Module 3
Erosion Theory
Erosion Theory
Module Presenter: Michael Chase CPESC, CPESC, CESSWI
1
Presentation Agenda

Erosion Theory


Erosion and Sedimentation Process
Erosion Prediction
2
Erosion can be beautiful…
3
But not on your project site!
4
What is erosion?
 Soil erosion is the process by which soil particles become detached by water, wind, or gravity and are transported from their original location
5
Geologic Erosion
 Natural process
 Created current features
 Tempered by natural forces
 Causes little damage (unless assisted by human activity)
6
Erosion Process
Accelerated Erosion = natural erosion x human activities
7
Erosion Process
What can accelerate erosion problems?
 Removal of surface cover  Increased imperviousness (i.e., paving) that increases runoff
Increased imperviousness (i e paving) that increases runoff
 Exposure of more erodible soil
 Uncontrolled offsite runoff flowing through DSAs
8
Erosion Process
Rain hitting the land
surface
can dislodge
Unchecked
erosion
will
significant
commonly
lead to amounts of
pollutants
formation of channels
Sheet flow overland
canreceiving
erode slopes
The
water
bears the impact of
quantity and quality
degradation
9
What is Sedimentation?
 Sedimentation is the deposition of the eroded material 10
Erosion and Sedimentation
Erosion
Sedimentation
11
What is Turbidity?
 Turbidity is a measure of the degree to which the water looses its transparency due to the p
presence of suspended p
particulates
 The more total suspended solids in the water, the murkier it seems and the higher the turbidity
 Turbidity measurements and Suspended Particle measurements are not the same
12
What is Turbidity?
 Turbidity is measured in Nephelometric Turbidity Units (NTUs)
 The instrument used for measuring it is called nephelometer or turbidimeter, which measures the intensity of light scattered at 90 degrees as a beam of light passes through a water sample
Turbidity standards of 5, 50, and 500 NTU
13
What is Suspended Particles
Electrostatic
charge
e-
 Sediment or other particles eee-
e-
e-
eee-
that will remain suspended due to the energy of the water, or as a result of water or as a result of “like” like charges carried by the particles that causes particles to repel one another.
eee-
14
Erosion Process
15
BMP Tool Box
Choose the correct BMP for the type of control needed
 If you choose from the correct category you’ll have an effective BMP
Erosion Control
BMPs
Runoff Control
BMPs
Sediment
Control BMPs
16
BMP Tool Box – Erosion Control Control
17
BMP Tool Box – Runoff Control
18
BMP Tool Box – Sediment Control
19
Which are More Effective… Erosion Control or Sediment Control?
 Erosion controls are preferred
 keep the soil in place
k
h il i l
 enhance the protection of the site resources
 When possible, use erosion controls as the primary protection, with sediment controls as a secondary system.
20
Types of Erosion
 Splash Erosion
 Sheet Erosion (Overland Flow)
 Rill Erosion
 Gully Erosion
 Channel Erosion
21
Splash Erosion
 Rain drops striking bare soil directly at 5‐20 mph
 Detaches soil particles
 Particles can then be transported by a t c es ca t e be t a spo ted by
the action of water and/or wind
22
Raindrop Erosion
•
•
•
Primary source of erosion
Raindrop erosion is often imperceptible Indicators • Pedestals
• Stains
• Gravelling or Lag
23
Pedestal
Splash detachment carries away soil fines except where gravel protects the soil
24
Sheet Erosion (Overland Flow)
The removal of a uniform thin layer of soil by raindrop splash or water run‐off
 Surface film of water 1/16” –
1/8” deep
”d
 This process may occur unnoticed on exposed soil even though raindrops are eroding large quantities of soil 

