Evaluating Riparian Vegetation Roughness Computation Methods Integrated within HEC-RAS

Abstract

Determining the roughness of riparian vegetation is often a challenge in river hydraulic modeling. A large number of quantitative methods have been developed, but none of them have been integrated into any one-dimensional (1D) hydraulic model for directly computing dynamic vegetation roughness. In this study, we evaluated the performance and applicability of eleven vegetation roughness computation methods by integrating them into a widely used 1D Hydrologic Engineering Center-River Analysis System (HEC-RAS). The enhanced HEC-RAS was applied to predict river stages of the San Joaquin River reach with dense and diverse riparian vegetation. Among the eleven vegetation roughness methods, the method of Freeman et al. clearly overestimates Manning’s roughness coefficient (n) values for flow depth greater than its original experimental flow depth (1.5 m). The method of Whittaker et al. considers the reconfiguration of flexible vegetation and uses measured vegetation projected area, resulting in a slightly more accurate prediction of river stages than all other methods. The methods that model vegetation as a rigid cylinder produce identical and reasonable river stage predictions. The Järvelä method includes the impact of velocity on vegetation roughness and uses an indirect metric leaf area index to represent the vegetation projected area, which produces a river stage prediction that is similar to other methods based on a rigid cylinder analogy. Compared with the original model with manually calibrated Manning’s n, the San Joaquin River HEC-RAS model integrated with dynamically computed roughness methods is able to predict observed river stages and performs notably better for flood flows exceeding the maximum calibration flow. The present study also demonstrates that methods that model vegetation as a rigid cylinder are applicable for computing dynamic n values in 1D hydraulic simulation for the areas in which most riparian vegetation is not fully submerged and dominated by trees and shrubs.