Lake Rotoiti fieldwork and modelling to support considerations of Ohau Channel diversion from Lake Rotoiti

Abstract

A detailed assessment of the water quality effects of the Ohau Channel inflow to Lake Rotoiti was carried out using field and computer modelling analyses. The modelling assessment evaluates the effects on Lake Rotoiti of partial (50%) or complete (100%) diversion of the Ohau Channel. A comprehensive bathymetric survey of Lake Rotoiti and the eastern end of Lake Rotorua was also carried out to provide information for the application of a three-dimensional model run by NIWA, so that different diversion wall positions and designs might be tested prior to possible implementation. Field measurements based on temperature, conductivity and chlorophyll fluorescence carried out from February to May 2004 captured instances when the Ohau Channel entered Lake Rotoiti as a buoyant overflow. These data and historical records suggest that approximately 20% of the buoyant surface inflow enters the main basin of Lake Rotoiti, while the remainder is short-circuited through the Kaituna arm to the outflow.