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Floods After Wildfires: How One Hazard Follows Another

By Cassie Masone
Vice President, Flood Operations

According to the Insurance Institute of Building and Home Safety (IBHS), the August 2020 Complex fire in northern California has burned more than one million acres, and California has seen more than four million acres burned this year. Every time I read about wildfires – such as the August Complex fire – in the news, I think of massive tracts of land and vegetation ablaze. I think of the roar of the fire and the snapping of branches. I think of exhausted firefighters battling unrelenting flames. I think of people displaced from their homes.

And then I think of the floods that could follow.

Surprised? Wildfires and floods occurring in the same location may seem contradictory, as wildfires typically arise out of dry conditions. However, flooding is one of the most dangerous hazards that can follow a wildfire, even in places far from any rivers or lakes. Here’s why...

Normally, soil and vegetation absorb rainwater and minimize runoff. But, for up to five years after a wildfire, the charred landscape may leave the soil unable to absorb water. The earth becomes about as impermeable as a parking lot. In these conditions, as little as half an inch of rain in less than an hour can cause flash floods. Steep terrain or severe burning would require even less rain to start a flood. Such floods rise up suddenly, and their fast, forceful flows can result in extensive damage.

In normal conditions, soil and vegetation help absorb rain water.

After a wildfire, baked and charred soil and burned vegetation form a water repellent layer, blocking water absorption.

During the next rainfall, water bounces off the soil, increasing risk of flooding.

(Source: National Flood Insurance Program; Selective Insurance)

That’s why, according to a recent article in the Sonoma County Gazette, the City of Santa Rosa and the County of Sonoma in California are encouraging residents living in and around recent wildfire burn areas to prepare for the rainy season now. The City and County are working to assess and reduce the risk of flooding and to prevent fire-related debris, pollutants and sediment from being carried into storm drains, creeks and the Russian River. In high risk areas, the City and County are installing post-fire hazard signs to alert residents and visitors to these potential hazards.

There have been several examples of post-fire flooding over the years. Most recently in June 2020, the Bighorn Fire burned almost 120,000 acres north of Tucson, AZ and a few weeks later, a light rain triggered a flash flood and mud flow in the area.

If there has been a wildfire near your home or business, you should take measures to prevent loss from a subsequent flood. A great first step, if you don’t already have one, is to purchase a flood insurance policy issued through the National Flood Insurance Program (NFIP). Since NFIP flood insurance policies have a 30-day waiting period, it's best to buy one well before the need arises.

What's more, under an imminent threat of flood, your flood insurance policy may allow reimbursement for any sandbags, plastic sheeting and labor costs you incur to prevent floodwaters from entering your property. The details may vary depending on the type of flood insurance you purchase, so take a few minutes now and review your policy coverages with your agent.

The bottom line is this: When rain begins to fall on a burned-out area, be prepared for flash floods.