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

GROUNDWATER FLOW MODELLING
The value of the FAT3D specific yield parameter derived for the piezometer P11 (0.03) is
lower than expected for an unconfined sand and gravel aquifer which usually have specific
yields > 0.15. There is no evidence of any horizons within the geological log that would
reduce the storage. An underestimation of Kx and Kz in the model may have caused an
underestimation in Sy but would not alone be sufficient to explain the low value. A likely
explanation is the effect of the capillary zone in reducing available storage adjacent to the free
water surface. Other evidence of the effect of unsaturated zone processes is found in the
variation in aquifer response to similar river flood events (Section 5.7) which must reflect a
variation in S/T. Different values of S/T for P10 and P11 were calculated by the analytical
model over two different time periods (Table 6.8) reflecting changes in the moisture content
profile which effects storage and flow processes above the water table. The variation in S/T
for P10 implies that the clay layer does not fully confine the aquifer as in that case S/T would
remain constant.
The transient analytical model indicates that a single value of S/T will not serve to fit the
modelled and observed data between events or for the entire duration of a single event (Figure
16a and b). The deviation of the model is most notable in the tails of the falling limbs of the
flood peaks where model values do not decline as rapidly as actual values indicating an over
estimation of S/T. The model also over-estimates lag times for peak arrivals which is an over
estimation of S/T. Maximum peak heights exceed actual values, indicating an underestimation
of S/T . The initial rise of the flood peak occurs more slowly in the model than in the actual
data, indicating an overestimation of S/T . Transmissivity is unlikely to vary greatly, owing to
an increase in saturated thickness which is small compared with the total. Small variations in
S/T may result from changes in geology across the fluctuation zone but these would not vary
220