These dangerous and deceptively risky weather events can be hardto predict. Here’s what you need to know to weather the storm.

Flash Floods in IllinoisStory By Stanley A. ChangnonPhotos By Paul Osman

Flash floods are one of the majorweather-caused dangers in Illi-nois. They serve as threats tohunters, boaters, campers oranyone caught along a river

course when the flooding occurs. Onaverage, floods in Illinois cause six

deaths a year. Flood damage producesproperty losses that average $257 mil-lion a year—larger than losses from anyother weather extreme—and the Illi-nois flood loss amount is the sixth-high-est state flood loss value in the nation.

What Are Flash Floods?Most flash floods result from sudden,

intense, short-duration rainstorms andaccount for 58 percent of all floods inthe state.An occasional break of a river ice

jam can cause a downstream flashflood, but this is an infrequent event. Inaddition to flash floods, Illinois experi-ences flooding from spring snow meltafter winters with heavy snows andfrom prolonged periods (weeks andmonths) of moderate to heavy precipi-tation, as occurred in 1993.

The intense rainstorms that causeflash floods occur sufficiently often tocreate 15 to 20 flash floods a year in Illi-nois. River basins, such as the BigMuddy, Sangamon, Vermilion, LittleWabash and Rock rivers, average one ortwo flash floods each year. Flash floodsoccur mostly during the April-Septem-ber period and are most frequent inJune and July.Two types of heavy rain events cre-

ate flash floods.One type consists of a difficult-to-

forecast, late-afternoon thunderstormthat develops in a near-stationary atmos-

Intense rainstorms causing flash

floods typically occur 15-20 times

a year in the Prairie State.

14 / OutdoorIllinois July 2009

July 2009 OutdoorIllinois / 15

pheric environment. A few more con-vective clouds merge with it and heavyrainfall typically begins around 6 p.m.The storm often lasts three to fourhours and often produces 6 to 8 inchesof rain. The area with moderate (1inch) to heavy rainfall usually covers500 to 650 square miles. The rainfallpattern produced usually has a west-east orientation with an elliptical shape,being 80 to 100 miles long and 40 to 50miles wide.The other type of flash-flood-produc-

ing rainstorm in Illinois is larger andtends to evolve from a large thunder-storm in a slowly moving squall line

with four or five storm cells ultimatelymoving from west to east across thefirst storm’s area. The resulting peakrainfall is usually 8 to 10 inches. Rainfallin excess of 1 inch typically falls over2,500 square miles, and the storm hasan elliptical shape, being 200 to 250miles long on a west-east orientationand 90 to 100 miles wide.Heavy rains quickly fall at the begin-

ning of both types of storms. Floodwater rapidly accumulates and beginsmoving downstream and along rivercourses. Some of the water accumu-lates in flat crop lands. The amount offlooding depends on pre-storm condi-

tions. If it has been dry, then pre-floodstream levels are low, and soils are ableto soak up some of the heavy rain.

What are the Impacts?One of the major dangers of flash

floods is that they are created at nightwhen visibility is poor and most personsare asleep and unaware of the event.The exact locations where the localizedsevere rainstorms develop are difficultfor meteorologists to forecast. Hence,flash flood forecasts are seldom, but

The majority of flash floods

occur at night when people are

asleep and unaware of developing

dangerous conditions.

Property losses due to flood

damage are higher than

those resulting from any other

weather-related event.

16 / OutdoorIllinois July 2009

intense rainfall is seen on weather radarsand, once in progress, reports of incipi-ent flood conditions can and are issued.Unfortunately, this often is late at nightwhen most in danger are asleep.Most deaths from flash floods are to

persons caught in autos, often trying todrive through a flooded street/road,campers or persons entering basementsof flooded homes.Environmental damages from flash

floods include uprooted trees and vege-tation, and sizable soil erosion. Not only

do these losses occur in the area wherethe storm occurs, but also along streamcourses draining the rapidly flowingflood waters.Property damages, including damage

to vehicles, can be sizable, particularlyto property in the heavy-rain area andalong the stream courses carrying floodwaters. Insured property losses fromflash floods in Illinois average $134 mil-lion annually. Much of this is a result offlood damage to houses and businesses.Flash floods in the Chicago metropoli-tan area also have done considerabledamage to vehicles and railroads, andled to the stoppage of traffic. In thespring and summer of 2008, flashfloods caused railroads in Iowa and Illi-nois to experience $123 million in loss-es due to damage to rail lines andbridges, and resultant train wrecks.Revenue losses included those fromdelayed train movements.Streets, roads and viaducts often are

badly flooded from flash floods, stop-ping traffic and damaging parked vehi-cles. In some instances, bridges andboats are damaged and considerablecrop damage occurs from flooded fieldsthat drain slowly.

What Can Be Done to ProtectLife and Property?Forecasts of flash-flood storms sel-

dom are issued in advance of a storm-flood event; however, warnings ofheavy rains and flooding can be issuedonce the event is in progress. Thismeans that one must utilize the media(radios and/or TV) to get warnings.Outdoor enthusiasts are advised tokeep a radio available.Having structures in flood plains can

be a dangerous situation. Propertylocated along or near streams and riversshould be covered by flood insurance.Never attempt to drive a vehicle

through a flooded city street, a viaductor a country road as the water may bedeeper than expected and the fast-mov-ing water may quickly push a car ortruck off a road. Also, avoid enteringflooded basements as they may bedeeper than realized, and electrical cir-cuits can lead to electrocutions.

Stanley A. Changnon is a senior scientist

with the Illinois State Water Survey,

Institute of Natural Resource Sustainabili-

ty at the University of Illinois in Urbana.

Deaths as a result of flash floods

usually are persons trapped in a

car or camper or those entering

the basement of a flooded home.

Recreational activities are popular

along Illinois’ rivers. Outdoor

enthusiasts should remain alert to

changing weather conditions.

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