Erosion Theory Module Rev1 Compressed


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  • Emphasize detachment. Point out the two primary detachment mechanisms: raindrop or from the tractive force of flowing water.
  • Instructor Comments: Saltation (bouncing) begins when the wind speed reaches 13 mph @ 1 foot above the soil surface. Larger particles are dislodged and “creep” along the surface, dislodging more particles. Fine particles are dislodged and “suspended.”
  • Classic rill erosion
  • Raindrop and sheet erosion
  • Channel erosion from impervious surfaces
  • Gully Erosion, be careful putting fiber rolls on the top of slopes, they are NOT preferred over gravel or sand because the concentrated flows can get under them
  • Sheet and rill erosion
  • Looks like RUN ON was the problem, hydroseeding will not fix it but looks like sandbags have been installed and hydromuching for temp fix. Need to provide downdrain system to convey concentratated flow to bottom of slope.
  • Erosion Theory Module Rev1 Compressed

    1. 1. Module 2 Erosion Theory
    2. 2. <ul><li>Erosion Theory </li></ul><ul><ul><li>Erosion and Sedimentation Process </li></ul></ul><ul><ul><li>Erosion Prediction </li></ul></ul>Presentation Agenda
    3. 3. Erosion can be beautiful…
    4. 4. But not on your project site!
    5. 5. <ul><li>Soil erosion is the process by which soil particles become detached by water, wind, or gravity and are transported from their original location </li></ul>What is erosion?
    6. 6. Geologic Erosion <ul><li>Natural process </li></ul><ul><li>Created current features </li></ul><ul><li>Tempered by natural forces </li></ul><ul><li>Causes little damage (unless assisted by human activity) </li></ul>
    7. 7. <ul><li>Accelerated Erosion = </li></ul><ul><li>natural erosion x human activities </li></ul>Erosion Process
    8. 8. <ul><li>Removal of surface cover </li></ul><ul><li>Increased imperviousness (i.e., paving) that increases runoff </li></ul><ul><li>Exposure of more erodible soil </li></ul>Erosion Process What can accelerate erosion problems?
    9. 9. Erosion Process Rain hitting the land surface can dislodge significant amounts of pollutants Sheet flow overland can erode slopes Unchecked erosion will commonly lead to formation of channels The receiving water bears the impact of quantity and quality degradation
    10. 10. What is Sedimentation? <ul><li>Sedimentation is the deposition of the eroded material </li></ul>
    11. 11. Erosion Sedimentation Erosion and Sedimentation
    12. 12. What is Turbidity? <ul><li>Turbidity is a measure of the degree to which the water looses its transparency due to the presence of suspended particulates  </li></ul><ul><li>The more total suspended solids in the water, the murkier it seems and the higher the turbidity </li></ul>
    13. 13. What is Turbidity? <ul><li>Turbidity is measured in Nephelometric Turbidity Units (NTUs) </li></ul><ul><li>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 </li></ul>Turbidity standards of 5, 50, and 500 NTU
    14. 14. <ul><ul><li>Splash Erosion </li></ul></ul><ul><ul><li>Sheet Erosion (Overland Flow) </li></ul></ul><ul><ul><li>Rill Erosion </li></ul></ul><ul><ul><li>Gully Erosion </li></ul></ul><ul><ul><li>Channel Erosion </li></ul></ul>Types of Erosion
    15. 15. Splash Erosion <ul><li>Rain drops striking bare soil directly at 5-20 mph </li></ul><ul><ul><li>Detaches soil particles </li></ul></ul><ul><ul><li>Particles can then be transported by the action of water and/or wind </li></ul></ul>
