What is flash flooding? What is the risk?
Ask someone which weather events cause them the most concern, and the top answers are bound to include lightning, tornadoes, and hurricanes. However, according to the National Weather Service, it’s floods that pose the greatest threat to life and limb. Consider the national 30-year average of deaths for each:
- Lightning – 41 deaths/year
- Tornadoes – 68 deaths/year
- Hurricanes – 45 deaths/year
- Flooding – 88 deaths/year
Flash floods are a particularly dangerous breed of flood because of the speed in which they develop and the tendency of people to underestimate their risk.
What is the difference between a flood and a flash flood?
The National Weather Service defines a flash flood as flooding that begins within six hours, and often three hours, of heavy rainfall or another floodwater source. It’s this rapid onset (that’s why it’s called a flash flood) that distinguishes this hazard from other floods. Other types of flooding can develop more slowly and last days or even weeks.
Flash flood hazards
Flash floods don’t just happen quickly, they can also move quickly. It’s these fast-moving currents of water that pose great danger to people and property. There’s a lot of power in moving water. Flash floods can easily move at 9 feet per second, and that flow is strong enough to move a 100-lb rock. It only takes six inches of fast-moving flood water to knock over a person or stall a car, and twelve inches of flowing water can sweep a vehicle away. It’s no wonder that almost half of flash flood deaths are vehicle-related.
What causes flash floods?
There are two sides to the flash flood equation: how quickly water moves in, and how quickly it drains out. The causes of flash floods impact one or both of these variables.
You don’t need a hurricane to cause a flood. Sudden or persistent rainfall from a front of thunderstorms can deliver enough water to start a flash flood. Ohio’s Independence Day Flood of 1964 is a prime example. Storms from Lake Erie parked themselves over Northeast Ohio, dropping 10 inches of rain overnight. The resulting floods left 41 people dead and destroyed 10,000 homes.
Storms can also impact the outflow of water. Depending on the soil type, the ground can become saturated by rainfall, impeding drainage and causing water to pool and runoff across surfaces instead of percolating downward.
When rivers freeze in the winter, chunks of ice can break off and block the flow of water. This can cause a flash flood upstream because the water pools behind the blockage and inundates the surrounding land. Once the ice dam breaks, the accumulated water can rush downstream, creating additional flash floods. Also, frozen ground doesn’t absorb water easily, which compounds the flooding problem by slowing drainage.
Levee and Dam Failure
These man-made structures are built to control and limit the flow of water. When they fail, water can pour through the breach, causing flash flooding. The levees protecting New Orleans failed in 2005 during Hurricane Katrina, flooding 80% of the city.
The wrath of 2021’s Hurricane Ida was amplified by the inability of New York’s and New Jersey’s sewage system to handle the intense rainfall. Improvements had been made since Superstorm Sandy, but most upgrades were meant to battle wind-driven flooding from the coasts, not flash floods due to rainfall from above.
While improvements to city infrastructures is essential to mitigate future floods, urban development can also cause flash floods. Increasing the acreage of impervious surfaces – think sidewalks, parking lots, and anything covered in concrete or asphalt – removes opportunities for water to drain naturally into the ground.
Who is most at risk for a flash flood?
A flash flood can happen almost anywhere: in urban and rural areas, along the coast and in the middle of the desert. FEMA designates special flood hazard areas in which flood insurance is mandatory. However, flash floods can impact low and moderate risk zones, as well. From 2015 to 2019, 40% of NFIP claims were for properties located in these lower risk areas. Because of the unexpected nature of flash floods, it may be worthwhile to talk with your clients about the benefits of protecting their property with a flood insurance policy.
Is flash flooding included in the definition of flood in private flood insurance?
The EZ Flood policy available from Aon Edge defines a flood in the same manner as the NFIP, and one of the water sources in that definition is “unusual and rapid accumulation or runoff of surface waters from any source”. Flash floods caused by heavy rainfall can find your clients wherever they live, so you may want to talk to them about a private flood insurance policy to help protect them from a potentially devastating event.
Do you have questions about Aon Edge or EZ Flood? Check out our FAQ or contact us to learn more.
This article is provided for general informational purposes only and is not intended to provide individualized advice. All descriptions, summaries or highlights of coverage are for general informational purposes only and do not amend, alter or modify the actual terms or conditions of any insurance policy. Coverage is governed only by the terms and conditions of the relevant policy.