This process eventually becomes more dramatic via the formation of rills and gullies
25
Rill Erosion
 Shallow surface flows that become condensed
 Well‐defined tiny channels
 Small enough to step across
 Often end part way up slope but can extend to crest by “headcutting”
 Increased velocity and turbulence
 The rate of rill erosion can be approximately 100 X greater than sheet erosion
26
Rill Erosion
• Rill Formation affected by: • Distance traveled
• Slope inclination
• Surface roughness
27
Gully Erosion
 Accumulating runoff becomes concentrated and forms small rills throughout the soil
 Several rills may form throughout a slope and eventually may join together to form Gullies
 The rate of gully erosion can be approximately 100 X greater than rill erosion
28
Gully Erosion
• Look for the following visual cues:
•
•
•
•
Large, deep cuts in soil
Single cuts
Branching cuts
Often too large to step across
• Often found in areas without evidence of other erosion types
Key Point – Gully and Rill erosion are caused by concentrated flows. Always treat the “problem” first – not the symptom.
29
29
Channel Erosion


Results from increased volume, velocity and or duration of flow, and concentration of flow ‐ primarily from increased impervious surfaces
Channel erosion occurs in areas where tributaries, storm drains and or culverts flow into unprotected channels Urbanization results in increases of impervious surfaces, which is reflected in incised and degraded stream channels
30
Channel Erosion
31
Channel Erosion
32
Wind Erosion

Depending on wind velocity and particle size, soil particles move by saltation, surface creep, and suspension.
33
Wind Erosion ‐ Stockpiles
34
Wind Erosion Control
 Control system for wind erosion work in one of two ways:
 Reduce wind speed on the soil surface
 Form a new, less erodible soil surface
Form a ne less erodible soil surface
35
Reducing Wind Speed at Soil Surface
 Covering the pile with a wind‐impervious fabric or g
p
p
other material
 Changing the pile orientation and shape
36
Forming a New Less Erodible Surface
 Spraying water to compact and weight the soil particles
 Applying a chemical dust suppressant or soil binder to form a crust or bind the surface soil particles together
 Establishing vegetation. Roots bind the soil together; stems and leaves reduce wind speed at soil surface
37
Junior Raindrop


It is often instructive to look at things from another perspective
Let’s get another perspective from “Junior Raindrop”, “Papa Cloud”, and “Mother Earth”
1948, U.S. Department of Agriculture, Forest Service
38
What did we learn from video?
How raindrop impact can infuriate Jr. or slowly let him infiltrate
 How Jr. will start to ‘run’ if nobody cares
“nobody cares”
 How rills and gullies form when Jr. starts to run as a “Gang”

• Key remedies:
• Mulch or Cover for raindrop impact
• Increase infiltration
• Decreased surface compaction ‐
organic matter incorporation as feasible
• Slope breaks, surface roughness, fiber rolls etc. to slow Jr. down
• Runoff reduction techniques to prevent concentrated flows and “gangster action”
39
What Did We Learn From “Junior”?
40
Pop Quiz – What kind of erosion is it?
41
Question #1
A ‐ Raindrop erosion
B ‐ Sheet erosion
C ‐ Rill erosion
y
D ‐ Gully erosion
E ‐ Channel erosion BMP Type Needed
A – Erosion Control
B – Runoff Control
C – Sediment Control
42
Question #2
A ‐ Raindrop erosion
B ‐ Sheet erosion
C ‐ Rill erosion
y
D ‐ Gully erosion
E ‐ Channel erosion BMP Type Needed
A – Erosion Control
B – Runoff Control
C – Sediment Control
43
Question #3
A ‐ Raindrop erosion
B ‐ Sheet erosion
C ‐ Rill erosion
y
D ‐ Gully erosion
E ‐ Channel erosion BMP Type Needed
A – Erosion Control
B – Runoff Control
C – Sediment Control
44
Question #4
A ‐ Raindrop erosion
B ‐ Sheet erosion
C ‐ Rill erosion
y
D ‐ Gully erosion
E ‐ Channel erosion BMP Type Needed
A – Erosion Control
B – Runoff Control
C – Sediment Control
45
Question #5
A ‐ Raindrop erosion
B ‐ Sheet erosion
C ‐ Rill erosion
y
D ‐ Gully erosion
E ‐ Channel erosion BMP Type Needed
A – Erosion Control
B – Runoff Control
C – Sediment Control
46
A ‐ Raindrop erosion
Question #6
B ‐ Sheet erosion
C ‐ Rill erosion
D ‐ Gully erosion
E ‐ Channel erosion BMP Type Needed
A – Erosion Control
B – Runoff Control
C – Sediment Control
47
Erosion Prediction‐ What We’ll Cover
 Factors that affect erosion
 Benefits of good site planning of temporary and permanent erosion control
48
Erosion Prediction







