Abstract
In order to design a vegetation structure to mitigate floods resulting from extreme events like tsunamis, vegetation density and thickness (width) are important parameters. Flow passing through vegetation faces great resistance, which results in a backwater rise on upstream (U/S) vegetation, increases the water slope inside the vegetation, and for some cases, forms a hydraulic jump downstream (D/S) of the vegetation, thus transforming a subcritical flow to supercritical [Pasha, G. A. and Tanaka, N. [2017] “Undular hydraulic jump formation and energy loss in a flow through emergent vegetation of varying thickness and density,” Ocean Eng.141, 308–325.]. Like the concepts of critical velocity and critical slope, this paper introduces the concept of “critical resistance of vegetation,” which is defined as “resistance offered by vegetation that transforms a subcritical flow to supercritical.” An analytical approach to find the water depths U/S, inside, and D/S of vegetation is introduced and validated well by laboratory experiments. Critical resistance was determined against vegetation of variable densities (, where of each cylinder in the cross-stream direction, of the cylinder), thicknesses (dn, where of a cylinder and of cylinders in a stream-wise direction per unit of cross-stream width), and the initial Froude number (Fro). A subcritical flow (, without vegetation) was transformed to a supercritical flow (D/S vegetation) with a range of Froude numbers of 1.6–1.9, 1.1–1.2, and 0.85–0.98 against ratios of 0.25, 1.09, and 2.13, respectively, thus defining as the critical resistance. However, altering vegetation thickness did not change the results.
