Step-pools are one of the major types of bed morphology prevalent in mountain streams. They have a unique flow structure as compared to low-gradient streams, in terms of large boundary elements and alternating super-critical and sub-critical flow conditions, which result in a non-uniform flow regime. Step-pools may also be constructed artificially to restore mountain incisions, and for creating close-to-nature fish passes. For hydraulic model development and various design considerations, the accurate prediction of flow phenomenon is required. This necessitates a detailed study of the turbulence phenomenon in natural step-pool reaches and its effect on the total flow. However, the influence of aerated conditions in step-pool hydrodynamics has not yet been adequately addressed. This paper presents a review of the mechanism of flow resistance and energy dissipation in step-pool mountain streams. Also, the significance of incorporating air entrainment in flow analysis, limitations and the way forward in modeling air–water flow in laboratory studies are discussed.

The morphology of step-pools is often implemented for ecological restoration and for the creation of close-to-nature fish passes. Step-pools display spatio-temporal variations in bed and flow characteristics due to meso-scale units such as step, tread, base of step, and pool. Exclusive research on the effects of bed variations in step-pools on the flow dynamics is limited. Here, we conducted laboratory experiments on a physical model downscaled from a field site in the Western Ghats, Kerala, India. The results of Kruskal–Wallis ANOVA show significant differences in the velocity and turbulent intensities for the morphological units. A regression equation of the form Power-Allometric1 has been proposed to relate the normalized turbulent kinetic energy with the velocity magnitude. The present study also estimated the range of Reynolds shear stress and energy dissipation factor existent in the step-pool systems. The normalized values of Reynolds shear stress in the x–z plane ranged from − 19.477 to 13.729, and energy dissipation factors obtained for the three step-pool systems are 321, 207, and 123 W/m3; both the results reveal insufficient pool volume for adequate energy dissipation. The study concludes that while designing close-to-nature step-pool fish passes, pool dimensions should be finalized with respect to the target aquatic species.

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