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A debris flow is a fast-moving mass of material -- slurries of water, rock, soil, vegetation, and even boulders and trees - that moves downhill by sliding, flowing, and/or falling. Debris flows range from a few square yards to hundreds of acres in area, and from a few inches to 50 feet deep. Even smaller debris flows can be locally dangerous. Imagine trying to walk through a 3-inch deep mass of wet concrete moving at 30mph.
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The post-fire landscape is especially susceptible to stormwater runoff-related hazards such as landslides, debris flow, flooding, and rockfall. Fire destroys vegetation and root systems that provide stability to the soil. Fire damage may also create hydrophobic soils, which could concentrate runoff into slopes that may already be prone to failure.
It is property owner’s responsibility to control stormwater runoff from their property. Property owners and contractors on burned lots and rebuild sites must take action to prevent pollutants, including sediment, from entering storm drains, creeks, rivers, and wetlands.
Property owners should evaluate their property for potential hazard areas and install erosion and sediment control Best Management Practices (BMPs) as required. The Napa County Debris and Ash Removal Stormwater Pollution Prevention Plan will help you evaluate your property and choose BMPs. BMP materials such as wattles, mulch, and silt fencing, are available for purchase at various agriculture, garden supply and hardware stores. BMPs are used to minimize erosion and control sediment to keep pollutants from entering storm drains and our natural water bodies like creeks and rivers.
Visit Napa County’s Watershed Recovery page for more information and resources.
Wildland fires are inevitable in the western United States. Expansion of human development into forested areas has created a situation where wildfires can adversely affect lives and property, as can the flooding and landslides that occur in the aftermath of the fires. Wildfires change the landscape, destroying root structure and creating top soil that could repel water instead of absorbing it.
Post-fire landslide hazards include fast-moving, highly destructive debris flows that can occur in the years immediately after wildfires in response to high intensity rainfall events, and those flows that are generated over longer time periods accompanied by root decay and loss of soil strength. Post-fire debris flows are particularly hazardous because they can occur with little warning, can exert great impulsive loads on objects in their paths, and can strip vegetation, block drainage ways, damage structures, and endanger human life. Wildfires could potentially result in the destabilization of pre-existing deep-seated landslides over long time periods.
Flood After Fire - Video from the Department of Water Resources
Intense forest and shrub land fires can burn soil organic matter, reducing the pool of nutrients in the soil, soil aeration and water infiltration/retention, and the soil’s ability to hold nutrients from organic material, ash, or fertilizer.
Hydrophobic soils form an impermeable crust and do not absorb moisture. When water falls on the soil, it will runoff or pond on the surface.
In severe, slow-moving fires, the natural litter layer burns and creates a gas that penetrates the soil layers. As the soil cools, the gas condenses and forms a waxy coating. This causes the soil to repel water - a phenomena called hydrophobicity. This hydrophobic condition increases the rate of water runoff. Less water percolates into the soil profile, making it difficult for seeds to germinate and for the roots of surviving plants to obtain moisture.
You can do a quick test to see if you have hydrophobic soils by scraping away the upper ash layer until you reach the mineral soil surface. Place a drop of water on the surface and wait at least one minute. If the water beads up, then the soil is hydrophobic. Often the upper few inches of soil are not hydrophobic and it may be necessary to go down 1/2 to 1 inch and repeat the test to find the hydrophobic layer.
A flash flood is a rapid flooding of low-lying areas in less than six hours, which can be caused by intense rainfall. Flash floods are known to roll boulders, tear out trees, and destroy buildings and bridges.
Mudflows are rivers of liquid and flowing mud on the surface of normally dry land, often caused by a combination of brush loss and subsequent heavy rains. Mudflows can develop when water saturates the ground, such as from rapid snowmelt or heavy or long periods of rainfall, causing a thick, liquid, downhill flow of earthen material. Mudflows are covered by flood insurance but are different from other non-covered earth movements where there is not flowing characteristic - such as landslides or slope failures.
Debris flows can take homes off of their foundations and can carry things like vegetation, trees, large boulders, and vehicles. Mudflows on the other hand are made of water and soil, and although they are more unlikely to move heavy objects than debris flows, both are fast moving and dangerous.
In a landslide, masses of rock, earth, or debris move down a slope. Debris and mud flows are rivers of rock, earth, and other debris saturated with water. While landslides may require lengthy rain events and saturated slopes, a debris flow can start on a dry slope after only a few minutes of intense rain.
Debris flows and mudslides are among the most numerous and dangerous types of landslides in the world. They are particularly dangerous to life and property due to high speeds and the sheer destructive force of flow.
Debris flow and landslides may occur in areas where surface runoff is channelized, such as along roadways and below culverts. Debris flows commonly begin in swales or other depressions on steep slopes, making areas downslope from swales particularly hazardous.
Flooding and debris flow can occur during storm events. The potential for debris flow can occur in and below areas where fire has denuded the slope and altered the characteristics of the topsoil. The extent and amount of flows will depend on the rainfall intensity and duration of the storm event, and on how much vegetation has regrown. The Watershed Emergency Response Team identifies risks by modeling a storm event with 0.25" of precipitation over 15 minutes.
Post-fire runoff flows can be highly destructive and can move large quantities of soil, rocks, brush, and trees which can cause property damage, block streets, and endanger occupants of structures. It can take four to five years for the burn area vegetation to significantly recover, and about ten years to fully recover.