Estimate of average soil loss, expressed as “A”
Usually calculated as an average loss over a site
Losses at various parts of the site may differ greatly from one area to another
Typically calculated on an annual basis but can also be calculated on T i ll l l t d l b i b t l b l l t d a less frequent basis
May be calculated on a storm basis
Should not be confused with sediment yield; the terms are not interchangeable
Apply erosion prediction models when
 planning temporary and permanent erosion control measures
 sizing sediment control BMPs
 comparing pre‐ and post‐project erosion rates
49
Sediment Yield
 Should not be confused with erosion; the terms are not interchangeable
 Amount of eroded soil that settles out at a point in the watershed that is remote from the origin of the g
detached soil particles
 Includes erosion from slopes, channels, and mass wasting, minus sediment deposited before it reaches the point of interest
 Calculate sediment yield to
 understand how much eroded material will settle out at a point of interest
50
Erosion Prediction

Models are available to predict erosion rate
 Universal Soil Loss Equation (USLE)  Revised Universal Soil Loss Equation (RUSLE) and RUSLE2

Most do not estimate sediment yield…RUSLE2 does
51
Erosion Prediction
Revised Universal Soil Loss Equation (RUSLE)
A = R x K x LS x C x P
There a 5 major factors influencing erosion:  A = Average Annual Soil Loss (tons/ac/yr)
 R = Rainfall Factor
 K = Soil Erodibility Factor
 L/S = Slope Length and Steepness Factors
 C = Soil Cover Factor
 P = Practice Factor
52
Rainfall Erosivity (R) Factor
 When factors other than rainfall are held constant, soil loss is directly proportional to a rainfall factor composed of total storm kinetic energy (E) times the maximum 30‐min intensity (I30) (Wischmeier and Smith, 1958)
53
Soil Erodibility (K) Factor
 Ease with which soil is detached by splash during rainfall or by surface flow, or both
 Fine‐textured soils with clay have low K values (about 0.05 to 0.15)…particles are resistant to detachment
 Coarse‐textured soils (e.g., sandy soils) have low K values (about 0.05 C
t t d il (
d il ) h l K l ( b t to 0.2)…high infiltration resulting in low runoff even though these particles are easily detached
 Medium‐textured soils (e.g., silt loam) have moderate K values (about 0.25 to 0.45)…moderately susceptible to particle detachment and they produce runoff at moderate rates
 Soils having a high silt content are especially susceptible to erosion and have high K values (can exceed 0.45) and can be as large as 0.65. 54
LS Factor
 Accounts for the effect of topography on erosion
 L factor represents the slope length
 S factor represents the slope steepness
55
RUSLE Slope Schematic
56
Average Watershed Slope (%)
Sheet
Flow
Length
(ft)
<3
6
9
12
15
25
50
75
100
150
200
250
300
400
600
800
1000
0.2
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.06
0.06
0.06
0.06
0.06
0.06
0.06
0.5
0.07
0.07
0.07
0.07
0.07
0.07
0.08
0.08
0.09
0.09
0.10
0.10
0.10
0.11
0.12
0.12
0.13
1.0
0.09
0.09
0.09
0.09
0.09
0.10
0.13
0.14
0.15
0.17
0.18
0.19
0.20
0.22
0.24
0.26
0.27
2.0
0.13
0.13
0.13
0.13
0.13
0.16
0.21
0.25
0.28
0.33
0.37
0.40
0.43
0.48
0.56
0.63
0.69
3.0
0.17
0.17
0.17
0.17
0.17
0.21
0.30
0.36
0.41
0.50
0.57
0.64
0.69
0.80
0.96
1.10
1.23
4.0
0.20
0.20
0.20
0.20
0.20
0.26
0.38
0.47
0.55
0.68
0.79
0.89
0.98
1.14
1.42
1.65
1.86
5.0
0.23
0.23
