Flash Floods: A Situational Analysis
By: Masters Student
University of Idaho
22 February 2013
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General Overview:
Flash floods are a growing concern to an increasing number of people in the U.S. A
greater number of people are settling in flood plain and in harms way due to poor and hasty
decisions in land development and urbanization. With increased vulnerability to flash floods, it
is important to advance knowledge and mitigation techniques to members of at risk
demographics as well as to the general population. Over the last two decades, an immense
amount of progress has been made through study and technological innovations in detection
and issuance of flash flood alerts. Unfortunately, however, “flash floods remain the #1 weather
related killer in the U.S ” (NFIP 2013 floodsmart.gov). “During the years 1985–2001, the total
annual number of floods or flash floods ranged from 361 (1988) to 3,376 in 1998. A total of
32,047 flash flood/flood events were recorded in 1985–2012” (weatherexplained.com). The
rise in flash floods reported can be credited mainly to greater efforts to learn about flash flood
hazards, to the clearing of land for urbanization. Population is always on the rise and so cities
are ever expanding to accommodate the influxes of people. Disseminating an understanding of
the causes, hazards, demographics at higher risk, and mitigation technique is fundamental to
managing flash floods and their associated hazards.
Background:
Causes and Hazards
According to the U.S National Weather Service, flash floods are floods that follow heavy
or excessive rainfall, dam or levee failure, or the sudden release of water impounded by an ice
jam (Glossary NWS 2010). Flash floods are characterized by passage of a large amount of water
over a short amount of time (approximately 6 hours) and can be caused by weather factors
such as hurricanes and thunderstorms or uncommon factors such as levee or damn failures.
According to the Weather Almanac Vol 1, “Typically, flash floods occur primarily at night and
when there is an abundance of atmospheric moisture, in addition, there is usually little, if any,
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vertical wind shear present. Not only do flash floods cause water damage, but the sheer power
of them yield the potential to tear trees from the ground and carry with them massive
boulders, thus inducing landslides and mudslides which pose great threat to life and property.
Flash floods can move at incredible speeds and form waves that reach over 20 feet high and
even carve out new channels for rivers. Sometimes the source of these waves of water is from
the failure of decades-old dams of levees in disrepair. An ice jam can also produce a tsunami of
water. An ice jam occurs when water builds up behind a blockage of ice such as a glacier
blocking a river. Ice jams are more likely to occur in the spring when water and snow that was
once frozen, thaws. With global warming becoming more of a concern, many people may be
living on flood plain or in low-lying areas at risk to such a hazard as an ice jam. Large plots of
land may be logged and cleared for development, or may be cleared because of fire, or to
harvest resources. When it rains on these cleared areas of land, where vegetation and
permeable soil are no longer available to absorb and catch the precipitation, mudslides and
landslides may result.
Damage
Damage caused by flash floods can be separated into two main types known as direct
and indirect damages. Direct damage is based on the immediate effects that high-speed
floodwaters can have on an affected location. Direct damage includes destruction of drainage
systems, the drowning of people and animals, and the destruction of farmlands, crops and
property. Floodwaters also wash up chemicals, paint, pesticides and gasoline that typically coat
plants, roads and buildings, causing water contamination.
Indirect damage can be
characterized as the result of direct damage. Indirect damage can occur, for example, when
transportation systems are disrupted, when roads or rail lines are washed away or flooded.
Likewise, electrical and gas lines can go down due to the lines getting broken (Bhattacharyya
2012).
Some people may lose their houses to the power of the water current or the
backpacking debris. Economic productivity often comes to a halt due to the destruction and
chaos during and following a flash flood. Time and money are required to rebuild and recover.
Judging by the image below, there appears to be a trend between flash floods and their effects
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on waterways and major rivers, thus affecting those who live in close proximity. Where there is
a higher concentration of blue and red dots, there is a major river located in the area.
U. S. Department of Commerce & National Oceanic and Atmospheric Administration & National Weather Service
(1992). Flash Floods and Floods. City and Publisher Unknown.
