A debris flow tore through Montecito, California, on January 9, 2018, three weeks after the Thomas Fire burned the slopes above town, destroying or damaging more than 400 homes the flames never reached, according to USGS research on the event. Investigators evaluated whether those losses were causally related to the wildfire and documented the technical evidence supporting those conclusions.
That pattern defines wildfire-related losses that may emerge long after containment. Soil hydrophobicity can cause burned ground to shed rainfall instead of absorbing it, creating conditions that may contribute to later runoff, erosion, and debris-flow events requiring technical evaluation.
Key takeaways: wildfire damage beyond the structure
Secondary wildfire losses can raise complex causation questions long after containment, particularly when contamination, runoff, and multiple environmental processes interact.
What drives secondary losses
- Structural ash can deposit heavy metals and other contaminants into soil and water
- Fire-altered soil can shed rainfall as runoff, raising debris-flow risk for years
- In some jurisdictions, determining the relationship between a wildfire and a subsequent debris flow or mudslide may require detailed causation analysis supported by technical evidence
How investigators establish the connection
- Burn severity classification, soil testing, and rainfall records typically anchor the attribution analysis
- Field records from emergency response teams often provide some of the earliest documentation available for later technical evaluation
For assistance evaluating secondary wildfire property claims, contact us to discuss specific requirements.
How can wildfire damage extend beyond the structure that burned?
A single wildfire can generate a structure loss, a smoke-damage claim on a standing home, a soil or water contamination dispute, and a debris-flow loss the following winter. Each presents its own technical causation questions and may require different investigative approaches.
Combustion deposits metals and organic compounds in ash and soil, while heat strips slopes of the vegetation that lets ground absorb rain. Secondary claims therefore may require evaluating fire effects on the site and surrounding land, not just the structure itself.
How can wildfire contribute to soil and water contamination?
Ash from a burned structure generally contains more harmful chemicals than ash from vegetation alone, according to EPA guidance, and post-fire storms can carry that residue into soil and waterways.
What physical and chemical changes occur in burned soil?
Fire consumes the litter and duff covering the ground, and losing that ground cover is widely recognized as a leading driver of increased post-fire erosion and runoff. Heat also breaks down the organic compounds that bind soil into stable aggregates, so soil in high-severity burn areas can become powdery and easily detached.
Burned area response teams commonly classify soil burn severity as unburned, low, moderate, or high, recording indicators such as remaining ground cover, ash depth, and water repellency testing. Those field records often provide valuable baseline documentation for subsequent forensic evaluation.
What contaminants may be present in burned soil and waterways?
Structural ash and underlying soil can carry heavy metals such as lead, arsenic, and chromium alongside polycyclic aromatic hydrocarbons (PAHs) and asbestos. Wildfire’s highly oxidizing conditions can also convert chromium to its more toxic hexavalent form, a distinction that may influence environmental assessment and remediation planning .
Volatile organic compounds from burned structures and melted plastic water-system components have contaminated drinking-water distribution networks after several major wildfires, a mechanism distinct from ambient smoke exposure. Investigators in these cases have traced contamination to heat-damaged plastic service lines and to depressurized mains drawing combustion gases backward into the system, rather than to the treated source water itself, which can shift both the remediation approach and the parties potentially responsible.
Debris removal after a major wildfire commonly proceeds in two phases: a hazardous-materials phase addressing items such as batteries, propane tanks, and asbestos-containing debris, followed by a phase removing ash, contaminated soil, and structural remains. Soil testing before and after this second phase can help determine whether contaminant concentrations have returned to background conditions before rebuilding and provide documentation to support subsequent environmental evaluations.
How can wildfires increase mudslide and debris-flow risk?
Severe fire can shift a watershed from an infiltration-dominated system to a runoff-dominated one. Rain the ground once absorbed may instead run off, entraining ash and loose sediment that can bulk into a debris flow.
How burned soil loses its ability to absorb water
Heat from burning surface litter vaporizes organic compounds, which then move downward and condense on cooler soil particles below, forming a water-repellent layer typically 1 to 3 centimeters beneath the surface. During a storm, the thin wettable layer above it can saturate quickly, and because the repellent layer blocks deeper infiltration, the water can become overland flow that carries saturated soil downslope.
Investigators generally weigh hydrophobicity alongside other concurrent mechanisms. Ash can clog soil pores, raindrop impact can crust bare soil, and lost ground cover can reduce features that slow runoff, so attribution work typically accounts for all of these rather than hydrophobicity alone.
