Page 311 - Hydrogeology Principles and Practice

HYDC08 12/5/05 5:31 PM Page 294

294 Chapter Eight

Fig. 8.8 Three hydrogeological situations
that represent flow between an aquifer and
a river. (a) The water table in the aquifer is
above the river stage and there is potential
for flow from the aquifer to the gaining
river (effluent condition) with the flux
generally proportional to the difference
between the elevations of the water table
and river stage. (b) The water table is
below the river stage and the losing river
(influent condition) potentially loses water
to the aquifer with the flux also generally
proportional to the difference between the
elevations of the river stage and water
table. (c) A common situation is shown in
which a partially penetrating river (where
the saturated aquifer extends beneath the
river) experiences a declining water table
below the base of the river. In this
situation, water will drain from the
perched river under gravity with a unit
head gradient. The unit head gradient
creates a limiting infiltration rate such that
river losses will not increase as the water
table falls further. In each case shown, the
nature of the river–aquifer interaction will
also depend on the properties of the river
bed sediments. Sediments with very low
permeability can result in a significant
resistance to flow. After Kirk and Herbert
(2002).

Chiltern Hills. In the River Ver catchment, about and this once typical Chalk stream has suffered
75% of the average annual recharge to the Chalk is substantial environmental degradation with major
allocated to licensed groundwater abstraction which changes in riparian habitat, the loss of naturally sus-
is now almost taken up (Owen 1991). Approximately tained fisheries, the loss of watercress farming and
half of the abstracted water is exported to supply reductions in the general amenity value of the river
areas outside of the catchment and is therefore effect- (Owen 1991).
ively lost. Of the remainder used within the catch- The above example illustrates the need for careful
ment, effluent returns via sewage treatment works management of catchment water resources and the
are typically in the lower reaches of the River Colne need to be able to predict potential environmental
valley and therefore are unavailable for supporting impacts. However, without very significant effort
river flows higher up in the catchment. towards field investigation and numerical modelling
The effects of the large demand for water, espe- (see Section 5.9), it is often difficult to evaluate the
cially from those boreholes situated towards the head impacts of abstractions on rivers with any degree of
of the River Colne valley, has been to dry up those confidence. A not unusual limitation is the availabil-
springs at the source of the perennial rivers (Fig. 8.9b ity of accurate data, particularly for the physical prop-
shows the hydraulic mechanism). In the case of the erties of the river bed and river bank sediments. As
River Ver, the upper 10 km section of originally discussed in the next section, a further approach is to
perennial or intermittent river is now normally dry employ an analytical solution in the hydrogeological
(Fig. 8.10b). Further downstream, the remaining assessment of river flow depletion caused by ground-
perennial section experiences much reduced flows water abstraction.