Los Angeles Under Siege: The Hidden Dangers Still Ahead

Over the past three years, Los Angeles has faced an unprecedented cycle of extreme weather events, leading to a destructive chain reaction of flooding, landslides, and wildfires. This is not just a historical account—it’s an urgent wake-up call for homeowners, developers, and policymakers to better understand the risks of living in disaster-prone areas and to take proactive steps toward resilience and recovery.

In 2023 and 2024, the weather included heavy rains driven by atmospheric rivers that dumped up to several inches a day.  These storms were amongst the worst of LA’s history, and put the hillsides and drainage systems of southern California to an almost Darwinian level of testing.  Thousands of occurrences of flooding –hundreds of mud and debris flows, and multiple landslides occurred.  Following these rains, the vegetation of the open space areas responded with strong growth. In early 2025, the southern California area, particularly the Santa Monica Mountains, the Valley and the LA Basin received strong offshore flow from gyres feeding storms to the northern California and northwest US area.  This resulted in very dry, warm flow following a long dry season – amongst the longest and hottest / driest in 130-years.  This offshore air flow is termed often as “Santa Ana” winds, and is a common condition. In the recent wind storm, there were prolonged storm to hurricane force winds with humidity falling in some cases to less than 10%.

The hillsides of the Santa Monica Mountains are made up of rock formations that typically have low inherent fertility, and include large areas soft and / or seamy and weak geology. An all important but typically thin to spotty layer of soil that aprons the rock is what supports much of the vegetation.

Chaparral type vegetation that is typical to the Santa Monica Mountains and southern California area has adapted to this type of weather pattern over the past millions of years and is partners with grasses and scrub.  These plants have evolved to adapt to the cyclic weather patterns of periodic drought by defensive methods that include “waxing up” using oily / tarry / wax secretions to plug up and limit evapo-transpiratiation that helps them conserve water.  Other mechanisms include simply drying up and going dormant.  Following wet weather, they grow, spread and flower.  Many of them drop a seed load as the weather dries back and they go into defensive mode again or die back. A portion of the seedload of chaparral community plants require being exposed to high temperatures to sprout.

These hillside and hillside adjacent areas have exceptional beauty under “normal conditions.”  This has made these areas very attractive for development.  Since the early 1900’s, these areas have been a focus of development of high end properties and infrastructure (including roads and utilities).  Since the last few decades, the development has pushed in tight with this terrain, and the terrain and vegetation has had little management.

Why is this important:

1. The hillsides of southern California, especially including the hills in the Los Angeles area have typically low to moderate inherent surficial stability.
2. The surface of much of this terrain has a precious apron or covering of topsoil-alluvial / colluvium soils that is typically thin and comprised largely of weathered rock and humus derived from vegetation litter and vegetation fire residuals.
3. Chaparral and grasses play major roles in watershed preservation and improve surficial stability and erosion potential.
4. These cover plants become incendiary in dry weather and can burn violently when exposed to an ignition source.
5. The effects of the winds dry out the vegetation further. Where fire occurs, the wind can spread the fire rapidly.  The front of these fires can exceed 2000-deg – similar to a forge furnace.  Paper spontaneously combusts at about 451-deg and wood at about 570-deg.
6. The fire fronts destroy literally everything in their path that is even remotely flammable. House framing and most plastic, wood, similar products are incinerated.  Hillside vegetation is reduced to ash. A portion of the seedload and root structures in the ground may often remain intact however, to sprout following the return of wet weather.  Some of the seedload for plants that play important roles in pioneering recovery are activated by exposure to high temperatures. This exposure to heat signals to them that a fire has occurred that likely removed the preexisting vegetation which would interfere with the growth of new seedlings.
7. Unless and until the seed load can be activated, the slope is subject to much higher risks of damage and development of instability from the hazards of rainfall. It may take some time for terrain to develop a new vegetative cover under good circumstances.  It may take much longer if the soil cover is damaged or destroyed.
8. Even with vegetative cover established, many areas developed instability following the 2023 and 2024 rain events. Where widespread fire has occurred and the slope is unprotected and heavy rains follow – the damage may be catastrophic.  Large mud and debris flows may occur, and the rate of runoff is increased.
9. Where the soil was damaged in fire events because seawater was used, the ground may be very slow to recover and the watershed will be damaged. Special tactics will be needed to protect and recover this ground.
10. One of the most important recovery concerns following a fire in hillside terrain is the preservation of the all-precious soil zone from erosion and contamination.
11. It will rain heavy again, and probably sooner than later.

Other considerations that can affect you and recovering your property following this series of disasters:

1. Runoff rates may be greatly increased.
2. The risk of hazards from mud and debris flows may be greatly increased.
3. The risks of deeper seated failures of the ground, such as landsliding, may be increased.
4. Vegetative recovery where the soil becomes damaged or lost will be slowed or essentially stopped.
5. The house foundation and superstructure that remain following a fire may have been damaged and weakened significantly, even where it “looks” intact to the layperson.
6. The utilities serving the site may have been damaged or destroyed.
7. Leaking wet utilities may be present that can create multiple geotechnical issues. Some of these issues may be latent to manifest.
8. The drain systems may be overrun and clogged with debris. Special temporary drainage and erosion controls may be needed.
9. The ash and debris may include a variety of hazardous / toxic substances. Where water from firefighting, broken water lines, and/or rain is added to the mix the result can be a toxic soup.
10. Special measures may be warranted to provide proper drainage, protect the slopes, facilitate revegetation and to limit risks from adverse slope hazards like flooding, and debris / mudflows.

What does this mean to me, who do I call, and why?

What this means to you as property owner affected by these disasters, or are concerned about hazards and risks of offsite conditions that may be increased and may affect your property: Your first steps once the emergency is over is to assess what has happened relative to the above considerations both onsite and adjacent areas that may adversely affect you, and/ or that your property may affect your neighbors. You will also want to assess the serviceability of the soils, structural, utilities, and drainage aspects, and formulate a plan to protect the areas from further damage and exposure.  You will want to have a foundational study that may be used by insurance, design and construction professionals to come up with a design that works.

Understand that the City and Code apply a broad brush to the world and are generalized – they have to be, given the spectrum of issues and situations these codes apply to.  Understand also that these preliminary assessments are not something for most civil engineers, architects, or insurance adjusters.  This requires specialized scientists and engineers – a major focus of G3Soilworks.