not consider the effects of momentum transfer. Ice
value to a particular ice jam event. Stage gages
jams may be grounded, particularly at the down-
may be affected by ice, contributing to inaccuracy
stream end, presenting modeling difficulty, because
in the discharge estimate. For this reason, gages
the theory behind ICETHK applies to floating
significantly upstream or downstream of the
jams. These limitations force the modeler to draw
jammed reach may provide the best estimate of
from other information sources, such as histori-
the average discharge during the event. A 15-
cal ice jam data and reports from field observers.
minute to hourly discharge record is useful for
If the toe of the jam is grounded, ICETHK may
estimating the flows at the time of breakup, jam
underpredict thickness in this area. Similarly,
initiation, and jam release. A stable discharge
ICETHK tends to overpredict ice thickness at the
while the jam is in place, sometime before the
jam's upstream end, where the ice supply may be
peak, is most appropriate for modeling equilibri-
limited. ICETHK may underpredict ice thickness
um conditions, rather than the peak discharge
along bends, because no provision is made for
itself. The peak may be associated with the release
the added friction along the outside of the bend.
of the jam. The relationship between the discharge
Finally, some uncertainty surrounds the model's
hydrograph and the timing of the ice event is dis-
ability to accurately reproduce ice thickness while
cussed in a later section on the construction of ice-
matching field-measured stage, because field-
affected stage discharge curves. Ice thickness dur-
measured ice-thickness data sets are rare.
ing a jam event is difficult to measure. A good
estimate of accumulation thickness may be pos-
sible, however, by observing shear wall height
Ice jam location
The user must decide on the location of the ice
along the channel sides, after the jam goes out.
jam being modeled. Historical records, eyewitness
accounts, and field observations are potential
Calibration parameters and variables
information sources on jam location. Lacking this
The effects of parameters and variables used to
kind of information, the user may have to infer
calibrate ICETHK to observed water levels are
likely ice jam locations from channel characteris-
described below.
The parameter is the coefficient related to the
tics, such as zones of reduced bed slope or chan-
nel bends. Deposition of sediment often occurs in
internal friction of the accumulation and the ice
shear along the banks. Lower means less inter-
these slope change areas, thereby further increas-
ing the ice jam potential. Once an observed or
nal strength and less bank shear, so the accumu-
potential jam location is chosen, the HEC-2 input
lation needs more thickness to withstand the
deck may be prepared, with its starting cross
downstream-acting forces.
section at the toe of the jam to be modeled.
The variable VEROS is the maximum non-
eroding velocity. Thickness is fairly sensitive to
this variable, particularly in reaches of high cur-
Calibration to
rent velocity.
observed ice conditions
Ideally some ice event field data will exist.
The variable RMULT is the roughness multi-
Important items, in addition to ice jam location,
plier. For all cross sections, roughnesses calcu-
are stage and discharge during the event period,
lated by ICETHK are multiplied by RMULT, unless
as well as estimates of ice piece size and accumu-
RMULT = 0. If RMULT = 0, then ICETHK refers
lation thickness.
to the external file RUFMULT.DAT for a list of
roughness multipliers, one RMULT value for each
Ice-affected stage
cross section.
ICETHK can be calibrated to match observed
DFACT is the depth factor, which limits thick-
or gaged water levels, or top of ice elevations,
ness due to shoving to a fraction of the total
along the ice-jam reach. Water levels downstream
water depth. This option may be useful if the user
and upstream of the jam are also important. Ide-
wants to limit ice thickening at cross-section
ally, a recording stage gage will be located near
locations.
The parameter Ci determines the cohesion be-
or within the ice-jam reach. Because this is rarely
tween ice pieces. This influences the internal
the case, however, the modeler must often rely on
strength and thus the thickness of a freezeup ice
photographs and the accounts of observers.
accumulation.
These calibration parameters are explained in
Discharge
greater detail in the section titled Inputs.
Care must be taken in assigning a discharge
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