Flooding

Floods are one of the most common hazards in the United States, however not all floods are alike. Some floods develop slowly, while others such as flash floods, can develop in just a few minutes and without visible signs of rain. Additionally, floods can be local, impacting a neighborhood or community, or very large, affecting entire river basins and multiple states.

Flash floods can occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a sudden release of water held by an ice jam. Flash floods often have a dangerous wall of roaring water carrying rocks, mud and other debris. Overland flooding, the most common type of flooding event typically occurs when waterways such as rivers or streams overflow their banks as a result of rainwater or a possible levee breach and cause flooding in surrounding areas. It can also occur when rainfall or snowmelt exceeds the capacity of underground pipes, or the capacity of streets and drains designed to carry flood water away from urban areas.

Be aware of flood hazards no matter where you live or work, but especially if you are in low-lying areas, near water, behind a levee or downstream from a dam. Even very small streams, gullies, creeks, culverts, dry stream beds or low-lying ground that appear harmless in dry weather can flood.

Flooding in Wisconsin

Although Wisconsin does not have exceptionally steep terrain, mountain slopes, or low-lying coastlands, significant areas of the state are flooded every year. Flooding in Wisconsin is generally caused by the accumulation of excessive surface runoff in low-lying flat areas or the overflowing of rivers and lakes. Routine annual flooding poses a danger to human life and safety, causes significant damage to property and infrastructure, and negatively affects the state’s economy. From 1990 to 2008, Wisconsin experienced eight flood-related fatalities¹ and countless injuries caused by responding to and recovering from flood events. Flooding in southwestern Wisconsin in 2008 was responsible for property damage, agricultural losses, and business losses with an estimated value of $764 million to $1 billion.2 Based on these data, preparing for flood events remains a priority for Wisconsin governmental units, citizens, and businesses.

Floods in Wisconsin

Floods in Wisconsin – Number of Floods, Injuries, and Fatalities by County 1982-2012

Major Causes of Flooding in Wisconsin

Flash Floods

Flash floods are exactly what the name suggests: floods that happen in a flash! Flash floods generally develop within 6 hours of the immediate cause. Causes of flash flooding include heavy rain, ice or debris jams, and levee or dam failure. These floods exhibit a rapid rise of water over low-lying areas. In some cases, flooding may even occur well away from where heavy rain initially fell. This is especially common in the western United States where low lying areas may be very dry one minute, and filled with rushing water from upstream the next.

There are many reasons that flash floods occur, but one of the most common is the result of copious amounts of rainfall from thunderstorms that cause flash flooding. This can also occur when slow-moving or multiple thunderstorms move over the same area. These sudden downpours can rapidly change the water levels in a stream or creek and turn small waterways into violent, raging rivers. Urban areas are especially prone to flash floods due to the large amounts of concrete and asphalt surfaces that do not allow water to penetrate into the soil easily.

Steep, hilly, or mountainous terrain produces rapid runoff and quick stream response, since the water will travel downhill at greater speeds into rivers and over land. Rocky terrain can exacerbate the development of flash floods and raging waters since rocks and clay soils do not allow as much water to infiltrate the ground. Steep, narrow valleys generate rapidly flowing waters that can quickly rise to considerable depth. For instance, a mountain creek that is usually only 6 inches deep can swell to a 10-foot depth in less than one hour.

July 23, 2010 Flash Flooding closes Milwaukee General Mitchell Airport

River Flooding

River flooding occurs when river levels rise and overflow their banks or the edges of their main channel and inundate areas that are normally dry. River flooding can be caused by heavy rainfall, dam failures, rapid snowmelt and ice jams. The National Weather Service issues Flood Warnings for designated River Forecast Points where a flood stage has been established.

River flooding is classified as Minor, Moderate, or Major based on water height and impacts along the river that have been coordinated with the NWS and local officials. Minor river flooding means that low-lying areas adjacent to the stream or river, mainly rural areas and farmland and secondary roadways near the river flood. Moderate flooding means water levels rise high enough to impact homes and businesses near the river and some evacuations may be needed. Larger roads and highways may also be impacted. Major flooding means that extensive rural and/or urban flooding is expected. Towns may become isolated and major traffic routes may be flooded. Evacuation of numerous homes and business may be required.

