Soil Erosion and Sedimentation Control Training Manual

Transcription

1 Soil Erosion and Sedimentation Control Training Manual Michigan Department of Environmental Quality Water Bureau Soil Erosion and Sedimentation Control Program

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3 Soil Erosion and Sedimentation Control Training Manual Dick Mikula and Hope Croskey, P.E. Additional Credits Units 1-5 Unit 5 Unit 7 Unit 9 Unit 12 Units 1-12 John Warbach, Ph.D., Planning and Zoning, Inc. Sean McNamara, MDEQ Marlio Lesmez, Ph.D., MDEQ Hubbell, Roth & Clark, Inc. Mark Fife and Steve Houtteman, MDEQ Cheryl Gates, MDEQ The Michigan Department of Environmental Quality (MDEQ) will not discriminate against any individual or group on the basis of race, sex, religion, age, national origin, color, marital status, disability, or political beliefs. Questions or concerns should be directed to the MDEQ Office of Personnel Services, PO Box 30473, Lansing, MI PRINTED BY AUTHORITY OF PART 91, 1994 PA 451, as amended TOTAL NUMBER OF COPIES PRINTED: TOTAL COST: $ COST PER COPY: $ Michigan Department of Environmental Quality EQC 4580 (Revised November 2005)

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5 TABLE OF CONTENTS Unit One: Erosion and Sedimentation Goals and Principles Introduction Background Definitions Erosion Process Geologic and Accelerated Erosion Major Categories of Erosion Physical Factors Affecting Erosion The Sedimentation Process Principles and Strategies Summary Unit One Review Answers to Unit One Review Unit Two: Controlling Runoff and Erosion on Construction Sites Introduction Construction Practices to Control Runoff Scheduling Seeding and Mulching Preserve Vegetative Buffers Surface Roughening Diversions Grade Stabilization Structures Check Dams Channel and Slope Stabilization Practices to Control Wind Erosion Windbreaks Watering Chemical Binders Maintenance Summary Unit Two Review Answers to Unit Two Review i

6 Unit Three: Vegetative Stabilization Introduction Critical, Non-critical and Sensitive Areas Vegetative Stabilization Temporary Vegetative Stabilization Permanent Vegetative Stabilization Selecting Plant Materials Climate Soils Slopes Site Use and Maintenance Soil Preparation Soil Additives Topsoil Correction of Droughty and Wet Soils Seedbed Preparation Methods to Establish Vegetation When to Plant Mulching Maintenance Summary Unit Three Review Answers to Unit Three Review Appendix 3A: Guidelines for Vegetative Erosion Control...Appendix 3A-1 Unit Four: Controlling Sediment Introduction Vegetative Sediment Control Structural Sediment Control Perimeter Barriers Diversions Rock Construction Exits Sedimentation Basins Storm Drain Inlet Protection Summary Unit Four Review Answers to Unit Four Review Appendix 4A: Filter Strip...Appendix 4A-1 ii

7 Unit Five: Developing a SESC Plan and Inspecting the Installation Introduction Soil Erosion and Sedimentation Control Plans Required Information Analysis Inspections Summary Unit Five Review Answers to Unit Five Review Unit Six: Legislation and Administrative Rules Legislation: Part 91, Soil Erosion and Sedimentation Control Definitions; A to W Rules; availability of information Administration and enforcement of rules; resolution; ordinance; interlocal agreement; review; notice of results; informal meeting; probation; consultant; inspection fees; rescission of order, stipulation, or probation Ordinances Notice of violation Permit; deposit as condition for issuance Agreement between public agency or county or municipal enforcing agency and conservation district; purpose; reviews and evaluations of agency's programs or procedures; agreement between person engaged in agricultural practices and conservation district; notification; enforcement Designation as authorized public agency; application; submission of procedures; variance; approval Earth change; permit required; effect of property transfer; violation; notice; hearing; answer; evidence; stipulation or consent order; final order of determination Injunction; inspection and investigation Additional rules Logging, mining, or land plowing or tilling; permit exemption; mining defined a Earth change activities not requiring permit; violations Reduction of soil erosion or sedimentation by owner iii

