First aid for Sonoma County's fire-damaged soil

What to do now to stop erosion after a fire.|

You are battered and fatigued, but the fight to save your property and community is far from over.

In the wake of recent wildfires on the North Coast, the risk of topsoil loss and the flow of debris has grown.

Erosion leaps as high as 200 percent following fires in urbanized areas. With this increase comes mass sedimentation, alteration of streambeds, property and infrastructure damage, and, in some cases, even injury and death.

We need to hold our ground.

The problem

Fires eliminate canopies, burn off leaf litter and expose the soil. When there is nothing to slow or stop them, wind and water gain leverage. Soil gets shoved around as a consequence.

But the problem is not just the lack of protective cover. Recently burnt landscapes also have to contend with repellency. Fires cook the waxes that are natural to our soils. When these waxes cool, they coat the first inch of soil with a repellency layer, stopping water from infiltrating.

The consequences can be dire when the lack of protective cover and repellency are combined. Fire-scarred communities can produce incredible amounts of runoff and debris flow.

This runoff and debris can overwhelm storm water drainage systems, leading to extensive erosion elsewhere. Worse still, debris flowing down slopes can overrun homes, businesses and small communities. These types of events can, and have, lead to personal injury and death.

We need to hold our ground this winter.

Repellency layer

A water repellency (hydrophobic) layer is created when a fire melts waxes and resins found naturally in Mediterranean soils. The melted waxes turn into a gas, rise and then cool and condense on top of a soil, coating it with an impermeable layer.

When rain hits a repellency layer it quickly runs off. As water runs its power grows and its ability to move things, like soil particles, grows proportionally. Hydrophobic soils are prone to topsoil loss and can produce severe debris flows.

Lightly watering an injured landscape can help break the repellency layer. If repellency persists after multiple light waterings, then you will have to physically break the barrier. Grass rakes, hoes and hula ho weeders can be lightly dragged across the top of the soil to break repellency.

Although most soils lose their repellency within a year, some may stay hydrophobic for up to six years. The amount of water repellence a fire creates is related to a fire's intensity and the size of a soil's particles. Larger soil particles, such as sand, will have greater rates of repellency.

Immediate first aid

A well-planned and quick response to the threat of erosion is needed. Below are the first six things that must be done after a wildfire.

Drain water: Drainage systems will be clogged with debris after a fire. Water skipping out of drains, such as culverts, is a leading cause of erosion, fire or no fire.

Roof gutters, street gutters, culverts, swales, infiltration and detention basins, small streams and concrete waterways will need cleaning. This is the very first thing to do.

Divert sheeting water: The chances of topsoil loss dramatically rise if a landscape receives sheeting water from nearby features.

Driveways roadways, sidewalks and parking lots are often designed to sheet their runoff to the landscape. If this is the case on your property, divert that runoff away from your landscape and toward the storm drain system.

Sandbags, diversion ditches, staked boards, dry stacked walls and bales are the most common diversion devices.

Directing runoff

Sandbags: Although labor intense to fill, move and stack, sandbags are an effective and steadfast method of diversion. Sandbags are prone to deterioration and after the threat of rain passes, they should be removed and a permanent solution installed.

Diversion ditches: Faster to construct than sandbags and stacked walls, ditches are a quick fix. Ditches (often called swales) should only be used for light to medium flows of runoff - earthen features are prone to rapid deterioration in large flows.

Staked boards: Often called check dams, the boards should be at least 4 feet long, 9 inches wide and ½ inch thick. Stakes made from No. 3 rebar are used to keep the boards in place. Stakes are 2 feet long.

Dry staked walls: Dry walls are inexpensive and quickly built, made from salvaged rock, broken concrete, and even bricks and roof tiles. Stacked walls are long lasting if built with care.

Bales: Used along roads, the base of slopes and around the perimeter of structures, bales are inexpensive and quickly placed. Stake bales with rebar to hold them. Bales are a fire hazard and should be removed before summer.

Minimize traffic

Keep foot and equipment traffic off burnt landscapes.

Activity on slopes will increase the likelihood of erosion by weakening a soil's bonds, dislodging soil particles and trampling just sprouted plants. Activity on flat ground can compact the soil and lower its water absorption rates, which increases runoff.

Instead, plan on working on an injured landscape only after on a plan of restoration has been developed and all the materials and tools are ready for use.


A recently burned landscape will absolutely need water, but there are two distinct types.

The first immediate watering is aimed at breaking the soil's repellency layer. This watering is super light - no more than 1 gallon per 10 square feet. The goal is to water only the top quarter inch of soil.

Once the repellency layer is broken, begin deeper waterings. The goal is to get the water 4 inches deep and encourage seeds, roots and surviving plants to sprout. Three to 5 gallons of water per 10 square foot will be required. Water again only when the first 2 inches have dried.

Leave the mess

Do not clean your landscape - the debris on your injured site provides much needed protection.

The charred remains of plants and garden features protect the landscape from wind and water erosion, slows sheeting water and helps prevent the surviving seeds and plants from drying out. Do not remove debris until a restoration plan has been developed.

Douglas Kent is a landscape architect and contractor specializing in landscape management and design. He is the author of “Firescaping,” a guide to planning and designing for fire danger in the western United States. He is based in Orange but much of his family lives in Sonoma County and was affected by the recent wildfires. He can be reached at 714-308-3547.

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