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

GROUNDWATER FLOW MODELLING
(headmodel-headobserved) 2 for all 17 piezometers was considered acceptable for the calibrated
model.
The calibration was performed in several stages. An initial calibration was carried out on a
trial and error basis with a two layer geology to obtain a suitable fixed head for the western
boundary of the system for which no field data were available. Further calibration and
sensitivity analyses was carried out on a trial and error basis by selecting varied blocks of
cells to represent individual geological units and varying values of vertical and horizontal
hydraulic conductivity for these blocks within a range suggested by field and literature data.
The complexity of the geology was gradually increased from uniform to incorporate a total of
five divisions. Several different geological types were required to calibrate the model
effectively and the riverbed sediments exerted a significant control for head values adjacent to
the river. A final calibrated model was selected for later sensitivity analyses and transient
modelling on the basis that the model incorporated all five geological divisions and produced
an acceptable error using conservative hydraulic parameters (Section 6.5.3.1).
Transient calibration was performed by varying the specific yield of the water table
fluctuation zone and comparing the observed riverbank piezometer (P10 and P11)
hydrographs with the model hydrographs. Specific yield was varied with minimum
increments of 0.001 to obtain the closest fit between the modelled and observed hydrograph
amplitudes. Details of the model calibration are presented in Appendix 24.
6.4.3.5 Sensitivity of parameters
The initial assessment of the sensitivity of the model was carried out indirectly during the
steady state calibration process. The general sensitivity of the system to varying complexities
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