8 Notice of determination Compliance; time Entry upon land; construction, implementation, and maintenance of soil erosion and sedimentation control measures; cost Reimbursement of county or municipal enforcing agency; lien for expenses; priority; collection and treatment of lien Violations; penalties Severability Training program; certificate; fees a Soil erosion and sedimentation control training fund; creation; disposition of funds; lapse; expenditures Unit Six Legislation Review Answers to Unit Six Legislation Review Part 91 Administrative Rules R Definitions R Earth change requirements generally R Soil erosion and sedimentation control plan requirements R Permit requirements R Permit exemptions and waivers R Application for permit R Application review and permit procedures R Soil erosion and sedimentation control procedures and measures generally R Earth change requirements: time; sediment removal; design installation, and removal of temporary or permanent control measures R Standards and specifications R Building permits R Enforcement R Periodic review R Availability of documents Unit Six Administrative Rules Review Answers to Unit Six Administrative Rules Review iv

9 Unit Seven: Soils and Runoff Introduction Soil and Runoff Characteristics Soil Texture Natural Drainage Soil Infiltration and Transmission Rates Surface Characteristics Rainfall Duration and Frequency Estimating Runoff Volume and Peak Discharge Runoff Problem #1: Undeveloped Site Runoff Problem #2: Site under Construction Runoff Problem #3: Developed Site Application of Runoff Calculations Summary Appendix 7A: Hydrologic Soil Groups for Michigan Soils...Appendix 7A-1 Appendix 7B: Practice Problems...Appendix 7B-1 Practice Problems A, B, and C...Appendix 7B-1 Answers to Practice Problems A, B and C...Appendix 7B-4 Unit Eight: Revised Universal Soil Loss Equation (RUSLE) Introduction Practice Problem A: Calculating Potential Soil Loss Practice Problem B: Calculating Potential Soil Loss Answers to Practice Problem A Answers to Practice Problem B Appendix 8A: Soil Erodibility K w Values for Michigan Soils...Appendix 8A-1 Unit Nine: Sedimentation Basins Introduction The Difference between Storm Water Basins and Sedimentation Basins Reviewing Sedimentation Basin Design Components Building Site Conditions Sedimentation Basin Location Sedimentation Basin Size Sedimentation Basin Configuration Sedimentation Basin Structures Other Design Considerations Sedimentation Basin Maintenance v

10 Sample Problems Problem A Problem B Problem C Appendix 9A: Glossary...Appendix 9A-1 Appendix 9B: Equations...Appendix 9B-1 Unit Ten: Diversions Introduction Design Considerations Soils Diversion Design Appendix 10A: NRCS Conservation Standards for Diversions...Appendix 10A-1 Unit Eleven: Map Interpretation and Plan Development and Review Introduction Map and Aerial Photo Evaluation U.S. Geological Survey (USGS) Topographic Maps Topographic Surveys County Soil Survey Aerial Photography Field Evaluation On-Site Field Visits Techniques and Tools for Field Review Other Field Observation Techniques Critical Erosion Areas Plan Development and Review Plan Development Plan Review Other Considerations Permit Issuance and Program Administration Summary Appendix 11A: Construction and SESC Measure Installation Schedule for Small Projects...Appendix 11A-1 Appendix 11B: Construction and SESC Measure Installation Schedule for Large Projects...Appendix 11B-1 vi

11 Unit Twelve: Michigan s Stormwater Program for Construction Sites Introduction Rules and Regulations Federal Clean Water Act Federal Storm Water Regulations Natural Resources and Environmental Protection Act Certified Storm Water Operator Site Stabilization and Permit Termination Summary Unit Twelve Review Answers to Unit Twelve Review Soil Erosion Inspection Log Appendix 12A: Notice of Coverage...Appendix 12A-1 Appendix 12B: Notice of Termination...Appendix 12B-1 vii

12 Unit One EROSION AND SEDIMENTATION GOALS AND PRINCIPLES INTRODUCTION Background The problem of erosion and sediment control has plagued society since colonial times. Early land clearing, logging, and farming damaged many streams and rivers (Figure 1-1). In the early 1930's, a nationwide soil conservation movement started which greatly reduced the agricultural erosion problem (Figure 1-2). Today, a typical farm conservation plan often reduces soil loss from 15 to 25 or more tons per acre per year, to less than five tons per acre per year. Figure 1-1 Figure 1-2 In recent years, construction activities have caused serious erosion and sedimentation problems. With modern equipment and technology, we create vast networks of highways (Figure 1-3), sprawling subdivisions (Figure 1-4), large industrial parks, and massive shopping centers. In many cases, these activities result in severe erosion and sedimentation damage to our land and water resources. Figure 1-3 Figure