    16. 16. Raindrop Erosion <ul><li>Primary source of erosion </li></ul><ul><li>Raindrop erosion is often imperceptible </li></ul><ul><li>Indicators </li></ul><ul><ul><ul><li>Pedestals </li></ul></ul></ul><ul><ul><ul><li>Stains </li></ul></ul></ul><ul><ul><ul><li>Gravelling or Lag </li></ul></ul></ul>
    17. 17. Splash detachment carries away soil fines except where gravel protects the soil Pedestal
    18. 18. Sheet Erosion (Overland Flow) <ul><li>The removal of a uniform thin layer of soil by raindrop splash or water run-off </li></ul><ul><li>Surface film of water 1/16” – 1/8” deep </li></ul><ul><li>This process may occur unnoticed on exposed soil even though raindrops are eroding large quantities of soil </li></ul><ul><li>This process eventually becomes more dramatic via the formation of rills and gullies </li></ul>
    19. 19. Rill Erosion <ul><li>Shallow surface flows that become condensed </li></ul><ul><li>Well-defined tiny channels </li></ul><ul><li>Small enough to step across </li></ul><ul><li>Often end part way up slope but can extend to crest by “headcutting” </li></ul><ul><li>Increased velocity and turbulence </li></ul><ul><li>The rate of rill erosion can be approximately 100 X greater than sheet erosion </li></ul>
    20. 20. Rill Erosion <ul><ul><li>Rill Formation affected by: </li></ul></ul><ul><ul><ul><li>Distance traveled </li></ul></ul></ul><ul><ul><ul><li>Slope inclination </li></ul></ul></ul><ul><ul><ul><li>Surface roughness </li></ul></ul></ul>
    21. 21. <ul><li>Accumulating runoff becomes concentrated and forms small rills throughout the soil </li></ul><ul><li>Several rills may form throughout a slope and eventually may join together to form Gullies </li></ul><ul><li>The rate of gully erosion can be approximately 100 X greater than rill erosion </li></ul>Gully Erosion
    22. 22. <ul><ul><li>Look for the following visual cues: </li></ul></ul><ul><ul><ul><li>Large, deep cuts in soil </li></ul></ul></ul><ul><ul><ul><li>Single cuts </li></ul></ul></ul><ul><ul><ul><li>Branching cuts </li></ul></ul></ul><ul><ul><ul><li>Often too large to step across </li></ul></ul></ul><ul><ul><ul><li>Often found in areas without evidence of other erosion types </li></ul></ul></ul>Key Point – Gully and Rill erosion are caused by concentrated flows. Always treat the “problem” first – not the symptom. Gully Erosion
    23. 23. Channel Erosion <ul><li>Results from increased volume, velocity and or duration of flow, and concentration of flow - primarily from increased impervious surfaces </li></ul><ul><li>Channel erosion occurs in areas where tributaries, storm drains and or culverts flow into unprotected channels </li></ul>Urbanization results in increases of impervious surfaces which is reflected in incised and degraded stream channels
    24. 24. Wind Erosion <ul><li>Depending on wind velocity and particle size, soil particles move by saltation, surface creep, and suspension. </li></ul><ul><li>May be estimated by: </li></ul><ul><li>E = f (I x K x C x L x V) </li></ul><ul><ul><li>E = the potential average annual soil loss in tons per acre, </li></ul></ul><ul><ul><li>f = a function of </li></ul></ul><ul><ul><li>I = the soil erodibility index. It is related to the percentage of non-erodible soil aggregates larger than 0.84 mm in diameter, </li></ul></ul><ul><ul><li>K = the surface roughness factor, </li></ul></ul><ul><ul><li>C = the climatic factor. It is based on the average wind velocity and surface soil moisture, </li></ul></ul><ul><ul><li>L = the unsheltered distance across a field or strip along the prevailing wind erosion direction, </li></ul></ul><ul><ul><li>V = the vegetative cover factor. </li></ul></ul>
