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

GROUNDWATER FLOW TO THE RIVER TAME
response at a single site between different flood events and within the same event, indicating
temporal changes in the S/T characteristics. The most likely explanation for this is that S/T
varies during an event as a function of unsaturated flow, and differences in available storage
across the capillary fringe as the water table fluctuates. The variation between events is
associated with the different wetting and drying histories leading to differences in the
moisture content profile.
During the period 11 th to 22 nd May 2001, a series of flood events occurred in rapid succession
after a period of dry weather flow conditions (Figure 5.22). Both piezometers at Profile 8
displayed damping and lag effects that decreased between each cycle indicating reduced
unsaturated storage due to moisture retention from the previous cycle. Piezometer P11 was
seen to give a greater response to the third peak than the second despite the maximum head in
the river being less on the third cycle. A different response was seen to the river forcing head
between the rising and falling limbs of any one event. Initial groundwater rise was rapid and
reduced towards the peak, and considerable lag between the piezometer and the river was
evident on the falling limb. This was due to available storage increasing progressively away
from the capillary fringe as the water table rose. The falling limb was subject to gravity
drainage and displayed a rapid initial drop in the water table which slowed with the onset of
delayed yield.
The groundwater discharge to the river varies in response to surface water fluctuations, the
soil moisture conditions and the local groundwater level. A detailed study of the area around
Profile 8 was undertaken to identify the local hydrogeological controls and provide data for
the construction of a numerical model to investigate groundwater/surface water interactions.
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