SHASTA COLLEGE EROSION CONTROL BLANKET STUDY
Research by Community College Quantifies Erosion from Slopes
By Vance Howard and John McCullah
(This article was published in "Land and Water" magazine)
To see more photos of the blanket study, 

Introduction

In the fall of 1998, Shasta Community College (Redding, California) implemented a comparative study of thirteen different erosion control blankets.  Greenfix America provided the funding for the study.  Greenfix America and Reed and Graham, Inc. (Sacramento, California) provided erosion control blankets.  Manufacturers included Greenfix America, American Excelsior, Xcel, North and American Green.  Instructors and students from various classes, including Watershed Restoration, Heavy Equipment Operation, Soils, Ecosystem Inventory, and Natural Resources Statistics, conducted the study from October 1998 through June 1999.  One of the most notable results of the study was that on average the blankets provided an 81% reduction in soil loss.  Also noteworthy was the average soil loss of the equivalent of nearly 25 tons/acre on the bare soil plots, with no observed rills or gullies. 

A site on the Shasta Community College campus in Redding, California was chosen for the study.  The study was conducted on the outer slope of the levee of a recently constructed water treatment detention pond.  The study area was graded and “track walked” to ensure a uniform soil surface.  Additionally, the slope was raked by hand to create a uniform seedbed.  The slope was set to a gradient of 1 ¾ : 1, or approximately 60%.  Slope length was approximately 16 feet.  The slope was de-compacted to a depth of 6-8 inches.  The soil type for the study area was classified by the Soil Survey for Shasta County as Churn Loam (CdA).  The slope aspect was northeast. 

The purpose of the study was to test the effectiveness of the blankets to prevent erosion, aid in grass seed germination, and also to test the durability of the blankets.  The durability test was not conducted, as we were unable to obtain the proper instrumentation necessary to perform this part of the study.  This paper will focus on the erosion (soil loss) portion of the study.  A total of 15 study plots were created at the site.  All plots were seeded with Elymus glaucus (blue wild rye), a local native grass, at a rate of 40#/acre.  A total of 12 different blankets were installed in October 1998 by Shasta College faculty and students.  The blankets were installed using the manufacturers instructions and under the supervision of a Certified Professional Erosion Control Specialist (CPECS).  All blankets were installed at a length of 14 feet and width of 8 feet.  The tops of the blankets were keyed-in at the top of the slope.  One plot was mulched with straw mulch and two plots were left as controls (bare soil with seed only).

Before the first winter storm of 1998-‘99, sediment collection troughs were installed at the base of each study plot.  The collection trough was a pre-manufactured 11” deep continuous/(seamless) gutter, 6 feet long, with end caps and a downspout nipple.  The 3” flange on the gutters was pounded into the slope to ensure the sediment-laden runoff went into the trough, not under.  The gutters were installed with a 1-2% drop towards the downspout.  A filter fabric screen was fastened to the outer end of the downspout to keep sediment from escaping.

The troughs were emptied by hand a total of four times throughout the winter season (specified by city ordinance to be Oct. 15th – June 1st for construction purposes).  Five gallon buckets were labeled and assigned to each plot for storage of the sediment collected from the troughs.  At the end of the winter season the depth of the sediment in each bucket was measured and a volume of sediment was calculated for each plot.  The dry weight for a sample of soil with a known volume was determined.  The dry weight for the sediment from each plot was calculated (in tons).  The area of each plot was the same (112 sq. ft.) and was converted to acres.  Soil loss was determined for each plot and is presented as tons/acre. 

Average soil loss from the blanketed and straw mulched plots was 4.71 tons/acre, with a range from 3.09 to 7.36 tons/acre.  The two unblanketed control plots showed an average soil loss of 24.83 tons/acre (23.04 and 26.61 tons/acre).  It is important to note that no rills or gullies formed on the control plots.  Also noteworthy is that the blankets resulted in an 81% reduction in soil loss compared to bare soil.

The results of this study indicate that without proper erosion control (blankets, mulching, etc.) on 60% slopes, approximately 25 tons/acre of soil (mostly fines) can be expected to be lost without the typical indicators of erosion (rills and gullies) even forming.  This could be important information for the development/construction industry, as well as the public agencies that monitor and enforce the rules regulating discharge of storm water off-site.