chor ice at a Froude number of about 0.37, within
10.16 cm in diameter in a refrigerated flume. They
the range reported by Hirayama et al. (1997).
observed a definite increase in effective bed eleva-
tion because of anchor ice formation, but the pre-
liminary results were inconclusive regarding
Jam erosion
Frazil ice and ice pieces deposited beneath an
roughness effects. Anchor ice was observed to ini-
ice cover can erode under certain conditions. Ero-
tiate and accumulate at 0.14 < F < 0.57. Three dif-
sion, like early efforts to quantify deposition be-
ferent types of anchor ice were observed: compact,
neath ice covers, is most often described in terms
relatively stable growth in the form of "balls" of
frazil attached to rocks at F ≥ 0.56; a loose, rela-
of an erosion velocity. Unlike HEC-RAS, ICETHK
and ICESIM do provide for erosion of deposited
tively unstable growth at F < 0.2 when loose, po-
ice through the use of a critical velocity criterion.
rous tails formed; and a transitional stage during
ICETHK uses the following equation, developed
which the tails either flattened out to form a mat
from continuity:
or were released (Fig. 14). According to Daly,* the
loose accumulations were very unstable and
tended to release in large sheets. The loose accu-
1
v
H - (H - si η)
ηt =
mulations also seemed to increase effective bed
(28)
ve
si
roughness compared to the compact and transi-
tional-type growth, which tended to smooth the
bed, thereby increasing velocities and decreasing
where ηt is the ice thickness after thinning by ero-
stage. White et al. (1999) performed a series of tests
sion and ve is the critical erosion velocity (Tuthill
in the same flume using cobbles of median diam-
et al. 1998). The critical erosion velocity is thought
eter 4 cm. They reported little growth at a Froude
to increase over the course of a winter and also to
number of about 0.21 and compact or transition-
vary by ice type. Tuthill et al. (1998) suggest val-
type growth that eventually formed a mat of an-
ues of ve between 0.9 and 1.5 m/s for freezeup jams
and 1.2 < ve < 2.4 m/s for breakup jams. Petryk
(1995) reports use of a critical erosion velocity of
*Personal communication, Steven F. Daly, Ice Engineer-
1.1 m/s for modeling erosion of frazil deposits on
ing Research Division, CRREL, Hanover, New Hamp-
the Peace River using the SIMGLACE model.
shire, 1999.
26
24
Loose
22
Transition
20
Compact
18
16
14
0.063 m3/s
12
m3/s
0.047
10
8
20
30
40
50
60
10
Depth (cm)
Figure 14. Anchor ice growth characteristics at different flow velocities and flow depths
from Kerr et al. 1998 (open markers) and White et al. 1999 (solid markers).
20
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