0.23
0.23
0.23
0.31
0.46
0.58
0.68
0.86
1.02
1.16
1.28
1.51
1.91
2.25
2.55
6.0
0.26
0.26
0.26
0.26
0.26
0.36
0.54
0.69
0.82
1.05
1.25
1.43
1.60
1.90
2.43
2.89
3.30
8.0
0.32
0.32
0.32
0.32
0.32
0.45
0.70
0.91
1.10
1.43
1.72
1.99
2.24
2.70
3.52
4.24
4.91
LS Factors
Average Watershed Slope (%)
Sheet
Flow
Length
(ft)
<3
6
9
12
15
25
50
75
100
150
200
250
300
400
600
800
1000
10.0
0.35
0.37
0.38
0.39
0.40
0.57
0.91
1.20
1.46
1.92
2.34
2.72
3.09
3.75
4.95
6.03
7.02
12.0
0.36
0.41
0.45
0.47
0.49
0.71
1.15
1.54
1.88
2.51
3.07
3.60
4.09
5.01
6.67
8.17
9.57
14.0
0.38
0.45
0.51
0.55
0.58
0.85
1.40
1.87
2.31
3.09
3.81
4.48
5.11
6.30
8.45
10.40
12.23
16.0
0.39
0.49
0.56
0.62
0.67
0.98
1.64
2.21
2.73
3.68
4.56
5.37
6.15
7.60
10.26
12.69
14.96
20.0
0.41
0.56
0.67
0.76
0.84
1.24
2.10
2.86
3.57
4.85
6.04
7.16
8.23
10.24
13.94
17.35
20.57
25.0
0.45
0.64
0.80
0.93
1.04
1.56
2.67
3.67
4.59
6.30
7.88
9.38
10.81
13.53
18.57
23.24
27.66
30.0
0.48
0.72
0.91
1.08
1.24
1.86
3.22
4.44
5.58
7.70
9.67
11.55
13.35
16.77
23.14
29.07
34.71
40.0
0.53
0.85
1.13
1.37
1.59
2.41
4.24
5.89
7.44
10.35
13.07
15.67
18.17
22.95
31.89
40.29
48.29
50.0
0.58
0.97
1.31
1.62
1.91
2.91
5.16
7.20
9.13
12.75
16.16
19.42
22.57
28.60
39.95
50.63
60.84
60.0
0.63
1.07
1.47
1.84
2.19
3.36
5.97
8.37
10.63
14.89
18.92
22.78
26.51
33.67
47.18
59.93
72.15
57
Cover (C) Factor
 Reflect the effect of plant cover and management practices on erosion rates
 Is the factor used most often to compare the relative impacts of management options on conservation plans
58
Table 3-4
COVER INDEX FACTOR C -- CONSTRUCTION SITES
C Factors for Construction Sites
Type of Cover
None (fallow ground)
Factor C
Percent1
1.0
0.0
Temporary Seedings (90 percent stand):
Ryegrass (perennial type)
Ryegrass (annuals)
Small grain
Millet or sudan grass
Field bromegrass
0.05
0.1
0.05
0.05
0.03
95
90
95
95
97
Permanent Seedings (90 percent stand):
0.01
99
Sod (laid immediately):
0.01
99
0.25
0.13
0.07
0.02
0.02
0.06
0.10
75
87
93
98
98
94
90
Application Rate
Tons Per Acre
Mulch:
Hay
Hay
Hay
Hay
Small grain straw
Wood chips
Wood cellulose
l
.50
1.00
1.50
2.00
2.00
6.00
1.75
Percent soil loss reduction as compacted/with fallow ground.
Source:
USDA-NRCS, Connecticut Technical Guide.
59
Practice (P) Factor
 Ratio of soil loss with a specific support practice to the corresponding soil loss with upslope and downslope disturbance
60
Table 3-5
PRACTICE FACTOR P
SURFACE CONDITION FOR CONSTRUCTION SITES
P Factors for Construction Sites
Surface Condition with No Cover
Note: P=0.48 for Track Walking
(Testing performed at San Diego State Erosion Control Laboratory)
1
Factor P1
Compact and smooth, scraped with bulldozer
or scraper up and downhill.
1.3
Same condition, except raked with bulldozer
root rake up and downhill.
1.2
Compact and smooth, scraped with bulldozer
or scraper across the slope.
1.2
Same condition, except raked with bulldozer
root rake across the slope.
0.9
Loose as a disked plow layer.
1.0
Rough, irregular surface equipment tracks in
all directions.
0.9
Loose with rough surface greater than 12”
depth.
0.8
Loose with smooth surface greater than 12”
depth.
0.9
Values based on estimates.
Source:
USDA-NRCS, Connecticut Technical Guide.
61
Erosion Prediction: RUSLE2
 RUSLE2 is a computer‐aided method for predicting erosion
 Helps document site data needed for analyses
 RUSLE2 can help the designer justify an erosion control strategy


