Post on 05-Jan-2016
description
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
Severe Weather:
Hail size: ¾ inch
Wind Speed: 58 mph
Tornado
Hurricane:
74 mph sustained winds
Hurricane Floyd, Sep 1999
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
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
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”
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, 1 August 1985
Continental Environment Tropical, Maritime
Environment
Precipitation Efficiency
23Z 5 May 1995 DFW Radar: Reflectivity and Cross Section
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Cross section
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Cross section
Dallas
42 kft
02Z 5 May 1995 Reflectivity and Cross Section
Dallas
42 kft
0815 UTC 16 December 1999, northern South America
Catastrophic storm
IR image with LTG
0330 UTC 29 July 1997
Radar reflectivity 9:30 PM, July 28, 1997
Issues With Flash Flood Warnings
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 time because the detailed guidance often requires reports of the flood
Better hydrologic tools are necessary to improve the Flash Flood Warning stage
Decision Making Regarding Flash Flood Events
Intense Precipitation• Possibly storm-specific
enhancementFast 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 AwarenessHigh Water Mark, Fort Collins, CO
18
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