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

GROUNDWATER FLOW MODELLING
would result from the introduction into the model of a more realistic heterogeneous geology.
The river is primarily gaining although groundwater discharge is reduced and surface water
infiltration to the aquifer does occur at the upstream end of the reach.
Large variations in groundwater discharge to the river between adjacent cells (e.g. cells 71 to
85) are a product of the coarse grid dimensions and the associated problems of representing a
diagonal feature across the grid. The ‘diagonal step’ method employed will overestimate the
number of river cells per unit length. For example, in the case of the diagonal section starting
from cell 50, a total of 23 cells are required to cover a straight line distance equivalent to 16
cells parallel to the grid. This represents a 44% increase in available channel length that may
lead to an overestimation of net discharge to the river but an underestimation of maximum
flow per unit length of channel. Nevertheless the model is still considered suitable to represent
the heterogeneity of the system dependent on the geometry of the river and the regional head
distribution, and a correction was not applied.
As discussed in previous sections, many different flow paths of different origin and
contaminant loading culminate and discharge through a particular section of channel. The
composition of the total flow to each river cell in terms of source (Figure 6.14) is highly
variable along the MODFLOW reach. Different portions of the reach are dominated by
groundwater (contaminant) discharge from a particular direction (bank) rather than an equal
inflow from each bank.
The steady-state modelling and field data indicate that the river is currently primarily gaining
groundwater but historical studies have indicated that substantial ingress of surface water to
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