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

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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