The impact of urban groundwater upon surface water - eTheses ...

GROUNDWATER FLOW MODELLING
The FAT3D model has been used to look at two-dimensional flow paths in the vertical plane
at profile 8 on a section of river where regional modelling (Figure 6.4) indicates groundwater
flow is nearly perpendicular to the river and flow parallel to the river can be ignored.
A series of steady-state model runs were carried out in which the fixed head at the western
boundary was varied. The geology in each case was simulated as two distinct units
comprising 5 m of superficial drift (Kx 5 md -1 , Kz 0.5 md -1 ) overlying 95 m of bedrock
sandstone (Kx 2 md -1 , Kz 0.2 md -1 ). The head distribution was contoured (Figure 6.8 a,b,c) to
visualise the flow paths and to locate the flow divide. The division of flow between
groundwater discharging to the river and underflow out to the flood plain is evident at depths
greater than 50 m. It is apparent that the location of the flow divide is dependent on the
Western head boundary (WHB). Depths of the flow divide increase with increases to the
WHB, from ~50 m when WHB is 91.5 m.a.o.d (Figure 6.8a) until all flow is directed towards
the river when WHB is 93.09 m.a.o.d. (Figure 6.8b) i.e when the regional head gradient is
zero.
The modelling indicates that flow through the overlying drift is primarily horizontal and
therefore dominated by Kx. Flow through the underlying bedrock aquifer is in general
horizontal until converging flow commences within 100 m of the river and Kz becomes
increasingly important. The shallow (young) groundwater is seen to flow towards the sides of
the river channel while deeper (older) groundwater flows towards the central portions of the
channel.
The effect of a complex four-layer geological system on flow rates across the east and west
fixed head boundaries was examined. Units representing the permeable weathered sandstone
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