Examining Trade-Offs in Piggybacking Flow Events while Making Environmental Release Decisions in a River System

AbstractHigh flow pulses (or spells or freshes) play a crucial role in maintaining the ecological health of a river system. Impoundment of water in a reservoir and release or diversion of water for human water needs has significantly altered the magnitude and frequency of flow pulses in many river systems, often reducing river ecological health. A limited volume of water is sometimes available for release into the river to reintroduce pulses specifically aimed at meeting ecological requirements (environmental water). If aiming to achieve maximum environmental benefit, such releases from the reservoir should be timed to augment or piggyback natural unregulated catchment flow events. These decisions must be made in presence of uncertainty of near-future unregulated catchment inflows entering the river. Making flow release decisions under this uncertainty poses the risk of either not achieving the benefit of the environmental flow release because too little environmental water is released, or of causing flood damage because too much is released. To date, assessment of risks associated with piggybacking environmental flows have focused solely on the flooding risks. This paper considers assessment of trade-offs between environmental risks and flooding risks while making piggybacking decisions. The key contribution of the paper is a risk framework that allows for the assessment of both flooding and environmental risks when piggybacking of natural flow pulses occurs. The risk framework is used to assess rules or rules with varying levels of piggybacking on the trade-offs between environmental outcomes and flooding risks when releasing piggybacking flows under these rules for flow events under near-future forecast uncertainty. Spawning flows for a key fish species in the Yarra River in southeast Australia is used as a case study to compare three piggybacking rules.