Selecting BMPs in RUSLE2 is an Iterative Process
RUSLE2 does not provide BMP specifications, cost, or absolute effectiveness
62
RUSLE2 is interactive (i.e., when input values are changed the soil loss and sediment delivery (yield) are re‐calculated)
63
Procedure for Estimating Gross Erosion
Gross Erosion = Sheet and Rill Erosion + Other Erosion
64
Sheet and Rill Erosion
 May be calculated using the USLE, RUSLE, or RUSLE2
65
Other Erosion
 Soil loss from gullies, channels, other concentrated flow may be determined by calculating the annual volume of soil removed from the eroded area
 Annual tons of soil loss can be determined by multiplying the volume by the density of the soil
66
Estimated Weight of Soils
Table 3-6
ESTIMATED WEIGHT OF SOILS1
Soil Textural Class
clay
silty clay, silty clay loam
sandy clay, loam, sandy loam
clay loam, silt loam
sandy clay loam, loamy sands, sands
Dry Density (lbs./ft
/ 3)
70-95
75-100
80-105
85-100
95-110
1
Data and estimates from published soil surveys, laboratory data and soil interpretation
records are to be used where available. Parent materials, soil consistency, soil structure, pore
space, soil texture, content of coarse fragments all have an influence on unit weight.
67
Review Problems
Review Problems
68
RUSLE Problem 1
 A new school development project will be constructed
 The Project will disturb 19.4 acres
 Project will have a duration of 1 year
 The average slope is ~25% (4H:1V)
The average slope is 25% (4H:1V)
 The average slope length is 100 feet
 Site will be compacted smooth and scraped with a bulldozer
69
RUSLE Problem 1
Determine the estimated soil loss from this site if it remains unprotected.
70
RUSLE Problem 1
A = R x K x LS x C x P
 A = Average Annual Soil Loss (tons/ac/yr)
g
y
 R = Rainfall Factor
 K = Soil Erodibility Factor
 L/S = Slope Length and Steepness Factors
 C = Soil Cover Factor
 P = Practice Factor
71
RUSLE Problem 1
 Obtain R from http://cfpub.epa.gov/npdes/stormwater/LEW/lewCalculator.cfm
 Result: R=51
Project location
j
72
RUSLE Problem 1
 Obtain K from http://websoilsurvey.nrcs.usda.gov/app/
 Result: K=0.20
73
RUSLE Problem 1
 Obtain LS from Table
 Result: LS=4.59
Average Watershed Slope (%)
Sheet
Flow
Length
(ft)
<3
6
9
12
15
25
50
75
100
150
200
250
300
400
600
800
1000
10.0
0.35
0.37
0.38
0.39
0.40
0.57
0.91
1.20
1.46
1.92
2.34
2.72
3.09
3.75
4.95
6.03
7.02
12.0
0.36
0.41
0.45
0.47
0.49
0.71
1.15
1.54
1.88
2.51
3.07
3.60
4.09
5.01
6.67
8.17
9.57
14.0
0.38
0.45
0.51
0.55
0.58
0.85
1.40
1.87
2.31
3.09
3.81
4.48
5.11
6.30
8.45
10.40
12.23
16.0
0.39
0.49
0.56
0.62
0.67
0.98
1.64
2.21
2.73
3.68
4.56
5.37
6.15
7.60
10.26
12.69
14.96
20.0
0.41
0.56
0.67
0.76
0.84
1.24
2.10
2.86
3.57
4.85
6.04
7.16
8.23
10.24
13.94
17.35
20.57
25.0
0.45
0.64
0.80
0.93
1.04
1.56
2.67
3.67
4.59
6.30
7.88
9.38
10.81
13.53
18.57
23.24
27.66
30.0
0.48
0.72
0.91
1.08
1.24
1.86
3.22
4.44
5.58
7.70
9.67
11.55
13.35
16.77
23.14
29.07
34.71
40.0
0.53
0.85
1.13
1.37
1.59
2.41
4.24
5.89
7.44
10.35