“The Center for Disease Control (CDC) report that over one half of all flood-related drownings
occur when a vehicle is driven into hazardous flood water. The next highest percentage of
flood-related deaths is due to walking into or near floodwaters” (NOAA). A widespread lack of
knowledge can cause people to underestimate the sheer power of a swift moving stream of
water. Underestimation of the power of water is not helped by advertisements for trucks and
SUVs boasting about these vehicles’ ability to transit water. Below are two pieces of
information that demonstrate the forces that are acting on a vehicle when it is in water.
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HOW CAN A FOOT OR
TWO OF WATER COST
YOU YOUR LIFE?
Water weighs 62.4 lbs per cubic foot and
typically flows downstream at 6 to 12 miles
an hour.
When a vehicle stalls in the water, the
water's momentum is transferred to the
car. For each foot the water rises, 500 lbs
of lateral force are applied to the car.
But the biggest factor is buoyancy. For
each foot the water rises up the side of the
car, the car displaces 1,500 lbs of water.
In effect, the car weighs 1,500 lbs less for
each foot the water rises.
Source: Up-NOAA’s National Weather Service TADD Campaign
TWO FEET OF WATER WILL
CARRY AWAY MOST
AUTOMOBILES.
(Right- http://www.scchealth.org/docs/ems/docs/prepare/floods.html
Scope and Urgency:
The United States Search and Rescue Task Force note the factors including “Topography,
soil conditions, and ground cover also play an important role” (USSRTF 2013). In other words,
environments most prone to flash floods are areas like the high desert that are dry, have sparse
vegetation, steep terrain and experience intense and infrequent rainfall.
In urban
environments where vegetation has been removed, bridges and culverts constrict flow, and
buildings and paving have greatly expanded impermeable surfaces, which likewise pose an
increasingly serious risk of flash flooding (Federal Emergency Management Agency, 1987).
Flood related deaths have risen in recent decades, and the U.S. Congress's Office of Technology
Assessment notes, "Despite recent efforts, vulnerability to flood damage is likely to continue to
grow" as populations in flood-prone regions steadily increase (Rowan 2010). As flash floods
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become a frequent occurrence, more people are likely to be killed and more property is likely to
be lost unless flash flood management means are discovered, implemented and spread.
The urgency of mitigating the risk of flash flooding for increasing numbers of people at risk to
flash flooding has not been lost on researchers and disaster relief agencies. Agencies have
made great gains in developing research technology, methodology and collecting data that
have resulted in better forecasting and management of flash floods, but “conditions that favor
flash flooding remain infrequent and unpredictable“ (Moen 2006). Furthermore, flash floods
were not as large of a problem in the past as they are today. These facts would explain why
past data is limited and rather inaccurate. It has been realized that best way to combat flash
flooding is through combined collaboration of a multitude of organizations and that is exactly
what is happening. Cooperation has reached an international level in the form of flash flood
conferences such as the Flash Flood Workshop sponsored by the NOAA in 2006, in which
experts congregate to discuss and share prediction technologies, and to discuss resulting
implications as well as possible solutions.
Target Population Data:
Flash floods do not discriminate based on age, gender or race. A demographic can be
discovered based on location but is vast and diverse. Populations most vulnerable to flooding
are those that typically live in the desert, on flood plains, downstream of a dam or levee or near
rivers. According to the NOAA, urbanization increases runoff by two to six times over what
would naturally occur (Rowan 2010). With all the concrete and pavement that covers the
ground in cities and large suburbs, runoff from storm waters is unable to absorb into the soil, as
would naturally occur elsewhere. Although these locations are the most prone to flooding
disasters, flash floods are not exclusively restricted to these locations.