High intensity storms may cause hazardous conditions. The Watershed Emergency Response Team identifies hazards by modeling a 0.25 inch in 15 minutes (or 1 inch per hour) storm event.
Monitor Napa OneRain for precipitation and stream monitoring data for early warnings of potentially hazardous storms.
After a fire and before and during the rainy season, property owners in impacted areas should examine their properties and be aware of the following landslide warning signs:
For more information, visit the USGS landslide preparedness page.
Wildfire damages the forest environment in several ways: it can kill trees outright, especially young trees; it may damage portions of the bark and kill the underlying cambium tissue, making the tree susceptible to insect or disease attack’ by destroying needles and leaves, wildfire retards tree growth and vigor; and fire can damage the soil directly or make it more susceptible to erosion.
There are many things to consider when deciding whether or not trees impacted by the fire should be removed. The first step should be to determine if the trees pose a health and safety hazard. To determine the hazard and health of trees impacted by the fire, contact a certified arborist or RPF to assess tree health. You may also contact RCD or NRCS to conduct a site assessment and provide additional guidance. Trees that pose a health and safety hazard, particularly in areas adjacent to or in proximity structures, should be removed.
Only trees that have determined by and arborist, and RPF or qualified professional that have been determined to be dead as a result of the fire may be removed. However, dead vegetation and their remaining roots systems continue to have value and provide protection from soil erosion and habitat and cover to wildlife. Please note that tree and vegetation removal within and near streams may require permits from the California Department of Fish and Wildlife (CDFW).
Most evergreen or conifer trees in the County are considered commercial timberland by the California Department of Forestry and Fire Protection (CalFire). These typically include Douglas fir, redwood, and ponderosa pine. Please contact Kim Sone, Unit Forester, at [email protected] or (707) 576-2344 for information and permit requirements related to the removal of timberland on your property.
Tree removal associated with a new vineyard, winery, or other project may be subject to additional approvals from Napa County. Allowed uses vary according to zoning, slopes, and other factors. New vineyards on slopes over 5% require an Agricultural Erosion Control Plan (ECPA), as detailed in the Water Quality and Tree Protection Ordinance Implementation Guide.
All residents who live on or below hillsides - especially in areas impacted by recent wildfires - should be aware that the rainy season increases the possibility of potentially dangerous debris flows, mudflows, or landslides. Additionally, your property may receive sediment via runoff from upslope properties. Keep an eye on drainage pathways and infrastructure on your property, like channels, storm drain inlets or culverts, for sediment loading.
The Napa County Resource Conservation District (RCD) has many staff members and helpful resources for landowners whose property burned and are looking for guidance on how to manage vegetation going into the winter and on a long-term basis. Their Post-Fire Website includes brochures regarding burned woodland, hazard trees, and tree removal. The RCD also offers site assessments to provide guidance regarding the installation of erosion control measures and the potential benefits of leaving trees in place. To schedule a site visit with the RCD, contact Bill Birmingham, RCD Conservation Program Manager, at [email protected] or (707) 252-4188.
The USDA-Natural Resources Conservation Service (NRCS) is another potential resource and can provide landowners financial assistance through their Environmental Quality Incentives Program (EQIP) and Farm Services Agency (FSA) programs. NRCS may also be able to offer advice on preventing erosion, covering and protecting exposed soil, directing water away from vulnerable areas and more. The local NRCS contact is Contact Wendy Rash, District Conservationist for Napa & Vacaville offices at [email protected] or Napa Office telephone (707) 252-4189 and Vacaville (707) 448-0106 ext. 111 for more information, or visit the NRCS Post-Fire Disaster Assistance website.
You can prepare yourself and your property by making sure that water can flow freely around your structure. That means clearing debris from any culverts or ravines that can help direct rainwater away from your residents. Channels, diversion barriers, and deflection walls can help to redirect flows. These must be installed carefully so that they don’t redirect the flow towards structures, foundations, or unstable slopes. Sandbags are a good temporary solution as they can slow the momentum of smaller mudslides. Other actions you may consider:
Click here for an update to the soil cover provision recommendations (June, 2021) including alternatives to straw mulch.
The purpose of the advice is to assist property owners in identifying risks and hazards, as well as site-specific emergency protection measures and erosion and sediment control treatments to protect their properties and our watersheds from mud and debris flows. The Engineering division is available to answer any questions, provide general advice and guidance, and conduct hazard assessment for potential risks to life and property.
Napa County is happy to provide advice and guidance to all residents of Napa County with concerns about post-fire impacts. Napa County is working to notify the owners of the properties with identified risks, since protection of the property is ultimately the owner’s responsibility. In cases where a fire has occurred right before or during the rainy season, Napa County may seek assistance from occupants in contacting the property owner and/or property manager to expedite assistance.
If you occupy a property that is owned by the U.S. Forest Service, protection measures are under the Forest Service’s authority and responsibility. Please contact the Pacific Southwest Region National Forest District Office at (707) 562-8737 for assistance.
No. Napa County is not authorized to expend public funds to perform work on private property.
You can retain a contractor to help install erosion control measures on your property. If you do not wish to hire a contractor, you can also look into inquiring at local volunteer organizations in your area, such as Boy Scouts of America troops, religious groups, etc. For guidance on the installation of erosion and sediment control treatments, visit Napa County’s post-fire erosion control page here.
Removal of debris from private property is the responsibility of the property owner. For more information on clearing debris, ash, and other hazardous materials, visit the Napa County post-fire debris removal page.