RIVER MAINTENANCE ACTIVITIES TO ADDRESS EARTHQUAKE DAMAGE ON THE STYX RIVER

The 2011 Canterbury earthquakes have had significant impacts on the Styx River and Kaputone Creek in Christchurch, including land damage, the heaving and subsidence of stream bed and banks, and the increase of flood depth. The impacts have been surveyed and assessed against the Christchurch City Council’s (CCC) six-values of water environment: ecology, landscape, recreation, heritage, culture, and drainage.

Using the results of a complaints data analysis, Operational Water Levels (OWL) have been calculated and proposed for the 5-year ARI as well as the base flow level. The 5- Year level is based on an analysis of complaints data and land damage.

To meet these proposed OWL targets, a range of maintenance activities have been investigated including the activities that are currently being used as well as potential new options. During this investigation phase, nonviable alternatives were removed to produce the following shortlist of options:

• Weed Harvesting
• Minor bank modifications
• Dredging
• Shading with riparian planting to limit aquatic weed growth

The project has carried out an assessment of the above options against several key criteria: effectiveness at water level reduction, environmental impact, and cost. The purpose of the project was to present the options to Council, not to indicate preferred options. After identifying the preferred option or options, Council could then perform hydraulic modelling to confirm effectiveness and to refine the maintenance trigger levels.

This study was able to quantitatively and qualitatively assess the earthquake impacts and then present a suite of potential maintenance options to mitigate those impacts. This required not just flood modelling, but also analysing complaints data, use of LIDAR, rating curve analysis, and the investigation of riparian shading to control aquatic weed growth.