Horseshoe Lake and the other Christchurch unsung heroes of draining a flat city
OPINION: Don't assume the languid shallow waters of Waikakariki or Horseshoe Lake make this an insignificant backwater. Technically an abandoned former meander of the Avon River, it is in fact a wahi taonga, a site treasured by local Maori, and is critical to the drainage of north west Christchurch.
There are now enormous opportunities to use adjacent red zone land to restore the waterway into the important wetland system it once was so it can better perform its critical functions.
During the recent storm, the lake worked overtime funnelling floodwaters from its extensive catchment into the Otakaro/Avon River through the New Brighton Rd pump station.
Comprising three impressive Archimedes screws, this is the largest pump station in Christchurch and is currently being upgraded to address earthquake effects and at the same time enable a higher lift to cope with increased future tide levels. It is able to lift flood waters into the Avon River at a hefty 13 cubic metres per sec.
In an ideal world we might allow the river to adopt its natural flood plain and remove the pump station at Horseshoe Lake.
However, because the catchments of the lake are very flat with minimal gradient, the pump is needed to maintain the hydraulic head to drain the upstream catchment and nearby green zone properties, especially during flooding conditions. This pumped system will become increasingly important with sea level rise.
Waikakariki receives waters from a myriad of streams, drains, diversions and stormwater pipes: Broomfield waterway is joined by drains from Hammersley and Marshland that run through Shirley golf course, and Snelling Drain from Prestons and Lakewood.
Then there is also the Lower Dudley Diversion from the mid reaches of the Dudley Creek catchment and Cranford Basin, which runs underground to enter the lake in the north west corner.
The Dudley Creek catchment, Cranford Basin and the box drains that service much of Marshland comprise very complex networks that ultimately drain Bishopdale, Papanui, Cranford, Mairehau, Fendalton, St Albans, Flockton, Shirley and Richmond.
So this is an extensive and complex drainage network with Horseshoe Lake playing a critical role in its function. But the influx of urban stormwater takes its toll. Water quality in the lake is degraded, biodiversity is compromised, and in State of the Takiwa assessments by Ngai Tahu the cultural health of Waikakariki was found to be poor.
However with red zoning, the now mostly vacant land in the interior and outer north east of Horseshoe Lake gives us an incredible opportunity to intercept incoming waters and address the water quality and flow before they enter Waikakariki, and thence the Avon River.
There is enormous potential to divert these inflows into constructed wetlands and detention ponds to clean up stormwater, dilute it with groundwater from springs in the area and, if necessary, hold flood water before discharge into the lake.
Wetlands slow water flows to allow suspended solids and sediments to settle out and provide habitat for the biofilms of organisms that help remove pathogens and dissolved contaminants like nitrogen, phosphorus and heavy metals present in urban stormwater run-off. Water processed through wetlands for a period of 48-72 hours is thus much cleaner or 'polished' when it eventually flows into the downstream water bodies.
It is for this reason that we find such wetlands and ponds in new subdivisions such as Wigram Skies and Prestons; with the red zone we now have an opportunity to retrofit stormwater remediation in older areas of Christchurch.
Treating stormwater in this way in Horseshoe Lake red zone before it enters the lake would give some measure of respect back to Waikakariki, and go some way toward returning it to its former condition with enhanced water quality and biodiversity that support mana whenua aspirations for improved mahinga kai values. In light of this it is noteworthy that the pump station upgrade is designed to provide a fish passage to allow for inanga (whitebait) migration.
At the same time, increased diverted flood water detention capacity in Horseshoe Lake red zone will have value in relieving some pressure on the pump station during flood events especially in the future with sea level rise.
These are priority land uses for consideration by Regenerate Christchurch as they develop their Regeneration Plan for the Otakaro/Avon River Corridor, including for Horseshoe Lake red zone. Any other land use proposals for the area will need to allow room for wetlands in a way that maximises their benefits and integration.
Only then will the importance of this waterway be fully acknowledged.
Evan Smith is the spokesperson for the Avon-Otakaro Network.