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COMPARISON OF THREE SOIL EROSION CONTROL TREATMENTS
A Research Paper Presented for the
Master of Science in Agriculture and Natural Resources Degree
The University of Tennessee at Martin
Eatedal Alqusaireen December 2012
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Acknowledgments
I would like to thank all the people that helped me finish this project. I am grateful to my
advisor, Dr. Sandy Mehlhorn, for her help and support during the project. Special thanks to Dr.
Barb Darroch for her close monitoring and editing of this work. I would also like to thank Dr.
Joey Mehlhorn and Scott Watson, the UT Martin Farm manager, for their help in the preparation
of rills and installation of treatments. I also thank the donors of the materials used in this project.
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Abstract
Soil erosion is a concern that affects agriculture, wildlife and bodies of water. Soil
erosion can be avoided by maintaining a protective cover on the soil, creating a barrier to the
erosive agent, or by modifying the landscape to control runoff amounts and rates. The objective
of this study was to compare the effectiveness of three erosion control treatments on rill erosion
in a hill-slope area. Sweetgum (Liquidambar styraciflua) fruits (sweetgum balls) were compared
to riprap and sod to determine their effectiveness in controlling erosion, compared to a control
(no treatment). A randomized complete block design with three blocks was used. The study was
conducted on a Loring silt loam at The University of Tennessee at Martin campus between
February and October of 2012. Twelve rills were created to simulate erosion channels on a
hillside of 4.3% slope. Visual observations and before-after measurements of the rills were used
to evaluate the erosion levels of each treatment based on changes of the rills’ shapes and amount
of sedimentation. SAS was used to conduct analyses of variance on before-after measurements of
the rills’ depths and widths. Visual observations were consistent through all blocks for each
treatment. Results indicated that the sod was the most effective erosion control treatment,
followed by the riprap, and then by the sweetgum balls. Statistical analysis showed a significant
difference in the means among the erosion control treatments for some measurements. There
was significantly (P<0.05) less erosion (as measured by rills’ depth and width) in the sweetgum
ball treatment than in the control; therefore sweetgum balls were an acceptable treatment for rill
erosion.
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Table of Contents
Chapter 1: Introduction ................................................................................................................................. 1
Objective ................................................................................................................................................... 2
Chapter 2: Literature Review ........................................................................................................................ 3
Sweetgum balls ......................................................................................................................................... 7
Sod ............................................................................................................................................................ 9
Riprap ...................................................................................................................................................... 10
Erosion Measurement ............................................................................................................................. 12
Chapter 3: Materials and Methods .............................................................................................................. 14
Location .................................................................................................................................................. 14
Treatment Materials and Experimental Design ....................................................................................... 14
Experiment Techniques .......................................................................................................................... 17
Data Collection and Assessment ............................................................................................................. 21
Chapter 4: Results and Discussion .............................................................................................................. 22
Visual Observation .................................................................................................................................. 22
February .............................................................................................................................................. 22
March .................................................................................................................................................. 22
April .................................................................................................................................................... 27
May ..................................................................................................................................................... 29
June, July and August ......................................................................................................................... 29
September ........................................................................................................................................... 29
October ................................................................................................................................................ 29
Statistical Analysis .................................................................................................................................. 33
Chapter 5: Conclusion ................................................................................................................................. 40
References ................................................................................................................................................... 41
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List of Figures Figure 1. The shape of rill erosion ................................................................................................................ 4
Figure 2. Mulch from trimmed small plants and branches. .......................................................................... 8
Figure 3. The spiky fruits produced by sweetgum trees. .............................................................................. 8
Figure 4. Sod ready to be placed. .................................................................................................................. 9
Figure 5. Riprap-Large pieces of limestone that can be used to control erosion. ....................................... 10
Figure 6. The location of the rills used in the study as seen from Google earth (36°20'56.56" N 88°51'41.61" W) ......................................................................................................................................... 15
Figure 7. A Magallen Mobile Mapper 6 GPS unit was used to map the rills used in the study. ............... 15
Figure 8. Map of the rills used in the study. ............................................................................................... 16
Figure 9. Layout of treatments in the experiment using a randomized complete block design. ................. 16
Figure 10. a. The experiment was located on 4.3% slope; b. A furrower was used to create the rills; c. Rills were approximately 30.4 cm (1 ft) deep and 60.8 cm (2 ft) wide and d. 6.08 m (20 ft) long. ....... 17
Figure 11. Placement of the sweetgum balls in the rill. .............................................................................. 18
