Turbulent hydraulics around habitat boulders

Authors: TSAKIRIS, ACHILLES - Northwest Hydraulic Consultants; Papanicolaou, Athanasios - Thanos

Submitted to: Joint Federal Interagency Sedimentation and Hydrologic Modeling
Publication Type: Abstract Only
Publication Acceptance Date: 9/22/2022
Publication Date: 5/8/2023
Citation: Tsakiris, A., Papanicolaou, A.N. 2023. Turbulent hydraulics around habitat boulders [abstract]. Joint Federal Interagency Sedimentation and Hydrologic Modeling.

Interpretive Summary:

Technical Abstract: Large habitat boulders are a frequently employed stream restoration practice in gravel bed rivers in mountainous watersheds. These habitat boulders generate a diverse turbulent flow field with ubiquitous localized acceleration and deceleration regions, which in turn create resting and shading areas as well as allow fauna growth and nutrient capture for aquatic organisms. Simultaneously, the habitat boulders regulate sediment transport patterns favouring size-selective entrainment, and increased hyporheic flow levels, thus improving spawning gravel quality. Despite their widespread use, optimal placement of habitat boulders is hampered by the poor understanding of boulder-flow-sediment interactions, especially of the generated vortices and their impacts on bed structure and biology. As a result, designs that include habitat boulders are either oversimplified or derived only on empiricism and “what works out there”. Hydraulic models (mostly 2D) can not fully capture the vortical structures, and their effects on vorticity, Turbulent Kinetic Energy and shear action. These shortcomings are in parts due to the variable relative submerge of these habitat boulders, which typically remain partially submerged during the summer and early fall lower flows, but become fully submerged by the higher flows. This study aims to: (1) describe the turbulent flow field, and; (2) identify the dominant vortex structures around a habitat boulder at high and low relative submergence conditions. To remove complexity found in nature and isolate the role of turbulence Particle Image Velocimetry (PIV) was used to interrogate the turbulent flow field around a spherical habitat boulder mounted atop a flat rough bed under high and low relative submergence conditions. For the high relative submergence condition an “S”-shaped mean velocity profile was documented within the habitat boulder near-wake region, gradually recovering to the typical for unobstructed flows logarithmic-shaped profile with distance downstream of the boulder. Under the low relative submergence condition, a predominantly linear mean velocity profile with increasing magnitude but decreasing slope was documented for the first time further downstream of the habitat boulder. The profiles exhibit an inflection point near the bed, which becomes more pronounced further downstream of the boulder. The experimental results revealed a shift in the magnitude and directionality of the bed shear stress vectors from high to low relative submergence, with the vectors being directed towards and away from the habitat boulder front face centerline, respectively. This shift was attributed to the distinct vortex systems that are generated from the interaction of the habitat boulder with the approach flow under high and low relative submergence. Specifically, a pair of arch and a pair of inboard vortices with the same sense of rotation dominated the habitat boulder wake under high relative submergence conditions. For low relative submergence, a pair of von-Karman vortices developed within the habitat boulder wake region. These findings offer an improved understanding of the key turbulent flow field characteristics around habitat boulders under different relative submergence conditions and constitute the first step towards improved approaches for simulating these habitat boulders in numerical models for restoration projects in gravel bed systems with implications to watershed management.