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Flooding on Russian River highlights a measurable exposure

By Marc Treacy  |  April 8, 2019

The near-record flooding of Guerneville, California, from the overflowing the Russian River in late February was a remarkable but foreseeable event. Extensive data supported classification of virtually the entire town as being at least at moderate risk of flooding, with the vast majority of locations at high or extreme risk.

To study the exposure faced by homeowners and businesses in and around Guerneville, aerial imagery from Geomni (a Verisk business) was used to determine which areas were flooded and examine those locations with Verisk’s flood risk assessment tool, WaterLine™. What follows is a review and analysis of this catastrophe and the lessons insurers can learn from it.

Russian River flood depth graphic

Russian River flood depth at Guerneville (Feb. 23 – Mar. 2). Source: NOAA

Russian River: An historic storm

On Monday, February 25, an atmospheric river* began moving slowly over northern and central parts of California, drenching Sonoma County and causing massive flooding of the Russian River. Some areas saw more than 10 inches of rain in as little as 24 hours, which caused the river to rise almost an inch an hour. The river crested on Wednesday, February 27, at 46.1 feet, nearly 14 feet above flood stage.

This left Guerneville, situated in a valley at a bend in the Russian River, almost totally inundated and completely isolated. Although approximately half of the 4,500 residents had evacuated, the floodwaters damaged or destroyed 2,572 homes and businesses.

Verisk analysis of Russian River flood graphic

Verisk analysis of Russian River flood in and around Guerneville. Source: Geomni and AIR

Recurring losses, damaging floods

This flood was nothing new for the Russian River or the town. Since accurate record keeping began in 1940, the river has overtopped its banks at Guerneville 38 times—meaning the town has faced damaging floods nearly every other year, on average.

This may seem staggering, but the town was built in this vulnerable location on purpose. Founded in the 1850s as a logging town, loggers took advantage of its proximity to the river to ferry great redwoods to the sawmills located downriver. The Russian courses through the Canyon Range and through the river valley where Guerneville is located. This valley, formed by earthquakes and associated geologic forces over millennia, created a unique geography that’s particularly conducive to flooding when heavy rains fall on the plains and vineyards of Mendocino and Sonoma Counties.

Distribution chart of flood risk scores for Guerneville area

Distribution of flood risk scores for the Guerneville area. Source: WaterLine. (Image courtesy of Geomni.)

Mapping flood risk

In our analysis, we geo-referenced imagery from the event provided by Geomni, digitized the observed flood extents based on this imagery, and then analyzed the WaterLine scores for all points inside that extent. For the more than 15,000 points inside the digitized flood extent, the average WaterLine score is 89, and just over 14,000 (or approximately 93 percent) of those points have a WaterLine score greater than 82, which classifies them as having high to extreme flood risk.

The remaining points within the flood extent have WaterLine scores indicating moderate to high flood risk. In addition, many of the areas just outside the flood extents also have scores that indicate this level of risk. The average WaterLine score for portions of Guerneville outside the flood extents is 75, so those areas could have flooded if the water levels had continued to or beyond their historical peak of 49.5 feet.

Questions for insurers

Verisk data and analysis show that historically, about 90 percent (and sometimes even more) of flood-damaged property locations are associated with moderate to extreme levels of risk. But even in high-risk areas such as Guerneville, many residents don’t have flood insurance, according to local news reports. Private insurers can step in to close this protection gap using tools such as WaterLine to make informed underwriting decisions.

In addition to large and high-severity losses such as the Guerneville event, floods produce other types of losses, including water damage, rot and contamination, additional living expenses due to evacuations, and business interruption. Therefore, risk management strategies—encompassing underwriting, pricing, and concentration management across both personal and commercial lines—should consider several questions:

  • Where are properties at risk located, and how can this information guide underwriting decisions?
  • How does concentration of risk within geographic areas influence loss outcomes?
  • Are premiums commensurate with the level of exposure to floods at individual property locations?

Insurers can pursue these answers without starting from scratch, relying on tools that help launch or refine private flood insurance programs. This can help identify new business opportunities and ways to better serve customers with more comprehensive coverage offered on a sound basis.


*An atmospheric river is defined by the National Oceanic and Atmospheric Administration (NOAA) as “relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics.” They are usually several thousand kilometers long, and according to two researchers at MIT, one can transport a greater flow of water than the Amazon.


Marc Treacy is managing director of flood insurance for ISO. You can contact him at MTreacy@verisk.com.