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Experimental investigation of flood energy reduction through vegetation at various angles

The efficiency of flow energy reduction past emergent vegetation has been typically studied assuming a right angle of the vegetated corridor to the flow direction. However, in many real‐world cases the riparian zones of natural, restored, or engineered rivers and waterways, are found at an oblique angle to the flood flow direction. In the current study, the effect of vegetation angle with respect to the flow direction is investigated experimentally in an open channel rectangular flume. The experiments are conducted under a range of subcritical steady flow conditions, with varying Froude number (Fro). The vegetation cover is placed at various angles to the flow direction (90°, 45°, and 30°), for a sparse and intermediate vegetation density, defined from the ratio of spacing of each vegetation element in the cross stream direction (B), and the diameter of vegetation element (d) (B/d = 2.13 and 1.09, respectively). Detailed water surface profiles are obtained for all those cases, demonstrating a considerable backwater rise, increasing with increasing vegetation density, Froude number, and flow approach angle. The energy reduction decreased by increasing the Froude number for the perpendicular (90°) and increased for oblique vegetation (45° and 30°). For the perpendicular vegetation, the average energy reduction rate for sparse (90VS) and intermediate (90VI) vegetation densities are 25.44% and 31.44%, respectively. The range of average energy reduction for sparse vegetation at 45° (45VS) and at 30° (30VS) are 18.3–19.8% and 18.7–19.7%, respectively. Similarly, range of average energy reduction for intermediate vegetation at 45° (45VI) and at 30° (30VI) are 21.4–22% and 18.8–22%, respectively.