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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