Environment Canterbury runs a network of 160 water level/flow recorders throughout the region these are represented by three main types of rivers; Alpine Rivers, Foothill Rivers and Springfed Streams.

The Selwyn River is a typical foothills river that flows across the Canterbury Plains to the south of Christchurch and the river where this presentation will concentrate on.

The Selwyn River has three water level recorders along its reach, Selwyn River @ Coes Ford is at the lower end of the catchment and the site that is of most interest for both low flow and flood analysis.

The site commenced in 1954 and records show significant events where the stage exceeded 8m in 1994, 2000, 2002, 2008 and 2017 note the stopbank is at 8.5m. Gaugings were done during the 1994, 2002 and the 2017 events the first two 5kms downstream due to the technology available at the time.

The results from the first two gaugings were transferred up to the recorder site using a velocity/area relationship. During this period our flood hazard engineers developed a flood model for the river, which was different from our extrapolated flow. We had many discussions about the difference but no resolution.

At the 2017 event we had new technology (ADCP and jetboat and ADCP and QBoat) that gave the opportunity to measure these higher flows and determine the flow when the river was bank full.

Conclusions: The results from our physical measurements and the estimate from the flood model both agreed within the uncertainties of both measurement. The learnings for me was that we should use all the tools in your toolbox to better determine/verify results.

The presentation will outline the calculations and extrapolations to obtain our initial flow and then the physical gaugings and the resulting changes and reasons why we got it so wrong initially.