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Decision Making Regarding Flash Flood Events

Decision Making Regarding Flash Flood Events. Definition Runoff Characteristics and Complexity Precipitation Characteristics Warning Issues Matt Kelsch WDM-III 01-3 19 July 2001. Fort Collins, CO 28 July 1997. Defining hazardous weather phenomena. Hurricane: 74 mph sustained winds.

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Decision Making Regarding Flash Flood Events

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  1. Decision Making Regarding Flash Flood Events Definition Runoff Characteristics and Complexity Precipitation Characteristics Warning Issues Matt Kelsch WDM-III 01-3 19 July 2001 Fort Collins, CO 28 July 1997

  2. Defining hazardous weather phenomena Hurricane: 74 mph sustained winds Severe Weather: Hail size: ¾ inch Wind Speed: 58 mph Tornado Hurricane Floyd, Sep 1999

  3. 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

  4. Avg ~5 in/hr Avg ~8 in/hr Basin Size: 5-50 miles2, avg=18 miles2 Accum.: 2-20+ inches Rainfall Rates: 3-12 inches/hr

  5. 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”

  6. Typical Basin Characteristics of Flash Floods Small, fast response Almost all <30 miles2 Enhanced by steep slopes Urbanization Deforestation, fire scars Flood Control **Detention Ponds** **Channelization** Know your flood control agencies Cheyenne, WY, 1 August 1985

  7. Precipitation Efficiency Tropical, Maritime Environment Continental Environment

  8. 42 kft ||| Cross section  ||| Cross section Dallas 23Z 5 May 1995 DFW Radar: Reflectivity and Cross Section

  9. 42 kft Dallas 02Z 5 May 1995 Reflectivity and Cross Section

  10. Catastrophic storm 0815 UTC 16 December 1999, northern South America

  11. IR image with LTG 0330 UTC 29 July 1997

  12. Radar reflectivity 9:30 PM, July 28, 1997

  13. Large 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 timebecause the detailed guidance often requires reports of the flood Better hydrologic tools are necessary to improve the Flash Flood Warning stage Issues With Flash Flood Warnings

  14. Decision Making Regarding Flash Flood Events Intense Precipitation • Possibly storm-specific enhancement Fast Response Basins • Need high-resolution hydrologic info and inter-agency communication Flood Control may reduce the frequency of floods, but can add complexity to those that still occur Public Awareness High Water Mark, Fort Collins, CO

  15. 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.htm http://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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