Figure 12. a. Geotextile filter placement within the rill, b. Riprap placement within the rill ..................... 19
Figure 13. Placement of Tifway Bermuda grass sod within the rill. ........................................................... 20
Figure 14. The four treatments in the first block at the beginning of the experiment: a. Sweetgum ball; b.Tifway Bermuda grass sod; c. Control; d. Riprap. ................................................................................... 20
Figure 15. Removing sweetgum balls (a) and filter under riprap (b) at the end of the experiment (October 2, 2012). ....................................................................................................................................... 20
Figure 16. Change in one sweetgum ball rill from February to June, 2012. ............................................... 23
Figure 17. Daily rainfall (inches) in February for Martin, TN (National Climatic Data Center, 2012)............................................................................................................................................... 24
Figure 18. Daily rainfall (inches) in March for Martin, TN (National Climatic Data Center, 2012). ........ 24
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Figure 19. Monthly rainfall in inches from February to October, 2012 for Martin, TN. ............................ 25
Figure 20. Change in the control rill during March, 2012. ......................................................................... 26
Figure 21. Change in the riprap rill during March, 2012. ........................................................................... 26
Figure 22. Change in the sweetgum balls rills during March, 2012. .......................................................... 27
Figure 23. Change in the Tifway Bermuda grass sod rills during March, 2012. ........................................ 27
Figure 24. Daily rainfall (inches) in April for Martin, TN (National Climatic Data Center, 2012). .......... 28
Figure 25. Change in all treatments rills as observed on April 8, 2012 a. Erosion at the edges of sweetgum ball rill, b. Tifway Bermuda grass sod rill, c. Soil on the surface of riprap rill and d. Sedimentation at the outlet of control rill. .................................................................................................. 28
Figure 26. Daily rainfall (inches) in May for Martin, TN (National Climatic Data Center, 2012). ........... 30
Figure 27. Change in all treatment rills as observed on May 9, 2012: a. Sweetgum ball, b. Tifway Bermuda grass sod, c. Control and d. Riprap. ............................................................................................. 30
Figure 28. Daily rainfall (inches) in June for Martin, TN (National Climatic Data Center, 2012). ........... 31
Figure 29. Daily rainfall (inches) in July for Martin, TN (National Climatic Data Center, 2012). ............ 31
Figure 30. Daily rainfall (inches) in August for Martin, TN (National Climatic Data Center, 2012). ....... 32
Figure 31. Final appearance of all treatment rills on September 29, 2012: a. Sweetgum ball, b. Tifway Bermuda grass sod, c. Control and d. Riprap. ............................................................................................. 32
Figure 32. Daily rainfall (inches) in September for Martin, TN (National Climatic Data Center, 2012). .. 33
Figure 33. Means of the final depth of the erosion control treatment rills. ................................................. 34
Figure 34. Means of the final outlet width of the erosion control treatment rills. ...................................... 35
Figure 35. Means of the difference between initial depth and final depth of the erosion control treatment rills. ............................................................................................................................................. 35 Figure 36. Means of the difference between final outlet width and initial outlet width of the erosion control treatments rills ................................................................................................................................ 36
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Figure 37. Sweetgum ball rill’s shape after removing sweetgum balls. a. Rill shape after removal, b. Sediments around sweetgum balls, and c. Erosion on the edge of rills. ................................................. 37
Figure 38. Riprap rill’s shape after removing riprap. a. Rill shape, b. Sediment on the filter, and c. Erosion at the outlet of the rill. ................................................................................................................ 39
Figure 39. Sweetgum ball properties: a. Spikes stick to the ground, and b. decay slowly. ........................ 39
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Chapter 1: Introduction
Soil erosion is the physical wearing of the earth’s surface by the action of water or wind.
It has been occurring for some 450 million years, since the first land plants formed the first soil.
There are two main types of soil erosion: geological and accelerated soil erosion. Geological soil
erosion happens at the same rate as soil is formed. Accelerated soil erosion is the loss of soil at a
much faster rate than it is formed (Favis-Mortlock, 2005)
Soil erosion is considered a serious problem all around the world. It is detrimental to
topsoil, which contains nutrients and organic matter. Loss of only 1/32 of an inch of topsoil can
represent a 5 ton per acre soil loss (USDA NRCS, 1996). This results in lower sustainability and
lower productive capacity of agriculture. During the last 40 years, nearly one-third of the world's
arable land has been lost by erosion and land continues to be lost at a rate of more than 24.7
million acres per year. In additions, soil erosion is considered to be the largest contributor to non-
point source pollution in the United States according to the federally mandated National
Pollution Discharge Elimination System (USEPA, 1997).
The rate of erosion from all cropland in Tennessee averaged 5.6 tons per acre per year
and 7.7 tons per acre per year on cultivated cropland (Denton, 2000). Compared to five years
earlier, this is down from 7.1 tons on all cropland and 9.1 tons on cultivated cropland. These
erosion levels are about half as high as 20 years ago. Tennessee has the highest rate of erosion of
cultivated cropland among the 50 states (Denton, 2000).
The rate and magnitude of soil erosion is affected by rainfall intensity and runoff, soil
erodibility, slope gradient and length, vegetation, and control treatments. Rill erosion results
when surface runoff concentrates, forming small yet well-defined channels (channels up to 30
cm deep). In many parts of the world, rill and gully erosion is the dominant form of water
2
erosion. Water erosion is most obvious on steep, convex landscape positions. (Shelton, 1987;
Agassi, 1996).
Soil erosion can be avoided by maintaining a protective cover on the soil, creating a
barrier to the erosive agent, and modifying the landscape to control runoff amounts and rates.
There are numerous treatments, combinations of treatments, and emerging products that may be
suitable for the site of erosion. Common treatments for erosion are mulching, vegetation,
terracing, riprap, matting, retaining walls, and reforestation (Rivas, 2006). There are different
ways to measure soil erosion. Visual, physical, chemical, and biological indicators can be used to
estimate soil surface stability or loss (USDA NRCS, 1996).
Objective
The objective of this study was to compare the effectiveness of three erosion control
treatments on rill erosion in a hill-slope area. Sweetgum (Liquidambar styraciflua) fruit
(sweetgum balls) was compared to riprap and sod treatments to determine their effectiveness in
controlling erosion, compared to a control rill (no treatment).
