Field and Numerical Investigations of Wave Effects on Groundwater Flow and Salt Transport in a Sandy Beach

Coastal dynamic forces, such as waves and tides, altogether drive groundwater circulation and salt transport in the intertidal zone. However, few numerical studies have ever considered the combined effects of waves and tides. In this study, the fluctuations of wave height are integrated with tidal level together using an iterative least squares fitting method, in which the wave height can be acquired from measured sea level in the surf zone, and this fitted wave height is further verified by wind speed. Groundwater flow and salt transport were then simulated using the MARUN code to evaluate the impacts after considering wave effect. The simulated equivalent freshwater head and salinity of the model with wave effect presented less difference with the measured data compared with the simulated results of the model without wave effect. After incorporating the wave effect in a model, submarine groundwater discharge (SGD) was increased, among which recirculated SGD grew more rapidly than the fresh, leading to the proportion of the fresh SGD accounting for a small proportion (1.1%). The water influx and efflux rates increased greatly especially during the period of high wave height. Most of the influx occurred in the intertidal zone, while a considerable amount of efflux occurred in the subtidal zone. The iterative algorithm to separate the wave height from the mixed field data can be employed to identify and quantify the respective effects of tides and the combined effects of waves and tides on the density-dependent beach groundwater flow.

Automated summary

Coastal dynamic forces, such as waves and tides, play a significant role in groundwater circulation and salt transport in the intertidal zone. This study integrates wave height fluctuations with tidal levels using an iterative least squares fitting method and simulates groundwater flow and salt transport considering the wave effect. The results show that incorporating the wave effect leads to a closer match between simulated data and measured data, and increases submarine groundwater discharge (SGD) with a higher proportion of recirculated SGD. The water influx and efflux rates also significantly increase, particularly during periods of high wave height.