    25. 25. Wind Erosion - Stockpiles <ul><li>E = 1.7 (s/1.5)( 365-p )(f/15) </li></ul><ul><li>235 </li></ul><ul><li>where </li></ul><ul><li>E = Total suspended particulates, lb/day/acre </li></ul><ul><li>s = silt content, percent </li></ul><ul><li>p = # days per year with > 0.01” rainfall </li></ul><ul><li>f = percent time with wind speed > 12 mph at mean pile height </li></ul>
    26. 26. Wind Erosion Control <ul><li>Control system for wind erosion work in one of two ways: </li></ul><ul><ul><li>Reduce wind speed on the soil surface </li></ul></ul><ul><ul><li>Form a new, less erodible soil surface </li></ul></ul>
    27. 27. Reducing Wind Speed at Soil Surface <ul><li>Covering the pile with a wind-impervious fabric or other material </li></ul><ul><li>Erecting a windscreen </li></ul><ul><li>Changing the pile orientation and shape </li></ul>
    28. 28. Forming a New Less Erodible Surface <ul><li>Spraying water to compact and weight the soil particles </li></ul><ul><li>Applying a chemical dust suppressant or soil binder to form a crust or bind the surface soil particles together </li></ul><ul><li>Establishing vegetation. Roots bind the soil together; stems and leaves reduce wind speed at soil surface </li></ul>
    29. 29. Play Jr. Raindrop video clip <ul><li>It is often instructive to look at things from another perspective </li></ul><ul><li>Let’s get another perspective from “Junior Raindrop”, “Papa Cloud”, and “Mother Earth” </li></ul>1948, U.S. Department of Agriculture, Forest Service Junior Raindrop
    30. 30. What did we learn from video? <ul><li>How raindrop impact can infuriate Jr. or slowly let him infiltrate </li></ul><ul><li>How Jr. will start to ‘run’ if “nobody cares” </li></ul><ul><li>How rills and gullies form when Jr. starts to run as a “Gang” </li></ul><ul><li>Key remedies: </li></ul><ul><ul><li>Mulch or Cover for raindrop impact </li></ul></ul><ul><ul><li>Increase infiltration </li></ul></ul><ul><ul><li>Decreased surface compaction - organic matter incorporation as feasible </li></ul></ul><ul><ul><li>Slope breaks, surface roughness, fiber rolls etc. to slow Jr. down </li></ul></ul><ul><ul><li>Runoff reduction techniques to prevent concentrated flows and “gangster action” </li></ul></ul>
    31. 31. What Did We Learn From “Junior”?
    32. 32. Pop Quiz – What kind of erosion is it?
    33. 33. Question #1 <ul><ul><li>A - Raindrop erosion </li></ul></ul><ul><ul><li>B - Sheet erosion </li></ul></ul><ul><ul><li>C - Rill erosion </li></ul></ul><ul><ul><li>D - Gully erosion </li></ul></ul><ul><ul><li>E - Channel erosion </li></ul></ul>
    34. 34. Question #2 <ul><ul><li>A - Raindrop erosion </li></ul></ul><ul><ul><li>B - Sheet erosion </li></ul></ul><ul><ul><li>C - Rill erosion </li></ul></ul><ul><ul><li>D - Gully erosion </li></ul></ul><ul><ul><li>E - Channel erosion </li></ul></ul>
    35. 35. <ul><ul><li>A - Raindrop erosion </li></ul></ul><ul><ul><li>B - Sheet erosion </li></ul></ul><ul><ul><li>C - Rill erosion </li></ul></ul><ul><ul><li>D - Gully erosion </li></ul></ul><ul><ul><li>E - Channel erosion </li></ul></ul>Question #3
    36. 36. <ul><ul><li>A - Raindrop erosion </li></ul></ul><ul><ul><li>B - Sheet erosion </li></ul></ul><ul><ul><li>C - Rill erosion </li></ul></ul><ul><ul><li>D - Gully erosion </li></ul></ul><ul><ul><li>E - Channel erosion </li></ul></ul>Question #4
    37. 37. <ul><ul><li>A - Raindrop erosion </li></ul></ul><ul><ul><li>B - Sheet erosion </li></ul></ul><ul><ul><li>C - Rill erosion </li></ul></ul><ul><ul><li>D - Gully erosion </li></ul></ul><ul><ul><li>E - Channel erosion </li></ul></ul>Question #5