3
Chapter 2: Literature Review
The effects of erosion impact two places: on-site (where the soil has become detached),
and off-site (where the eroded soil goes). Subsequent harm results in damage to plants, animals,
and humans (Mamo and Hain, 2005). Erosion removes topsoil, reduces levels of soil organic
matter, and contributes to the breakdown of soil structure (Figure 1). This creates a less favorable
environment for plant growth. In soils that have restrictions to root growth, erosion decreases
rooting depth, which decreases the amount of water, air, and nutrients available to plants (on-site
effect). Erosion removes surface soil, which often has the highest biological activity and greatest
amount of soil organic matter. Nutrients removed by erosion are no longer available to support
plant growth onsite, but can accumulate in water (off-site effect) where such problems as algal
blooms and lake eutrophication may occur (Favis-Mortlock, 2005). Deposition of eroded
materials can obstruct roadways and fill drainage channels. Sediment can damage fish habitat
and degrade water quality in streams, rivers, and lakes. In addition, blowing dust can affect
human health and create public safety hazards (USDA NRCS, 1996).
The simplest mathematical model for prediction of soil loss is the Universal Soil Loss
Equation (USLE), which has been frequently used around the world since it was developed by
American statistician W. H. Whichmeier in the 1960s. The USLE describes average annual soil
loss rates based on estimated and measured input data. The input data is divided into five
different factors: rainfall erosivity, soil erodibility, topography, crop management and
conservation practice (Agassi, 1996). The Revised Universal Soil Loss Equation (RUSLE),
which is a computerized version of USLE with improvements in many of the aforementioned
factors, was initially released for public use in 1992. RUSLE is used by numerous government
agencies and private businesses and individuals to assess the magnitude of rill
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Figure 1. The shape of rill erosion
Source: http://www.erosionpollution.com
erosion, to pin point situations where erosion is serious, and to guide development of plans to
control soil erosion (USDA ARS, 2009).
The rate and magnitude of soil erosion is affected by rainfall intensity and runoff, soil
erodibility, slope gradient and length, vegetation, and erosion control treatments (Wall et al.,
2003). Both rainfall and runoff factors must be considered in assessing a water erosion problem.
Raindrops on the soil surface can break down soil aggregates. Light soil aggregates can be easily
removed by raindrop splash and runoff water, while large soil aggregates need more raindrop
energy or runoff amounts to be moved. Raindrop splash, which is the movement of soil by
rainfall, is noticeable during short-duration, high-intensity thunderstorms. Runoff can occur
whenever there is excess water on a slope that cannot be absorbed into the soil or trapped on the
surface. Runoff from agricultural land may be greatest during spring months when the soils are
usually saturated, snow is melting and vegetative cover is minimal (Favis-Mortlock, 2005).
Soil erodibility is another factor that controls the rate and magnitude of soil erosion.
Erodibility is an estimate of the ability of soils to resist erosion, based on the physical
characteristics of each soil. Soils with faster infiltration rates, higher levels of organic matter and
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improved soil structure have a greater resistance to erosion. The soils on eroded sites tend to be
more erodible than the original topsoils, because of their poorer structure and lower organic
matter content. Slope gradient and length also affect soil erosion: the steeper the slope of a field,
the greater the amount of soil erosion. As the slope length increases, the soil erosion by water
increases due to the greater accumulation of runoff (Agassi, 1996). However, vegetation cover
and plant residue protect the soil from raindrop impact and splash, which slows down the
movement of surface runoff and allows excess surface water to infiltrate. The effectiveness of
vegetative covers depends on the type, extent and quantity of cover. Partially incorporated
residues and residual roots are also important, as these provide channels that allow surface water
to move into the soil (Wall et al., 2003).
Signs of soil erosion include rills or cuts visible on the soil surface after rain or snowmelt,
soil accumulated at the bottom of slopes or depressions, soil on knolls that is lighter in color,
stones visible on hilltops, and crops buried with soil (Shelton, 1987). Soil erosion can be
categorized into several types: splash erosion, sheet erosion, rill erosion, gully erosion, and
tunnel erosion. Water erosion is most obvious on steep, convex landscape positions (Wall et al.,
2003; Agassi, 1996). In many parts of the world, rill and gully erosion are the dominant forms of
water erosion. Rills are shallow drainage lines resulting when surface runoff concentrates,
forming small yet well-defined channels (channels up to 30 cm deep) (Rivas, 2006). They
develop when surface water concentrates in depressions or low points and erodes the soil. Rill
erosion is common in bare agricultural land, particularly overgrazed land, and in freshly
cultivated soil where the soil structure has been loosened. The rills can usually be removed with
farm machinery. Rill erosion is often described as the intermediate stage between sheet erosion
and gully erosion (Heubeck and Schütt, 2007).
6
Soil erosion can be avoided by maintaining a protective cover on the soil, creating a
barrier to the erosive agent, and modifying the landscape to control runoff amounts and rates.
Protective cover can absorb some of the energy of the water that causes the erosion. (Mamo and
Hain, 2005). There are numerous treatments, combinations of treatments, and emerging products
that may be suitable to control erosion. The general erosion-control treatments can be
categorized into grade-related, seed, fertilizer, soil amendments, soil stabilizers and tackifiers,
mulch, rolled erosion-control products, hard armor, and soil bioengineering. Common treatments
for erosion are mulching, vegetation, terracing, riprap, matting, retaining walls, and reforestation
(Rivas, 2006).