How long does the elevated risk of debris flow last after a fire?
The USGS postfire hazard FAQ reports that a design storm with a 15-minute rainfall intensity of 24 mm/h, roughly a quarter inch, is the threshold commonly used to assess debris-flow likelihood at a burn area, and that the elevated hazard generally persists for 2 to 5 years after a wildfire. Triggering thresholds rise as vegetation recovers, so a storm that would trigger a debris flow in the first winter after a fire may not do so several years later.
What is the efficient proximate cause doctrine?
When covered and excluded perils combine to cause a loss, courts applying the efficient proximate cause doctrine may assign coverage to the predominant cause rather than denying the claim outright because an excluded peril appears in the chain of events. California codifies a version of this rule for certain wildfire-related events: Insurance Code Section 530.5, illustrating how legal treatment of secondary wildfire losses may vary by jurisdiction.
Legal treatment of secondary wildfire losses varies among jurisdictions. Regardless of the applicable legal framework, technically supported causation analysis remains an important component of evaluating wildfire-related secondary losses.
How do investigators evaluate fire-related debris flows and distinguish them from landslides?
No single indicator settles the question. Investigators typically weigh converging evidence: the source area’s relationship to the burn scar, field signs of how the flow initiated, soil hydraulic testing, and the rainfall record for the storm in question.
A post-fire runoff-generated debris flow is typically shallow, bulks with sediment as overland flow moves downslope, leaves widespread rilling across the burned surface, and shows no discrete slip surface; the source area usually lies within or just below the burn scar after a short, intense rain burst.
A deep-seated landslide instead shows a distinct shear surface, a displaced coherent soil mass, and scarps or tension cracks at the head of the slide, and it typically follows prolonged rainfall on already saturated ground rather than a brief burst.
At Montecito, investigators documented moderate-to-high burn severity across the source watersheds, widespread surface rilling, and only sparse shallow landsliding, a combination consistent with a runoff-generated post-fire debris flow rather than a deep-seated slope failure.
That distinction is important because accurately identifying the failure mechanism supports appropriate engineering, geotechnical, and hydrologic evaluation of the event.
How does forensic investigation support these secondary wildfire claims?
A wildfire-related secondary loss may call for fire science, environmental testing, hydrology, or geotechnical evaluation, often more than one discipline on the same file. Investigators can test contamination, evaluate debris-flow attribution, and document post-fire site conditions before repairs or remediation alter them, preserving important evidence such as ash deposits, hydrophobic soil layers, and contamination patterns that may change rapidly during cleanup.
Reports typically identify the materials reviewed, describe the analytical methods used, explain the evaluation of reasonably likely alternative causes, and clearly document the basis for the conclusions reached.
Because these claims may arise months or even years after a wildfire, early coordination between claims professionals and technical experts can help preserve site conditions, documentation, and environmental evidence needed for later forensic evaluation.
Rimkus offers multidisciplinary resources across fire investigation, environmental testing, hydrologic evaluation, geotechnical engineering, and related forensic disciplines to support investigation of secondary wildfire losses. For a qualified review of a secondary wildfire property claim, contact us to discuss specific requirements.
Frequently asked questions about secondary wildfire property claims
Does my policy cover a debris flow if a wildfire caused it?
Whether a debris flow is related to a prior wildfire depends on the available technical evidence, including burn severity, rainfall, soil conditions, and site observations. The legal treatment of those findings varies by jurisdiction and policy language.
How is a debris flow different from a landslide for claims purposes?
A debris flow is typically shallow, forms from overland runoff after brief intense rain, and shows no distinct shear surface, while a landslide involves a displaced coherent mass along a discrete slip surface after prolonged rainfall. That physical distinction can matter because some policies and statutes treat the two perils differently.
How long after a fire can a secondary property claim still arise?
Elevated debris-flow risk commonly persists for 2 to 5 years after a wildfire, and contamination or water-system claims can surface even later depending on when testing occurs. Each type of secondary claim generally requires its own timing and causation analysis rather than assuming the same clock applies across all of them.
This article is intended to provide general information and insights into prevailing industry practices. It is not intended to constitute, and should not be relied upon as, legal, technical, or professional advice. The content does not replace consultation with a qualified expert or professional regarding the specific facts and circumstances of any particular matter.