Water Quality Modelling on the Nepean-hawkesbury River

The Nepean-Hawkesbury River over recent years has felt the impact of Sydney's urban sprawl. The Hawkesbury and Nepean Rivers are the receiving waters for urban runoff and treated sewage effluent associated with a population of almost one million which is planned to grow to 1.5 million over the next 20 years. This paper describes the development of a mathematical Model (SALMON-Q) of the Hawkesbury and Nepean River System for use as a predictive water quality management tool for the Sydney Water Board. SALMON-Q (formerly known as TIDEWAY) is a fully dynamic water quality model developed by Hydraulics Research, Wallingford, UK. It has successfully simulated water quality processes in many applications around the world during the last 15 years. The paper describes the model set-up, calibration, verification, sensitivity analysis and provides examples of recent model applications. The Nepean-Hawkesbury model has been able to successfully simulate eutrophication of both the freshwater and estuarine channels of the system. The model simulates the recycling of nutrients by algae and interactions between the water column and the bed. It was used to successfully simulate an algal bloom which occurred during October- December, 1991. The paper also examines the potential of elevated freshwater flows to flush algal masses from the river system.