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

GROUNDWATER FLOW MODELLING
increases from 15m to 100m along the river reach. The actual effective thickness of the
aquifer in relation to groundwater flow to the river is unknown owing to the presence of
possible confining mudstone horizons within the sandstone. Aquifer units comprising alluvial
flood-plain deposits and made ground, of unknown conductivity, overlie the sandstone. These
exhibit a maximum thickness of 5 m in borehole geological logs. A uniform transmissivity for
the layer was set in the range of 100 m 2 d -1 to 600m 2 d -1 . No allowance was made in the model
for any occurrence of lateral anisotropy as a result of depositional processes and buried
palaeo-channels.
Recharge
An average recharge estimate of 0.00045 md -1 for the entire aquifer was obtained by Greswell
(1992) amounting to 18% recharge from average rainfall of 736 mm year -1 plus additional
urban sources such as mains leakage. The effective rainfall (precipitation minus
evapotranspiration) was multiplied by a modification factor incorporating drift type and
thickness, and a modification factor for housing density and industry. Added to this were
urban return flows multiplied by a drift modification factor. A later model that was available
(Robinson, 2001) based on the earlier work, was used to determine the average recharge
(0.00069 md -1 ) over the 1.8 x 1.8 km 2 area of the current investigation. The level of recharge
was probably higher than the average for the aquifer due to the limited thickness of permeable
drift on the flood plain compared with the surrounding hills.
River-aquifer relationship.
Initial estimates of riverbed conductance were based on a cell length of 36 m, an average river
width of 10 m and a conductivity of 2 md -1 . A riverbed thickness of 2 m was obtained from
geological logs. The conductance was increased from 360 to 720 m 2 d -1 during calibration of
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