    38. 38. <ul><ul><li>A - Raindrop erosion </li></ul></ul><ul><ul><li>B - Sheet erosion </li></ul></ul><ul><ul><li>C - Rill erosion </li></ul></ul><ul><ul><li>D - Gully erosion </li></ul></ul><ul><ul><li>E - Channel erosion </li></ul></ul>Extra credit: How would you fix it ? Question #6
    39. 39. <ul><li>Estimate of average soil loss, expressed as “A” </li></ul><ul><li>Usually calculated as an average loss over a site </li></ul><ul><li>Losses at various parts of the site may differ greatly from one area to another </li></ul><ul><li>Typically calculated on an annual basis but can also be calculated on a less frequent basis </li></ul><ul><li>My be calculated on a storm basis </li></ul>Erosion Prediction
    40. 40. <ul><li>Should not be confused with erosion; the terms are not interchangeable </li></ul><ul><li>Amount of eroded soil delivered to a point in the watershed that is remote from the origin of the detached soil particles </li></ul><ul><li>Includes erosion from slopes, channels, and mass wasting, minus sediment deposited before it reaches the point of interest </li></ul>Sediment Yield
    41. 41. <ul><li>Models are available to predict erosion rate </li></ul><ul><ul><li>Universal Soil Loss Equation (USLE) </li></ul></ul><ul><ul><li>Revised Universal Soil Loss Equation (RUSLE) and RUSLE2 </li></ul></ul><ul><li>Most do not estimate sediment yield…RUSLE2 does </li></ul>Erosion Prediction
    42. 42. <ul><li>There a 5 major factors influencing erosion: </li></ul><ul><li>A = Average Annual Soil Loss (tons/ac/yr) </li></ul><ul><li>R = Rainfall Factor </li></ul><ul><li>K = Soil Erodability Factor </li></ul><ul><li>L/S = Slope Length and Steepness Factors </li></ul><ul><li>C = Soil Cover Factor </li></ul><ul><li>P = Practice Factor </li></ul><ul><li>Universal Soil Loss Equation (USLE) </li></ul>A = R x K x LS x C x P Erosion Prediction
    43. 43. Rainfall Erosivity (R) Factor <ul><li>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) </li></ul>
    44. 44. Rainfall Erosivity (R) Factor <ul><li>R is the average annual sum of EI30 for storm events during a rainfall record of at least 22 years </li></ul><ul><li>&quot;Isoerodent&quot; maps developed for R values </li></ul><ul><li>R can also be obtained from </li></ul>
    45. 45. Soil Erodibility (K) Factor <ul><li>Ease with which soil is detached by splash during rainfall or by surface flow, or both </li></ul><ul><li>Fine-textured soils with clay have low K values (about 0.05 to 0.15)…particles are resistant to detachment </li></ul><ul><li>Coarse-textured soils (e.g., sandy soils) have low K values (about 0.05 to 0.2)…high infiltration resulting in low runoff even though these particles are easily detached </li></ul><ul><li>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 </li></ul><ul><li>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. NRCS soil data </li></ul>
    46. 46. Soil Erodibility (K) Factor <ul><li>K can be obtained from </li></ul><ul><ul><li> </li></ul></ul><ul><ul><li>Site-specific data </li></ul></ul>
    47. 47. LS Factor <ul><li>Accounts for the effect of topography on erosion </li></ul><ul><li>L factor represents the slope length </li></ul><ul><li>S factor represents the slope steepness </li></ul>
    48. 48. RUSLE Slope Schematic
    49. 49. LS Factors
    50. 50. Cover (C) Factor <ul><li>Reflect the effect of plant cover and management practices on erosion rates </li></ul><ul><li>Is the factor used most often to compare the relative impacts of management options on conservation plans </li></ul>
