Test for Chances of Erosion and Topsoil Loss
Video Clip

Test for Chances of Erosion and Topsoil Loss

The somewhat predictable chance of water runoff and topsoil loss are influenced by six factors: length and steepness of a slope, amount of rain, type of vegetation burned, site’s type of soil, amount of activity by animals and humans, and lastly, severity of the fire. 

Go through the list of contributing factors, mark the box that best describes the burnt landscape and then add the points together. The tallied score corresponds to an approximate level of risk. 

The erosion test provides indicators to the likelihood of erosion, but cannot provide accurate data. If accurate data is needed, consult a local Certified Professional Soil Erosion and Sediment Control Specialist (CPESC). Also, different parts of a landscape will score differently. Always prioritize work around the highest risk areas. 

Slope

A slope’s degree of incline has the greatest influence on its chances of producing erosion. The incline and length of a slope are two measurable factors.

Although slopes have a higher risk of erosion, flat ground is susceptible too. Scorched and bare landscapes are easily compacted and prone to puddling and sheeting water, contributing to erosion elsewhere.

Steepness of Slope

o  1 Point: Between 0 -16%: Not Likely

o  2 Points: Between 17% - 34%: Low Likelihood

o  4 Points: Between 35% - 51%: Likely

o  8 Points: Between 52% - onward: Most Likely

Length of Slope

o  1 Point: 0 - 25 feet

o  2 Point: 26- 50 feet

o  4 Points: 51 - 100 feet

o  8 Points: 101 - 200 feet

Rain

How a burnt landscape reacts to storms and rain depends on the amount of time between the fire and the first rain, the amount of recovery by the landscape, the amount of debris littering the landscape and the rain’s intensity and duration. A heavy autumn and winter downpour should always be planned on when testing a landscape’s chances of erosion.

o  1 Point: A late autumn sprinkle with light to moderate storms through remaining season.

o  2 Points: Late autumn sprinkle with heavy winter down pour.

o  4 Points: No autumn rain and heavy winter down pour.

o  6 Points: Heavy early autumn and winter downpour. 

Type and Density of Plants Burnt

This is a forest versus grasslands comparison. You are looking at two measures: the amount of debris littering the landscape and the amount of plants remaining that can resprout. 

o  1 Point: Formerly densely forested landscapes. Trees have shrubs and possibly ground covers growing below them.

o  2 Points: Formerly a landscape with scattered trees and no understory shrubs, or a landscape with only shrubs and ground covers. Oak woodland and coastal scrub communities are examples.

o  3 Points: Formerly a grassy landscape with scattered perennials.

4 Points: Formerly a landscape with diificult growing conditions and plants were shallow rooted and sparse. 


Type of Soil

The structure, density and size of a soil’s particles influences its likelihood of erosion. Clay soils are the least erodible, sand and gravel the most. 

But clay soils have other problems: they are much faster to produce runoff due to poor infiltration, and this leads to the washing of fine particles and the siltation of streams. Sand and gravel do not travel like clay.

·     1 Point: A soil dominant in clay that has silt, sand and/or organic matter. 

·     2 Points: A sandy soil mixed with silt and organic matter. 

·     3 Points: A clay soil with little or no organic matter.

·     6 Point: Sandy soils with little or no organic material. Loose and gravely rock. 

Amount and Type of Activity 

Activity by animals and humans has a large affect on likelihood of erosion. The more activity, the more erosion. Tunneling animals, such as gophers and ground squirrels, are a threat to stability, more so if the fire has displaced their predators.

People neglect drainage systems and improperly clear landscapes, and both increase chances of erosion greatly. But even walking on a burnt landscape can slow its recovery; trampling lowers germinations rates, redistributes seeds, and crushes new seedlings.

o  1 Point: Animals and people walking on the site.

o  3 Points: Storm drains and gutters clogged. Tunneling and browsing animals lack predators and their populations are large. 

o  4 Points: An area that was cleared sometime before the fire, and never replanted, allowing shallow rooted opportunists to grow. 

o  6 Points: A barren landscape. Also, massive cuts into a hill and/or fill brought in on a slope. 

Fire Intensity

A low temperature fire can cleanse and waken a dynamic landscape. A high temperature will do just the opposite—not much survives 2,000 degrees. 

Fire intensity not only affects a landscape’s rate of recovery, but also the amount of repellency a soil has. Hotter fires produce more repellency. This test assumes a fairly hot fire, slow recovery, and high repellency.

Offsite Water

Roadways, sidewalks, driveways and parking lots can sometimes deposit their runoff on surrounding properties. If your burnt landscape is receiving sheeting water, the risks of erosion are high.

o   0 Points: The site is not receiving sheeting water from nearby properties.

o   6 Points: The site is receiving sheeting water and it is flowing into the landscape.

Approximate Level of Risk

Between 6 - 13 points = Fairly Low Risk of Erosion

Between 14 - 20 points = Medium Risk of Erosion

Between 21 - 28 points = Fairly High Risk of Erosion

Between 29 - 44 points = High Risk of Erosion

* This test was developed in February 1996, after the Vision Fire in Pt. Reyes and Inverness, October 1995. It was developed in partnership with Robert Crowell of Cagwin and Dorward, a landscape architect and engineering firm in San Rafael, CA. This test is a fire-modified version of the Universal Soil Loss Equation, a nationwide standard developed for farmers.



Enter supporting content here

CREATING ECOLOGICALLY MEANINGFUL and MEASURABLE LANDSCAPES

This site  The Web

Website hosting by Web.com

Designed by Debbie Dunne * saildunne@juno.com * Dunne Designs 2009