Environmental Engineering Reference
In-Depth Information
of time over which measurements are made.
If streamflow is monitored with recording
gauges, then seepage estimates can be obtained
throughout the year. Careful planning of a
seepage run may be required to ensure that the
neglected terms in Equation (
4.2
) are small in
magnitude.
during part of the year and loses during other
parts of the year. Within any particular reach,
subreaches could be gaining or losing.
4.3 Streambed seepage
determination
4.3.1 Seepage meters
Exchange of water between streams, or other
surface-water bodies such as lakes, and ground-
water can be directly measured with seep-
age meters (Lee,
1977
; Lee and Cherry,
1978
;
Rosenberry
et al
.,
2008
). These devices consist of
a measurement chamber that isolates an area
of the surface-water bed and a variable-volume
reservoir that is used to quantify the volume of
flow across the measurement area over a spe-
cific time interval. Chambers range from the
size of a coffee can to a cattle-watering tank but
are typically cylinders about 0.2 m in height
and 0.5 m in diameter, open only at the bot-
tom (
Figure 4.4
). Seepage meters are commonly
constructed by cutting the end off of a 208-L
(55-gal) steel or plastic storage drum. After care-
ful insertion of the bottom of the chamber into
the streambed, a collapsible plastic reservoir
(commonly a plastic bag) containing a known
amount of water is attached to a fitting so that
the chamber is completely sealed from the
stream (a tube vented to the atmosphere may
be attached to allow dissolved gases to escape).
Flow into or out of the chamber is determined
by measuring the change in volume of water
in the reservoir during the time the reservoir
is connected to the chamber. Taniguchi and
Fukuo (
1993
) and Rosenberry and Morin (
2004
)
described automated seepage meters in which
the flux to or from the meter is measured elec-
tronically. Rosenberry (
2008
) described a seep-
age meter designed for use in streams and
rivers where fluvial forces would corrupt meas-
urements made by standard meters.
Seepage meters are inexpensive, easy to
use, and capable of measuring exchange rates
ranging from less than 1 mm/d to more than
2 m/d (Rosenberry
et al
.,
2008
). Because of the
small measurement area (typically 0.25 m
2
),
seepage meters provide a point measurement
Example: Lemhi River, Idaho
Donato (
1998
) described a set of seepage runs
that were made on the Lemhi River in east-
central Idaho to provide information to local
and federal agencies to aid in watershed man-
agement. Seepage was measured in 14 reaches
extending over approximately 100 km of
river length (
Figure 4.2
). Discharge measure-
ments were made with current meters using
the standard method described by Rantz
et al
.
(1982). Because of the large number of tributar-
ies, diversions for irrigation, and return flows
of irrigation, discharge measurements were
made at 117 locations. Five days were required
to conduct all of the measurements. Two seep-
age runs were made: August 4-8 and October
27-31, 1997.
August was a period of high groundwater
levels due to irrigation return flow; hence,
groundwater discharged to almost all river
reaches during the first seepage run (
Fig u re 4.3
).
Groundwater levels were lower in October,
after irrigation had ceased for the year; during
the October seepage run, stream loss (focused
recharge) occurred in 6 of the 14 reaches
(
Figure 4.3
).
The Lemhi River study illustrates several
aspects of the stream water-budget method for
determining
Q
seep
. Rates of exchange between
groundwater and the river were relatively high.
Of the 28 measurements over the two periods,
only five of the calculated values of
Q
seep
were
less in magnitude than the stated acceptability
criterion of 5% of measured streamflow at the
downstream end of the reach. The large number
of discharge measurements shows the intense
amount of data collection required when apply-
ing the method in a highly managed watershed.
Finally, the results demonstrate the dynamic
nature and complexity of hydrologic systems in
space and time; in some reaches the river gains
