The effects of capillary fringe on solitary wave induced groundwater dynamics

Abstract

The capillary fringe is an important factor influencing the groundwater hydrodynamics in the coastal aquifer. The mechanism of the water table changes with the effect of capillary fringe under the wave action is still unclear. In this study, laboratory wave flume experiments were conducted to investigate the groundwater hydrodynamic features, paying particular attention to the capillary effects, in different areas of the beach under the solitary wave action. In the present experiments, medium sands were used to form the beach, resulting in a large capillary fringe height. Accordingly, the entire swash process occurred within the capillary truncated area. Based on the experiment measurements, three mechanisms are confirmed with respect to the coastal water table fluctuations, i.e., the flux mechanism, the gradient mechanism and the newly-proposed meniscus mechanism. The meniscus mechanism in the capillary truncated zone and the gradient mechanism in the capillary untruncated zone contribute to the non-hysteresis of the pressure head variation in the beach landward of the swash zone. The capillary truncated zone can be deemed as an extent of the swash zone since the effect of the swash process plays roles beyond the swash zone through a truncated capillary fringe. In addition, detailed observation of the evolution of the meniscus profile among the beach surface particles suggests that both vertical and horizontal seepage flows are important in the capillary truncated zone. In the swash zone, temporal variation of the hydraulic head can be classified into four/five stages in the case that the measuring position is near to/far from the uprush limit, and the effects of shoreline, water surface elevation, seepage face and exit point on the groundwater dynamics are discussed. Findings from the present study provide useful insights into the wave-induced coastal groundwater dynamics. • Capillary fringe effects on solitary wave induced groundwater dynamics are investigated through wave flume experiments. • Observation of the meniscus dynamics among beach surface particles provides new insights into the capillary fringe behaviors. • Flux mechanism, gradient mechanism and the newly-proposed meniscus mechanism for groundwater fluctuations are confirmed. • Meniscus mechanism dominates the hydrodynamic process in the capillary truncated zone. • Hydrodynamic process in swash zone can be divided into four or five stages with different dominant influencing factors.