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Flash Floods:Defining and Forecasting
Matt KelschCOMAP Symposium 02-1, 02-2
Heavy Precip/Flash Flood
October 2001
kelsch@comet.ucar.edu
Fort Collins, CO28 July 1997
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Defining hazardous weather phenomena
Severe Weather:
Hail size: ¾ inch
Wind Speed: 58 mph
Tornado
Hurricane:
74 mph sustained winds
Hurricane Floyd, Sep 1999
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Defining hazardous weather phenomena
FLASH FLOOD:
Rainfall: 2-20 inches
Stream Level:
1-20 feet above bankfull
Time lag from rainfall to flood:
15 min- 6 hours
NWS currently claims an average of 52-min lead time on warnings
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Defining a flash flood (NWS)
A flood which follows within a few hours (usually less than 6 hours) of heavy or excessive rainfall, dam or levee failure, or the sudden release of water impounded by an ice jam.
An Urban Flood Advisory is issued to cover non life-threatening flooding of streets and low-lying areas (such as underpasses). If the flood should become life threatening, then it is considered a flash flood
Aftermath of 9 June 1972 flash flood in Rapid City, SD
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Defining a Flash Flood: (from the NATO Advanced Study Institute, “Coping With Flash
Floods,” November 1999 in Ravello Italy
A flood in which the causative rainfall and subsequent runoff are occurring on the same time and space scales.
Impact related to the sudden increase in water momentum, rather than the final depth, velocity, or extent of floodwater.
MANY ARE NOT A “WALL OF WATER”
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Avg ~5 in/hr
Avg ~8 in/hr
Accum.: 2-20+ inches Basin Size: 5-50 miles2, avg=18 miles2
Rainfall Rates: 3-12 inches/hr
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Typical Basin Characteristics of Flash Floods
Small, fast responseAlmost all <30 miles2
Enhanced by steep slopes
Urbanization
Deforestation, fire scars
Flood Control
**Detention Ponds**
**Channelization**
Know your flood control agencies
Cheyenne, WY, after flash flood and hailstorm, 1 August 1985
1-h Accumulation, 12 July 1996
Purples=50-75mm
Fire Scar
Buffalo Creek, CO, flash flood
Damage from 12 July 1996 flash flood that occurred just downstream of a fire scar.
Buffalo Creek, Colorado
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Issues With Flash Flood Warnings
Improvements have occurred mainly in the Flash Flood Watch stage since the 1970’s, but there are two ways in which warnings are sometimes inadequate:
1) In an attempt to ensure everyone in the threat area is warned, long-duration warnings are issued for large areas
2) In a conscientious attempt to provide specific detail in space and time, there is little or no lead time because the detailed guidance often requires reports of the flood
Better hydrologic tools and more specific 1-6h QPF are necessary to improve the Flash Flood Warning stage
IT DOESN’T HAVE TO ROTATE TO KILL YA!
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Flash Floods:Defining and Forecasting
Precipitation• Intensity, Duration, Area• Need better 1-6h QPFHydrologic Response• Understand Hydrology
– Altered Basins– Natural Basins
• Better, high-res toolsInteragency Communications*Need to come up with a real
definition and remember that flash floods are not just a precipitation issue
Cheyenne, WY after flash flood on 1 August 1985
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Flash Floods: Forecasting and Decision Making
for more information from COMET residence training you can view flash flood material presented at AMS meetings and the WDM workshops in 2001, or from
the COMAP Symposia on Flash Floods in 2000:http://www.comet.ucar.edu/presentations/index.htm
http://www.comet.ucar.edu/class/comap_symposium/index.htm
for more information on the Fort Collins flash flood and a detailed case summary with links:
www.comet.ucar.edu/resources/cases/c11_28jul97/lab1.htmhttp://meted.ucar.edu/qpf/urbanf/indexm.htm
link to our Venezuela flash flood and landslide page:http://www.comet.ucar.edu/resources/cases/venezuela/index.htm
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The Fort Collins, Colorado (USA) Flash Flood of 28 July 1997
• Terrain-locked thunderstorm system increased rainfall duration
• Unusual maritime-tropical characteristics increased rainfall efficiency
• Orographic influences increased rainfall prodcution
• Urbanization increased hydrologic response (volume and speed of runoff)
• Flood control structures increased runoff complexity
0000 UTC 29 July 1997
ETA 500 mb
0100 UTC
700 hPa wind and dewpoint, 0000 UTC 29 july 1997
Radar accumulation at 0430 UTC 29 July 1997
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Timetable of Flood Events
5:30
8:00
8:30
9:00
9:40
10:30
11:00
1:20
Rain begins
EOC Activated
Ponds Overflowing, rapid water rescues begin
Most intense rain commences
NWS Warning
Storm begins to dissipate and move northeast
Trailer Park Flooding, Fires, Train Derailment
Declared City Disaster
October 1999 Hydromet class at Spring Creek
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