Geoscience Reference
In-Depth Information
be diverted underground in well structured limestone
(
Plate 14.3
).
Storage and transmission capacity is more consistent
than in soil and a function of porosity and
hydraulic
conductivity
- the ease with which porous rock transmits
water. This is a component of
Darcy's law
, which shows
groundwater flow
Q
g
as:
Q
g
=
kA
h/l
where
k
is hydraulic conductivity,
A
is the discharge cross-
sectional area,
h
is the hydraulic head and
l
the distance of
flow. It is applicable only to homogeneous porous media,
whereas most soil and rock transmits water through
fissures. Typical porosities of principal rock types are
given in
Table 13.1
but we need to know the impact of
ground water on river flow.
Specific retention
, reminiscent
of field capacity, and
specific yield
show the mobility of
ground water under gravity flow, with losses declining
over time (
Figure 14.7
).
A more direct measure is the
base
flow index
, the proportion of stream flow contributed by
ground water, shown in
Table 14.3
.
River flow and hydrographs
River flow offers one of the most direct measures of the
water balance of part or all of the catchment.
Stream
gauging
attempts to quantify the water volume discharged
over a fixed time period are normally given in cumecs or
cubic metres per second. It is measured by
stage
, the
depth of water passing a flume or weir of known
dimensions and water velocity (
Plate 14.4
),
since it is
virtually impossible to collect and measure total flow
sandstone for Carboniferous limestone, on the western flank
of Pen-y-Ghent, North Yorkshire.
Photo: Ken Addison
Permeability Coefficient k (md
-1
)
10
1
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
2
10
3
10
4
10
5
10
6
10
7
50
40
Specific yield
Porosity
30
20
10
Specific retention
0
6
200 mm
0.002
0.006
0.01
MEDIUM
0.02
0.06
0.1
0.2
0.6
1
2
10
20
60
100
FINE
COARSE
FINE
MEDIUM
COARSE
FINE
MEDIUM
COARSE
PEBBLES
BOULDERS
CLAY
SILT
SAND
GRAVEL
