A few researchers have developed means of
velocity to average channel velocity. Another ex-
measuring frazil ice at low concentrations in the
isting problem with AVMs in stream flow mea-
laboratory (e.g. Lever et al. 1992), but none of the
surement that must be corrected is the disruption
instruments appear promising for field use. As ice
of the acoustic signal during periods of slushice
concentration measurements were rated very low
flow. The instrumentation would also need to be
in the survey, this type of instrument would see
installed and protected to reduce equipment losses
limited demand until field users have a need to
during ice breakup. If the problems with this in-
measure frazil mass concentrations.
strument can be worked out, and it can be perma-
A method that may be capable of interpreting
nently installed at a site, it holds great potential
ice conditions is that of monitoring active and pas-
for allowing (if desired) continuous real-time
sive microwaves from an ice surface. Melloh and
stream flow gaging measurements.
Gatto (1990a) and Melloh et al. (1991) describe the
Tsang (1974) proposes a multipurpose instru-
use of passive microwave imagery to monitor river
ment for winter stream metering, including col-
and lake ice conditions near Fairbanks, Alaska. The
lection of flow velocity at a point, concentration
imagery was obtained from a Ka-band radiomet-
of frazil ice at a point, and velocity of the frazil ice
ric mapping system (KRMS) mounted from the
at that point. Flow velocity would be measured
bomb bay of an RP-3 aircraft. The KRMS differen-
by releasing a small volume of conductive solu-
tiates between wet and dry snow conditions, and
tion into the stream and measuring the conduc-
open water areas within ice covered rivers and
tivity between two consecutive pairs of electrical
lakes (Melloh and Gatto 1992). Although the
poles. As frazil ice causes a decrease in current
KRMS was not able to readily distinguish freezeup
between two electrical poles, the instantaneous
ice jams from smooth ice, it could be useful for
concentration of frazil could be measured as well.
determining large-scale areal ice coverage. The
Average velocity of the frazil could be determined
KRMS also appeared capable of imaging fractures
by measuring the time displacement of recordings
in the ice cover of a lake. Active microwave imag-
at two points. No development of an instrument
ery was obtained with synthetic aperture radar
prototype appears to have been carried out.
(C-, L-, and P-band) aboard a DC-8 aircraft. Melloh
GPS units could be used in the field in conjunc-
and Gatto (1992, 1990a, 1990b) report that active
tion with portable instruments for obtaining posi-
microwave imagery can distinguish between
tioning in the absence of other information. Use
rough and smooth ice covers and of detect open
of a single unit yields a horizontal accuracy as poor
water areas within an ice cover. They conclude the
as 100 m. However, the use of two units, or differ-
C- and L-bands were better at determining sur-
ential GPS (DGPS), with one unit stationary at a
face roughness. In both instances, the systems
known location, can yield horizontal accuracy as
tested by Melloh and Gatto were in development
small as a few centimeters. The U.S. Coast Guard
states. Each system may be potentially useful in
is establishing a network of DGPS stations to aid
the future, but further refinement of the instrumen-
navigation along coastal areas and the Mississippi
tation and further investigation into usability in
River. If a user is within range of one of these DGPS
other regions is needed. Additionally, a more con-
stations, then horizontal position can be deter-
venient and less expensive platform than the RP-
mined with the use of a single GPS unit in the field.
3 and DC-8 aircraft is needed.
Vertical position can be determined within a few
Wagner (1994) states that the U.S. Geological
centimeters as well, but the unit must be station-
Survey and Environment Canada have both dem-
ary for as long as 45120 minutes. A major draw-
onstrated that acoustic velocity meters (AVM)
back is that the GPS unit must have a clear line of
have potential for collection of stream flow data.
sight to the orbiting satellites; trees may block the
AVMs have been successfully used to collect line
signal.
velocity between transducers, and both agencies
plan to continue evaluation of AVMs. Acoustic
Additional instrumentation configurations
flow meters are already in fairly widespread use;
to consider
one example of their use in the USACE would be
for discharge measurement in a power plant pen-
Discharge
stock to detect decreases in flow due frazil buildup
There are a few instrumentation configurations
on trash racks. However, implementation of AVM
not mentioned above that appear technically fea-
in the field for stream flow measurement needs a
sible. One would be to combine a short-pulse ra-
great deal more work, especially in adjusting line
dar and MMW FMCW radar to make a discharge
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