Cedar Rapids, Iowa, June 13, 2008 (David Greedy/Getty Images)

Floods

Learning Objectives

Of all the natural disasters we study this semester, the one you are most

likely to be impacted by is flooding. Floods happen in all 50 states and

in all climates, in all kinds of landscapes, and at all times of the year.

Your goals in studying this chapter are to:

• Learn the impact flooding has on the United States each year.

• Learn what agencies monitor and issue flood warnings.

• Understand the types of floods and their causes.

• Understand river systems and how they evolve over time.

• Learn how to recognize a floodplain.

• Understand how floods are monitored and measured.

• Understand what a “100-year flood” is, and what it is not.

• Understand the pro’s and con’s of various flood mitigation measures.

• Understand how other factors affect flooding.

(NPS)

Flooding is a coast-to-coast threat to the United States and its

territories in all months of the year. Melting snow can combine with

rain in the winter and early spring; severe thunderstorms can bring

heavy rain in the spring and summer; or tropical cyclones can bring

intense rainfall to coastal as well as inland states in the summer and

fall.

Flooding typically occurs when prolonged rain falls over several days,

when intense rain falls over a short period of time, or when an ice or

debris jam causes a river or stream to overflow onto the surrounding

area. Flooding can also result from the failure of a water control

structure, such as a levee or dam. The most common cause of flooding

is water due to rain or snow melt that accumulates faster than soils can

absorb it or rivers can carry it away.

Three fourths of all presidential disaster declarations are associated

with flooding. Year in and year out, flooding has a significant

economic impact on the Nation. In most years it causes more damage

in the United States than any other weather-related event – an average

of $5.3 billion a year for the 30-year period, 1975-2004. During this

same period, an annual average of 93 people lost their lives due to

flooding. In 2005, flooding from Hurricanes Katrina and Rita took

nearly 2000 lives and caused tens of billions of dollars in damages,

dwarfing the long-term averages.

Because flooding can occur any place and at any time, NOAA’s

National Weather Service (NWS) monitors conditions that lead to

flooding 24 hours a day, 7 days a week, and issues forecasts, watches

and warnings.

Web Resources fema.gov ready.gov/floods floodsmart.gov usgs.gov/water

NOAA text modified and supplemented by BYUI faculty

NWSRFS also provides estimates of soil moisture, which is a critical

factor in predicting flash flooding. Soil type and moisture content

determine how much water the ground can absorb and are used to

develop what is known as flash flood guidance. When rainfall is

expected to exceed flash flood guidance, flooding can be

expected. NEXRAD precipitation estimates, combined with detailed

flash flood thresholds, are used in an application known as flash flood

monitoring and prediction, which has increased the precision and lead

time for flash flood forecasting.

Flash flooding typically occurs when intense rains fall faster than the

ground can absorb it. Flash floods do not always affect streams, but

can impact low-lying areas and urban areas where drainage systems

can be overwhelmed by torrential rains.

In northern areas, snow accumulates over the winter. This reservoir of

water is released when it melts. If accompanied by warm rains, the

snowmelt and rain can combine to cause flooding. In flat areas without

sloping terrain to remove it, snowmelt alone can cause

flooding. Knowing the amount of water stored in the snow pack is key

to providing accurate snowmelt flood forecasts. The NWS’ National

Operational Hydrologic Remote Sensing Center (NOHRSC) uses

satellite and ground-based snow observations in a snow modeling

system to produce detailed snow water equivalent estimates. In areas

of particular concern, the NOHRSC deploys aircraft with gamma ray

sensors that provide high quality estimates of snow water

equivalent. This information is used by a component of NWSRFS to

provide snow melt forecasts.

Aircraft used to perform snow surveys

In the case of flash flooding, rapid dissemination of these forecasts, watches and warnings is especially

critical. NOAA Weather Radio provides up-to-the-minute flood warnings. Receivers can be set to provide audible

alarm even when they are turned off. This technology is critical to saving lives, particularly during nighttime

disasters.

In addition, NWS works closely with national, state and local emergency managers to disseminate forecasts and

warnings as well as to support their flood response activities. Another key partner in communicating weather

threats is the media, including television, radio and the Internet.

Unfortunately, most flood fatalities are not due to limitations in the forecast system. All too often, people in

vehicles literally drive into harm’s way. As little as two feet of water can float an average car. Currently

popular sports utility vehicles (SUVs) require only slightly more water to be swept away. While it may appear that

water is not deep enough to cause problems, there is almost no way of knowing if the roadbed itself has been

eroded or undermined. The NWS urges people to AVOID WATER, no matter how benign it may look: Turn

Around, Don’t Drown.