13 It is estimated that from all sources, over 4.5 billion tons of sediment pollute the rivers of this country each year. This is the equivalent to a volume the size of 25,000 football fields, 100 feet high (Figure 1-5). It costs $8 to $12 per cubic yard to remove sediment from waterways. It is estimated that $6 billion to $13 billion per year are spent in the United States to correct the effects of erosion and sediment. Figure 1-5 Source: John Warbach, Planning and Zoning Center, Inc. Damage from erosion and sediment affects nearly every citizen. Erosion and sedimentation result in: Loss of fertile topsoil (Figure 1-6) Clogged ditches, culverts, and storm sewers that increase flooding (Figure 1-7) Muddy or turbid lakes and streams (Figure 1-8) Damage to plant and animal life (Figure 1-9) Filled-in ponds, lakes, and reservoirs (Figure 1-10) Damage to aquatic habitats and reduced recreational value and use (Figure 1-11) Structural damage to buildings, roads, and other structures (Figure 1-12) Figure 1-6: Loss of fertile topsoil Figure 1-7: Clogged ditches, culverts, and storm sewers that increase flooding 1-2

14 Figure 1-8: Muddy or turbid streams Figure 1-9: Damage to plant and animal life. Figure 1-10: Filled in ponds, lakes, and reservoirs Figure 1-11: Damaged aquatic habitats and reduced recreational value and use Figure 1-12: Structural damage to buildings, roads, and other structures Definitions What are erosion and sedimentation? Erosion and sedimentation are two separate, but inter-related processes. Both processes cause different types of environmental damage and require different control measures to minimize the impacts. 1-3

15 Erosion is the process by which the land surface is worn away by the action of wind, water, ice, or gravity. In simple terms, it is the process where soil particles are dislodged or detached and put in motion (Figure 1-13). Figure 1-13 Sedimentation is the process whereby the detached particles generated by erosion are deposited elsewhere on the land or in our lakes, streams, and wetlands (Figure 1-14). Together, the two processes result in soil being detached, carried away, and eventually deposited elsewhere (Figure 1-15). Figure 1-14 Figure 1-15 EROSION PROCESS Geologic and Accelerated Erosion There are two types of erosion: geologic and accelerated. Geologic erosion or natural erosion is the action of the wind, water, ice, and gravity in wearing away rock to form soil and shape the ground surface. Except for some stream and shore erosion, it is a relatively slow, continuous process that often goes unnoticed. Geologic erosion is reported to account for about 30 percent of all sediment in the United States each year. Accelerated erosion is a speeding up of erosion due to human activity. Whenever we destroy the natural vegetation or alter the contour of the ground without providing some sort of surface protection, we greatly increase the rate of erosion. This type of 1-4

16 erosion is reported to account for about 70 percent of all sediment generated in this country each year. Accelerated erosion can be minimized through careful planning and by implementing appropriate control measures. Farming, construction, logging, and mining are the principal causes of accelerated erosion. These activities radically upset the delicate balance that nature has developed between rainfall and runoff. Although all the sources mentioned above generate sediment, we will focus on construction. There are two major reasons that erosion is often increased during and after construction. The first one is the removal of protective natural vegetation. The second is the placement of impermeable surfaces like paving and rooftops on the soil. This prevents water infiltration and increases runoff. These two factors increase the likelihood that soil will be exposed to the erosive forces of water and wind. Major Categories of Erosion Wind erosion is common on agricultural lands and large construction sites. Soil that is piled and left unprotected is especially vulnerable to wind erosion. In some areas, more soil is lost from wind erosion than from water erosion. The Natural Resources Conservation Service (NRCS) recently estimated that wind erosion is responsible for 42 percent of the erosion damage occurring in Michigan annually. The amount of soil lost from wind erosion may not be realized because the soil particles disperse over a large area where they are not visible. In an urbanizing area, the most damaging aspect of wind erosion is dust. It creates traffic hazards, adds to cleaning costs, is abrasive to plant tissue, and blights the appearance of structures and other surfaces. The effects of water erosion are usually more visible than wind erosion. One can readily see gullies, turbid or muddy water, and sediment build-up. Runoff causes both stream channel erosion and overland erosion. Channel erosion occurs both in intermittent and permanent waterways and streams. Three causes of channel erosion are: increased runoff, removal of natural vegetation along the waterway, and channel alterations resulting from construction activities. It includes both stream bank and stream bed erosion. Overland erosion occurs on bare slopes as a result of rain splash and runoff. It is the predominate type of erosion and source of sediment from construction sites. Overland erosion is generally separated into three categories: sheet erosion, rill erosion, and gully erosion. Sheet erosion is the removal of a uniform layer of soil from the land surface as a result of rain splash and runoff. Rain splash is the impact of raindrops on a soil surface. The splash dislodges soil particles, making them more susceptible to movement by overland water flow. The loosened particles that are not washed away can form a muddy slick that clogs pores in the ground surface. The sealed surface 1-5

