Fragmentation of rivers due to anthropogenic interventions and recovery of river connectivity are topics of paramount interest in the present day. A larger proportion of the instream obstructions are caused by low head or partial dams. The adverse effects of large dams (height above 15 m) are lower than the cumulative effect of small dams that are a hundred to thousand folds greater in number than large ones. One of the immediate effects of damming the river system is the disruption of upstream migration of aquatic species for spawning, dispersal, or seeking refuge from predators. In India, restoration of fish passage across river obstructions through the construction of fishways is often overlooked or ineffective due to a sheer lack of knowledge, experience, and research. The vision of ensuring environmental flows and provision of fishways across high and low head dams are vital aspects that should be incorporated for sustainable development.
The present study evaluates the hydraulic performance of step-pool nature-like fishways (NLFs) for different bed and flow conditions. The concept of NLFs is to design a structure that mimics the natural riverine ecosystem and enables the passage and inhabitation of diverse aquatic organisms in the most efficient manner. One method of implementing a step-pool NLF is by adapting the step-pool morphology in mountain streams. The other method entails designing the fishway as a rock weir according to the body dimensions of the largest target species. The hydraulic characteristics of both methods have been evaluated through laboratory experiments in terms of the flow velocity, flow depth, turbulent kinetic energy, and energy dissipation factor of the pools. It was found that the self-formed step-pool NLF failed to provide sufficient pool volume for adequate energy dissipation. At the same time, the performance of the designed NLF varies with respect to the rock weir dimensions. Flow over step-pool NLF for varying rock weir dimensions has been simulated using computational fluid dynamics software FLOW-3D HYDRO. The variation in the rating curves and energy dissipation factors for different weir dimensions are presented in the form of design charts for the benefit of practicing engineers.