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

GROUNDWATER FLOW MODELLING
The river hydrograph may be approximated as a series of stepped increases and decreases in
head (h0i) occurring at times ti after time zero (the beginning of the event). The effect of each
increment, hoi, of the river stage on the head in a monitoring piezometer at distance x, is
calculated using Equation 6.2, and combined by superposition to give a final distribution of
head changes within the piezometer. The model is fitted to the field data from the piezometer
by varying storage and transmissivity (S/T). An Excel spreadsheet was constructed to carry
out the fitting automatically (Appendix 22). Error residuals of the form headobserved – headmodel
were calculated for each time step. An optimum fit for S/T was obtained using the solver
function in Excel. Optimisation was based on minimisation of the sum of the squared
residuals.
6.2.3 MODFLOW
The three-dimensional, finite difference, numerical flow model program MODFLOW
(original version 88/96) was used in conjunction with Groundwater Vistas (version 3). The
MODFLOW code (McDonald et al., 1988) was selected as it has been extensively validated
in many groundwater studies including a regional scale model of the Birmingham Aquifer
(Greswell, 1992). Details of the model setup and calibration are given in Appendix 23.
6.2.4 FAT3D
The computer code FAT3D (Mackay, 2001) is a 3-dimensional block centred finite difference
numerical model which solves the equation for confined flow.
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