Author(s): Hugh Macmurray
Linked Author(s): Hugh MacMurray
Keywords: Weir simulation; Reversing flow
Abstract: A gated weir structure was built at the outlet of Lake Wakatipu in 1926, and a good quality database of gauging data is available. An apparent shift in the rating meant that gauging data after 1983 formed a consistent set. Water levels are recorded in the lake and a short distance downstream of the outlet gates. Low flows are critical in the gates section, but at higher flows the resistance to flow in the Kawarau River raises the tailwater and causes partially drowned flow. The Shotover River joins the Kawarau River about 3km downstream of the outlet gates, and when the Shotover is in high flood, reverse flow to the lake can occur. Webby and Waugh (2006) developed a rating formula based on the data collected since 1983, which takes account of high tailwater levels by means of a drowned flow reduction factor, and is reversible to allow calculation of return flows to the lake from recorded water levels. In the context of an investigation into the Lake Wakatipu/Kawarau River/Shotover River system, and flood mitigation measures for Queenstown, an accurate unsteady flow model of the outlet gates was required. This paper describes the model, which is based on an energy analysis through the gates, and compares the model results with those given by the rating formula. The semi-analytical model is shown to compare well with the empirically derived rating formula.