Forecast information is provided to 122 NWS Weather Forecast Offices that translate the forecast guidance into

watches and warnings tailored to the local areas they serve.

(NOAA)

Flash flood videos

Types and Causes of Floods

The variety of floods is as diverse as the nation’s natural resources, ranging from localized flash flood covering

several city blocks to massive flooding encompassing up to a quarter of the area of the lower 48 states. Causes and

characteristics of flooding include:

Flash floods: A flash flood is one that arises quickly. Flash floods are typically caused by short, intense rainfall

over areas as small as a city to larger than a state. Flash flooding risks can occur well away from rivers and

streams. Water pools in low spots such as underpasses and basements because rain falls faster than the ground can

absorb it. Year in and year out, flash floods take more lives than any other type of flooding – most flood fatalities

involve motorists. The cumulative effect of widespread, prolonged flash flooding can lead to flooding of smaller

streams and, if enough rain falls, major river systems.

River flooding: When runoff from widespread rain events flows into rivers and streams faster than the water moves

downstream, excess water causes flooding along these waterways. Once a river reaches flood stage, flood severity

categories are used by the NWS to communicate the expected flood impact:

Minor Flooding: minimal or no property damage, but possibly some public threat or inconvenience.

Moderate Flooding: some inundation of structures and roads near streams. Some evacuations of

people and/or transfer of property to higher elevations are necessary.

Major Flooding: extensive inundation of structures and roads. Significant evacuations of people

and/or transfer of property to higher elevations.

Tropical cyclones: Hurricanes consistently capture the attention of the nation because of their destructive power. While the effects of extreme

winds and coastal storm surge flooding are widely recognized, the potential for severe flooding inland is not always appreciated. Rainfall is

typically heavier with slower moving storms that allow heavy rain to persist over a location. In fact, weaker systems, while often not the focus of

intense attention, can lead to some of the most serious flooding associated with tropical systems. Inland freshwater floods accounted for more

than half (59%) of U.S. tropical cyclone deaths over the past 30 years.

Coastal flooding: Driving winds associated with hurricanes pushes water inland, causing storm surges that can exceed 20 feet, causing

devastation in coastal areas. Storm surges can be caused by any strong storm system that moves slowly. We will study these more in our lesson

on hurricanes. Severe coastal erosion and flooding are threats along all coastlines.

Snow melt floods: Snow melts slowly enough that, by itself, it seldom causes flooding in most parts of the country. However, warm, moist

conditions and heavy rain can combine with snow melt to cause dramatic winter and spring flooding. In relatively flat areas in the Midwest, river

beds drop very slowly along the length of the river. As a result, the water in the river glides slowly downstream. In such areas, accumulation of

melt water from extensive snow-covered areas can cause significant flooding. This situation is often compounded by the effects of ice jams.

(State of California)

An alluvial fan is a fan-shaped apron of gravel deposited by flash floods and debris flows at the mouth of a canyon. Flood water velocity at the canyon mouth can be extreme, transporting large boulders onto the fan and posing a significant hazard to anyone or anything on the fan.

Ice jam floods: When rivers clogged with ice rise rapidly due to rainfall and/or snowmelt, the ice breaks up into chunks, some larger than an

automobile. These chunks of ice move downstream and can jam at constrictions in rivers such as bends, bridge abutments or shallow

areas. The effect is much like a traffic jam that occurs if travel lanes are closed due to an accident. The ice jam can act as a dam, causing

water to back up behind it. River levels behind the ice jam can rise rapidly. On occasion, the ice jam can release quickly, sending huge

chunks of ice downstream in the torrent, destroying everything in its wake.

Structural failure: Flooding is sometimes also caused by dam breaks or levee failures, resulting in a sudden release of water.

Mudflow: Once the smoke clears from a wildfire, the danger is not over. Because of the lack of vegetation, moderate amounts of rain can

cause flash floods and debris flows – or mudflows – on burned hill slopes. The powerful force of rushing water, soil, and rock, both within the

burned area and downstream, can destroy culverts, bridges, roadways, and structures, and can result in injury or death.

Slot canyons: Slot canyons are formed over the centuries by water eroding narrow slits into sandstone. These are common in the U.S. desert

southwest. Some are more than 100 feet deep and so narrow that one has to turn sideways to pass through them. Their uniqueness and beauty

make them an increasingly popular destinations for outdoor lovers. They can also be death traps, as water, sometimes falling miles away, can

surge though them with ferocious power.

(NOAA)

Ice jam flood in New York.

Minor flash flood in a slot canyon, Arizona.

(State of Arizona)

Ice jam flood videos