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

GROUNDWATER FLOW MODELLING
of 0.5md -1 but considerable flow (>40%, Section 6.8) was also seen to occur horizontally
through the sides of the river dependent on higher Kx values. Therefore the value of riverbed
conductance of 720 m 2 d -1 used in the calibrated MODFLOW model includes estimates of
riverbed conductivity of 2-4 md -1 which incorporate values of both Kx and Kz for the riverbed
sediments and the overlying drift.
The steady state FAT3D model was used to examine the effect of a series of evenly
distributed impermeable obstructions simulating cobbles (dia. 30 cm) on the surface of the
riverbed. This is representative of the ‘armouring’, by large clasts of the surface of the
riverbed that occurs due to the erosion of finer material under high river velocities, which may
lead to a significant reduction in riverbed conductivity. Riverbed coverage of 23% caused a
5% drop in discharge to the river. The overall importance of the riverbed sediments in
controlling groundwater discharge in comparison to the other geological units of the FAT3D
model was assessed during sensitivity analyses performed under transient conditions. The
riverbed sediments were found not to be a significant control with a Kz maximum of 10 md -1
and a Kz minimum of 0.01 md -1 producing a 7% increase and a 10% reduction in discharge
to the river respectively.
The FAT3D transient sensitivity analyses showed the dominant controls on groundwater
discharge to the river in terms of conductivity to be the sandstone and gravel units. Discharge
to the river was decreased by 40% with a reduction in sandstone Kx to 0.5 md -1 , and a fall of
37% occurred with a reduction of Kx for the gravel to 0.1 md -1 . Increased groundwater
discharge to the river was most sensitive to an increased value of Kz to 1 md -1 in the
sandstone that produced a 48% rise in groundwater discharge. An increase in discharge of
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