A Hydraulic Roughness Model for Submerged Flexible Vegetation

Abstract

Submerged vegetation is a key component in natural and restored rivers. It preserves the ecological balance yet has a hydraulic impact on the flow carrying capacity. 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 Spalar t 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 av ailable 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.