| Garfield County Dam Break: Ross Dam #2 on June 23, 2002 |
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| Rainfall |
| Extremely heavy rainfall on the evening on June 22nd helped pave the way for a dam
failure during the early morning hours of June 23rd. The
following precipitation maps show the radar estimates a max value of 9.7" (the "A"
on the precip map is the location of the dam). Rainfall reports near the dam matched NWS
Doppler Radar estimated storm total precipitation with several spotters reporting 4 to 5 inches
of rain. There were no gages where the most intense rainfall fell, just to the north and
west of the dam, but there were numerous other reports from spotters 5-15 miles away from
the dam, and their reports also correlated well with the radar estimates. Forecasters
considered the possibility of hail contaminating the max value, but there was only one report
of any hail, and that was penny size hail at Cohagen, which is about 15-20 miles south
of the dam. |
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| Ross Dam #2 |
| The owner of the dam stated that the dam was nearly empty on Friday the 21st (it's
had very little water in it during the past two years), and that it filled up to a pool
level of about 30 feet by the time the dam broke, just from the rain we had on Saturday.
He also estimated that volume of the dam is 2200 acre-feet. There are 37 sections that
drain into the lake. The owner believed that the intense sudden rainfall, coupled with
problems of gopher holes 3/4 of the way up the dam, caused the dam to fail around 6 a.m.
Sunday morning. The width of the breach was approximately 60 feet
wide. The dam was originally built 45-50 years ago, and has failed once before, in
1979 after a really heavy snowmelt from the previous winter. |
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| Van Norman River Gage |
| There is one river gage site that we had access to that showed the crest of the rainfall
from the night before, and then again from the dam break. The
site is Van Norman, located on the Big Dry Creek, just above the intersection with the
Little Dry Creek. At 6 pm MDT on the 22nd, the gage was at .5 feet. It rose to 2.2 feel
with the rainfall, and continued to rise to just over 9 feet at 2 a.m. on the 23rd, where
it crested. By 6 am, the rise caused by the rainfall had dropped down to 6.3 feet. The
dam failed close to this time, and the gage had another peak at 11 a.m. around 9 feet.
(Flood stage at Van Norman is tentatively at 10 feet.) |
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| Flash Flood Guidance |
| Flash Flood Guidance that is issued by the Missouri Basin River Forecast Center in Kansas
City, Missouri did well, having a 1 hour Flash Flood total of 3.7", a 3 hour total of
4", and a 6 hour total of 4.2" Forecasters on duty the evening on the 22nd, issued a
Flash Flood Warning at 243 p.m. for the southwestern portion of the county, and reissued it for
the eastern portion of the county at 830 p.m.
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| Damage |
| Damage from the event included flooding of a home downstream of the dam, and several
older vehicles that were swept downstream. It was reported that 4 of 5 smaller stock
dams downstream were breached because of the Ross Dam failing. The
U-All road also had substantial damage in places, with gravel being completely washed
off, and the road washed out in one location. County Commissioner Brent McRae stated that
one portion of the U-All Road had water for 350 yards, with a max depth at least 4 to
5 feet deep. Not related to the dam failure, but to the heavy rains the night before, Highway
200 had to be closed for several hours as water went above the road. State of Montana
Dept. of Transportation crews had to make repairs on the highway the next day |
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