Webby, M.G.; Waugh, J.R. Hydraulic behaviour of the outlet of Lake Wakatipu, Central Otago, New Zealand
Outflows from Lake Wakatipu are normally controlled by the geometry of the gated weir across the outlet completed in 1926 and by the hydraulic characteristics of the Kawarau River, including the delta of the Shotover River, which joins the Kawarau River 3 km downstream of the outlet. On rare occasions, outflow from the lake is reversed when a large flood occurs in the Kawarau catchment and the peak response of Lake Wakatipu lags that of the Shotover River by several days. This paper describes a re-evaluation of the discharge rating for the modified lake outlet that takes account of the reverse flow phenomenon and the gradual drowning of the outlet weir leading up to the occurrence of reverse flow. The revised outlet rating utilises measurements of Lake Wakatipu level (NIWA site no. 75277) and the downstream Kawarau at Frankton river level (NIWA site no. 75263) and is based on standard hydraulic principles of weir behaviour.