Flash Floods and Other Flooding
Two of Baltimore County's worst natural disasters—Hurricane Agnes and Tropical Storm Isabel—both did their damage through flooding.
Flash floods—caused by an intense rain event—are common in our region, because of the frequent thunderstorms that move across the area. Flash floods are extremely dangerous, largely because they catch people unaware. They are the #1 cause of weather-related deaths in the U.S.
Flash flooding occurs within six hours of the rain event. Flooding is a longer term event and may last a week or more. The precautions you should take and the safety measures you should take afterwards are basically the same regardless of the type of flooding.
The Power of Water
Water weighs 62.4 lbs. per cubic foot and typically flows downstream at six 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 pounds 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 pounds of water. In effect, the car weighs 1,500 pounds less for each foot the water rises.
Two feet of water will carry away most automobiles. Don't attempt to drive through standing water!
Especially if you live in flood-prone areas, familiarize yourself with these precautions.
Before a flood:
- Review your insurance policies to make sure you have flood insurance. Regular homeowner's insurance does not cover flood damage. You must purchase flood insurance.
- Keep your automobile fueled; if electric power is cut off, gas stations may not be able to operate pumps for several days.
- Assemble a comprehensive supply kit containing: a flashlight with extra batteries, a battery-powered radio and enough water to last for three days (one gallon per person per day).
- Keep first aid supplies on hand.
- Install check valves in building sewer traps to prevent flood water from backing up into the drains of your home.
During a flood:
- Do not attempt to drive through standing water!
- If your vehicle stalls, leave it immediately and seek higher ground. Rapidly rising water may engulf the vehicle and its occupants and sweep them away.
- Know your flood risk and elevation above flood stage. Do your local streams or rivers flood easily? If so, be prepared to move to a place of safety. Know your evacuation routes, and identify in advance where to go if you need to seek higher ground.
- Avoid walking through flowing water. If you come upon a flowing stream where water is above your ankles, STOP! Turn around and go another way.
After a flood:
- Floodwaters are polluted. Do not let children play in floodwaters.
- If fresh food has come in contact with floodwaters, throw it out.
- Boil drinking water before using. Wells should be pumped out and the water tested for purity before drinking. If in doubt, call the Health Department.
- Seek necessary medical care at the nearest hospital. Food, clothing, shelter, and first aid are available from the Red Cross.
- Do not visit disaster areas. Your presence might hamper rescue and other emergency operations.
- Electrical equipment should be checked and dried before being returned to service.
- Use flashlights—not lanterns, torches or matches—to examine buildings. Flammables may be inside.
- Report broken utility lines to appropriate authorities.
About Flash Flooding
The two key elements when it comes to flash flooding are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts. Topography, soil conditions, and ground cover also play an important role.
Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms. The Baltimore region regularly experiences these conditions.
Flash floods 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 can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels.
Rapidly rising water can reach heights of 30 feet or more. Flash flood-producing rains can also trigger catastrophic mud slides. You will not always have a warning that these deadly, sudden floods are coming. Most flood deaths are due to flash floods.
Terms You Should Know
Stay informed about possible flooding by listening to NOAA Weather Radio, commercial radio or television for the latest flash flood/flood watches, warnings, and advisories. Make sure you understand these terms:
- Flash flood or flood watch: Flash flooding or flooding is possible within the designated watch area. Be alert and ready to evacuate.
- Flash flood or flood warning: Flash flooding or flooding has been reported or is imminent. Take necessary precautions at once. Follow local authorities' instructions regarding evacuation.
- Urban and small stream advisory: Flooding of small streams, streets and low-lying areas such as railroad underpasses and urban storm drains is occurring.
- Flash flood or flood statement: This is follow up information provided after flash flood/flood event
Winds generated from tropical storms and hurricanes or intense offshore low-pressure systems can drive ocean water inland and cause significant flooding. This is the meteorological phenomenon that caused the damage following Tropical Storm Isabel.
Flooding along rivers is a natural and inevitable part of life. Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.
As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall.
Urbanization increases runoff two to six times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements can become death traps as they fill with water.
Revised April 12, 2012