ABSTRACT
Bae, J. and Sherman, D.J., 0000. Microscale morphologic changes caused by groundwater seepage on a macrotidal beach.
Beach groundwater dynamics and groundwater seepage (exfiltration) are important agents of morphologic change in the intertidal zone and are controlled mainly by the relationships between local water level, the elevation of beach water table, and the permeability of beach sediments. Groundwater seepage can alter sediment transport processes on beaches through regulating saturation on the beach face and generating shear stresses sufficient to initiate sediment transport. This study investigates seepage dynamics in the context of a macrotidal beach and consequent erosion, deposition, and seepage-induced morphologic changes. Data were collected from the west coast of South Korea, at Hakampo Beach, where the spring, mean, and neap tide ranges are 6.5, 4.6, and 2.8 m, respectively. Beach morphology and changes were measured using terrestrial LiDAR. Beach groundwater levels were measured with a series of wells. Sand samples were taken and analyzed to characterize beach face texture and to estimate beach permeability. The water table and sediment data were used to estimate seepage flows at low water, using Darcy’s law. The relationship between seepage rates and morphologic changes was examined with geographically weighted regression analysis. The results show that seepage-induced offshore transport occurred in the intertidal zone below groundwater seepage lines during spring tide, whereas transport was concentrated along seepage-induced tidal rills during neap tide. This highlights the importance of recognizing the variability in seepage discharge with different tidal ranges, especially in the context of potential flow rates exceeding the threshold for sediment transport. This study also points to the control of breaks in beach face slope and concomitant changes in grain size in the location of the groundwater seepage line in macrotidal environments.
