Fire and Then Flood: Why Wildfires Create Further Danger
It is the kind of thing that keeps emergency managers awake at night. If there is fire, then there is often flood. And it is not just any kind of flood, but often intense fast-moving mudflows that sweep through areas causing cataclysmic damage. It happens in communities around the world and is actually testament to the solidifying power of trees. It also shows what happens to the land when they are destroyed.
The risk of a flood and debris flow after a fire increases due to vegetation loss and soil exposure, according to the U.S. Geological Society (USGS). Well documented instances, particularly in coastal California, show how powerful post-fire floods can be.
“The flows are a risk to life and property because they can occur with little warning, can exert great impulsive loads on objects in their path, and may strip vegetation, block drainage ways, and damage infrastructure,” per the USGS.
As little as seven millimeters or 0.3 inches of rainfall in 30 minutes has triggered dangerous debris flows in California. As the USGS noted, any storm that has greater than 10 millimeters or 0.4 inches per hour presents the risk of creating debris flows.
The threat does begin to dissipate with the passage of time. The largest and most dangerous debris-flow events typically occur during the first post-fire storm season.
“Wildfire-related flooding and increased runoff may continue for several years in a burn area, but it is unusual for post-fire debris flows to occur beyond the second rainy season,” according to the website.
Those in emergency management positions are understandably on edge after a fire.
Fires usually reduce the rate at which water can seep into the soil, which means surface-erosion runoff is typically the most common contributing factor. Less common is the landsliding process that comes about from rainfall seeping into the ground. Prolonged heavy rains can increase soil moisture after a wildfire. The saturated soil can then fail and produce landslides that can be shallow or quite deep, sometimes greater than 10–15 feet.
What is actually happening in the broadest sense is the hydrologic response of a watershed is significantly altered by wildfire so that even the slightest rainfall can create hazardous debris flows and flash floods.
Many countries have entities that monitor those impacts to help emergency managers prepare for floods.
“Scientists use geospatial data to understand the size and shape of the watershed, and information on burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to various types of storms,” according to the USGS.
Communities that have seen their fair share of fire and flood have some suggestions for those in wildfire/flood-prone areas: sign up for alerts from emergency management — it is a way for government entities to convey real-time information about weather conditions and pressing safety issues to the public.
Also, if a fire has impacted your area, it would be wise to get flood insurance immediately. Afterall, it is too late to get flood insurance after a flood.
Protect your structure as best as possible with sandbags: “The use of sandbags in the doorways, or small diversions just upstream of the residence is usually the most effective way to protect the residence while leave the rest of the property open to flow,” Pima County told residents after a fire, further explaining that protecting more than just your structures may cause harm to neighbors’ properties and can leave you open to civil lawsuits. Such over-vigilance can also be in violation of the Floodplain Ordinance, which could leave you open to enforcement action and fines.
It is important to understand risks and potential protections for your property in this world of weather extremes and catastrophic events.
