and allowing only excess stormwater to feed into the various streams and drains that dewater this low-lying area.  As a consequence, retention ponds, swales and wetlands are visible features of these sub-divisions.

Have a look at the accompanying map of the area. Within the area bounded by Sparks Road, Hendersons Road, Cashmere Road and Sutherlands Road, the City Council is building a number of enormous stormwater retention ponds to slow the passage of stormwater from the Hoon Hay Valley and from Halswell.

The first set of ponds (1) that has been completed is on the corner of Hendersons Road and Sparks Road.  A series of inter-connected shallow ponds and wetlands have been constructed and planted; the areas fill with stormwater progressively in a rain event, delaying movement of the water and controlling its release.  Another consequence of the retention ponds is that the water released contains much less sediment.

There are other large connected retention pond areas being developed by the City Council further upstream.

• Off Sutherlands Road, upstream of the new Halswell Downs sub-division, (2) the Sutherlands Wetland and Stormwater Basins have been constructed.
• Near the south end of Cashmere Road, (3) the Hoon Hay Wetlands and Stormwater Basins are being developed with the permanent diversion of the Hoon Hay Valley Drain into this system.
• The Sutherlands Wetlands & Basins and the Hoon Hay Wetlands & Basins have been designed so that in significant rain events, when they reach capacity, they will spill water into an extensive area, (4) the Eastman Wetlands and Stormwater Basins.  To do this, a spillway has been developed with a barrier that will allow the overflow from these two areas to cross the Cashmere Stream.
• On Sparks Road, the Eastman Wetlands will also treat and slow the water from the recently (2019) naturalised Milns Stream.  This stream takes stormwater from land in Halswell that is currently being developed.

In a significant addition to the above, the Cashmere Stream Care Group and the CCC have received Ministry for the Environment funding to enhance the whole of the Cashmere Stream which passes through and along the border of these retention ponds, carrying spring and surface water to the Ōpāwaho Heathcote River.  This project, commencing in 2022, will include riparian planting, reshaping some of the embankments to enhance and restore the ecology of the stream and the provision of walking/cycling tracks.  Perhaps the most significant stage of this enhancement programme is (5) the return of this stream into part of its original streambed and the creation of a length of new, naturalised stream bed.  To find out more about this project, a community information-sharing event will take place on 22 November.

Further downstream of these retention ponds, and around the corner, are the Cashmere and Worsley Valleys where the CCC has already created significant retention ponds and wetlands and is in the midst of planting them.  Further significant earthworks will commence here in 2022 as the last stage of this development, a low earth dam, is built across the valley floor to retain stormwater and help reduce sediment.

Flow through the culvert under the dam to release any water held by the dam will be controlled in conjunction with the gates on the Cashmere Valley Drain.  In fact, the intention is that all control mechanisms on all of the retention ponds mentioned above will ultimately be controlled remotely by a sophisticated computerised system based at the control room at the Christchurch Sewage Treatment Plant.  The intention is that during significant rain events, stormwater released will managed in real time to minimise the risk of flooding downstream.  

There is a real hope that the system will also reduce the amount of sediment entering the Ōpāwaho Heathcote River.  However, given the loess soils in the hills, the most likely outcome is reduced sediment but longer periods of murky water following a rain event as the stormwater is gradually released from the storage basins.

There is therefore, work to be done in the hills to reduce sediment at source and we will be focusing the attention of all agencies on this; but we should celebrate as the wetlands, storage basins and naturalisation of the Cashmere Stream begin to play their significant roles in improving the health of the Ōpāwaho Heathcote River.

To keep up-to-date about progress on these projects, visit the CCC website.