    51. 51. C Factors for Construction Sites
    52. 52. Practice (P) Factor <ul><li>Ratio of soil loss with a specific support practice to the corresponding soil loss with upslope and downslope disturbance </li></ul>
    53. 53. P Factors for Construction Sites Note: P=0.48 for Track Walking (Testing performed at San Diego State Erosion Control Laboratory)
    54. 54. <ul><li>RUSLE2 is a computer-aided method for predicting erosion </li></ul><ul><li>Helps document site data needed for analyses </li></ul><ul><li>RUSLE2 can help the designer justify an erosion control strategy </li></ul><ul><ul><li>Selecting BMPs in RUSLE2 is an Iterative Process </li></ul></ul><ul><ul><li>RUSLE2 does not provide BMP specifications, cost, or absolute effectiveness </li></ul></ul>Erosion Prediction: RUSLE2
    55. 55. <ul><li>RUSLE2 is interactive (i.e., when input values are changed the soil loss and sediment delivery (yield) are re-calculated) </li></ul>
    56. 56. <ul><li>Gross Erosion = </li></ul><ul><li>Sheet and Rill Erosion + Other Erosion </li></ul>Procedure for Estimating Gross Erosion
    57. 57. <ul><li>May be calculated using the USLE, RUSLE, or RUSLE2 </li></ul>Sheet and Rill Erosion
    58. 58. <ul><li>Soil loss from gullies, channels, other concentrated flow may be determined by calculating the annual volume of soil removed from the eroded area </li></ul><ul><li>Annual tons of soil loss can be determined by multiplying the volume by the weight of the soil </li></ul>Other Erosion
    59. 59. Estimated Weight of Soils
    60. 60. Review Problems
    61. 61. <ul><li>A new school development project will be constructed in San Diego </li></ul><ul><li>The Project will disturb 19.4 acres </li></ul><ul><li>Project will have a duration of 1 year </li></ul><ul><li>The average slope is ~25% </li></ul><ul><li>The average slope length is 100 feet </li></ul><ul><li>Site will be compacted smooth and scraped with a bulldozer </li></ul>RUSLE Problem 1
    62. 62. <ul><li>Determine the estimated soil loss from this site if it remains unprotected. </li></ul>RUSLE Problem 1
    63. 63. <ul><li>A = Average Annual Soil Loss (tons/ac/yr) </li></ul><ul><li>R = Rainfall Factor </li></ul><ul><li>K = Soil Erodability Factor </li></ul><ul><li>L/S = Slope Length and Steepness Factors </li></ul><ul><li>C = Soil Cover Factor </li></ul><ul><li>P = Practice Factor </li></ul>A = R x K x LS x C x P RUSLE Problem 1
    64. 64. <ul><li>Obtain R from </li></ul><ul><li>Result: R=51 </li></ul>RUSLE Problem 1 Project location
    65. 65. <ul><li>Obtain K from </li></ul><ul><li>Result: K=0.20 </li></ul>RUSLE Problem 1
    66. 66. RUSLE Problem 1 <ul><li>Obtain LS from Table </li></ul><ul><li>Result: LS=4.59 </li></ul>
    67. 67. RUSLE Problem 1 <ul><li>Obtain P from Table </li></ul><ul><li>Result: P=1.3 </li></ul>
    68. 68. <ul><li>A = (51)(0.20)(4.59)(1)(1.3) = </li></ul><ul><li>61 tons/acre/year </li></ul><ul><li>x 19.4 acres = </li></ul><ul><li>1,183 tons per year </li></ul>RUSLE Problem 1 - Solution
    69. 69. <ul><li>What if we track walk the slope and spread straw mulch at 2 tons/acre? </li></ul>RUSLE Problem 2
    70. 70. C Factors for Construction Sites RUSLE Problem 2
    71. 71. RUSLE Problem 2 Note: P=0.48 for Track Walking (Testing performed at San Diego State Erosion Control Laboratory)
    72. 72. <ul><li>A = (51)(0.20)(4.59)(0.02)(0.48) = </li></ul><ul><li>0.4 tons/acre/year </li></ul><ul><li>x 19.4 acres = </li></ul><ul><li>~8 tons per year!! </li></ul>RUSLE Problem 2 - Solution
    73. 73. Questions Answers Discussion