Characteristics of a Hydraulic Jump Formed on Upstream Vegetation of Varying Density and Thickness

The effectiveness of coastal vegetation as a barrier to mitigate a tsunami greatly depends on the magnitude of tsunami and vegetation structure. This paper summarizes a series of laboratory experiments that investigated the upstream flow structure and energy loss due to a hydraulic jump in a steady super-critical flow. The characteristics of the jump were determined against vegetation of variable density (G/d, where G = spacing of each cylinder in cross-stream direction, d=diameter of cylinder), thickness (dn, where d = diameter of cylinder, n = number of cylinders in the stream-wise direction per unit of cross-stream width), and initial Froude number (Fr-o, where Froude number is obtained from a model without vegetation in the flume). In super-critical flow (Fr-o-1.67-1.83), a weak hydraulic jump formed on upstream side of vegetation. The height of the jump, its location, and the resulting energy loss were increased by increasing both the vegetation density and thickness. Due to reduced reflection at vegetation front, the drag force against sparse vegetation (G/d=2.13) was higher compared to intermediate (G/d=1.09) and dense (G/d=0.25) vegetation. Under these conditions, the maximum energy reduction due to a weak hydraulic jump reached 9.4% for dense vegetation while it was 8.1% and 7.8% for intermediate and sparse vegetation, respectively.