Experimental study of the solitary wave induced groundwater hydrodynamics

Abstract

Coastal groundwater hydrodynamics are affected by the swash uprush and backwash processes. In this study, laboratory experiments were carried out to investigate the solitary wave induced groundwater hydrodynamics over a coarse-grained sand beach. Pore water pressures at 27 cross-shore positions (covering both the subaqueous and subaerial areas) along 3 different beach depths were measured, and the synchronized shoreline position as well as the local water surface elevations (WSE) were also recorded. Temporal variation of the total water head (TWH) was found almost hydrostatically controlled by the instantaneous WSE in the subaqueous area. In the capillary truncated zone, the capillary effect contributes to the non-hysteresis phenomenon between the TWH and WSE. In other subaerial area, time-varying features of the TWH and WSE show clear differences in terms of their magnitudes and phases. Regarding the experimental observations, four representative regions could be identified along the cross-shore direction, i.e., Region I corresponding to the initially subaqueous area, Region II in between the still water shoreline and the top of capillary fringe (saturated beach), Region III in between the upper boundary of Region II and the maximum swash run-up, and Region IV landward beyond the Region III. With respect to the cross-shore distribution characteristics of the maximum TWH and the maximum WSE, as well as the time instants when the maximum TWH and the maximum WSE occur, different groundwater hydrodynamics were observed during the swash event in these four regions. In addition, spatiotemporal characteristics of the TWH were scrutinized and discussed. Both the vertical and horizontal TWH gradients in different regions were considered to reveal the sophisticated groundwater hydrodynamics. Finally, the groundwater movement pattern and its percolation circulation under the solitary wave swash were also specified. • Solitary wave induced groundwater hydrodynamics are investigated through detailed laboratory experiments. • Four regions are identified regarding the spatial characteristics of the maximum total water head and its occurrence time. • Hydrostatic effect, capillary effect and the wetting front are significant for the swash groundwater hydrodynamics. • Horizontal groundwater seepage flow is evident, even larger than the vertical one, during the entire swash process. • Two groundwater circulations are observed in the subaqueous and subaerial areas during the swash cycle.