People who live in areas prone to flash floods generally know their area is at risk, or have seen
or heard of the effects of flash flooding first hand. However, many others, particularly people
living in urban locations, who have not even considered themselves and their property as
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potential victims. Those who have not experienced a flood or flood warning are likely to draw
upon other past ‘available’ experiences or images which appear to them to be similar to an
imagined flood (Tversky and Kahneman, 1973. There is a lack of trust and a lot of ignorance
among people who have not seen or felt the devastating effects a flash flood can have. Perhaps
flash floods were never a problem before something was built, expanded or destroyed. Until an
individual experiences the consequences of a flash flood personally, ignorance will likely
continue. According to Parker et al, “Those who underestimated flood risk were those with no
flash flood experience.” This does not mean that there are not people out there who are
simply clueless. This lack of trust and awareness is not due to the amount of information, but
misinformation and the idea of invincibility. The preconceived notion that “it would never
happen to me” leads at risk demographics to underestimate flash floods. Many resist taking
any preventative measures to the potential that a flash flood may happen simply due to
stubbornness, laziness or lack of personal funds. For example, one method of protecting
oneself and one’s personal property is to purchase life and flood insurance. However, in areas
that are known for hurricanes and flooding like the Gulf Coast, the insurance is so expensive
that many people cannot afford it or unable to move. In some cases, there is too much trust in
the drainage systems that are currently in place in certain locations to mitigate the increase of
water under normal circumstances. The Las Vegas Wash is an example. The immense volume
of water in short periods of time characteristic of flash floods is not normal and many don’t
consider this fact. Flash floods build within minutes of heavy rain and many are unable to get
out of their way.
Thanks to technological advancements over the last several decades, the majority of people
that live in affected areas can be reached through mass media. Internet, television, newspaper
and radio are four main means by which flash flood warnings and messages can be spread. If
more information were disseminated via mass media and through flash flood survivors and
victims’, people in “at risk” demographics would have access to information about preventative
actions that they can and should take when flash flooding is imminent. In more rural areas
subject to flash floods, word of mouth, billboards, local paper, signs and radio may be more
appropriate and trusted means for communicating about this issue.
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Management Techniques:
The greatest preventative step that can be taken to manage the devastating effects of
flash floods is to educate oneself. Get to know the area that one lives in or is moving to. Talk
to neighbors and get first hand experience about where one lives and its history and then use
the Internet to find out additional information. Key questions to ask are: How much rainfall
does this area get and typically during what months? Is it located on a flood plain, what is the
elevation of the house in comparison with the surrounding terrain? Is the house located
downstream of a damn or levee? The time and effort needs to be made to answer questions
such as these, and the means to attain them are available. The images below show areas
where flash floods many be more likely.
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The Council of State Governments. (1979). The States and Natural Hazards. Lexington KY: Author.
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http://www.today.com/id/35891373/ns/today-weather/#.USMBcY52tUQ
Protect Yourself
If you find that you are in a flood risk area, consider purchasing flood insurance for your
home, obtain a battery powered radio or TV, and develop an evacuation plan to discuss with
your family. In addition consideration should be given to putting together a Disaster Supplies
Kit. Kit basics should include: A portable, battery-powered radio or television and extra
batteries, flashlight and extra batteries, first aid kit and first aid manual, supply of prescription
medications, credit card and cash, personal identification, an extra set of car keys, matches in a
waterproof container, signal flare, map of the area and phone numbers of places you could go,
special needs. For example, diapers or formula, prescription medicines and copies of
prescriptions, hearing aid batteries, spare wheelchair battery, spare eyeglasses, or other
physical needs (Disaster Supplies Kit 1999).
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Warning Signs
The National Weather Service releases two kinds of alerts concerning flash floods. One
is a flash flood watch, which means that current conditions favor the likelihood of a flash flood
within the next 12-36 hours. The other is a flash flood warning, which means that there is a
flash flood occurring or going to occur in your area in the next 30 minutes to an hour (Disaster
Supplies Kit 1999). Flash flood alerts will typically be issued through television and radios
broadcasts and interrupt current programming and are most likely during heavy rainstorms.
