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

GROUNDWATER FLOW MODELLING
6.4.3 Groundwater flow to the river channel - FAT3D cross sectional model
The FAT3D code was selected to represent groundwater flow to the river through a vertical
cross section (323 m wide and 100 m deep) with conditions set to be representative of profile
8 (Figure 6.5 and Section 6.8), grid reference O.S. 082913. The model was chosen to simulate
flow through a multiple layer aquifer system in two-dimensions in the vertical plane, under
steady state and transient conditions. The model allows fixed head boundaries to be
incorporated at either edge of the model to simulate the regional head gradient and fixed head
boundary conditions at the top centre of the grid to simulate the river. Boundary conditions
could also be set on the bank cells for the simulation of the seepage face. The user-defined
grid allowed a fine mesh size to be specified adjacent to the river to resolve the groundwater
head distribution. The model allowed the representation of multiple geological units,
including the riverbed, by assignment of aquifer properties on a cell by cell basis. Head data
from each cell, and groundwater flows across each cell face and to each boundary condition,
were used to define the groundwater flow paths to the river. The model also allowed
simulation of transient river level conditions such as groundwater/surface water interactions
during a river flood event, that are discussed in more detail in Section 6.10.
6.4.3.1 Boundaries and grid layout
A non-uniform, 2-dimensional grid of 100 x 100 cells was used to represent a vertical cross
section 323 m wide and 100 m deep. The cell sizes in the x and z directions range from 0.3 to
100 m in the x direction and 0.1 to 5 m in the vertical z direction. Cells have a uniform
thickness in the y direction of 1 m. The finest mesh size corresponds to the area immediately
167