Resistance of Flexible Emergent Vegetation and Their Effects on the Forces and Runup Due to Waves

Abstract

Field surveys of post great Indian Ocean tsunami of 2004 revealed that coastal vegetation can greatly improve the safety of coastal infrastructure. Buildings exposed to direct attack of the giant wave suffered greater damage than those fronted by vegetation. Thus, understanding the effect of vegetation on the hydrodynamic impact of such long waves on the coastal structures has become an important issue among the coastal engineering community. In the backdrop of such requirement, a comprehensive experimental program was taken up to investigate on the dynamic pressures exerted on a wall due to cnoidal waves in the presence and absence of a patch of elastic vertical cylinders. The patch of vertical cylinders is expected to replicate typical characteristics of vegetation of emerged type. The resistance offered by vegetation to flow is essentially of drag in nature (Fischenich, 2000 and Darby, 1999) and is generally termed as, Green Belt effect. However, the nature of drag may be largely influenced by the hydro-elastic interaction of the current with the vegetal stems (Freeman, 1997). As such, stiffness of the vegetation appears to be one of the major parameters that govern the net drag offered to the flow. The percentage of reduction in the energy due to vegetation is also governed by the average diameter of the members within the vegetation, its height, distribution, density and type (Darby, 1999; Fischenich, 2000). In the past, several researchers have studied the flow characteristics past vegetal patches in open channels in order to evaluate friction factors for quantifying the resistance. However, measurements and quantification of resistance in the practical range of flow and vegetal parameters applicable to coastal protection are rather limited due to problems relating to scaling of all the fluid-structure physics.