Dredging to Restore Resilience to a Critical Christchurch Waterway

Natural waterways and drainage networks are important features within the urban landscape. They provide a critical link by managing stormwater runoff and offering opportunities for enhancing community amenities. Restoring, protecting and managing surface water has become a critical focus for Christchurch as result of the Canterbury Earthquake Sequence (CES, 2010 – 2011), particularly in combatting increasing sedimentation rates within various watercourses.

The Christchurch City Council (CCC), through the Land Drainage Recovery Programme (LDRP), has initiated a series of investigations and ensuing projects into floodplain management in Christchurch, including in the Ōpāwaho / Heathcote River. The river has a long history of flooding, which has increased both in severity and frequency since the CES. Dredging was known to have little impact in larger events, and with a projected forecast of up to 1 m sea level rise. However, it was found to be effective at reducing the severity of smaller flood events, and with sea level rise of less than 0.25 m. Therefore, the lower Heathcote River Dredging project was developed to provide immediate relief to frequently flooded properties adjacent the Heathcote River. Many of these properties had flooded under- or above-floor multiple times since the CES.

The scope of the project was to fast-track design of the dredging profile and associated bank and landscape works for the lower Heathcote River. As well as flood relief, it was also important to minimise the environmental impact from the works and to enhance the riparian margins where possible. The work area is defined as the reach of river from the Woolston Tidal Barrage to the Beckford Road Bridge and is approximately 3.4 km long.

This paper will discuss the nature of the project and how the project team (CCC and Stantec) has developed a process to facilitate the dredging works, incorporating bank stability pre- and post-dredging and seismic performance analyses. Impacts from the dredging works on urban waterways and local communities will be discussed, including protection of existing drainage values, recreational enhancement, and improvements in ecological and landscape features where possible. These enhancements will contribute to a healthier waterway system and improved hydraulic capacity along this reach of the river. The paper will also discuss consideration and methodologies for restoring resilience to the community, as this has become a key element for people of Christchurch since the CES.

Working near waterbodies escalates risks associated with undertaking the dredging works and maintenance thereafter. This paper will describe; how the design mitigates construction risks; some of the site-specific constraints and challenges encountered during the design and construction phase; and highlight the benefits of bank treatment options adopted.