Author(s): Yonggang Zhang; Zi Wu; Ping Wang
Linked Author(s):
Keywords: Vegetation drag; Sediment transport; Emergent canopy; Mobile sand bed; Bed-load transport model
Abstract: Vegetation plays key roles in waterways ecology and bed scour control, interacting with the water flow and sediment. However, the physical processes governing these interactions are still poorly understood especially for the nonuniform flow over a mobile sand bed. This paper investigates how bed-load transport responds to the presence of dense emergent canopy, and how this response is influenced by the vegetation drag coefficient Cd over a mobile bed. Emergent vegetation was modeled with rigid cylinders arranged in staggered arrays of different vegetation coverage φ. Laboratory flume tests were conducted to measure variations in both the water and bed surfaces along the canopy on a sand bed. Based on the experimental and theoretical analyses, a dimensionless drag model integrating both terms of flow properties and bed effects is proposed to predict the drag coefficient Cd, and a bed-load transport model was proposed to predict the sediment transport rate Qs based on the turbulence theory. The results reveal that the calculated values of Cd exhibit two different trends, i.e., nonmonotonically or monotonically increasing along the streamwise direction, due to the combined effect of water surface gradient and bed slope. The morpho-dynamic response of the mobile bed to nonuniform flow manifests as an evolution in the bed slope within the canopy. The sediment transport rate Qs increases with downstream streamwise distance. The increase of the vegetation-generated turbulence can in turn enhance the bed-load transport, thereby affecting the sediment transport process and bed slope. Finally, the new bed-load transport model can predict the Qs measurements for the canopy in nonuniform flows. This study advances the existing understanding of the sediment transport process over a mobile bed within the emergent canopy.
Year: 2024