Using hydraulics to evaluate ecological benefits, risks, and trade-offs from engineered flooding

Regulation and water extraction have reduced river flow worldwide. In some rivers, this has led to reductions in the frequency and duration of floodplain inundation and, in turn, declining condition of flood-dependent vegetation. One management response has been the construction and modification of infrastructure, or 'regulators', to engineer floodplain inundation at discharges that do not otherwise cause overbank flooding. Such actions have the potential to benefit floodplain vegetation, yet the atypical hydrodynamics of engineered floods may threaten riverine ecological processes and biota and increase the risk of adverse water quality. We investigated the influence of engineered flooding, and associated reductions to in-channel hydraulics, on key ecological processes. Specifically, the transport and retention of larvae of an iconic fish species (Murray Cod, Maccullochella peelii), and a common microinvertebrate Trichocerca. Analysis of different flow management scenarios for floodplain inundation indicates that maintaining in-channel velocities greater than 0.2 m s-1 produces higher Trichocerca densities, while velocities greater than 0.3 m s-1 creates suitable habitat for Murray cod larvae, thereby providing targets for avoiding detrimental effects of engineered floods on in-channel biota. To evaluate responses to regulator operation, velocity was represented by a hydrological model using pre-computed results from detailed hydrodynamic models. These results were used to relate upstream discharge and downstream water level to the proportion of each reach in the hydrological model meeting each velocity criterion. The model also represents changes in discharge, inundated area and a water quality parameter, dissolved oxygen. Model scenarios for engineered floodplain inundation were used to demonstrate the potential benefits, impacts and trade-offs between the different metrics identified. The model framework enables a more holistic evaluation of infrastructure operation, extending analysis beyond discharge and inundated area to risks and benefits to key indicators of the ecosystem. This refined integrated approach to the management of regulated river systems may become critical in the future where water resources are projected to further decline under a changing climate. Relationship between mean water velocity and channel-floodplain (lateral) connectivity for different floodplain inundation mechanisms. Increases in lateral connectivity using infrastructure, including engineered flooding and weir pool raising, decouple the natural relationship with water velocity and can lead to deleterious in channel outcomes. Relationships are developed between water velocity and ecological responses to inform the assessment of undertaking the different floodplain inundation actions.image