There is an additional level of flooding known as record flooding. In many cases this falls into the major flood category, but it doesn’t have to. A record flood is simply one where the water reaches a level higher than it ever has been recorded before. Therefore, record flooding can cause extensive damage or even no damage or other negative impacts at all.

FEMA - 45683 - DR-1944-WI.jpg
Black River Falls, WI, November 5, 2010- Brockway Road is collapsed and totally breached by September rains. “FEMA – 45683 – DR-1944-WI” by Ed Edahl – This image is from the FEMA Photo Library.. Licensed under Public domain via Wikimedia Commons.

Ice Jams

Ice jams are common during the winter and spring along rivers, streams and creeks in the higher latitudes of the continental U.S. as well as in Alaska. Many of the record flood events along major rivers in Alaska are the result of ice jams Debris jams can occur at any time of year and have the same implications as an ice jam. As ice or debris moves downstream, it may get caught on any sort of obstruction to the water flow. When this occurs, water can be held back, causing upstream flooding. When the jam finally breaks, flash flooding can occur downstream.

Typically, an ice jam is resolved when the ice melts. With debris jams, the options are to take measures to remove the jam or wait for the debris to break free. In addition to causing flooding, these jams may also have economic and ecological implications. They might delay or suspend navigation along a waterway, affect hydropower operations, or cause damage to vessels. Jams can cause riverbank erosion, impede migration of aquatic creatures and adversely impact wildlife habitats. Loss of life has also been attributed to flooding caused by ice and debris jams.

Snowmelt

Snowmelt and the breakup of river ice often occur at about the same time. Ice jams often form as a result of the sudden push exerted on the ice by a surge of runoff into the river associated with snowmelt. Ice jams can act as dams on the river that result in flooding behind the dam until the ice melts or the jam weakens to the point that the ice releases and moves downstream. A serious ice jam will threaten areas upstream and downstream of its location. Six inch thick ice can destroy large trees and knock houses off their foundations. Once an ice jam gives way, a location may experience a flash flood as all the water and debris that was trapped, rushes downstream.

Snowmelt flooding occurs when the major source of water involved in a flood is caused by melting snow. The northern tier states and mountainous areas of the U.S. are particularly susceptible to snowmelt flooding. Unlike rainfall that can reach the soil almost immediately, the snowpack can store the water for an extended amount of time until temperatures rise above freezing and the snow melts. This frozen storage delays the arrival of water to the soil for days, weeks, or even months. Once it begins to melt and does reach the soil, water from snowmelt behaves much as it would if it had come from rain instead of snow by either infiltrating into the soil, running off, or both. Flooding can occur when there is more water than the soil can absorb or can be contained in storage capacities in the soil, rivers, lakes and reservoirs.

High soil moisture conditions prior to snowmelt can contribute to snowmelt flooding. Rainfall during the late fall is particularly important because there is less evapotranspiration and less time for the soil to drain and dry before it freezes. Ground frost or frozen soil is another contributor. Deep, hard ground frost prevents snowmelt from infiltrating into the soil. Cold temperatures prior to heavy snowfall and normal or above normal soil moisture contribute to this.

Deep snow cover can worsen snowmelt flooding since there is more water stored and available for snowmelt. Also, when snow cover is widespread, it usually keeps air temperatures cooler and delays spring warming, which increases the potential for more rapid snowmelt. Rain falling while snow is still on the ground contributes more water for flooding and helps to melt the snowpack, thus rain-on-snow events are watched carefully.

Most often, snowmelt is a relatively slow phenomenon. Snowmelt rates are usually comparable to light or moderate rainfall.  Important exceptions to this can occur, especially during unusually warm periods with high dew point temperatures, and when nighttime temperatures remain above freezing. Snowmelt rates can be much higher than normal under these conditions, which can increase the risk of snowmelt flooding.

Dam Failures

According to FEMA and the National Inventory of Dams (2007), there are more than 80,000 dams in the United States. Approximately one third of these pose a “high” or “significant” hazard to life and property if failure occurs. Dam failure or levee breaches can occur with little warning. Intense storms may produce a flash flood in a few minutes or hours while other failures and breaches can take much longer to occur, from days to weeks.

