A combined approach of experimental and numerical modeling for 3D hydraulic features of a step-pool unit

Abstract. Step-pool systems are common bedforms in mountain streams and have been utilized in river restoration projects around the world. Step-pool units
exhibit highly nonuniform hydraulic characteristics which have been reported to closely interact with the morphological evolution and stability of
step-pool features. However, detailed information on the three-dimensional hydraulics for step-pool morphology has been scarce due to the difficulty
of measurement. To fill in this knowledge gap, we established a combined approach based on the technologies of structure from motion (SfM) and
computational fluid dynamics (CFD). 3D reconstructions of bed surfaces with an artificial step-pool unit built from natural stones at six flow rates
were imported to CFD simulations. The combined approach succeeded in visualizing the high-resolution 3D flow structures for the step-pool unit. The
results illustrate the segmentation of flow velocity downstream of the step, i.e., the integral recirculation cell at the water surface, streamwise
vortices formed at the step toe, and high-speed flow in between. The highly nonuniform distribution of turbulence energy in the pool has been
revealed, and two energy dissipaters of comparable magnitude are found to co-exist in the pool. Pool scour development during flow increase leads to
the expansion of recirculation cells in the pool, but this expansion stops for the cell near the water surface when flow approaches the critical
value for step-pool failure. The micro-bedforms (grain clusters) developed on the negative slope affect the local hydraulics significantly, but this
influence is suppressed at the pool bottom. The drag forces on the step stones increase with discharge (before the highest flow value is
reached). In comparison, the lift force consistently has a larger magnitude and a more widely varying range. Our results highlight the feasibility and great
potential of the approach combining physical and numerical modeling in investigating the complex flow characteristics of step-pool morphology.
