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

GROUNDWATER FLOW MODELLING
unsaturated zone was incorporated. Cells were considered to contain the water table if they
were intersected by straight lines projected from the eastern and western model boundaries to
the nearest river bank. The elevations of the endpoints of the lines were equal to the observed
steady state water levels in the adjacent riverbank piezometer at one end and the fixed head
boundary conditions at the other. Cells above this level were assigned no flow conditions, as
were all cells above ground level.
For transient conditions cells are assigned a specific storage coefficient. In addition, a specific
yield component was set for cells that may contain the water table at some point during the
event (otherwise set to zero). The field data indicated all water level fluctuations occurred
within the gravel for the modelled event and only this unit was assigned a value of specific
yield. For transient conditions the base of the unsaturated no flow zone must be raised. In
order to estimate how much it had to be raised a transient simulation using the geology from
the calibrated steady state model with specific yield set as zero was used to determine the
likely maximum water levels. No flow was then assigned to levels one cell above the greatest
head obtained in the riverbank observation piezometer (P10) (91.7 m.a.o.d). In all the
subsequent transient simulations which incorporated values of >0 for specific yield the heads
in the riverbank piezometers were >10cm below the base of the no flow zone. The model is
not ideal for the simulation of transient unsaturated conditions but does provide an insight into
the processes that occur during groundwater/surface water interaction.
6.4.3.3 Model Parameters
Hydraulic parameters
Initial model runs were undertaken with a uniform geology and more complexity was added
in later runs to include five geological units identified in borehole logs representative of
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