and working upstream. Once this cumulative ice
Ice jam volume
volume exceeds the previously estimated parent
and ice supply
ice volume, the user may want to zero out the ice
As discussed in the section on limitations,
cover at upstream sections. This is done by remov-
ICETHK calculates the thickness of an equilibri-
ing the IC lines from the appropriate cross sections
um ice accumulation reach-by-reach and does not
of the final HEC-2/ICETHK run. A search-and-
address ice motion or transport, assuming an un-
replace function, available with many text editors,
limited ice supply. In a natural river system, the
is useful for this task. A final HEC-2 run then gives
ice supplying a jam is finite, however. There may
the combined profile of ice jam and open water
be an ice-jam site or dam upstream, which limits
reaches.
the ice supplying a jam downstream. Through
field observation or a process of assumptions, the
Construction of
modeler may want to define the source reach
ice-affected rating curves
supplying ice to the jam being modeled. For a
The ICETHK model may be used to construct
breakup event, the volume of the pre-breakup
ice-jam rating curves. The accuracy of this effort
(parent) ice cover may be calculated by multiply-
depends on many factors, some of which have
ing the source reach area by a pre-breakup ice
already been mentioned. It is important to have a
thickness. The pre-breakup ice volume may be
knowledge of the range of discharges at which an
found by running HEC-2 with a sheet ice cover
equilibrium jam will exist. The case of the 11
of the desired thickness, at a typical midwinter
March 1992 ice jam at Montpelier, Vermont, is
base discharge. It is important to note that there
used here to illustrate.
may be losses. A significant portion of the parent
The jam initiated at 0700 hr when the discharge
ice volume (Vp) may melt during the run or be de-
hydrograph (Fig. 6) showed a flow in the 2000- to
posited as debris along the banks. With this in
3000-cfs range. The jam released at 1700 hr, at a
mind, the user can estimate the volume of ice in
discharge of roughly 8000 cfs. The ICETHK
the jam being modeled (Vj), accounting for the ice
model was calibrated to observed water levels
pack porosity:
along the ice-jam reach using the daily average
Vp (1 - % losses)
flow of 4700 cfs. After model calibration, ice-jam
Vj =
.
(9)
stages were found for discharges within the
1- e
known ice-jam range, and an ice-jam rating curve
was constructed (Fig. 7). The open water and
ICETHK's FIN.DAT output file (App. G) accu-
sheet-ice rating curves were found using HEC-2
mulates ice volume, starting at the toe of the jam
12 103
Jam Releases
10
8
6
Daily Average Flow = 4700 cfs
4
Jam Forming
(0700 hr)
2
0
6
12
18
6
12
18
6
12
18
March 10
March 11
March 12
Figure 6. Discharge hydrograph for the Winooski River at Montpelier, Ver-
mont, during the 11 March 1992 breakup ice jam.
12
Previous Page
