the previous 32 years (19691991) of 1266F-day
(Fig. 17).
Typically, breakup is initiated by an increase
in stage due to runoff from rain and snowmelt.
To quantify the potential runoff in the form of
snow cover, the accumulated snowfall is plotted
in Figure 16, as well as in Appendix A.
Ice force measurements
The 1992 and 1993 ice-outs were not as dra-
matic as in previous years (Fig. 4). In 1992, breakup
in the lower reaches of the White River was initi-
ated by the heavy rains of March 910 (Fig. 16a).
The ice concentration during the ice runs was
Figure 18. Ice conditions during breakup, March 10,
primarily single layer, covering 7595% of the
1992.
water surface (Fig. 18). The maxi-
24 x 103
mum floe sizes were on the order of
15 ft in diameter, with a norm of 6
ft. Prior to the ice-out, random ice
20
thickness measurements upstream
varied from 10 to 18 in. thick, with a
16
norm of 12 in. Because of the open
water, ice that impacted the struc-
ture typically stopped and then piv-
12
oted around the structure. When
crushing was observed, it occurred
8
as the floe was stopping and was of
short duration.
Figure 19 is a typical ice force
4
record reconstructed using digitized
data points. Ice forces are calculated
using the simply supported beam
0
0
4
8
12
16
equation with the measured reaction
Time (s)
load and the calculated moment arm. Figure 19. Typical ice force record for the Bridge Street bridge, 1992.
The water level reading is assumed
to be the impact elevation for the ice. An algo-
beam and a nominal ice thickness of 12 in., the
rithm was used to identify the maximum load
maximum effective pressure was 45 psi.
associated with each impact. To ignore minor loads
During February 1993 the White River valley
associated with localized crushing, the algorithm
received a heavy snowfall. With sunny days and
used the following logical tests. If the current
warm nights in March, the snowpack slowly
load Fn was greater than the local maximum load
melted, with a slow rise in stage. The slowly ris-
(Fn > Fmax), Fn was identified as the new local
ing hydrograph and warm weather allowed the
Fmax. The end of the impact event was defined
ice sheet to structurally deteriorate or melt in place.
by either of two conditions:
The weakened, unstable sheet ultimately collapsed,
The local load (Fn) was less than 10% of the
with very little increase in stage. The limited
maximum load encountered during the event
strength was reflected in the nominal floe sizes
(Fn < 10% Fmax), or
on the order of 3 ft in diameter, smaller than the
Open water was detected between floes by
1992 breakup. During the 1992 breakup a floe
the load dropping below the 1000-lb thresh-
impacting the instrument beam was accompa-
old.
nied by a loud percussion. This was not the case
During the 20-hour period of the ice-out, 8,056
of the 1993 breakup, when the ice flushed out of
impacts were identified, with the maximum load
the system with hardly a sound. The force record
being 26,071 lb. The impact load frequency dis-
was nearly constant at 6000 lb, or 10 psi, with a
tribution is plotted in Figure 20. For a 4-ft-wide
few distinctive impacts in the 60-psi range.
13
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