Experimental study on reduction of scouring and tsunami energy through a defense system consisting a seaward embankment followed by vertically double layered vegetation

Abstract To countermeasure the scouring and tsunami energy reduction, this study conducted a series of laboratory experiments where vertically double layered vegetation (VDLV) was introduced behind the seaward embankment on a gravel bed. The experimental results demonstrated that the overtopping flow from the embankment experienced a submerged hydraulic jump over the embankment, against all the considered cases. The dense VDLV i.e. dense (G/d = 0.041, where G is the clear gap between the cylinders in the cross-stream direction, and d is the cylinder diameter) short submerged vegetation (L1) incorporated within a sparse (G/d = 2.125) tall emergent vegetation (L2) influenced the scour phenomena. It reduced the relative scoured depth by approximately 29 − 41% at the embankment toe, the maximum scoured depth by approximately 29 − 37%, and the scoured length by approximately 28 − 34%, as compared to that of single embankment system. Furthermore, dense VDLV provided the maximum i.e. 49 − 53%, loss to the total flow energy. The increase in the density of L1 in VDLV offered a larger drag to the flow by shifting the position of the submerged hydraulic jump more towards the embankment top; hence, consequently resulted in significant reduction of scoured depth, scoured length and energy of the tsunami flow.