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Hydrometeorlogical Disasters

Matt Kelsch, Hydrometeorologist & InstructorUniversity Corporation for Atmospheric Research

(UCAR)Cooperative Program for Operational Meteorology,

Education and Training (COMET)Boulder , Colorado

Wednesday, 5 September 2001kelsch@ucar.edu

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Hydrometeorlogical Disasters

How are flash floods different from other weather disasters?

•Tropical Cyclones/Hurricanes•Winter Storms

•Slow rise floods •Severe Weather/Tornadoes

What exactly IS a flash flood?Why is the public so vulnerable to flash floods?

Why is flash flood forecasting so difficult?

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Tropical Cyclones: what specific problems do tropical cyclones (hurricanes) present?

Like Flash Floods:• Increasing population in vulnerable areas• Even in relatively “easy” forecast events, uncertainty exists

about the precise location likely to be hit hardest• Public does not understand uncertainty well

Unlike Flash Floods:• Objective definitions about the phenomenon (> 65 kt wind)• General understanding of location of greatest impact of

wind and storm surge (along and near coast)• Longer life spans allow tracking and detail monitoring

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Hurricane Floyd 16 Sep 1999

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What specific problems do tropical cyclones present?

Impact larger areas than severe weather or flash floods

Most intense winds and storm surge are likely in a relatively small coastal area near eyewall:

• Uncertainty can be significant for forecast periods > 24h

• Coastal communities may need more than 24h to prepare

• Consequently, warnings take uncertainty into account and typically encompass larger areas than necessary

• Post-landfall inland rainfall can be extreme

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In the last 30 years, inland flooding has been the most disastrous part of tropical cyclones

• Hurricane Camille 1969 (Virginia)– Category 5 storm at Mississippi landfall, but does more damage as

a historic rainstorm in Virginia after the winds decrease

• Hurricane Agnes 1972 (Pennsylvania/New York)• Hurricane Tico 1983 (Texas/Oklahoma)

– Came in from Pacific and set rainfall records as an ex-hurricane

• Hurricane Fran 1996 (NorthCarolina/Virginia)• Hurricane Mitch 1998 (Central America)• Hurricane Floyd 1999 (South Carolina to Massachusetts)• Tropical Storm Allison (Texas to East Coast)

– Weak storm, but potent rainfall producer

Tar River near Tarboro, NC in the aftermath of Floyd

Tarboro after Hurricane Floyd, Sep 1999

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Winter Storms: an evolving and complex relationship between nature and society

Like flash floods:

• Problem is from precipitation, not wind damage

• Societal infrastructure is heavily impacted

• Risk taking puts people in wrong place at wrong time

Unlike flash floods:

• Winter Storms have objective definitions

• They generally occur over larger space and times scales (with the exception storms near mountain ranges and in the Great Lakes)

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Front Range Snowstorm:

8-9 March 1992

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Winter Storms: an evolving and complex relationship between nature and society

Prior to the 1880s, snow was often welcome for commerce because it ensured good sleighing & efficient transportation

Then came:

• Power & communications lines– National security threatened by downed lines during 1888 Blizzard

from Washington DC to New York City

• Inflexible work hours– 1888 Blizzard killed > 200 people in NYC commuting to work

• The automobile, and snow became transportation’s enemy– Transportation SNAFUs like Colorado’s October blizzard of ‘97

– Longer commutes, expensive & difficult road maintenance

NOOA/LDAD Display: Eastern Colorado Blizzard, 24-26 October 1997

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Main Stem River Floods

Like flash floods:• Too much water for natural or human-made channels• Societal infrastructure heavily impacted, can enhance the

problem (dam and levee failures, blockages at bridges)• Risk taking puts people in wrong place at wrong time• Toxic materials, firesUnlike flash floods:• Occur over larger space and times scales—allows

preparation time• Victims drown in river floods, but may suffer fatal

traumatic injury in flash floods

April 1997 Red River Flood, Grand Forks, ND

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Severe Weather: rapid evolution of dangerous weather phenomena

Like Flash Floods:• Rapid storm-scale evolution makes for very difficult

forecast problem• Small scale nature difficult to convey to the publicUnlike Flash Floods• Objective definitions for threshold and verification

– 50 kt winds and/or 0.75” diameter hail– Tornado

• Trained spotters help with field reports• Reasonable correlation between meteorological data

(radar/satellite) and severe weather

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Kansas City Brush Creek Flash Flood and Severe Winds 4 October 1998

4 Oct 1998 0022 UTC

4 Oct 1998 0107 UTC

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When severe weather turns into a hydrologic disaster

Severe weather can evolve into a flash flood situation, especially in urban environments where it does not take as much rainfall

• Cheyenne 1985

• Dallas 1995

• Kansas City 1998

Cheyenne, WY, 2 August 1985

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Flash Flood: what is it?

A flood where the rainfall and the subsequent runoff are occurring on the same time and space scales.

The problem is the rapid change in floodwater momentum, not the final depth, velocity, or extent. Fort Collins, CO, 28 July 1997

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Flash Flood: what is it?

Wall of Water?• These typically only

occur when a structural failure is involved

Can Define by Velocity• It’s really the rapid

change in velocity rather than the actual value that’s important

Cheyenne,WY, 1 August 1985

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Flash Flood: why are they so difficult to forecast and warn for?

Unlike severe weather, hurricanes, and winter storms:• Involves important and complex non-meteorological

processes• No reliable relationship between meteorological data

(rainfall) and flash flood potential• Hydrologic response may dominate, causing major

flash floods with relatively unimpressive rainfall – Especially small basins, steep basins, urban basins, and

altered basins (fire scars deforestation)

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Flash Flood: why are they so difficult to forecast and warn for?

Unlike hurricanes, winter storms, and slow-rise floods:

• Occurs on very small time and space scales

• In fast-response basins, the rainfall may not be the most important factor Cheyenne, WY, August 1985

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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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Rapidan, VA

Big Thompson, CO

Las Vegas, NV

Las Vegas, NV

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Flash Flood: Public Vulnerability

Rainfall is necessary for life– water just doesn’t seem that threatening

Difficult to know when a flood goes from a nuisance to a threat

Unrealistic feeling of safety in our vehicles, especially SUVs!

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What are the typical scales of the flash flood process?

• Almost always < 75 km2 (30 mi2), and sometimes less than 25 km2 (10 mi2).

• Less than 6 hours, and typically the real intense precipitation burst and subsequent flash flood are occurring in the 1-2 hour time frame.

• Rainfall rates >150 mm/h (6 in/h), perhaps only ~ 100 mm/h (4 in/h) dry climates.

1-h Accumulation, 12 July 1996

Purples=50-75mm

Fire Scar

Buffalo Creek, CO

Omaha, Nebraska

0500 UTC 7 Aug 1999

0700 UTC 7 Aug 1999

Fort Collins, CO Flash Flood

28 July 1997

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Timetable of flash flood event

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

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October 1999 Hydromet class at Spring Creek

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Hydrometeorlogical Disasters

It Doesn’t Have to Rotate to Kill Ya!Flash Floods take more lives in a typical year than

Severe Weather, Hurricane Winds & Storm Surge, or Winter Storms

Flash Floods involve complex and rapidly evolving meteorological and hydrologic processes– difficult to

define with objective quantitative criteria

Floods are not viewed by many as being as violent as tornadoes or hurricanes