Roughness zoning and calibration of channel cross-section based on vegetation distribution

The synthesis roughness of natural rivers varies with cross-sections, as well as water level and discharge. However, there are a few methods to parameterize the roughness curves (e. g., a roughness-water level curve). In order to improve the model accuracy, a significant effect of vegetation on channel roughness is considered in this research to parameterize the roughness. The channel cross-section is divided into several zones with different partition roughness based on vegetation distribution. After calibrating partition roughness values, the roughness-water level curve is obtained by using a water level-discharge relationship method. By analyzing the impact of channel cross-section characteristics and vegetation cover on the roughness curve, a formula is given for calculating the synthesis roughness of channel cross-section based on partition roughness values. This formula quantitatively describes the relationship between partition roughness and comprehensive roughness. The proposed method was used to calibrate the roughness of the hydrodynamic model of the Lijiang River. The results show that the synthesis roughness of the main channel varies in relation to water level ranging from 0.022 to 0.180. The critical depth was set at 1.5 m to divide the cross-section of the channel into the bottom-bed vegetation zone (n=0.210) and non-vegetation zone (n=0.006). Thus, the roughness-water level curve can be inverted, causing reasonable water level simulation results to be obtained. Dense vegetation in the Lijiang riverbed is the main factor causing the synthesis roughness changing with water level. The slope change of the channel cross-section is the main driving factor inducing the gradient change of the roughness-water level curve. These two factors change the cross-section synthesis roughness non-linearly in relation to water level. The results can provide a reliable reference for the hydrological forecasting of rivers in mountainous areas.