Seyed Mohammad Hossein Jazayeri Shoushtari(a), Nick Cartwright(b), Pierre Perrochet(c), Peter Nielsen(d)
(a, b) Griffith School of Engineering, Gold Coast Campus, Griffith University, Queensland 4222, Australia
(c) Centre d’hydrogéologie, Rue Emile-Argand 11,Case postale 158,2009 Neuchâtel, Switzerland
(d) School of Civil Engineering, The University of Queensland, 4072, Australia
Corresponding author: Seyed Mohamad Hossein Jazayeri Shoushtari, Griffith School of Engineering, Griffith University, Gold Coast, Queensland, 4222, Australia.
([email protected], Tel: +61(0)7 5552 7608)
This paper presents a new laboratory dataset on the moisture-pressure relationship above a dispersive groundwater wave in a two-dimensional vertical unconfined sand flume aquifer driven by simple harmonic forcing.
A total of five experiments were conducted in which all experimental parameters were kept constant except for the oscillation period, which ranged from 268 s to 2449 s between tests.
Moisture content and suction head sensor pairings were co-located at two locations in the unsaturated zone both approximately 0.2 m above the mean water table elevation and respectively 0.3 m and 0.75 m from the driving head boundary.
For all oscillation periods except for the shortest (T = 268 s ), the formation of a hysteretic moisture-pressure scanning loop was observed.
Consistent with the decay of the saturated zone groundwater wave, the size of the observed moisture-pressure scanning loops decayed with increasing distance landward, and the decay rate is larger for the shorter oscillation periods.
At the shortest period (T = 268 s ), the observed moisture-pressure relationship was observed to be non-hysteretic but with a capillary capacity that differs from that of the static equilibrium wetting and drying curves.
This finding is consistent with observations from existing one-dimensional vertical sand column experiments.
The relative damping of the moisture content with distance landward is higher than that for the suction head consistent with the fact that transmission of pressure through a porous medium occurs more readily than mass transfer.
This is further supported by the fact that observed phase lags for the unsaturated zone variables (i.e. suction head and moisture content) relative to the driving head are greater than the saturated zone variables (i.e. piezometric head).
Harmonic analysis of the data reveals no observable generation of higher harmonics in either moisture or pressure despite the strongly non-linear relationship between the two.
In addition, a phase lag of moisture content relative to the suction head was observed indicating that the response time of the moisture content to water table motion is greater than that of the pore water 3 pressure.
The observed moisture-pressure dynamics are qualitatively reproduced using a hysteretic Richards’ equation model.
However, quantitative differences exist which are likely to be due to previous findings that demonstrated that the Richards’ equation model is unable to accurately reproduce the observed water table wave dispersion, particularly at shorter period oscillations.
Keywords: Groundwater, Hysteresis, Richards’ equation, Oscillatory flow, Unsaturated flow.