One of the most common erosion control treatments is mulching. Mulching is an
effective and cost efficient management practice for stabilizing construction sites, eroded banks,
and topdressing buffer plantings. The primary function is to protect the underlying soil from
erosion and keep soil out of a water body. Erosion control mats (ECM) were developed to
protect soil from erosion in areas that receive high traffic, exposure to the elements or occur on
steep slopes. One key to the effectiveness of ECM is the size and shape of the material. When
applied, the long and fairly thin material essentially weaves itself together and creates a kind of
blanket over the soil (USDA NRCS, 2012a). ECM trap plenty of moisture, similar to standard
bark mulch. Mulching treatments have a wide range of material application. The materials used
depend on the availability, cost, appearance and effect of materials on the environment and can
include organic residues, compost, rubber mulch, and plastic mulch. Organic residue might be
grass clippings, leaves, hay, straw, shells, woodchips, wool, and animal manure (Dehan 1996).
Mulch material varies from artificial material to plant residue. Figure 2 shows trimmed small
7
plants and branches that are used as mulch. Some fruits of hardwood trees, such as sweetgum
fruits, have the potential to be used as a mulch material.
Sweetgum balls
Sweetgum (Liquidambar styraciflua) is one of the most common hardwoods in the
eastern United States (USDA NRCS, 2012b). It is a shade tree that can be found from
Connecticut to Florida and in the mountains of Central America ranging from Mexico to
Panama. The trees range west all the way to Texas, Iowa and Oklahoma. Fruits of sweetgum are
spiky green balls that turn brown with age (Figure 3). They are just over one inch wide and
dangle on a long stalk. The sweetgum trees are known for their porcupine-like fruit or the
sweetgum pods that are often called “Sweetgum balls.” The mature spiny sweetgum balls remain
on the tree branches through the winter. They drop off intermittently in the spring, and need to be
raked up for several months. Many consider it a major annoyance in the Southern US, where the
trees are plentiful (USDA NRCS, 2012b). Awkward to feel underfoot, the sharp, spiky brown
orbs with stems attached cover otherwise tidy lawns and driveways in this part of the country and
fall into the street to be smashed flat by cars. They can also cause damage to lawn mowers (Jett,
2006). The sweetgum balls can be considered as a candidate for mulching material. Some
gardeners use them to help fill the bottom third of large flower pot containers to save potting soil
and to allow better drainage. Many people use them to keep cats and dogs from ruining their
landscapes. The seed pods take so long to decompose that they can be used as a soil amendment.
For lawns that are mostly clay or rocky, mixing the soil with sweetgum balls can help keep the
soil loose and aerated (Jett, 2006).
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Figure 2. Mulch from trimmed small plants and branches.
Source: http://www.cuckooforcoconuts.com
Figure 3. The spiky fruits produced by sweetgum trees.
Source: http://en.wikipedia.org
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Sod One of the most effective erosion control materials available is grass sod. Sodding is a
permanent form of erosion control that involves laying a permanent cover of grass sod on
exposed soils. Because of the high density of grass blades, thick root mass and total weight, sod
stops erosion immediately. In addition to stabilizing soils, sodding can reduce the velocity of
storm water runoff. Sodding can provide immediate vegetative cover for critical areas and
stabilize areas that cannot be vegetated by seed. It can also stabilize channels or swales that
convey concentrated flows and can reduce flow velocities. Selection of grass in the sod is
primarily determined by region, availability, and intended use. For example, in Alabama, the
choice of species is limited to Bermuda grass (Cynodon dactylon), zoysia (Zoysia), centipede
(Eremochloa ophiuroides), St. Augustine (Stenotaphrum secundatum), tall fescue (Festuca
arundinacea), and bahiagrass (Paspalum notatum ) (Pitt, 2003). According to a project at The
Pennsylvania State University in 1986, grassed areas established with turfgrass sod are up to 15
times more effective in controlling runoff than seed established grass, even after three years
(Beard and Green, 1994). Figure 4 shows a pallet of sod ready to be used in landscaping or
erosion control.
Figure 4. Sod ready to be placed. Source: http://www.sodding.com
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Riprap
An efficient erosion control treatment for sloped sites (such as river banks, shorelines,
creeks, highway ditches, etc) is riprap. Riprap is permanent erosion control –a resistant
protective layer of rock intended to prevent soil erosion in areas of concentrated flow, turbulence
or wave energy. There are many specifications for riprap. The amount and size of rock needed
depend on each site's exposure to erosion. Many states recommend using 30.48 cm (12-inch)
diameter, or larger, natural rock for most moderate erosion areas. Large pieces of limestone are
commonly used to control erosion (Figure 5) (PESC, 2010). A filter must be placed under the
riprap to prevent water from removing the underlying soil material through the voids in the
riprap. The filter material under the rock can be fabric, gravel, crushed stone or small rock. The
size of the filter material depends on the site, but it should be smaller than the riprap. If gravel or
other material isn't available, a filter cloth can be used (Best Management Practices Manual.
2010).
Figure 5. Riprap-Large pieces of limestone that can be used to control erosion. Source: https://www.landscapemulch.com
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There are many soil erosion treatment studies that are found in the literature. Treatments
vary based on type of soil, topography, climate, and land use. Materials used depend on type and
abundance of useful raw material.
Benik et al. (2003) applied five treatments on a slope of a newly constructed highway:
wood fiber blanket, straw with coconut blanket, straw blanket, a bonded-fiber matrix, and disk-
anchored straw mulch. The largest soil erosion was observed in a bare area with no treatment.
The soil erosion there was ten times more than that from the straw-mulch treatment.
