Frictional swash hydrodynamics under the improved seaward boundary condition

Abstract

Effects of the improved seaward boundary condition on the swash zone hydrodynamics with bottom resistance are investigated using the method of characteristics. The characteristic equations including the Chezy-type resistance term are solved numerically by using the specified time interval scheme. Following Guard and Baldock (2007), the improved seaward boundary condition is introduced after incorporating a parameter k. Spatiotemporal distribution of the forward and backward characteristic lines under different bottom frictions and k parameters are specified. Large k parameter postpones the swash flow reversal time and extends the entire swash duration, which enhances/reduces the function of pressure force/bottom friction in swash hydrodynamics. During the late backwash stage, the bottom resistance effect becomes less influential with the increase of parameter k. Shoreline movement is mainly controlled by the bottom friction initially, then by gravity around the swash flow reversal, and by the joint actions of bottom friction and gravity (or hydrostatic pressure and gravity) in the case of k = 0 (or k = 1) near the late backwash stage. With respect to the entire swash duration, the flow reversal time at the shoreline, the maximum run-up height and the maximum backwash velocity, it is confirmed that the effects of the boundary condition parameter k on the swash hydrodynamics become more significant under the large bottom resistance. The present study extends Deng et al. (2016)'s work with the improved seaward boundary conditions and makes it more applicable to the actual swash process. Findings from the present study could further improve our understandings of the frictional swash hydrodynamics.