The devastating firestorm that has ravaged Southern California will eventually come to an end, perhaps aided by some much-needed rainfall. Yet, for emergency and public works teams, any respite may be brief. The anticipated rain brings with it the threat of mudslides and debris flows, posing a robust challenge to the region’s infrastructure.
Once wildfires decimate mountain slopes, vegetation is lost and the intense heat significantly alters the surface soils. Even a moderate amount of rain on these landscapes can cause substantial runoff, moving swiftly downhill, eroding, and gathering soil and debris into a potent mix.
These natural occurrences, depending on their components like sediment, rock, and wood, are referred to as mudslides or debris flows. They present a grave danger to both people and property, and their threat can linger for years until vegetation and soil fully recover.
In response to a series of deadly post-fire floods in the 1930s, Southern California made substantial investments in infrastructure aimed at managing such flows. Large dams and smaller debris basins were constructed to capture potential hazards before they reached the densely populated areas at the foot of mountains. Today, more than 150 debris basins are in place throughout Southern California. These serve as the frontline defense for communities below the Santa Monica and San Gabriel mountains, affected by recent fires like the Palisades and Eaton.
Nevertheless, these structures don’t offer foolproof protection. The smaller basins, in particular, can be overwhelmed.
Consider the aftermath of the Thomas fire, which started in December 2017 in the Santa Ynez Mountains above Ventura and Santa Barbara counties. This past blaze set the stage for some of the state’s deadliest floods in a century. Torrential rain led to streams of water, mud, boulders, brush, and trees cascading down from the canyons. The debris overtopped containment basins and surged across several miles, wreaking havoc on the town of Montecito and blocking Highway 101. Tragically, 23 lives were lost, and 400 structures were demolished.
One crucial aspect of prevention is maintaining debris basins by clearing out the built-up sediment to ensure they’re ready for subsequent storms. The task of cleansing these basins between storms is challenging and costly, due to soil saturation, but highly necessary. Recent studies from UC Irvine show that the domino effect of consecutive storms can lead to basins being overwhelmed.
Governor Gavin Newsom’s executive order, which calls for preparedness against mudslide and debris flow risks, is a welcomed move. Removing debris from burn areas is vital to lessen the volume that could contribute to these flows. Additionally, regular cleaning of debris basins and flood channels will enhance their ability to protect communities. However, it is equally important for policymakers to allocate sufficient funds to manage flood control infrastructure, regularly update flood hazard maps, notify nearby residents and businesses of the risks, and prepare for swift evacuations as needed.
Climate change is exacerbating California’s cycle of fires and floods, placing more people in harm’s way and straining infrastructure designed for a previous climate and population size. As wildfire dangers increase, so too does the urgent threat of post-fire mudslides and debris flows. Immediate action and public cooperation are crucial to safeguard lives and protect communities.
Jeffrey Mount is a senior fellow at the Public Policy Institute of California’s Water Policy Center. Brett Sanders is a professor of civil and environmental engineering, urban planning, and public policy at UC Irvine.
