Dike. The third significant change in slope is at
Open-water, freeze-up and stable ice-cover
the confluence of the Loup and Platte Rivers,
profiles were developed for a number of dis-
where the slope decreases from approximately 6
charges ranging up to 3500 cfs, which is about the
to 4.6 ft/mile. This location coincides also with a
maximum discharge experienced during freeze-
historical jam location on the Platte River.
up. Open-water conditions show a flattening of
Using available HEC-2 models, we performed
the profile near the downstream end of Wagners
a hydraulic analysis of two reaches within the
Lake and about 3000 ft downstream of the High-
study area to determine whether any obvious hy-
way 81/30 bridge, as did profiles for a stable ice
draulic conditions conducive to ice jam formation
cover and freeze-up conditions. The freeze-up ice
could be detected. The two locations were the
thicknesses computed by ICETHK ranged from
Platte and Loup Rivers at Columbus and the Platte
0.5 to 3.6 ft thick, with Manning's n values rang-
River near its confluence with the Elkhorn River.
ing from 0.02 to 0.036. The stable ice cover was
We first developed open-water profiles, which can
represented by 1-ft-thick ice and a Manning's n
be used to identify critical changes in water sur-
value of 0.015. As can be seen in Appendix A, the
face slope often associated with ice jam initiation.
open-water profiles were lowest and the freeze-
We then modeled freeze-up jam conditions be-
up profiles were highest, as expected. This ap-
cause frazil ice accumulations can result in areas
pendix includes profiles for 750, 2500 and 3500
of localized thick ice that may be significant dur-
cfs, although many other intermediate discharges
ing breakup jam formation. Finally a stable ice
were modeled.
cover was modeled in both locations, and an equi-
The thickest accumulations of ice computed
librium breakup ice jam was modeled for the lower
for the freeze-up case, up to 3.3 ft, were generally
Platte River location. Open-water and stable ice
located between the Highway 81/30 bridge and
cover profiles were modeled with the HEC-2 step
the water treatment plant at all discharges. This
backwater program (USACE 1990). Freeze-up and
might be expected since the stream slope flattens
breakup jam profiles were modeled using HEC-2
out slightly near the highway bridge. Significant
and the ICETHK utility (Wuebben and Gagnon,
ice thickening was computed for all discharges
in prep.).
about 0.5 mile downstream of the highway bridge.
This location would correspond closely to the toe
Loup River near Columbus
of the 1969 ice jam. Another area of ice thickening
The reach modeled extended from a point on
approximately 2000 ft upstream from the water-
the Platte River from 0.5 mile downstream from
treatment plant was seen at discharges between
the Loup confluence to the Loup confluence and
2000 and 3000 cfs. This location corresponds close-
then up the Loup River to about 7.25 miles up-
ly to the toe of the 1993 jam. Other discharges
stream of the confluence. The HEC-2 model used
showed slightly thicker ice in this area, but the
available cross-sectional geometry for a limited
increase in stage was most noticeable for discharg-
map and maintenance study of the Loup River at
es between 2000 and 3000 cfs. Field observations
Columbus, Nebraska, performed for FEMA.*
of the November 1993 freeze-up jam showed that
However, while overbank information was rela-
frazil accumulations upstream of the actual jam
tively detailed, for many cross sections only one
were thickest from about 0.25 mile upstream of
point below the surveyed water surface was used
the water treatment plant to just downstream of
to describe the channel bottom. The computation
the UPRR bridge west of Wagners Lake, with ac-
of water surface profiles at high discharges is not
cumulations of up to 3 ft on top of sandbars and
significantly affected by the lack of detailed chan-
along shore common. Ice accumulations of great-
nel geometry but may decrease accuracy at low
er than 3 ft in thickness may be possible during
discharges such as those used in this study. Chan-
freeze-up due to river level fluctuations associat-
nel geometry (cross-sectional area, top width, etc.)
ed with Loup River Power Canal operations. The
fluctuates between adjacent sections within the
formation of thick ice accumulations in the areas
reach but not significantly. The overall bed slope
with reduced slope would appear to contribute to
decreases from about 5 to 4 ft/mile downstream
later breakup ice-jam formation near Columbus.
of the Highway 81/30 bridge as the Loup enters
the Platte River valley, although local bed slopes
Platte River from
differ significantly from that.
its mouth to near Highway 92
The area modeled on the Platte River extended
from its confluence with the Missouri River up-
* LMMP study not yet published.
21
Previous Page
