Preprints
https://doi.org/10.5194/esurf-2022-5
https://doi.org/10.5194/esurf-2022-5
 
18 Mar 2022
18 Mar 2022
Status: this preprint is currently under review for the journal ESurf.

Hybrid modeling on 3D hydraulic features of a step-pool unit

Chendi Zhang1, Yuncheng Xu1,2, Marwan A. Hassan3, Mengzhen Xu1, and Pukang He1 Chendi Zhang et al.
  • 1State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing, 100084, China
  • 2College of Water Resources and Civil Engineering, China Agricultural University, Beijing, 100081, China
  • 3Department of Geography, University of British Columbia, 1984 West Mall, Vancouver BC, V6T1Z2, Canada

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 non-uniform hydraulic characteristics which have been reported to closely interact with the morphological evolution and stability of step-pool features. However, detailed information of 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 hybrid model based on the technologies of Structure from Motion (SfM) and computational fluid dynamics (CFD). The model used 3D reconstructions of bed surfaces with an artificial step-pool unit built by natural stones at six flow rates as inputs for CFD simulations. The hybrid model succeeded in providing high-resolution visualization of 3D flow structures for the step-pool unit. The results illustrate the segmentation of flow regimes below the step, i.e., the integral jump at the water surface, streaky wake vortexes near the bed, and high-speed jets in between. The highly non-uniform distribution of turbulence energy in the pool has been revealed and two energy dissipaters with comparable capacity are found to co-exist in the pool. Pool scour development under flow increase leads to the expansion of the jump and wake vortexes but this increase stops for the jump at high flows close to the critical condition for step-pool failure. The micro-bedforms as grain clusters developed on the negative slope affect the local hydraulics significantly but this influence is suppressed at pool bottom. The drag forces on the step stones increase with discharge before the highest flow is used while the lift force has a larger magnitude and wider varying range. Our results highlight the feasibility and great potential of the hybrid model approach combining physical and numerical modeling in investigating the complex flow characteristics of step-pool morphology.

Chendi Zhang et al.

Status: open (until 02 Jun 2022)

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Chendi Zhang et al.

Chendi Zhang et al.

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Short summary
Step-pool morphology is commonly formed in mountain streams. The geomorphic processes of step-pool features are closely interacted with hydraulic properties, which have limited access due to measurement difficulties. We established a hybrid model approach combining physical flume experiments and numerical simulations to acquire detailed three-dimensional hydraulics for step-pool morphology, which provides new understanding on the links between hydraulics and morphology for a step-pool feature.