In California and many other places in the United States, climate change and land-use patterns intensify disaster dynamics, where one catastrophe leads to another. Wildfires can drastically increase the risk of flood and flood damage, making it crucial to understand how these disasters are interconnected.
Normally, soil and vegetation effectively absorb rainwater. However, after a wildfire, burned vegetation and scorched soil prevent water absorption. When heavy rain follows, water runs off the surface instead of seeping into the ground, significantly increasing the risk of flash floods. This post explores how wildfires alter landscapes, heightening flood risks and exacerbating disaster impacts.
It never rains but it pours: California’s flood challenge
The sunny state of California is no stranger to floods. With 38 major rivers, river flooding is the most common type of flood event. All 58 counties in California have experienced significant flood damage in the past 25 years, leading to loss of life and billions of dollars in damages.
Beyond river flooding, flash floods from heavy downpours increasingly threaten Californian communities. In just the past year, repeated flood events have devastated the state. The Federal Emergency Management Agency (FEMA) declared flood-related disasters in January 2023 and again from March to July 2023. Media outlets, as illustrated by the CNN news feed, reported extensively on these widespread floods.
Burned landscapes: The dual threat of wildfires and floods
Though somewhat counterintuitive, California struggles with both floods and wildfires. The environmental pollution, property losses and casualties caused by wildfires in the state are getting worse by the year. Add to this that the extent and severity of wildfire is expected to increase in the future, mainly because of fuel accumulation and climate change. Already now, the wildfire season has lengthened and the peak months have been advanced from August to July.
Looking at historic trends, both frequency and total burned area of all wildfires have increased significantly since the turn of the century.
Wildfires have burned through large areas in recent years, leaving burn scars behind as lasting impact on the landscape – and further increasing the likelighood of flooding.
Fire-flood dynamics: How wildfires increase flood risk
A wildfire consumes vegetation and leaves behind a landscape which is covered in soot, ash and charred stumps and stems. It abruptly changes the hydrologic and soil properties of watersheds decreasing infiltration capacity and changing soil surface structure. For example, an ‘ash crust’ can be formed or the underlying soil can become water repellent due to a hydrophobic layer of burned organic matter.
Intense rainfall after wildfire can result in substantial overland flow and potential for flash floods. Furthermore, the risky dynamics involves elevated chances of mudslides which may be even more impactful than extreme water flows.
In California, studies have shown that wildfires enhance flood risk in the first two years after wildfire; however, the effects diminish after another two years.
Severe dynamics: The cascading efffect of wildfires and floods
The transition from wildfire to flood can have severe consequences. When the Tropical Storm Hilary stroke California in August 2023, the authorities issued a flood warning specifically to those living in wildfire burn scar areas encouraging them to evacuate.
In the Arroyo Seco watershed in Southern California, detailed studies have identified how great the fire-flood dynamics can be. Following moderate to high burns, the first year since fire demonstrates that a 100-year flood event can be three times larger than non-fire-affected years. When a rain heavy weather system passes, a flood peak flow three times over can make the difference between no issues at all and severe flash floods with detrimental consequences for affected communities.
3x – in the first year since fire, floods can be three times larger.
The evidence is clear—within the first year after a wildfire, floods occur at three times their usual intensity. The increased runoff, combined with weakened soil stability, creates a dangerous combination that significantly raises flood risks.
To mitigate flood damage, communities, governments and businesses must be proactive in flood safety measures, from securing flood insurance, to consulting flood maps to assess their vulnerability and making use of real-time flood warning tools. Understanding the different types of floods—from flash floods to river flooding—is essential for preparedness, and at the core of what we do at 7Analytics.
As climate change continues to fuel extreme weather patterns, the relationship between wildfires and floods will only become more pronounced. By staying informed and taking preventive action, individuals and communities can better protect themselves from the devastating effects of these interconnected disasters.
