First aid for Sonoma County’s fire-damaged soil
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.
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.
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.