Causes of dam failure vary from natural causes such as prolonged rainfall, landslides, earthquakes, or erosion to human causes such as improper maintenance and design, negligent operation, or sabotage and terrorism. Dam failures are categorized into three groups: overtopping, in which the water level exceeds the top of the dam; excessive seepage, in which water seeps through the ground; and structural failure, where part of the dam doesn’t complete its job sufficiently.

New Dell Creek Channel draining Lake Delton with wrecked homes.jpg
New Dell Creek Channel draining Lake Delton with wrecked homes” by U.S. Air Force photo by Master Sgt. Paul Gorman/Released – www.defenseimagery.mil 080609-F-6967G-196. Licensed under Public domain via Wikimedia Commons.

Turn Around … Don’t Drown

Turn Around Don't Drown

Turn Around Don’t Drown

The Centers for Disease Control report that over 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 ear flood waters. Why? The main reason is people underestimate the force and power of water. Many of the deaths occur in automobiles as they are swept downstream. Of these drownings, many are preventable, but too many people continue to drive around the barriers that warn you the road is flooded. Most flood-related deaths and injuries could be avoided if people who come upon areas covered with water followed this simple advice: Turn Around Don’t Drown®.

The reason that so many people drown during flooding is because few of them realize the incredible power of water. A mere six inches of fast-moving flood water can knock over an adult. It takes only two feet of rushing water to carry away most vehicles. This includes pickups and SUVs.

If you come to an area that is covered with water, you will not know the depth of the water or the condition of the ground under the water. This is especially true at night, when your vision is more limited.

Play it smart, play it safe. Whether driving or walking, any time you come to a flooded road, Turn Around Don’t Drown®.

Prepare for Floods

BEFORE A FLOOD (When Flooding is Forecast)

Flooding Myths and Facts

Flooding Myths and Facts

Be alert.

  • Monitor your surroundings.
  • Monitor NOAA Weather Radio, local television and radio stations, or go to www.weather.gov.

If a flash flood warning is issued for your area: Climb to safety immediately.

  • Flash floods develop quickly. Do not wait until you see rising water.
  • Get out of low areas subject to flooding.
  • If driving, do not drive through flooded roadways!

Assemble disaster supplies:

  • Drinking water – Fill clean containers.
  • Food that requires no refrigeration or cooking.
  • Cash.
  • Medications and first aid supplies.
  • Clothing, toiletries.
  • Battery-powered radio.
  • Flashlights.
  • Extra batteries.
  • Important documents: insurance papers, medical records, bank account numbers.

Be prepared to evacuate.

  • Identify places to go.
  • Identify alternative travel routes that are not prone to flooding.
  • Plan what to do with your pets.
  • Fill your car’s gas tank.
  • If told to leave, do so quickly.

Review your Family Disaster Plan.

  • Discuss flood plans with your family.
  • Decide where you will meet if separated.
  • Designate a contact person who can be reached if family members get separated. Make sure every family member has the contact information.

Protect your property.

  • Move valuables and furniture to higher levels.
  • Move hazardous materials (such as paint, oil, pesticides, and cleaning supplies) to higher locations.
  • Disconnect electrical appliances. Do not touch them if you are wet or standing in water.
  • Bring outside possessions indoors or tie them down securely. This includes lawn furniture, garbage cans, and other movable objects.
  • Seal vents to basements to prevent flooding.

DURING A FLOOD

Be alert.

  • Monitor your surroundings.
  • Monitor NOAA Weather Radio, local television and radio stations, or go to www.weather.gov.

Don’t drive unless you have to.
If you must drive, travel with care.

  • Make sure your vehicle has enough fuel.
  • Follow recommended routes. DO NOT sightsee.
  • Avoid disaster areas. Your presence might hamper rescue or other emergency operations and put you at further risk.
  • Watch for washed out roads, earth slides, and downed trees or power lines.
  • Be especially cautious at night, when it is harder to recognize flood dangers.
  • If the vehicle stalls, abandon it.
  • If water rises around your car, leave the vehicle immediately. Climb to higher ground as quickly as possible.

NEVER drive through flooded roadways. STOP! Turn Around Don’t Drown.

  • The roadbed may be washed out.
  • You can lose control of your vehicle in only a few inches of water.
  • Your car may float. Vehicles can be swept away by less than 2 feet of water.
  • Do not drive around a barricade. Turn around and go another way!

