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

GROUNDWATER FLOW MODELLING
The FAT3D model indicates that discharge is primarily concentrated through the sides of the
channel (Figure 6.11) with discharge through the central channel bed and the seepage face
being of secondary importance. For the calibrated steady-state model, 25% of the total
discharge occurs within 0.3 m of each riverbank. This is a general finding, as under the same
head conditions using a single isotropic media, 22% of the total discharge occurs within 0.3 m
of each riverbank. This implies that the results of water-quality samples taken from the sides
of the channel are more significant when calculating total contaminant flux to the river than
samples from the central section of the riverbed. Also, this may explain the lower-than-
expected total groundwater discharge estimates that were based on field measurements taken
from piezometers within the riverbed that do not sample the high flow zones around the river
banks.
The high flow through the river bank cells reflects the higher conductivity associated with
horizontal flow through the bank side as opposed to the lower conductivity associated with
vertical flow through the river bed. Under real conditions the vertical conductivity of the
riverbed may be further restricted by the clogging of pore spaces with fine sediments when
downward flow may otherwise have occurred, such as in the case of some tidal rivers where a
‘one way valve’effect is observed (Rivett, pers com). The vertical conductivity of the river
bed will also be dependent to some extent on the flow conditions within the river. Low
velocity conditions will facilitate the deposition of fines and a reduction of conductivity, high
velocity flood conditions may lead to the stripping of fine bed sediments and an increase in
conductivity.
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