In coastal aquifers, the tide-induced groundwater fluctuations make the interactions between surface water (SW) and groundwater (GW) more complicated than the SW-GW interactions at inland regions. In general, the tidal signal is bichromatic, manifested in its amplitude variation during every month; and such feature can be described by spring-neap tide, which consists of two primary dominant wave signals with two different frequencies. Previous research has shown that, due to the nonlinear effect and the interactions between spring and neap tides, a special fluctuation signal (called SNWTF) with lower frequency can be generated in coastal aquifers. The SNWTF can propagate further and promote the mass transport and hyporheic flow formation far away from the seaward boundary.
Despite the above findings, previous research did not considered the effects of unsaturated zone, which has been found to play an important role in affecting groundwater propagation. Therefore, the objective of this study was to investigate the combined effects of spring-neap tide and unsaturated zone on groundwater propagation. This paper introduced a new governing equation and corresponding analytical solution for describing the groundwater propagation in a sloping beach, influenced by spring-neap tide and unsaturated zone. Based on the analytical solution, this study revealed that, near the seaward boundary, the enhancement of water retention capacity of unsaturated zone would weaken the formation of the highest amplitude of spring-neap tide; whereas the existence of unsaturated zone far away from the seaward boundary is in favour of the slow decay of the amplitude of spring-neap tide. In addition, this study also examined the effects of water retention parameters, unsaturated zone thickness, aquifer thickness, hydraulic conductivity and beach slope on the propagation of low-frequency wave. The results show some situations which inhibit the formation of groundwater wave induced by spring-neap tide, including lower level of unsaturation, thicker unsaturated zone, thinner aquifer, lower hydraulic conductivity, and higher beach slope.
Furthermore, this study also investigated the effects of unsaturated zone on the over-height of groundwater. The results indicate that, when considering the impacts of unsaturated zone, there is a dramatic reduction in the groundwater over-height near the inland boundary. As a consequence, the low-frequency wave can propagate further. Finally, this study found that the formation of spring-neap tide is negatively correlated with the propagation of primary dominant wave signal. More specifically, when the soil characteristics are in favour of the propagation of primary dominant wave signal, it is more difficult for spring-neap tide to form. Such a mechanism indicates that, at muddy coastal beaches, the formation of spring-neap tide is more prominent, and this would exert profound influences on the solute transport near the inland boundary.
Jiangsu Key Laboratory of Coast Ocean Resources Development and Environment Security, Hohai University, Nanjing, China
Wednesday, January 4
10:30 AM – 12:30 PM