17 further reduces infiltration and increases runoff. The magnitude of soil loss resulting from rain splash can best be seen on a gravelly or stony soil (Figure 1-16). As runoff water moves down a slope, it increases in velocity and increases the potential for erosion. The volume of sediment also increases because the transported particles scour and dislodge more soil particles. Rilling is another form of overland erosion. Evidence of rill erosion is the development of small grooves spaced fairly uniformly along the slope. It is caused when runoff is heavy and water concentrates in rivulets (Figure 1-17). Individual rills range in depth and width up to several inches and reflect a tremendous loss of soil. If rilling is not corrected immediately, it will develop into gully erosion. Figure 1-16 Figure 1-17 The depth of erosion defines the difference between rills and gullies. Although there are no formal definitions for rills and gullies, it is commonly accepted that rills can be easily obliterated by normal tillage practices, whereas gullies cannot (Figure 1-18). Figure 1-18 Gullies do not always represent the culmination of unchecked rill erosion. Gullies can form wherever ground or paved surfaces concentrate water into an area that cannot handle the flow. Proper planning and construction practices prevent this from happening. 1-6

18 Physical Factors Affecting Erosion Erosion is affected by several physical factors; the common ones are: Climate Vegetative cover Soil Slope characteristics Climate The climatic factors that influence erosion are rainfall amount, intensity, and frequency. Rainfall amount is usually measured in inches. Rainfall intensity is the rate at which the rain falls. It is measured in inches of water falling in an hour of time. The infiltration rate is the rate that water is absorbed into the soil. It is also measured in inches per hour. When rainfall exceeds the infiltration rate, runoff occurs. The frequency of rainfall is the number of separate rainfall events occurring during a specific period of time, such as a week or month. During periods of frequent rainfall a greater percentage of the rainfall will become runoff because of high soil moisture or saturated soil conditions. Temperature is another climatic factor influencing erosion. While frozen soil is highly resistant to erosion, rapid thawing of the soil surface brought on by warm rains can lead to serious erosion. Temperature also influences the type of precipitation. Falling snow does not erode. However, heavy snowmelts in the spring can cause considerable runoff damage (Figure 1-19). Temperature also influences the amount of organic matter that collects on the ground surface and incorporates with the topsoil layer. Organic matter is plant and animal residue in various stages of decomposition (Figure 1-20). Areas with warmer climates have thinner organic cover on the soil because decomposition is more rapid. For example, states in the southwest, such as Arizona, have a thinner organic layer than northern states such as Michigan. Figure 1-19 Figure 1-20 Organic matter protects the soil by shielding it from the impact of falling rain and by soaking up rainfall that would otherwise become runoff. Organic matter also provides essential nutrients for plant growth. 1-7

19 Vegetative Cover Vegetation is probably the most important physical factor influencing soil erosion. A good cover of vegetation shields the soil from the impact of raindrops. It also binds the soil together, making it more resistant to runoff. A vegetative cover provides organic matter, slows runoff, and filters sediment. On a graded slope, the condition of the vegetative cover will determine whether erosion will be stopped or only slightly halted. A dense, robust cover of vegetation is one of the best protections against soil erosion. Soils Physical characteristics of soil have a bearing on erodibility. Soil properties influencing erodibility include texture, structure, and cohesion. Texture refers to the size or combination of sizes of the individual soil particles. Three broad soil size classifications, ranging from small to large, are clay, silt, and sand (Figure 1-21). Soils having a large amount of silt-sized particles are most susceptible to erosion from both wind and water. Soils with clay or sand-sized particles are less prone to erosion. Figure 1-21 Structure refers to the degree to which soil particles are clumped together, forming larger clumps and pore spaces. Structure influences both the ability of the soil to absorb water and its physical resistance to erosion. Organic matter influences the structure of most soils. In clay soils, it loosens the structure and allows more water to infiltrate. In granular structured sand or silt soils, organic matter tends to bind the soil into clumps that are more resistant to erosion. Soils with organic matter absorb and store more water than soils without organic matter. 1-8

