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The hydrodynamics of partially saturated coastal sediments under periodic forcing are investigated through a multi-disciplinary approach combining semi-analytical modeling (Moving Multi-Front) and laboratory experiment (Tide Machine). The Moving Multi-Front (MMF) method is presented as a robust Lagrangian semi-analytical approach for analyzing the response of partially saturated flow to periodic tidal forcing within a vertical porous column through a sand beach. By solving the nonlinear Richards equation through a system of nonlinear ordinary differential equations, the MMF method generalizes the classical Green–Ampt piston flow approximation.
This study evaluates the method’s efficiency in capturing complex subsurface dynamics, including water table fluctuations Z_s (t), bottom flux fluctuations q_0 (t), and the complex evolution of zero-flux planes Z_0 (t). A systematic error analysis demonstrates that the MMF approach achieves second-order accuracy for space–time water content profiles, and a fractional 4/3 order of accuracy for temporal water table elevation Z_s (t). While accuracy increases with the number of moving fronts (N), results indicate that twenty fronts are sufficient to capture most hydraulic features, with as few as just two fronts providing satisfactory results for sandy soil substrates.
In parallel, an experimental investigation was conducted using a Darcy-scale sand column apparatus equipped with a hydro-mechanical "Tide Machine" designed to impose an oscillatory harmonic pressure at the column basis. High-resolution tensiometers were used to calibrate and measure both positive and negative pore water pressures (positive pressures and suctions) across various elevations, providing a comprehensive data set for comparison. Results from both the MMF model and experimental observations reveal critical phenomena such as pressure signal attenuation, phase lag, and non-harmonic behavior.
Furthermore, a parametric study of the mean water table height versus forcing frequency underscores the MMF method’s usefulness as an efficient tool for exploring the frequency response of the unsaturated zone.
By bridging Lagrangian modeling with experimental validation, this work provides a streamlined approach for predicting the impact of periodic forcing on coastal groundwater systems undergoing partially saturated / unsaturated flow regimes.
REFERENCES:
Alastal K., R. Ababou, D. Astruc, N. Mansouri (2025): One-dimensional Oscillatory flows in Partially Saturated Media with Moving Multi-Front. Physics of Fluids, 37(2), 026626 (2025). https://doi.org/10.1063/5.0251587
Alastal K., R. Ababou (2019): Moving Multi-Front (MMF): A generalized Green-Ampt approach for vertical unsaturated flows. J. of Hydrology, 579, 124184 (2019). https://doi.org/10.1016/j.jhydrol.2019.124184
Alastal K. (2012): Oscillatory Flows and Capillary Effects in Partially Saturated and Unsaturated Porous Media: Applications to Beach Hydrodynamics. PhD thesis. Institut National Polytechnique de Toulouse, France.
| References | Alastal K., R. Ababou, D. Astruc, N. Mansouri (2025): One-dimensional Oscillatory flows in Partially Saturated Media with Moving Multi-Front. Physics of Fluids, 37(2), 026626 (2025). https://doi.org/10.1063/5.0251587 Alastal K., R. Ababou (2019): Moving Multi-Front (MMF): A generalized Green-Ampt approach for vertical unsaturated flows. J. of Hydrology, 579, 124184 (2019). https://doi.org/10.1016/j.jhydrol.2019.124184 Alastal K. (2012): Oscillatory Flows and Capillary Effects in Partially Saturated and Unsaturated Porous Media: Applications to Beach Hydrodynamics. PhD thesis. Institut National Polytechnique de Toulouse, France. |
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| Country | France |
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