Myth:
Hydraulic mulching application rates change as slope gradient increases.


Point of View:
In recent months there has appeared on some product packaging recommendations for hydraulic mulch application rates based on gradient of slope. For example, one manufacturer provides the following departures from a standard 3,000 lbs per acre application rate for their bonded fiber matrix (BFM):

< 4H to 1V                              2500 lb/ac

> 4H to 1V and < 3H to 1V       3000 lb/ac

> 3H to 1V and < 2H to 1V       3500 lb/ac

> 2H to 1V and < 1H to 1V       4000 lb/ac

> 1H to 1V                              4500 lb/ac

Steepness is probably not the only factor that needs to be considered in the erosion potential of slopes; slope length is generally considered to play a related and complementary role. For example, in all derivations of the Universal Soil Loss Equation (USLE) length and steepness of slope are considered as a joint “LS” factor for predicting soil erosion:

A = R x K x LS x C x P

where:
A = The annual rate of erosion
R = Rainfall factor
LS = Slope factor
C = Cover factor
P = Practice factor

There appears to be no research to support the conclusion that additional mulch (and costs) is needed to maintain erosion control performance of hydraulic applications on steeper slopes. Conversely, there doesn’t appear to be any quantitative basis for thinking one can apply less material on less steep slopes.

And while it is probably just fine for a manufacturer to designated how much of its product should be applied to meet its own expectations, the great fear – and history of the erosion control industry - is that unsupported, proprietary specifications sometimes find their way into ASTM or DOT protocols without the quantitative data to support them.