China is one of the countries that suffer most from erosion. The Chinese have conducted
a large amount of research and work in this area. For example, Xu et al. (2010) evaluated the
effects of revegetation on eroded soil biochemical and biological properties in China. Artificial
revegetation can effectively enhance the productivity of degraded soil caused by erosion in
subtropical areas. Six artificial revegetation treatments were used in the experiment: crenate
gugertree (20 years), Chinese fir (20 years), Chinese fir (12 years), grapefruit (12 years), annual
ryegrass (12 years) and the sixth was without treatment. Among the species used in the study,
crenate gugertree and ryegrass enhanced soil biological properties better than the other species.
Faucette et al. (2007) examined different erosion control practices such as wood mulch
blankets, yard waste compost, blending wood and compost at different percents, and straw
blanket with polyacrylamide (PAM). Faucette et al. (2007) concluded that the greater the
percent of compost used in an erosion control blanket, the lower the total runoff and slower the
rate of the runoff. In addition, the compost blanket was the best erosion control practice,
followed by the blended compost and wood blanket.
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Erosion Measurement
There are many reasons to measure erosion: to assess erosion for an erosion inventory,
for scientific erosion research, to develop and evaluate erosion control technology, to develop
erosion prediction technology, to allocate conservation resources and to develop policies and
regulations (Stroosnijder, 2003).
Soil erosion can be measured in different ways. Visual, physical, chemical, and biological
indicators can be used to estimate soil surface stability or loss (USDA NRCS, 1996). Visual
indicators include comparisons of aerial photographs taken over time, presence of moss and
algae (crypotogams), presence of crusts in desert or arid soils, changes in soil horizon thickness,
and deposition of soil at field boundaries. Physical indicators include measurements of aggregate
stability, and increasing depth of channels, rills, and gullies. Chemical indicators of erosion
include decreases in soil organic matter content, increases in calcium carbonate content at the
surface (provided large amounts of calcium carbonate exist in subsurface layers) and changes in
cation-exchange capacity (CEC). Finally, the biological indicators of erosion are decreased
microbial biomass, lower rate of respiration in the soil, and slower decomposition of plant
residues (USDA NRCS, 1996).
Shit and Maiti (2012) designed a facility to quantify the effects of grass roots on the
erodibility of lateritic topsoil by concentrated flow in Paschim Medinipur, West Bengal, India.
The slope gradient and flow rate were controlled. The concentrated flow erosion was tested for
three lateritic topsoils (bare, scattered grass-root-permeated and densely grass-root-permeated)
and exposed to two slopes (25 and 35%) by using a hydraulic flume. Their findings showed that
there was a significant negative exponential relationship between relative soil detachment and
13
root density. They concluded that the formula based on root density has the potential to improve
methodology for assessing soil detachment rate in concentrated flow for lateritic topsoil.
Geographic information system (GIS) techniques can also be used to evaluate and
monitor soil erosion. Digital analyses of spatial and climatic data were used to estimate soil
erosion on a basin wide scale for Mourganis catchment, Kalabaka area, in cental Greece (Tsimi
et al. 2012). The GIS database included the following geospatial data: land elevation, cities,
river network, road network, geology, and land use. The spatial features were digitized:
catchment boundaries, streams, geology formations, elevation contours, and elevation. The input
data included slope angle of the ground surface, elevation, rainfall, land cover, and geology. The
digital elevation model was constructed by combining elevation information from photogram-
metrically extracted contours and elevation points. The results from the soil erosion model were
compared with a photo-interpretation from Google Earth (Tsimi et al. 2012).
14
Chapter 3: Materials and Methods
Location
This project was conducted at The University of Tennessee at Martin campus between
February and October of 2012. The soil type for the study area was a Loring silt loam with 2 to 5
percent slopes; it is considered to be an eroded soil (USDA NRCS, 2012c). The selected study
area is located at 36°20'56.56" N 88°51'41.61" W north of campus next to the motor pool
(Figure 6).
A geographic information system (GIS) was used to map the 12 rills used in the
experiment. The global positioning coordinates of the rills were measured using a Magallen
Mobile Mapper 6 (Figure 7) with Arc Pad software. The data were taken on March 25, 2012. The
XY coordinates of the rills were measured using points features. Each rill has four points located
at each corner. After entering these feature points into the GIS software, they were saved as
polyline features and edited. A map of the experiment was created using ArcGIS (Figure 8).
Treatment Materials and Experimental Design
Four treatments were used: sweetgum ball mulch, Bermuda grass sod, riprap, and no-
treatment as a control. Sweetgum balls were collected locally from nearby trees and farms and
stored in bins near the site. A net was used to wrap the sweetgum balls inside the rills to prevent
sliding or moving. Riprap was donated and delivered to the site. The average diameter of the
riprap was 25.4 cm (10 inches) and it was composed of limestone rock. Geotextile fabric filter
was placed below the riprap to prevent erosion below the rocks. Tifway Bermuda grass sod was
donated by McCurdy Sod Farm and was placed within the rills. A randomized complete block
design (RCBD) with three blocks was used for this project (Figure 9).
15
Figure 6. The location of the rills used in the study as seen from Google earth (36°20'56.56" N 88°51'41.61" W)
Figure 7. A Magallen Mobile Mapper 6 GPS unit was used to map the rills used in the study.
Motor Pool
16
Figure 8. Map of the rills used in the study.
Figure 9. Layout of treatments in the experiment using a randomized complete block design.
