Stormwater System with no Apparent Runoff

The Christchurch Southern Motorway: Stage Two (CSM2) has just received Resource Management Act permissions from the Board of Inquiry. The 17 km route commences at Halswell Junction Road and travels southwest to Rolleston.

The CSM2 Project provided a range of challenges for dealing with stormwater, groundwater and flow in stockwater races. To a casual observer the nearly flat plains have no obvious water features. Road names such as Marshes, Waterholes and Springs Road however give some indication of historic water features; and the current Canterbury Plains from a water take perspective are over allocated leading to depressed groundwater.

From a water perspective, our areas of interest were:

• The collection and treatment of road runoff and discharge of this treated stormwater to ground as there are no adjacent natural water courses.
• The project crosses the plains and also crosses nine Selwyn District Council stockwater races for which no discharge was permitted.
• There was potential runoff from upstream of the project to be passed beneath the project in a series of inverted siphons. However there is no evidence of recent ponding or runoff within these catchments.
• The Central Plains Water Irrigation Scheme is consented and as part of that scheme, groundwater levels are predicted to rise to an extent where parts of the CSM2 project would be impacted during critical rain events in conjunction with groundwater highs.

This paper sets out the issues, challenges and solutions.