13.07
15.67
18.17
22.95
31.89
40.29
48.29
50.0
0.58
0.97
1.31
1.62
1.91
2.91
5.16
7.20
9.13
12.75
16.16
19.42
22.57
28.60
39.95
50.63
60.84
60.0
0.63
1.07
1.47
1.84
2.19
3.36
5.97
8.37
10.63
14.89
18.92
22.78
26.51
33.67
47.18
59.93
72.15
74
Table 3-5
RUSLE Problem 1
PRACTICE FACTOR P
SURFACE CONDITION FOR CONSTRUCTION SITES
 Obtain P from Table
 Result: P=1.3
Surface Condition with No Cover
Compact and smooth, scraped with bulldozer
or scraper up and downhill.
1
Factor P1
1.3
Same condition, except raked with bulldozer
root rake up and downhill.
1.2
Compact and smooth, scraped with bulldozer
or scraper across the slope.
1.2
Same condition, except raked with bulldozer
root rake across the slope.
0.9
Loose as a disked plow layer.
1.0
Rough, irregular surface equipment tracks in
all directions.
0.9
Loose with rough surface greater than 12”
depth.
0.8
Loose with smooth surface greater than 12”
depth.
0.9
Values based on estimates.
Source:
USDA-NRCS, Connecticut Technical Guide.
75
RUSLE Problem 1 ‐ Solution
A = (51)(0.20)(4.59)(1)(1.3) =
61 tons/acre/year
x 19.4 acres =
1,183 tons per year
76
RUSLE Problem 2
What if we track walk the slope and spread straw mulch at 2 tons/acre?
77
RUSLE Problem 2
Table 3-4
COVER INDEX FACTOR C -- CONSTRUCTION SITES
Type of Cover
Factor C
None (fallow ground)
1.0
Percent1
0.0
Temporary Seedings (90 percent stand):
0.05
0.1
0.05
0.05
0.03
95
90
95
95
97
Permanent Seedings (90 percent stand):
0.01
99
Sod (laid immediately):
0.01
99
0.25
0.13
0.07
0.02
0.02
0.06
0.10
75
87
93
98
98
94
90
Ryegrass (perennial type)
Ryegrass (annuals)
Small grain
Millet or sudan grass
Field bromegrass
Application Rate
Tons Per Acre
Mulch:
Hay
Hay
Hay
Hay
Small grain straw
Wood chips
Wood cellulose
C Factors for Construction Sites
l
.50
1.00
1.50
2.00
2.00
6.00
1.75
Percent soil loss reduction as compacted/with fallow ground.
Source:
USDA-NRCS, Connecticut Technical Guide.
78
Table 3-5
RUSLE Problem 2
PRACTICE FACTOR P
SURFACE CONDITION FOR CONSTRUCTION SITES
Surface Condition with No Cover
Compact and smooth, scraped with bulldozer
or scraper up and downhill.
Note: P=0.48 for Track Walking
(Testing performed at San Diego State Erosion Control Laboratory)
1
Factor P1
1.3
Same condition, except raked with bulldozer
root rake up and downhill.
1.2
Compact and smooth, scraped with bulldozer
or scraper across the slope.
1.2
Same condition, except raked with bulldozer
root rake across the slope.
0.9
Loose as a disked plow layer.
1.0
Rough, irregular surface equipment tracks in
all directions.
0.9
Loose with rough surface greater than 12”
depth.
0.8
Loose with smooth surface greater than 12”
depth.
0.9
Values based on estimates.
Source:
USDA-NRCS, Connecticut Technical Guide.
79
RUSLE Problem 2 ‐ Solution
A = (51)(0.20)(4.59)(0.02)(0.48) =
0.4 tons/acre/year
x 19.4 acres =
x 19 4 acres ~8 tons per year!!
Demonstrates the benefits of good site planning of both temporary and permanent erosion control
80
Questions
Answers
Discussion
81