For many years Melbourne Water has released water for the environment on behalf of the Victorian Environmental Water Holder (VEWH) across the Tarago, Yarra, Maribyrnong and Werribee systems to enhance water-dependent environmental assets across the Port Phillip and Westernport region. One of the major risks that Melbourne Water assesses when planning the use of an environmental entitlement is the potential for impacts, including flooding, on third parties. Currently, risk and contingencies of third-party impacts are assessed and developed individually for each priority watering action (PWA), rather than systematically across all PWAs.

The importance and challenges of changing to a new systematic approach for managing PWAs prompted Melbourne Water to seek external expertise in formulating this shift. Alluvium was engaged to assist in developing a systematic approach to risk management of environmental water releases; in particular, the potential risk of flooding associated with PWAs.

Building on work already undertaken by the environmental water team, we undertook statistical analysis of available flow gauge data to identify the likelihood of a PWA causing a minor flood event during an environmental water delivery. For some reaches, the likelihood of a minor flood event occurring was deemed acceptable and therefore Melbourne Water can implement a process to deliver these PWAs without a specific approval each time.

For reaches where this likelihood was not deemed to be at an acceptable level, we developed a detailed method to reduce the risk of causing minor floods while optimising the ability to deliver environmental flows. The first detailed assessment was undertaken on the Watts River, part of the Yarra system. After analysis of many potential parameters, modelled soil moisture data and seasonal streamflow forecasts were proven to meet the criteria of data availability and ability to suitably predict events of interest. Using these metrics, a set of rules were developed to guide PWA delivery – where these simple rules are followed, the likelihood of causing a minor flood is reduced to an acceptable level, while still keeping available a significant proportion of days where the rules allow PWA delivery without further approval.

This project drew on Alluvium’s skills in understanding hydrologic processes and statistical analysis combined with an appreciation of environmental water management in practice.