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

GROUNDWATER FLOW MODELLING
by the FAT3D model. For P11, estimates of storage from the analytical solution would be
0.68 to 0.31 (T = 450 – 206 m 2 d -1 ) versus the FAT3D estimate of Sy=0.03. The difference
results from an overestimation of T used to derive the analytical storage values. Pressure
changes generated by the river may be assumed to propagate in a straight line from the bottom
corner of the river channel to the centre of the piezometer screen. The centre of the screen for
piezometer P10 is 13.27 m in horizontal distance and 2.5 m in vertical distance below the
eastern corner of the riverbed (gradient 19%). The centre of the screen for piezometer P11 is
8.9 m horizontal distance and 2 m vertical distance below the western corner of the riverbed
(gradient 22%). There is therefore a significant vertical component of T to consider which is
already incorporated within the FAT3D model, which simulates convergent flow and
anisotropy. An order of magnitude of difference exists between the average values of Kx
(2.14 md -1 ) and Kz (0.209 md -1 ) calculated for the FAT3D model (Section 6.6). An order of
magnitude reduction in the value of T used to estimate storage values for the analytical model
would give greater consistency with the FAT3D storage values. While the analytical model
provided estimates of S/T it required estimates of T containing a vertical component to
calculate meaningful storage values. The model would be better suited to obtain storage
values under confined conditions, and at greater distances from the river where the vertical
component of T would be less significant.
The results of the FAT3D sensitivity analyses indicated that the specific yield of the gravel
was the dominant control of groundwater-surface water interaction under transient conditions.
An increase in the Sy of the gravel from 0.005 to 0.35 caused an 83% decrease in the net
groundwater discharge for the event and maximum losses from the river of 0.73 m 3 d -1 over a
short (40 minute) time period. A reduction in the sandstone thickness to 10 m and an increase
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