Flash flooding is more prone to occur during a thunderstorm, but can also happen after a
number of days or hours of moderate rain showers.
Means of Detection
In the past there has been little historical data to rely on to measure the probability of a
flash flood. Prior to the 1980’s few rivers and basins were gauged and continually monitored for
water rise and there was little known of the initial conditions of soil moisture and retention.
Over the past 10 years, lead times for flash flood warnings have increased by more that 40
minutes and the probability of detection has more than doubled (Sokich 1999).
A multitude of joint organizational programs have made the increase in flash flood detection
possible. Studies in precipitation forecasting such as project KONRAD, and in hydrological
modeling like projects NAXOS-PRAEDICT and PAI-OFF are three such programs. There is also a
study for flash flood management in urban areas known as project URBAS (Meon 2006). These
studies have made flash flood prediction and detection faster and more accurate. New
observation systems include satellites, weather radar (NEXRAD Doppler Radar), and velocity
sensors in combination with gauge station data to facilitate the evolution of forecasting models.
The utilization and 24 hour monitoring of these information and data collection tools is the
source for flash flood alerts.
The Advanced Hydrologic Prediction Service (AHPS) is a joint organizational effort led by the
National Weather Service to collect and disseminate the most current and accurate flood and
flash flood information across the country. The AHPS endeavors to continually monitor and
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evolve its means of predicting and alerting the public of a potential or impending flash flood
and strives to answer the following questions: “How high will the rivers rise? When will the
river reach its peak? Where will the flooding occur? How long will the flood last? How long
will the drought last? How certain is the forecast?” (Weather Predictions for Life Decisions).
Campaign
The NOAA National Weather Service and U.S Department of Commerce have adopted
the “Turn Around Don’t Drown (TADD)” campaign to keep
people from walking and driving through floodwaters. This
campaign provides information to educate the public
about the hazards associated with flooding and flash
flooding. TADD urges people to walk or drive away any
time they come to a flooded road no matter how low the
water appears to be; the safest thing to do is turn around
and find another way. Other than finding alternate means around flooded areas TADD advises
that the safest thing to do is avoid flooded areas and get to higher ground (Flooding Ahead Turn
Around Don’t Drown). A few organizations that are collaborating to share, and fund the
dissemination of reliable information to the general public includes: NOAA, NWS, Federal
Emergency Management Agency, and the American Red Cross. The TADD campaign is just one
effort to spread knowledge that can help people stay safe from floods and flash floods. Funds
are available to finance further research and efforts to investigate prediction means and
management tools for flash flooding. Currently, colleges such as the University of Colorado,
Colorado Springs and Technical University Braunschweig, Germany are receiving funding to
conduct research in the area of flash floods. The Hydraulic Research Center is a non-profit
research corporation that has acquired over $2 million from private and public sectors for their
flash flood initiative in developing countries. Disaster Relief Emergency Fund (DREF), the
Adaptation Fund, and the National Science Foundation working with National Center for
Atmospheric Research (NCAR) are three additional organizations, working with, many others to
fund additional research with a desire to advance flash flood prediction technologies.
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Summary:
Flash floods are becoming more of a hazard to a great number of people. This natural
disaster is only going to become an increasing concern in the future and needs to be addressed
accordingly. Those who live in flood plains and near rivers need to understand the risks they
are taking by living in such areas. The general public should be made aware of the awesome
power of water and give it the respect and wide berth it deserves. The death toll that flash
floods leave in their wake is avoidable through communication. To effectively spread
knowledge of flash flooding and its hazards, the attitude of invincibility needs to be overcome.
There is an enormous amount of funding going into and making information available
concerning flash floods, but people don’t seem to care and access it until it is too late. Notably,
the National Oceanic and Atmospheric Administration and the National Weather service, have
been making considerable efforts to help the U.S. population understand the risks of flash
floods. Flash flooding cannot be prevented everywhere and in many places they are
unavoidable, but the effects they bring can be dampened and managed.
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