Fish passage modelling for environmental flows: Hawkesbury‐Nepean River, NSW, Australia

Abstract

AbstractRobust, objective, and repeatable approaches that define flow thresholds for fish passage across critical natural barriers such as riffles, rapids, and waterfalls are required for determining environmental flow strategies. These approaches also provide an opportunity to garner community sector backing for environmental flow releases from dams in support of tangible environmental beneficiaries—native fish. This paper outlines the results of a two‐dimensional hydraulic modelling approach to fish passage assessment for Australian bass (Percalates novemaculeata) that was used to inform the development of an environmental flow regime downstream of Warrragamba Dam, NSW, Australia. Flow rates of ≥500 MLd−1 were found to facilitate depth‐limited upstream passage through a 20‐km river reach that contained 19 natural passage barriers to adult Australian bass up to 400–450 mm in length. Ideal passage conditions were determined at flow rates of ≥1,000 MLd−1. Juvenile bass passage was found to be inhibited by high velocities at flow rates >250 MLd−1, with flows of 100–250 MLd−1 providing ideal conditions for juvenile passage. Fish length, body depth, and caudal fin depth data, as used in this study for Australian bass, provided more precise fish passage depth thresholds. Precision in fish passage assessments is important as each centimetre of additional flow depth influences cost–benefit analyses of environmental flow releases versus consumptive water uses. Although hydraulic modelling and field‐based approaches to fish passage assessment are well established, there is currently a lack of published data on native Australian freshwater fish length, body depth, and caudal fin depth data for use in fish passage assessments and for inclusion in “fish‐friendly” government policy initiatives.