A hydraulic roughness model for submerged flexible vegetation with uncertainty estimation

Abstract

Submerged vegetation is a key component in natural and restored rivers. It preserves the ecological balance yet has an impact on the flow carrying capacity of a river. The hydraulic resistance produced by submerged flexible vegetation depends on many factors, including the vegetation stem size, height, number density and flow depth. In the present work a numerical model is used to generate synthetic velocity profile data for hydraulic roughness determination. In the model turbulence is simulated by the Spalart-Allmaras closure with a modified length scale which is dependent on the vegetation density and vegetation height to water depth ratio. Flexibility of vegetation is accounted for by using a large deflection analysis. The model has been verified against available experiments. Based on the synthetic data an inducing equation is derived, which relates the Manning roughness coefficient to the vegetation parameters, flow depth and a zero-plane displacement parameter. Furthermore, the uncertainty of the inducing equations in the estimation of the Manning roughness is assessed and the propagation of the uncertainty due to the variability of the vegetation and flow parameters existed in nature is investigated by using the method of Unscented Transformation (UT). The UT is found efficient and gives a more accurate estimation of the mean Manning roughness coefficient and provides information on the covariance of the roughness coefficient.