Block 1
Gu
mb
all
Sod
Con
trol
Riprap
Block 3
Rip
rap
Sod
Con
trol
Gum
ball
Block 2
Gum
ball
Rip
rap
Con
trol
Sod
G
radient D
irection
17
Experiment Techniques
The experiment was located on a hillside with an average slope of 4.3% (Figure 10a).
Erosion channels were simulated on February 24, 2012 by creating several rills along the hill
slope using a furrower attached to a tractor (Figure 10b). Each rill was 30.4 cm (1ft) deep and
60.8 cm (2ft) wide (Figure 10c). The rills were 6.08 m (20 feet) long (Figure 10d).
Figure 10. a. The experiment was located on 4.3% slope; b. A furrower was used to create the rills; c. Rills were approximately 30.4 cm (1 ft) deep and 60.8 cm (2 ft) wide and
d. 6.08 m (20 ft) long.
a b
c d
18
Sweetgum balls were collected locally and held in place using a plastic net fixed to the
bed of the rill. The sweetgum balls were placed by putting a net on the ground of the rill and
filling with more than 0.17 m3 (six cubic feet) of sweetgum balls for each rill. The net was then
tied together, and four stakes were put along the rill to hold the netted sweetgum ball to the
ground (Figure 11).
Riprap treatment installation followed Tennessee division of water control and United
States army corps of engineers guidelines (Tennessee Department of Transportation, 2001). The
average diameter of rocks used along the rill was 2504 cm (10 inches). Geotextile fabric filter
from Preen was placed below the rocks to prevent erosion below the rocks (Figure 12).
Figure 11. Placement of the sweetgum balls in the rill.
Net around
Sweetgum ball
19
Figure 12. a. Geotextile filter placement within the rill, b. Riprap placement within the rill.
The third treatment in the experiment was the use of Tifway Bermuda grass sod that was
laid along the bed of the rill (Figure 13). Three wooden stakes were placed through the rill to
hold the sod in place until a root system could be established.
Figure 14 shows all four treatments in the first block at the beginning of the experiment
(on February 24, 2012).
The final step in the project was removing two treatments in all blocks at the end of the
experiment. Sweetgum balls and riprap were removed from the rills on October 2, 2012 to
observe the change of the rills’ bed. Figure 15 shows removing the Sweetgum ball and the riprap
with the fabric filter.
a b
20
Figure 13. Placement of Tifway Bermuda grass sod within the rill.
Figure 14. The four treatments in the first block at the beginning of the experiment: a. Sweetgum ball; b.Tifway Bermuda grass sod; c. Control; d. Riprap.
Figure 15. Removing sweetgum balls (a) and filter under riprap (b) at the end of the experiment (October 2, 2012).
a b c d
a b
21
Data Collection and Assessment
Data were collected to assess the erosion caused by rainfall and runoff from February to October
of 2012. Rill erosion treatments were observed on a weekly basis and immediately after rainfall.
Visual observation for erosion were conducted and documented. Observations included taking
photos for each treatment at critical locations along the rill. The treated rills were compared to
the control rill (no treatment) for all blocks. In addition to visual observations, measurements of
depth and width of the rills were taken at both beginning and end of the project period. The
width of the rills was measured at both inlet and outlet of the rill. The depth of the rill was
measured at five different sections along the rill and then averaged. The measurements were
taken for sweetgum ball, riprap, and control rills. No measurements were taken for Tifway
Bermuda grass sod because it was hard to remove at the end of the project. ANOVA tests for
RCBD were run for all measured variables for sweetgum ball, riprap, and control rills. The tested
variables were initial depth, final depth, initial inlet width, final inlet width, initial outlet width,
final outlet width, the difference between initial depth and final depth, the difference between
final inlet width and initial inlet width, and the difference between final outlet width and initial
outlet width.
22
Chapter 4: Results and Discussion
Visual Observation
Visual assessment to evaluate erosion levels was based on the observed change of the
rills’ shapes. Also, assessment was based on the relative amount of sedimentation at the outlet of
the rills for each treatment compared to that of the control rills (no treatment). The final
assessment for each rill treatment was rated from high to no change relative to the erosion level
of the control rill. The results were summarized by treatment in each block separately and an
average for all blocks during the whole period. All treatments had consistent changes throughout
all blocks. The block number in the discussion will not be mentioned because all the blocks had
similar changes.
The photos in Figure 16 show an example of the changes in sweetgum ball rills over the
data collection period. Pictures were taken at different time periods after heavy rain falls. Similar
photos were used to compare the other treated rills.
February
Since rills were prepared on February 24th, there was no change in the rills’ shapes and
sedimentation levels during February, although there was one heavy rainfall during that period
(Figure 17).
March
March was the critical period for this study where most changes in the rills occurred,
especially for the control rill. There were three storms with heavy rainfall and more than four
times of low intensity rainfall or scattered showers (Figure 18), and the maximum rainfall
occurred during this month (5.57 inches) (Figure 19). The control treatments had the biggest
change in the shape of the rill and in the amount of sedimentation. By the first week of March,
23
Figure 16. Change in one sweetgum ball rills from February to June, 2012.
Feb 29 Mar 6 Mar 10 Mar 17
Mar 25 Apr 8 Apr 17
May 9
May 1
May 15 Jun 3
24
Figure 17. Daily rainfall (inches) in February for Martin, TN
(National Climatic Data Center, 2012).
Figure 18. Daily rainfall (inches) in March for Martin, TN
(National Climatic Data Center, 2012).
25
Figure 19. Monthly rainfall in inches from February to October, 2012 for Martin, TN.
the sedimentation was obvious in the outlet of the control rill (Figure 20d). Also, on March 17th
and 25th, sedimentation increased and a small groove was formed in the middle of the rill
(Figure 20c,d).
