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

GROUNDWATER FLOW MODELLING
6.4 Investigation of groundwater flow paths at the near channel scale in the
vertical plane.
6.4.1 Conceptual model of groundwater flow to the river channel
The river meanders across an urbanised alluvial flood plain overlying the unconfined
sandstone aquifer. The channel has been engineered in places to a uniform width of 12 m. The
river banks are vertical to a height of 1.5 m above the riverbed. They are supported in places
by permeable rock filled baskets (gabions). Dry weather river levels range from 0.3 to 0.5 m
above a natural riverbed composed of sand and gravel. The regional modelling (Section 6.3)
indicates that groundwater flow occurs towards the river from both sides of the channel in the
selected model area. Flows discharging to the river may have many different origins and ages.
Some will be recently recharged shallow groundwater flowing through the drift. Older,
slower-moving, groundwater will be derived from deeper within the sandstone aquifer. Flow
direction may change from horizontal to convergent in proximity to the river with a flow
divide between flow to the river and underflow which discharges further downstream.
6.4.2 Application of the MODFLOW flood plain model to assess underflow
The calibrated regional MODFLOW model allows the examination of flows within each cell
containing a river boundary condition. Flows across each cell face and discharge/recharge
from the river were obtained. Calculations indicate the proportions of the total flow leaving
the cell as discharge to the river and as underflow. The MODFLOW model gives estimates of
the proportion of underflow occurring but cannot provide information on the distribution of
flow in the vertical plane. For this, a detailed cross sectional model was created using FAT3D.
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