A comprehensive flood frequency report has provided Council with a current and robust evidence base for understanding flood risk across the Wairau Catchment.
The Wairau Catchment Flood Frequency Assessment Report 2025, prepared by Earth Sciences New Zealand, is the first wide-ranging update of flood frequency information for the catchment since 1993 and incorporates more than three decades of additional river monitoring data.
The assessment, presented to August’s Environment and Planning Committee, provides flood frequency analysis across major sub-catchments of Marlborough’s largest river system, which is central to the region’s flood management framework.
For the first time, sub-catchments, such as the Taylor, Omaka, Tuamarina and Goulter rivers have been assessed within a consistent flood frequency framework, improving Council’s understanding of flood risk across the wider catchment.
The work has undergone a robust technical assurance process through Earth Sciences New Zealand and an internal review by Council’s Hydrological Working Group, providing confidence in its future application.
The updated information strengthens the evidence base that supports Council’s future planning, investment and risk management decisions.
The Wairau Valley Floodways Scheme was established in 1960 to provide flood protection against a one-in-100-year flood event in the Wairau and Taylor/Ōpaoa Rivers. It was reviewed in 1994 when the spatial extent of levels of service were adjusted.
The 1993 flood frequency analysis established peak flows of 5,500 cubic metres per second for the Wairau at Barnetts Bank and 170 cumecs for the Taylor River at Hutcheson Bridge.
The updated assessment now estimates one-in-100-year peak flows of 5,850 cumecs for the Wairau and 200 cumecs for the Taylor River.
While these updated flow estimates do not change Council’s existing level of service, they indicated improvements to parts of the flood protection scheme are likely to be required. A detailed assessment is now underway which will recommend immediate, interim and longer-term mitigation measures.
The assessment is based on historical hydrological records and does not incorporate future climate change effects.