The rate of the erosion in the other treatments was not as obvious as in the control rills.
The erosion rate in the riprap rill was low in March. There were only small changes at the outlet
of the rills (Figure 21). The same observation was noted for the level of erosion at the sweetgum
ball rills (Figure 22). In the sweetgum balls rills, there were some grooves at the outlet of the rills
by the end of March. There were no signs of erosion in the sod rills in all blocks and they did not
show any changes to the shape of the rills nor any sedimentation (Figure 23).
26
Figure 20. Change in the control rill during March, 2012.
Figure 21. Change in the riprap rill during March, 2012.
Mar 6 Mar 10
Mar 17 Mar 25
Mar 6 Mar 25
27
Figure 22. Change in the sweetgum balls rills during March, 2012.
Figure 23. Change in the Tifway Bermuda grass sod rills during March, 2012.
April
April had one heavy rainfall storm and three events with low rainfall intensity (Figure
24). The total rainfall in April was 2.1 inches (Figure 19). There were only slight changes in all
treated rills in April (Figure 25). In the control rill, sedimentation increased and the depth of the
grooves in the rills’ beds also increased. There were some changes in the sweetgum ball rills
with some erosion noted, especially at the edges. The soil on the surface of riprap rocks was
evidence of some erosion, especially at the edges of the rills. No observed changes were noticed
in the sod rills.
Mar 6 Mar 25
Mar 6 Mar 25
28
Figure 24. Daily rainfall (inches) in April for Martin, TN
(National Climatic Data Center, 2012).
Figure 25. Change in all treatments rills as observed on April 8, 2012 a. Erosion at
the edges of sweetgum ball rill, b. Tifway Bermuda grass sod rill, c. Soil on the surface of riprap rill and d. Sedimentation at the outlet of control rill.
a b
c d
29
May
Three rainfall events occurred in May with low intensity (Figure 26). The rate of erosion
was high, similar to that in April. The erosion of the sweetgum ball rill at the edge and outlet
was noticeable (Figure 27). The sod rills had no signs of erosion. On the other hand, there was a
change at the bottom part of the edges of the riprap rills. One third of the control rill was filled
with sediment and the groove in the middle of the rill was bigger than the previous month.
June, July and August
During June and July, there were scattered showers of rainfall with low intensity (Figure
28 and 29). August had few events of low intensity rainfall and one medium intensity rainfall
event that occurred at the end of August (Figure 30). The mean temperature in the Martin area
was 5.8 ᵒF above normal for the entire summer. This broke the previous record set in 1936.
Precipitation was 12.41" below normal. It was third driest summer since 1936 (Tennessee
Climatological Service, 2012). There was no change in the rills after the month of May until the
end of August.
September
In September there were minor changes in the rills’ shapes. The final shape of the rills
can be seen in Figure 31. Weeds had grown tall during the summer and there was a need for
application of weed killer and trimming. Because of the weeds, changes in the rills’ appearance
were difficult to detect even though there were high rainfall intensity events during this month
(Figure 31).
October
The data collection process ended on October 2nd where the sweetgum ball and the riprap
were removed.
30
Figure 26. Daily rainfall (inches) in May for Martin, TN
(National Climatic Data Center, 2012).
Figure 27. Change in all treatment rills as observed on May 9, 2012: a. Sweetgum ball, b. Tifway Bermuda grass sod, c. Control and d. Riprap.
a b
c d
31
Figure 28. Daily rainfall (inches) in June for Martin, TN
(National Climatic Data Center, 2012).
Figure 29. Daily rainfall (inches) in July for Martin, TN
(National Climatic Data Center, 2012).
32
. Figure 30. Daily rainfall (inches) in August for Martin, TN
(National Climatic Data Center, 2012).
Figure 31. Final appearance of all treatment rills on September 29, 2012: a. Sweetgum ball, b. Tifway Bermuda grass sod, c. Control and d. Riprap.
a b
c d
33
Figure 32. Daily rainfall (inches) in September for Martin, TN
(National Climatic Data Center, 2012).
The following discussion of the results is a comparison between the data collected in
February (beginning of study) and that in October (ends of study). This comparison was done by
statistical analysis. Sod couldn’t be removed and showed no change in the rills shape, which
means no observed erosion happened or the erosion was controlled.
Statistical Analysis
The results of the ANOVA tests indicated that four variables had a significant (P<0.05)
difference (Table 1). These variables were the final depth (Figure 33), final outlet width (Figure
34), the difference between initial depth and final depth (Figure 35), and the difference between
final outlet width and initial outlet width (Figure 36). Sweetgum ball rills had less erosion than
the control rills but more erosion than the riprap rills. The mean of the difference between final
outlet width and initial outlet width for sweetgum ball (4.1 cm) was significantly less than the
control (10.2 cm) but not significantly different from the mean difference for riprap (6.1 cm;
Table 1, Figure 36 ). Also, there was no difference between initial depth and final depth for
34
Table 1. Means of all variables for the sweetgum ball, riprap, and control treatments.
