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

GROUNDWATER FLOW MODELLING
transmissivity and riverbed conductance terms. The model employs a lumped estimate of
transmissivity for the geological sequence as a single unit across the model area. More
detailed modelling of the geology was required to investigate the control of the individual
geological formations and the FAT3D model was used for this purpose.
6.5.3 Application of the FAT3D channel scale model.
The geological controls on groundwater discharge to the river include the spatial distribution
of individual geological units and their associated hydraulic parameters of Kx and Kz. The
FAT3D near channel model is well constrained by head data, and the expected groundwater
discharge to the river. The geology is known from borehole logs, and the model incorporates
five geological units representative of sandstone, weathered sandstone, gravel, made ground
and riverbed sediments. The model was used to carry out sensitivity analyses for the hydraulic
parameters of Kx and Kz under both steady-state and transient conditions.
6.5.3.1 Hydraulic parameters
The conductivity values for the five geological units incorporated into the calibrated model
along with the maximum and minimum values used for the sensitivity analyses are presented
in Table 6.2. Initial values of Kx = 5 md -1 and Kz = 0.5md -1 for the Kidderminster Sandstone
were reduced during the calibration process as additional geological units were added.
Weathered sandstone was assigned a higher Kz value than the bedrock due to its likely
decementation, increased fracture density and the low frequency of mudstone units. Estimates
for Kx (5md -1 ) in the sands and gravel were based on rising head tests conducted within
environment agency flood defence boreholes in the Tame Valley.
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