Numerical Analysis of Groundwater Ridging Processes Considering Water‐Air Flow in a Hillslope

Abstract

In this study, a water-air two-phase flow model was employed to investigate the formation, extension, and dissipation of groundwater ridging induced by recharge events in a hypothetical hillslope-riparian zone, considering interactions between the liquid and gas phases in soil voids. The simulation results show that, after a rain begins, the groundwater table near the stream is elevated instantaneously and significantly, thereby generating a pressure gradient driving water toward both the stream (the discharge of groundwater to the stream) and upslope (the extension of groundwater ridging into upslope). Meanwhile, the airflow upslope triggered by the advancing wetting front moves downward gradually. Therefore, the extension of groundwater ridging into upslope and the downward airflow interact within a certain region. After the rain stops, groundwater ridging near the stream declines quickly while the airflow in the lower part of upslope is still moving into the hillslope. Thus, the airflow upslope mitigates the dissipation of groundwater ridging. Additionally, the development of groundwater ridging under different conditions, including rain intensity, intrinsic permeability, capillary fringe height, and initial groundwater table, was analyzed. Changes in intrinsic permeability affect the magnitude of groundwater ridging near the stream, as well as the downward speed of airflow, thereby generating highly complex responses. The capillary fringe is not a controlling factor but an influence factor on the formation of groundwater ridging, which is mainly related to the antecedent moisture. It was demonstrated that groundwater ridging also occurs where an unsaturated zone occurs above the capillary fringe with a subsurface lateral flow.

Dynamics of groundwater flow and upwelling pressure heads at a wetland zone in a headwater catchment

“…This explanation was supported by subsequent studies of laboratory experiments [4], field experiments [1,5] and numerical simulations [6]. Furthermore, some researchers [7,8] have attempted to describe the phenomenon using mass balance equations. However, it is worth noting that in the capillary fringe, which is also descriptively referred to as the zone of tension saturation, every pore is fully occupied with water (saturated) whose pressure head is below atmospheric pressure (in tension).…”

“…Since at U3 the ground surface was already saturated, the intense rainfall could not infiltrate the soil profile to cause the observed rapid responses in pressure head. Note that the difference between a capillary fringe and a phreatic zone is only in the pressure energy content, and the conversion of the former into the later requires only an additional energy, although some researchers [7,8] have used mass-based models to simulate the phenomenon. In the groundwater ridging phenomenon, the kinetic energyladen intense rainfall induces additional pressure head into the pressure energy-deficient capillary fringe (tension pore water) at the ground surface.…”

“…Abdul and Gilham [4] used a laboratory physical model to demonstrate that the rapid rise of the water table in groundwater ridging phenomenon can result in the rapid delivery of pre-event groundwater into a nearby stream channel. More recently, Zang et al [8] used a numerical two (water-air) phase flow mass balance model to demonstrate that Lisse effect pressurized airflow in the upslope mitigates the dissipation of groundwater ridging into a hillslope. They also demonstrated that groundwater ridging can also be observed where an unsaturated zone is above the capillary fringe with a subsurface lateral flow.…”