Numerical Calibration of Flow Roughness for Vegetated Channel

Vegetation roughness is an important parameter for simulating flow and sediment transport in vegetated channels and streams using numerical models. When excessive vegetation grows in the floodplains of regulated rivers, flood levels can increase under high-flow conditions. Vegetation roughness is the main calibration parameter for simulating flood level changes in vegetated channels. Therefore, many studies have been conducted to improve on the description of the relationship between flow resistance and the presence and spatial distribution of vegetation. The use of unscaled experimental data and field observation data is crucial for better predicting vegetated flow using numerical models. In this study, the spatial distribution of roughness values for vegetation patches, vegetated bank, and nonvegetated channel sections was calibrated with velocity data measured from large-scale experiments.

Delft3D with different hydraulic roughness setups was applied to simulate experimental conditions for large-scale outdoor flumes. The experimental channel section consisted of seven vegetation patches—four dense and three sparse—of young willow saplings in an alternate bar arrangement. Flow discharge of 2.6 m³/s was calculated with the measured velocity and cross-sectional area in the approach channel section. For the boundary condition of numerical modeling, a downstream water level of 1.1 m was applied. Numerical modeling results indicated that vegetation patches had over 50% higher roughness than nonvegetated areas and that the use of different roughness values for bed and bank produced better consistency compared with the measured data. A final combination of Manning’s n for bed and vegetation patches in a large-scale outdoor channel was presented.