2002 Annual Report
Research
Characteristics of Erosion Control Measures and Their Impact on Erosion
Bruce Wilson, Associate Professor
Bradley Hansen, Assistant Scientist
Udai Singh, Research Associate
Hung Nguyen, Research Associate
Anita Thompson, USDA National Needs Fellow
Funding Source
Minnesota Department of Transportation
Objective
The goals of the project are to develop a better understanding of the
interactions of erosion control measures and fundamental erosion principles
and to extend the existing database to longer hillsides and to sandy soils.
The specific research objectives are:
- To determine the vegetative growth and erosion using blankets for sandy soils and for relatively long slopes.
- To determine the vegetative growth and hydraulic properties of turf reinforcement mats using field plots and measurements in a laboratory flume.
- To link the blanket and turf characteristics to vegetative growth,
and to detachment and transport of sediment using previously published
data.
Project Description
Erosion control blankets and synthetic turf mats are widely used to reduce
erosion from construction sites. Their effectiveness is currently evaluated
by empirical investigations. Although this approach is useful, each new
product requires expensive and tedious field experiments. Long-term progress
in selecting erosion control measures requires a better understanding
of the interactions of the control measures with the detachment and transport
of sediment. The effectiveness of blankets and turf mats can then be evaluated
using simulation tools instead of field experiments.
The first goal of the project is to extend the current Minnesota database
on the effectiveness of erosion control blankets. This will be accomplished
by using field plots on long hillsides with sandy soils. The second goal
is to obtain a better understanding of the interactions of erosion control
measures and fundamental erosion principles. The impact of blankets/mats
on the shear stress acting on soil particles will be investigated using
a laboratory flume. Additional insight will be obtained by compiling and
analyzing previously published data.
Results
Twelve plots with slope lengths of 30.5-m and twelve plots with slope
lengths of 18.3 m were installed to study erosion from a highway embankment.
Four erosion control treatments with three replicates were studied for
each set of plot lengths. The treatments for the 30.5-m plots were straw
blanket, wood blanket, spray emulsion product, and straw mulch. The treatments
for the 18.3-m plots were straw blanket, spray-emulsion product, straw
mulch, and no erosion control measures (bare soil). Data were collected
for two different stages of vegetative growth (spring and fall) and for
two different initial moisture contents (dry and wet). A low-impact sprinkler
system was designed and used to apply water to the plots. Runoff and sediment
data were collected for ninety-six different runs. Above-ground biomass
was also measured.
Above-ground biomass data were measured for each plot twice during the
growing seasons: once in the spring and once in the fall after the completion
of the wet runs. The spray-emulsion plots had the smallest amount of biomass
in the spring. No other trends in biomass data were apparent. Different
rainfall depths among plots made direct analyses of runoff depths and
sediment loads more difficult. To account for differences in rainfall
depths, the curve number was used as an index of runoff characteristics.
The curve number appeared to be most strongly influenced by the initial
moisture content of the plots. Curve numbers for the fall dry-runs were
typically smaller than those observed for the spring dry-runs. Vegetative
cover and initial moisture contents were different between these two sets
of runs. The curve numbers for the wet runs in spring and fall were roughly
equivalent. With the possible exception of smaller runoff depth from the
spray-emulsion plots during the spring runs, no trends in runoff depths
with treatment was apparent. Sediment load was substantially smaller for
the fall runs than the spring runs, likely the result of greater vegetative
cover. Relative sediment loads for the bare treatment were roughly eight
times larger than those observed for the other treatments. This trend
was observed for both wet and dry conditions and for spring and fall seasons.
There were no other apparent trends for the spring runs. For the fall
runs, the blankets and spray-emulsion relative sediment load was consistently
smaller than that observed for the straw mulch plot.
Particle detachment by surface runoff is fundamentally dependent on the
shear forces acting on them (particle shear). The effect of erosion control
blankets on reducing particle shear was explored in this study using a
laboratory flume and hot-film anemometry techniques. Experiments were
conducted to measure the impact of blanket type on the percentage of the
total shear acting on the bed. Another set of experiments investigated
particle shear as a function of blanket height above the flume floor.
These experiments showed a slight increase in particle shear as the distance
from the bottom of the blanket to the flume floor increased. Experiments
were also conducted to study the effect of fastener spacing on particle
shear. The data suggests that stapling density plays an important role
in reducing the shear stress responsible for particle detachment. The
percentage of total shear acting on the bed was less than 13.2% in all
experiments, indicating the importance of shear partitioning in the design
of erosion control systems.
The project also attempted to develop predictive relationships of blanket
performance using a regression analysis. Sediment load and vegetative
density data from TxDOT test facility were compiled and blanket characteristics
were requested from manufacturers. The number of blanket characteristics
obtained was too limited to evaluate the usefulness of possible regression
equations.