Abstract
The influences of tides on a coastal environment with a sloping shore are investigated by means of field observations and groundwater flow modelling. The Belgian western coastal plain consists of a wide shore, dunes and polders where diurnal tides with large amplitude occur. The effects of tides on the groundwater flow are studied using the MOCDENS3D code. First, MOCDENS3D is validated to accurately simulate the propagation, attenuation and lag of a tidal wave in an aquifer. Then groundwater flow and influences of tides are modelled for a cross-section along the French–Belgian border. This gives an exhaustive insight into the spatial and temporal varying groundwater flow and propagation of the tidal wave in the aquifer. Simulation shows that there are two interfering flow cycles. The first is a shallow tidally fluctuating flow cycle on the shore due to the interaction of the gently sloping shore and the tidally oscillating sea level. The second is a deeper flow cycle from the dunes towards the sea. Further, it is indicated that the propagation and attenuation of the tidal wave follows a complex pattern with lateral as well as vertical components. The interaction between tides and shore topography also influences the salinity distribution.
Résumé
L’influence des marées sur un environnement côtier à côte pentue, est étudiée via des observations de terrain et la modélisation de l’écoulement des eaux souterraines. Le littoral belge occidental consiste en une large côte, des dunes et des polders où l’on peut observer des marées diurnes de grandes amplitudes. Les effets des marées sur l’écoulement des eaux souterraines ont été étudiés en utilisant le code MOCDENS3D. En premier lieu, MOCDENS3D a été validé pour simuler précisément la propagation, l’atténuation et le décalage d’une vague de marée dans un aquifère. Par la suite, l’écoulement des eaux souterraines et les influences des marées sont modélisés sur un profil au travers de la frontière franco–belge. Ceci apporte une connaissance poussée et exhaustive des variations spatiales et temporelles de l’écoulement des eaux souterraines et de la propagation de la vague de marée dans l’aquifère. La simulation montre qu’il y a deux cycles interférents. Le premier est un cycle d’écoulement fluctuant, lié aux marées et situé en surface, due à l’interaction de la côte en pente douce et du niveau oscillant des marées marines. Le second est le cycle d’écoulement plus profond, des dunes vers la mer. Ultérieurement, il est montré que la propagation et l’atténuation des vagues de la marée suit un schéma compliqué, comprenant des composantes latérales et verticales. Cette interaction entre les marées et la topographie côtière influence également la distribution de la salinité.
Resumen
Se investiga las influencias de las mareas en un ambiente costero con una playa inclinada mediante observaciones de campo y modelizado de flujo de agua subterránea. La planicie costera occidental de Bélgica consiste de una playa ancha, dunas y tierras bajas ganadas al mar donde ocurren mareas diurnas de gran amplitud. Se estudian los efectos de las mareas en el flujo de agua subterránea usando el código MOCDENS3D. Primero se valida MOCDENS3D para simular con precisión la propagación, atenuación, y el retardo de una onda de marea en un acuífero. Luego se modelan el flujo de agua subterránea y las influencias de las mareas para una sección transversal a lo largo de la frontera entre Bélgica y Francia. Esto aporta un entendimiento exhaustivo acerca de la variación temporal y espacial en el flujo de agua subterránea y la propagación de la onda de marea en el acuífero. La simulación muestra que existen dos ciclos de flujo interferentes. El primero es un flujo cíclico somero que fluctúa con la marea en la playa debido a la interacción de la playa de suave pendiente con el nivel del mar que oscila con la marea. El segundo es un ciclo de flujo más profundo de las dunas hacia el mar. Además, se indica que la propagación y atenuación de la onda de marea sigue un patrón complejo con componentes laterales y verticales. La interacción entre mareas y la topografía de la playa también influye en la distribución de la salinidad.
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Acknowledgements
The authors would like to thank Dr. Peter Nielsen and Dr. Hailong Li for their constructive review of the paper.
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Vandenbohede, A., Lebbe, L. Effects of tides on a sloping shore: groundwater dynamics and propagation of the tidal wave. Hydrogeol J 15, 645–658 (2007). https://doi.org/10.1007/s10040-006-0128-y
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DOI: https://doi.org/10.1007/s10040-006-0128-y