The Dight’s Falls fishway complex, located in Melbourne, is a hybrid fishway design incorporating both rock ramp and vertical slot sections. Monitoring of the Dights Falls fishway has shown that the performance of the fishway decreases at higher flows. Based on the results of the fishway performance monitoring, the owner and operator of the fishway (Melbourne Water) has decided to pursue options to improve fishway performance at high flows.
The hydrodynamic conditions at the fishway entry were identified as a likely cause for the observed reduction in fishway performance with flow. An investigation was undertaken involving flow observations, Computational Fluid Dynamics (CFD) modelling, and one-dimensional modelling of the internal fishway hydraulics. The results of the modelling studies agreed with observations and indicated that excessive water velocities and turbulence near the fishway entry are likely to exclude fish at high flows. In addition, higher than desirable flows within the vertical slot section of the fishway may also contribute to the observed reduction in functionality at high flows.
The models developed to characterise the existing hydraulic conditions were subsequently used to evaluate potential modifications to the fishway. Following the evaluation of 8 options, a preferred option was selected. This option involves extension of the existing fishway and the addition of a third entry slot. The third slot is placed downstream of high turbulence zone and is oriented to intercept fish moving upstream along the bank and in the wake of the fishway structure.
This case study demonstrates the benefits of post-construction condition and performance monitoring as part of a fishway maintenance program, as well as the need for adaptive fishway management. The project illustrates the value of simulation tools in the evaluation of rectification options. The results of this study also demonstrate the significant role that fishway approach hydraulics play in the successful operation of fishways, and provides guidance for future designs.