Expert says West Coast flooding, mudslides remain threat from multiple large winter storms

As California and the west coast stare down yet another major winter storm this week, the threat of devastating flooding and mudslides is even more urgent.

Drew Ellis, a climate science and meteorology expert at Virginia Tech, explains that in drought-stricken areas, the ground is less able to absorb large influxes of water, and this is complicated by soil saturation from other recent storms. “This makes an additional major storm that much more dangerous. And while it brings much needed water to the region, it won’t fully solve the area’s freshwater resource needs.”

“Whenever you get a major deviation from the normal amount of rainfall in a short period of time, the possibility of weather-related natural disasters goes up markedly,” says Ellis. “And the concern is much greater when these storms come in quick succession.”

Ellis shares his expertise on how winter storms are impacting drought and water supply on the west coast.

Q: What leads to increased chances of flooding?

The risk of flooding is predicated on the rate at which rainfall can infiltrate the soil. Exceedance of the infiltration rate can be caused by a high rainfall rate, or a lowered infiltration capacity due either to exposed soil from vegetation loss, or to saturation of the soil such that it has no room for additional water.

In California, El Niño years are more likely to bring more frequent rainfall events, as well as events with high rainfall rates, like atmospheric rivers. When heavy rainfall events follow recent wildfires that have increased the direct exposure of soil to rainfall, overland flow and flash flooding are much more likely.

More broadly, heavier rain events are symptomatic of climate warming – the capacity of the air to hold moisture increases exponentially with an increase in air temperature. So, we are generally seeing greater precipitation volatility in recent years.

Q: What areas are particularly susceptible to mudslides?

Heavy rain on areas of steep, complex topography – such as that of the western United States – is a recipe for overland flow and flash flooding that is capable of moving soil, vegetation, and rocks downslope. The reduced protective and anchoring vegetation from wildfires that occur during the frequent drier periods of the more arid western U.S. climate often accentuates this.

Q: What are the biggest vulnerabilities for infrastructure during these events?

The infrastructure most vulnerable to flooding events in California are hillside homes and roadways – the power of flowing water being easily enough to scour away sections of roads and breakaway homes from foundations.

Q: What are some warning signs people can look out for, or areas they should avoid?

A positive for California is that the heavy rain-producing weather systems are very large in scale, so they can be anticipated and forecast days in advance. This affords time for preparation. During an event, it’s critical to reduce travel and obey travel restrictions.

Among the complex topography of California, flooding is largely confined to areas susceptible to channeled flow, with gravity taking the water to immediate areas of lesser elevation. These preferred areas are semi-permanent, so experience with previous flooding is valuable to help anticipate a coming flood event.

Q: How else can those impacted try to prepare in advance?

If in a flood-prone area, prepare for a quick evacuation, and plan ahead on where to go and the route to take. Reducing threats to property can be more difficult, as the power of water is hard to combat. In the case of minimal flooding, prepositioning resources for sandbagging is helpful. In the case of severe flooding, it’s important to prepare high-valued possessions and important documents for relatively quick evacuation.

About Ellis
Professor Andrew Ellis teaches meteorology and climate science in the Department of Geography within Virginia Tech’s College of Natural Resources and Environment. His research focuses on snowfall variability, understanding and monitoring the occurrence of drought, and assessing the sustainability of freshwater resources in arid and semi-arid climates.

Schedule an interview
To schedule an interview with Drew Ellis, contact Noah Frank in the media relations office at nafrank@vt.edu or 805-453-2556.