Li
Vload = 0.5ρaV 2 Dc + Di
.
(29)
heat-balance model hindcast a 0.5-cm-thick uni-
100
form radial accretion with 26-cm-long icicles (as-
The factor Li/100 converts the wind load per length
suming 45 icicles/m), which is equivalent to a 1.3-
of icicle to wind load per meter of wire. The wind
cm uniform radial ice accretion. According to Storm
load results are useful for identifying the combina-
Data this ice storm affected most of Iowa and
tion of wind and ice in each event that causes the
resulted in 1.3 to 7.6 cm (0.5 to 3 in.) of ice on
highest horizontal load. This combination is inde-
exposed surfaces and 7.6- to 12.7-cm (3- to 5-in.)-
pendent of drag coefficient as long as it can be
thick ice on some power lines. Central Iowa from
assumed to be the same for both the wireice
near Des Moines and up to the north and west,
accretion and the icicle.
particularly Carroll and Crawford counties (about
80 miles from Des Moines' Polk County), was
hardest hit.
According to the model, the 31 October 1991 ice
9. SAMPLE RESULTS
storm was less severe than the 1990 storm in the
ZRAIN was run for the freezing-rain events
Des Moines area. The hindcast uniform radial ice
that occurred at the Des Moines, Iowa, airport
accretion is only 0.2 cm thick with 3-cm-long icicles.
between 1948 and 1993. The equivalent radial ice
Storm Data describes this ice storm as affecting
thicknesses on a 2.54-cm-diameter wire 10 m above
about a third of Iowa, including the forecast zone
the ground, hindcast using the heat-balance model,
where Des Moines is located. From 2.5 to 5.1 cm (1
are shown in Figure 6 for the 316 freezing-rain
to 2 in.) of ice accumulation was reported "in many
events in those 45 years. For many events either no
areas," and much of the damage was attributed to
ice or very little ice accreted, even though freezing
the high winds that accompanied the storm. How-
rain or drizzle was observed, typically because
ever, Des Moines' Polk County was not included
there was little or no measured precipitation. The
in the Presidential Disaster Declaration for 43 Iowa
largest accretion predicted by the model occurred
counties following that storm (FEMA 1991), and
in January 1982. That event consisted of two freez-
the measured wind speeds at Des Moines airport
ing-rain storms separated by 19 days of cold
were lower during this storm than in the 1990
weather with temperatures as low as 30C. In the
storm.
first 12 hours of freezing rain, 0.75 cm of ice accreted,
The simple flux model results are compared
and in the second 19-hr period of freezing rain
with the heat-balance model results in Figure 7.
there was an additional 0.75-cm uniform radial
The models agree well for the vast majority of the
accretion.
freezing-rain events at Des Moines. In the cases for
It is interesting to compare the modeled ice
which the results from the simple model are sig-
thicknesses with the Storm Data descriptions of the
nificantly higher than those from the heat-balance
model, the air temperature was at or above 0C
severe ice storms in 1990 and 1991. For the 78
March 1990 storm at the Des Moines airport, the
with relative humidity near 100%, and moderate
1.5
1.0
0.5
0
Figure 6. Uniform radial ice thicknesses hindcast by the heat-balance model for freezing-rain events at the Des Moines
airport from 1948 to 1993.
15
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