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

GROUNDWATER FLOW MODELLING
adjacent to the river (Figure 6.6) and the origin corresponds to the eastern end of the section.
The top elevation of the grid was set at 95 m, which is 2 m above the maximum fixed head
values, at approximate ground level for the flood plain. The boundaries set for the model are
as follows:
1. Fixed head boundaries at the east and west edges of the model. The eastern boundary lies
175 m from the river bank and a head value of 93.25 m.a.o.d was set, based on average
water levels from borehole 19. The western boundary lies 130 m from the river bank and a
head value was set by calibration (Section 6.4.3.4) at 92 m.a.o.d.
2 Fixed head boundaries for the river cells. The steady-state river was represented using
fixed head cells (90.3 m.a.o.d.) across the river bed (38 cells x 0.3 m in width) and in the
adjacent banks (3 cells x 0.1 m in height). Under transient conditions additional fixed head
cells were added to the banks to reflect the increase in river depth.
3 Seepage cells were located in the banks above the river cells. These cells can only
discharge groundwater when the groundwater head is above the cell centre elevation.
4 No flow boundary at the base of the model.
6.4.3.2 Adaptation of model to represent the unsaturated zone
The FAT3D modelling code assumes fully saturated flow and requires some adaptation to
model the unsaturated zone. Field data on the location of the water table are needed to
constrain the model (Section 5.8). Areas containing the unsaturated zone have been classed as
no flow areas to prevent lateral flow through this zone. Tests performed under steady state
conditions using a uniform geology (Kx,Kz = 5 md -1 ) indicated a 7% reduction in flow to the
river and a 7 cm rise in modelled river bank piezometer heads when the no flow zone for the
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