Get to high ground – Climb to safety!

  • Get out of low areas that may be subject to flooding.
  • Avoid already-flooded areas and do not attempt to cross flowing water.
  • Stay away from power lines and electrical wires.

Evacuate immediately, if you think you are at risk or are advised to do so!

  • Act quickly. Save yourself, not your belongings.
  • Move to a safe area before access is cut off by rising water.
  • Families should use only one vehicle to avoid getting separated and reduce traffic jams.
  • Shut off water, gas, and electrical services before leaving.
  • Secure your home: lock all doors and windows.
  • If directed to a specific location, go there.

Never try to walk or swim through flowing water.

  • If flowing water is above your ankles, STOP! Turn around and go another way.
  • If it is moving swiftly, water 6 inches deep can knock you off your feet.
  • Be aware that people have been swept away wading through flood waters.
  • NEVER allow children to play around high water, storm drains, creeks, or rivers.

Shut off the electricity at the circuit breakers.
If someone falls in or is trapped in flood water:

  • Do not go after the victim!
  • Use a floatation device. If possible throw the victim something to help them float, such as a spare tire, large ball, or foam ice chest.
  • Call 911. Call for assistance and give the correct location information.

AFTER A FLOOD

Wait until it is safe to return.

  • Monitor NOAA Weather Radio or local television and radio stations.
  • Do not return to flooded areas until authorities indicate it is safe to do so.
  • Do not visit disaster areas following a flood. Your presence may hamper urgent emergency response and rescue operations.

Travel with care.

  • Follow recommended routes. DO NOT sightsee.
  • Watch for washed out roads, earth slides, and downed trees or power lines.
  • Stay away from downed power lines.

If a building was flooded, check for safety before entering.

  • Do not enter a building if it is still flooded or surrounded by floodwater.
  • Check for structural damage. Inspect foundations for cracks or other damage.
  • Turn off any outside gas lines at the meter tank.
  • Do not enter a building that has flooded until local building officials have inspected it for safety.

Use extreme caution when entering buildings.

  • Wear sturdy shoes. The most common injury following a disaster is cut feet.
  • Use ONLY battery-powered lighting. Flammable material may be present.
  • Look for fire hazards (such as damaged gas lines, flooded electrical circuits, or submerged furnaces).
  • Check for gas leaks. If you smell gas or hear a blowing or hissing noise, open a window and quickly leave the building. If possible turn off the gas at the outside main valve. Call the gas company.
  • Report broken utility lines to appropriate authorities.
  • Check for electrical system damage (sparks, broken or frayed wires, or the smell of burning insulation). Turn off the electricity at the main circuit breaker if you can reach it without stepping in water.
  • Examine walls, floors, doors, windows, and ceilings for risk of collapsing.
  • Watch out for animals that might have entered with the floodwaters.
  • Let the building air out to remove foul odors or escaping gas.

Take pictures of the damage, both of the building and its contents, for insurance claims.

Get professional help.

  • Seek necessary medical care. Do not neglect minor wounds or illnesses.
  • Food, clothing, shelter, and first aid are available from the American Red Cross.
  • If the gas has been turned off for any reason, it must be turned back on by a professional.
  • Have an electrician check the electrical system and appliances.
  • Wells should be pumped out and the water tested for purity before drinking.

Your home is no longer a safe place.

  • Throw away medicine, food, or water that had contact with floodwaters (including canned goods).
  • If water is of questionable purity, boil drinking water for 10 minutes.
  • Restrict children from playing in flooded areas.
  • Keep windows and doors open for ventilation.
  • Pump out flooded basements gradually (removing about 1/3 of the water volume each day) to avoid structural damage.
  • Keep the power off until an electrician has inspected the system for safety. All electrical equipment should be checked and dried before being returned to service.
  • Clean and disinfect everything that got wet.
  • Service damaged sewage systems as soon as possible.

When making repairs, protect your property from future flood damage.

  • Follow local building codes.
  • Use flood-resistant materials and techniques.
  • Elevate electrical components above the potential flood height.
  • Elevate utilities (washer, dryer, furnace, and water heater) above the level of anticipated flooding.
  • Consider elevation of the entire structure.
  • Install a backflow valve in the sewer system.

 More Information

Click on the image above for an informative brochure about flood safety.

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