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Periodic forcing of surface water-groundwater interaction: modelling in vertical section

Smith, Anthony John (1999) Periodic forcing of surface water-groundwater interaction: modelling in vertical section. PhD thesis, Murdoch University.

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Sinusoidal variations in recharge can induce cyclical flows in surface water and groundwater. In this thesis, such time-dependent flows are explored in a coupled lakeaquifer system. The modelling extends previous steady state results and introduces new flow-visualisation techniques. Local responses in a 2D vertical section are illustrated for lakes within a 1D regional groundwater mound. The theory employs complex variables to decouple the periodic groundwater flows into separate steady state and fluctuating components.

The time dependent behaviour causes the lake-aquifer flow to change between flowthrough, recharge and discharge regimes. Corresponding fluctuations between inflow and outflow across the lakebed allow interchange of lake water with the aquifer (recycling and recapture). This also gives rise to sinuous flowpaths that can result in apparent dispersion; the number and size of waves, cusps and loops is characterised by a nondimensional waviness ratio. Streakline plots are introduced and provide an intuitive impression of the time-dependent groundwater motion. Such plots are enhanced by animation and illustrate the complex and potentially dispersive nature of the flows.

Interplay between the steady state and fluctuating responses determines the type and strength of flow regime transition. Importantly, there is an inverse relationship between head and flow in the fluctuating response. This is characterised by a dimensionless response time; a function of the aquifer geometry, hydraulic properties and period of fluctuation. During fast response, the recharge propagates mainly as fluctuation in flow, with small phase lags; particle trajectories form elliptical paths in the visualised flows. With a slower aquifer response, variation in recharge is manifest mostly as fluctuation in water level; cyclical perturbations in the flows are small and flows are nearly in steady state.

The position of a lake within the regional setting, size of the lake, and ratio of lake to aquifer recharge are important to the steady state response. Flow-through regimes occur throughout the regional setting, but dominate when the lake is lower in the system and groundwater flow is greater. Discharge and recharge regimes occur higher in the flow system, when the ratio of lake to aquifer recharge is large in magnitude.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental Science
Supervisor(s): Scott, William and Townley, Lloyd
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