Erosion Control after Wildfire
The potential for severe soil erosion exists after a wildfire because as a fire burns it destroys plant material and the litter layer. Shrubs, forbs, grasses, trees, and the litter layer break up the intensity of severe rainstorms. Plant roots stabilize the soil, and stems and leaves slow the water to give it time to percolate into the soil profile. Fire can destroy this soil protection. There are several steps to take to reduce the amount of soil erosion. A landowner, using common household tools and materials, can accomplish most of these methods in the aftermath of a wildfire. More specific information on how to implement the soil erosion control techniques that follow can be found by accessing the electronic links found in the NRCS Fire Recovery Tips section at the end of this document.
In severe, slow-moving fires, the combustion of vegetative materials creates a gas that penetrates the soil profile. As the soil cools, this 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. Percolation of water into the soil profile is reduced, making it difficult for seeds to germinate and for the roots of surviving plants to obtain moisture.
Hydrophobic soils do not form in every instance. Factors contributing to their formation are: a thick layer of litter before the fire; a severe slow-moving surface and crown fire; and coarse textured soils such as sand or decomposed granite. (Finely textured soils such as clay are less prone to hydrophobicity.)
The hydrophobic layer can vary in thickness. There is a simple test to determine if this water repellant layer is present:
In addition, freezing and thawing, and animal activity will help break up the hydrophobic layer.
The first step after a wildfire is reseeding grass in
the severely burned areas. Remember many plants can recover after fire
depending on the severity of the burn. It is important to leave existing
vegetation if the plants do not threaten personal safety or property
(hazardous trees in danger of falling should be identified first).
Seed can be purchased in Arizona. Obtain certified seed this guarantees the variety, that it was tested under field conditions, that it is recommended for the state, and that it is certified weed free to prevent the introduction of noxious and invasive weeds.
Sun/Shade: S = full sun, PS = partial
sun, Sh = shade
Weeds are among the first plants to recolonize after a fire. In many
instances they are not a problem. However, if the weeds are listed as
noxious, they must be controlled. Noxious weeds displace native plants
and decrease wildlife habitat, plant productivity, and diversity. They
can spread downstream or into agricultural areas, resulting in high
control costs. Control of noxious weeds is best accomplished through
an integrated plant management system that includes chemical, biological,
mechanical, and cultural controls.
Straw provides a protective cover over seeded areas to reduce erosion and create a suitable environment for revegetation and seed germination. If possible, the straw should be crimped into the soil, covered with plastic netting or sprayed with a tacking agent. If you can only broadcast the straw, do so; its better to have some coverage than none at all. The straw should cover the entire reseeded section and extend into the undamaged area to prevent wind and water damage. Care must be taken to insure that you use only certified weed-free hay straw to avoid spreading noxious weeds. (Contact the State Department of Agriculture for a listing of Certified Weed Free Hay growers.)
Straw should be applied to a uniform depth of two to three inches. When applied at the proper density, 20 to 40 percent of the soil surface is visible. One typical square bale will cover about 800 square feet. (Figure 1.)
Log terraces provide a barrier to runoff from heavy rainstorms. Dead trees are felled, limbed, and placed on the contour perpendicular to the direction of the slope. Logs are placed in an alternating fashion (Figure 2.) so the runoff no longer has a straight down slope path to follow. The water is forced to meander back and forth between logs, reducing the velocity of the runoff, and giving water time to percolate into the soil. Felling of trees can be dangerous and is best done by a professional logger or arborist.
Logs should be 6 to 8 inches in diameter (smaller logs can be used) and 10 to 30 feet long. The logs should be bedded into the soil for the entire log length and backfilled with soil so water cannot run underneath; backfill should be tamped down. Secure the logs from rolling by driving stakes on the downhill side. It is best to begin work at the top of the slope and work down. (It is easier to see how the water might flow by looking down on an area to better visualize the alternating spacing of the logs.
Straw wattles are long tubes of plastic netting packed with excelsior, straw, or other material. Wattles are used in a similar fashion to log terraces. The wattle is flexible enough to bend to the contour of the slope. Wattles must be purchased from an erosion control material supplier.
Silt fences are made of woven wire and a fabric filter cloth. The cloth traps sediment from runoff. These should be used in areas where runoff is more dispersed over a broad flat area. Silt fences are not suitable for concentrated flows occurring in small rills or gullies. Silt fences are made from materials available at hardware stores, lumberyards, and nurseries. (Figure 3.)
Straw bales placed in small drainages act as a dam collecting sediments from upslope and slowing the velocity of water traveling down the slope. Bales are carefully placed in rows with overlapping joints, much as one might build a brick wall. Some excavation is necessary to ensure bales butt up tightly against one another forming a good seal. Two rows (or walls) of bales are necessary and should be imbedded below the ground line at least six inches or held in place by stakes or rebar driven into the soil. Again, make sure the straw bales that are purchased for use are certified as weed free. (Figure 4.)
Bare ground and hydrophobic soils left after a fire increase water runoff. This requires intervention to channel water off of the burned area and release it to the streams below. The two most common structures to do this are culverts and water bars. Determining the type of drainage practice to use depends on the soil, type of road use, slope, speed of vehicles, season of use, and amount of use.
It is best to engage a professional engineer to assist with the design and construction of culverts. These professionals are able to determine the size of the drainage area and the amount of runoff for rainfall events of varying intensity that require culverts. Once sized, culverts must be installed properly at the correct locations. Installing more culverts than previously existed before the fire may be required. The inlet sides must be regularly maintained to prevent sediment and trash from plugging the pipe. It is common practice to armor the ground at the outlet end with rock riprap in order to dissipate the energy of the discharged water and to spread it over the slope below. The inlet side can have a drop inlet so as to allow sediment to settle out before water enters the pipe. Armoring the inlet side with rock will also prevent water from scouring under and around the pipe and flowing under the road.
Water bars are berms of soil or bedded logs that channel water off roads and trails to avoid the creation of gullies. Water bars are angled down the slope to the outlet side. These bars can divert water to a vegetated slope below or redirect it to a channel that will take it to a culvert. On-site soils and the road grade will dictate spacing. (Figure 5.)
Burlap Bag Check Dams
Establishing Grasses and Legumes
Straw Bale Check Dams
Straw Bale Dikes
Arizona FIREWISE Communities Cooperators
University of Arizona, Northern Arizona University, Arizona State Land Department, Arizona Fire Chiefs Association, Arizona Fire Districts Association, Arizona Emergency Services Association, Arizona Planning Association, Bureau of Indian Affairs, Bureau of Land Management, Bureau of Reclamation, Inter-Tribal Council of Arizona, National Park Service, USDA Forest Service, USDA Natural Resources Conservation Service, U.S. Fish and Wildlife Service
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.
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