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Friction factor for turbulent open channel flow covered by vegetation
Wei-Jie Wang1,2, Wen-Qi Peng3,4, Wen-Xin Huai5
1State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing, 100038, China.
Scientific Reports
|March 28, 2019
Summary
A new model quantifies the friction factor in vegetated streams by accounting for vortical structures. This provides a more accurate understanding of shallow inertial flows in eco-hydraulics.
Area of Science:
- Hydraulics and Fluid Mechanics
- Eco-hydraulics
- Environmental Engineering
Background:
- The friction factor (f) is crucial for hydrological and hydraulic applications, especially in shallow inertial flows.
- Existing models like Darcy-Weisbach are effective for uniform roughness but struggle with natural stream complexities.
- Vegetation in streams introduces intricate vortical structures that significantly impact flow dynamics and friction.
Purpose of the Study:
- To develop an operational model for the friction factor in vegetated streams.
- To incorporate the effects of coherent vortical structures within and above vegetation canopies.
- To provide a more accurate friction factor formulation for eco-hydraulics.
Main Methods:
- Introduced a novel friction factor expression for vegetated flow: f_v = 4Cd(U_v/U_b)^2.
- Derived the local drag coefficient (Cd) to encompass layer-wise vortical structures and vegetation properties.
- Compared the proposed model with empirical relations and extensive datasets for emergent and submerged vegetation.
Main Results:
- The new model expresses the friction factor (f_v) using spatially averaged canopy velocity (U_v) and a derived drag coefficient (Cd).
- The formulation successfully integrates the influence of von Karman vortex streets, Kelvin-Helmholtz instabilities, and attached eddies.
- Validation against diverse datasets shows the model's applicability across various vegetation types and conditions.
Conclusions:
- The proposed friction factor model offers a significant advancement for understanding flow dynamics in vegetated streams.
- This formulation is expected to be widely adopted in eco-hydraulics for improved flow-vegetation interaction analysis.
- The model provides a robust tool for predicting friction in complex natural water bodies.
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