The project
The Wairiri Valley Land Drainage Scheme in the central Canterbury foothills plays an important role in supporting productive farmland while also providing opportunities to improve water quality, biodiversity, and the health of waterways flowing towards Te Waihora.
As part of a wider catchment management planning process, Selwyn District Council engaged EOS Ecology and Aqualinc to develop a Catchment Management Plan for the Wairiri Valley catchment, identifying both current challenges and opportunities for long-term improvement.
The catchment experiences regular flooding, with parts of the valley floor prone to prolonged inundation due to poorly drained soils and downstream channel constrictions.
What we did
Aqualinc undertook 2D rain-on-grid flood modelling to better understand the causes and extent of flooding within the catchment and to assess the effectiveness of potential solutions. Using HEC-RAS modelling, we simulated flood behaviour under existing conditions and evaluated how changes to catchment management could influence flood extent and duration.
The modelling focused on key constraints within the system, including:
- Downstream channel constrictions caused by dense willow and blackberry growth.
- Raised channel margins that prevent floodwaters from draining back into waterways.
- The interaction between drainage channels, surrounding farmland, and overland flow paths.
- Potential management actions to improve drainage performance across the catchment.
Key findings and solutions
The modelling identified several factors contributing to flooding within the Wairiri Valley catchment and helped assess practical solutions for reducing flood impacts.
Key findings included:
- Dense vegetation in downstream reaches of Wairiri Stream was restricting flow and causing floodwaters to back up into the upper catchment.
- Removing these channel constrictions could significantly reduce flood extent and allow floodwaters to recede more quickly.
- Raised channel margins were preventing floodwaters from returning to waterways, prolonging inundation of surrounding farmland.
- Targeted channel restoration and regrading of raised banks would improve drainage performance while supporting wider environmental outcomes.
The project demonstrated how integrated catchment management planning, supported by hydraulic modelling, can identify and test practical interventions that reduce flood impacts while supporting wider environmental outcomes. The work provides an evidence base for future investment in drainage, flood mitigation and catchment restoration across the Wairiri Valley.
The approach used in Wairiri Valley demonstrates how hydraulic modelling can be integrated into catchment management planning to support decisions in rural catchments facing flooding, drainage and environmental challenges.
View the full suite of Selwyn District Council Catchment Management Plans on the EOS Ecology website.
