Rolled Erosion Control Products (RECPs) specification based on slope gradient.
Point of View:
Diagrams have been drawn and specifications written that recommend the use of various rolled erosion control products based solely on slope gradient. These unsupported recommendations were made by blanket manufacturers nearly two decades ago. This generally has resulted in more expensive products being specified and designed for steeper slopes when less expensive products would perform just as well.
For example, here’s a table of recommendations excerpted from one RECP manufacturer’s website: 4:1 to 3:1 Single-netted straw blanket 3:1 to 2:1 Double-netted straw blanket 2:1 to 1:1 Double-netted straw/coconut fiber blanket 1:1 and greater Coconut fiber blanket
Most RECP manufacturers and indeed, the Erosion Control Technology Council (ECTC) should concede that current research appears to support the idea that all netted and stitched RECPs, regardless of infill (straw, coconut fiber, wood fiber, synthetic fiber, etc.) have a high erosion control performance (i.e., 97% or better) when evaluated on slopes under rainfall simulation testing at Purdue University, Utah State University, Texas Transportation Institute or San Diego State University. Almost all products have been evaluated on slopes of at least 3:1 or greater, but most often the testing has occurred on the more steeper 2:1 slopes.
There is a logical flow to the conclusions one can draw from much of this testing, and it’s not that RECPs should be specified by slope gradient. On the contrary:
1. When one looks at testing data from a variety of sources, there appears to be no separation of performance based on the infill of the RECPs; i.e., straw, straw-coconut, coconut fiber and synthetic fibers all manage the same high performance standard on slopes of 3:1 or greater;
2. It is generally accepted that high performance levels of RECPs on the steepest of slopes (2:1) do not generally diminish on slopes of lesser gradient, i.e., < 2:1;
3. Furthermore, if there is some separation of performance between various RECPs on less steep slopes, it is statistically insignificant (i.e., at 97% erosion control effectiveness, any improvement based on lower slope gradient couldn’t amount to much);
4. The Texas Transportation Institute (TTI) and some Departments of Transportation (DOTs) do differentiate RECP specification on a combination of slope gradient and some other factor (e.g., both TTI and the Oregon DOT differentiate product application based on slope gradient and soil texture); therefore,
5. Although slope gradient is a contributing factor for RECP product specification, complementary variables such as material and installation cost, longevity, durability, environmental compatibility and others should be considered as proportionally important in the decision making process when selecting a BMP, and this principle applies not only to RECPs but to other non-rolled BMPs as well.
Finally, why is this important? Quite simply, to illustrate that many ingrained specifications are not supported by science and also that higher-priced technologies do not necessarily infer increased erosion control performance.