Treatments Initial depth
Final depth
Initial inlet
width
Final inlet
width
Initial outlet width
Final outlet width
Initial depth-Final depth
Final inlet width –
initial inlet width
Final outlet width-
initial outlet width
-------------------------------------------------------- cm ------------------------------------------------------- Sweetgum ball
21.7a* 16.7ab 42.7a 46.7a 43.7a 47.8c 5.0 a 4.1 a 4.1 b
Riprap
19.8 a 19.8 a 53.9a 56.9a 48.8a 54.9b 0.0 b 3.1 a 6.1 b
Control
20.8 a 14.5 b 44.7a 48.8a 49.8a 59.9a 6.3 a 4.1 a 10.2 a
*Within the columns, means followed by the same letter are not significantly different (at P = 0.05) using Ryan-Einot-Gabriel-Welsch multiple range test.
Figure 33. Means of the final depth of the erosion control treatment rills. Means labeled by the same letter are not significantly different (P=0.05)
using Ryan-Einot-Gabriel-Welsch multiple range test.
35
Figure 34. Means of the final outlet width of the erosion control treatment
rills.Means labeled by the same letter are not significantly different (P=0.05) using Ryan-Einot-Gabriel-Welsch multiple range test.
Figure 35. Means of the difference between initial depth and final depth of the erosion
control treatment rills. Means labeled by the same letter are not significantly different (P=0.05) using Ryan-Einot-Gabriel-Welsch multiple range test.
36
Figure 36. Means of the difference between final outlet width and initial outlet width of
the erosion control treatments rills. Means labeled by the same letter are not significantly different (P=0.05) using Ryan-Einot-Gabriel-Welsch multiple range test.
riprap, and there was a difference in depth for both sweetgum ball and control rills. The
difference in depth for sweetgum ball (5 cm) was less than control rill (6.3 cm) but these means
were not significantly different (Table 1, Figure 35).
Control rills showed a difference in the shape of the rills and the amount of sediments
when compared at the beginning and at the end of this project. The average depth of the control
rill was 14.5 cm (0.48 ft) at the end of the project compared to the 20.8 cm (0.68 ft) at the
beginning of the project period (Table 1).
The pictures in Figure 37 show the shape of the bed of the rills after removing the
sweetgum balls. The average depth of the rills after removing the sweetgum balls was decreased
in all blocks. For example, the average depth of the rill before placing the sweetgum ball was
21.6 cm (0.7 ft) and the outlet depth of the rill after removing the sweetgum balls was 16.7 cm
(0.55 ft; Table 1). There was some sediment around the removed sweetgum balls as shown in
37
Figure 37. Sweetgum ball rill’s shape after removing sweetgum balls. a. Rill shape after
removal, b. Sediments around sweetgum balls, and c. Erosion on the edge of rills.
Figure 37b. Also, Figure 37c shows the breakdown or the erosion of the soil at the edges of the
rill. There was some erosion at edges of sweetgum ball rills where soil was transported to the
bottom of the bed.
The pictures in Figure 38 show the shape of the bed of the rills after removing the riprap.
There was sediment in the riprap fabric filter after removing the riprap (Figure 38b). The width
of the riprap rill was increased at the outlet from 48.8 cm to 54.9 cm (1.6 ft to 1.8 ft; Table 1).
Figure 38c shows the erosion on the edge of the rill at the outlet. There was no difference in the
depth and the shape of the bed of the riprap rill. The amount of sediment in the fabric filter
indicates that erosion occurred at the edges since the depth had not changed.
The observed and measured erosion changes that occurred in the sweetgum ball rills were
smaller than the changes in the control (no treatment) rills. Similar studies in the literature
confirmed the effectiveness of mulch material similar to sweetgum balls as an erosion control
ba c
38
treatment. Benik et al. (2003) observed that soil erosion was ten times larger in bare areas than in
straw-mulch treated areas. Also, Faucette et al. (2007) concluded that the greater the percent of
compost used in an erosion control blanket, the lower the total runoff and slower the rate of the
runoff, which leads to reduced erosion.
Sweetgum balls had stuck to the bed of the rill and stayed in its place during the entire
period of the experiment. The spikes on the Sweetgum ball helped the netted mulch to stick to
the ground and prevented it from sliding (Figure 39a). After more than six months, the
Sweetgum balls were not decayed (Figure 39b), which proved the property of slow decay (Jett,
2006). Sweetgum balls showed desirable characteristics as a mulch material suitable for erosion
control. The sweetgum ball treatments had an advantage over riprap because agricultural
operations can be done on top of the treatment. This can include such operations as mowing and
placing desired vegetation cover on site. Also, grass was able to grow up through the sweetgum
balls to further stabilize the soil and help reduce erosion.
At the end of this study, the rate of erosion in all rill treatments was ranked from highest
to lowest as follows: control, sweetgum ball, riprap and finally sod.
39
Figure 38. Riprap rill’s shape after removing riprap. a. Rill shape, b. Sediment on the filter, and
c. Erosion at the outlet of the rill.
Figure 39. Sweetgum ball properties: a. Spikes stick to the ground, and b. decay slowly.
a b c
a b
40
Chapter 5: Conclusion
In conclusion, this study explored the effectiveness of three erosion control treatments. A
new proposed treatment was the use of sweetgum balls as a mulch-based erosion control
treatment. It was compared to riprap, sod, and to the control (no treatment). All three erosion
control treatments were effective in reducing erosion compared to the control (no treatment).
According to the visual observations of the rills, the sod was the most effective erosion treatment
followed by the riprap, and then followed closely by the sweetgum balls. The measurements of
rill depth and width before and after the installation of the treatments were tested statistically
using SAS. The statistical results showed a significant difference among the erosion control
treatments. Sweetgum ball treatment was considered as an acceptable and efficient treatment to
control erosion, similar to the other treatments in this study.
41
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