20 The last soil property to consider is cohesion. Cohesion refers to the binding force between soil particles and influences the structure. When moist, the individual soil particles in a cohesive soil cling together to form a doughy consistency. Clay soils are in this category. Clay soils are very cohesive, while sand soils are not. Slope Steepness and Length The last physical factor we will discuss is slope. Slope steepness, length, and roughness affect erodibility. Generally, the longer the slope the greater the potential for erosion. The greatest erosion potential is at the base of the slope, where runoff velocity is the greatest and runoff concentrates. To avoid this problem, long slopes are often "broken up" so that they function as a series of short slopes rather than one long slope. Runoff is also slowed by using various runoff control structures, including diversions and terraces. These structures function to intercept runoff and thereby reduce the flow of water over the lower portion of the slope. Slope steepness, along with surface roughness, and the rainfall amount and intensity control the speed at which runoff flows down a slope. The steeper the slope, the faster the water will flow and the greater potential for erosion. Steepness of slope is expressed in several ways. The most common ways are as a ratio of the difference in the vertical and horizontal distance or as a percentage. For example, a slope with a 100-foot horizontal change for every 10 feet of vertical distance would be called a 10 to 1 or a 10 percent slope. Although we have little control over soil features and other natural factors, we do have control over how we develop a site and what measures we use to prevent or minimize erosion. After every effort has been made to prevent erosion, efforts should then be directed to controlling sediment. THE SEDIMENTATION PROCESS As previously explained, sedimentation is the process whereby eroded soil particles settle out or are deposited. Deposition of sediment occurs when soil-laden water slows enough to allow the different sized particles to settle out. Sediment deposition may result in one or more of the following: Increased flooding due to reduced channel or storm drain capacity Reduction of fish spawning areas Less desirable fish communities Reduction of aquatic insect communities Impaired or destroyed terrestrial habitats Reduced recreational opportunities Increased costs to keep harbors and marinas navigable There are important physical factors that influence the sedimentation process just as there were those affecting erosion. The interactions of these factors will determine how sediment is transported and deposited. 1-9

21 The velocity and turbulence of the runoff water are key factors in determining the fate of sediment. The greater the velocity and turbulence of flow, the greater will be the amount of sediment transported in suspension in the water or carried along the stream bottom as bedload. The lesser the velocity and turbulence of flow, the greater the amount of sediment deposited. The size, shape, and density of the transported particles also influence the rate at which they settle out. Smaller, lighter particles, such as clay-sized particles, are more easily transported. They stay suspended and are slow to settle out. Larger, heavier particles, such as sand, are harder to transport and thus are more quickly deposited. It may be useful or necessary to estimate potential loss of soil. Estimates are necessary, for example, when determining maintenance schedules for removing sediment from sedimentation basins or for prioritizing sites for periodic inspections. The NRCS has equations for estimating soil loss from both wind and water erosion. A more time consuming technique to determine soil loss is by sampling. Although sampling is not often done, sediment volume being transported in a stream can be calculated by measuring stream flow and sediment concentration in the stream. PRINCIPLES AND STRATEGIES The goal of erosion and sediment control is to protect land and water resources by minimizing erosion and off-site sedimentation, using the best practical combination of procedures, practices, and people. Let's look at each segment of this goal more closely: Protect land and water resources. Responsible people seek to be stewards of all our natural resources, including land and water. A balance must be met between resource protection and the other activities of the construction project. Minimizing erosion and off-site sedimentation. During construction activities, everything possible should be done to prevent the erosion of soil from the site and its deposition off-site and into surface waters and wetlands. Using the best practical combination of procedures, practices, and people. To control erosion and sediment we need workable laws, regulations, and procedures; up-to-date practices and techniques; and responsible people working together. The effective control of erosion and sedimentation requires the application of the following five principles of erosion and sediment control: One, plan the development to fit the particular topography, soils, waterways, and natural vegetation at a site. Think stewardship or a partnership with nature. When structures and grading are designed to fit the site less soil is exposed to erosive forces. The result can be both reduced environmental damage and savings in project costs. 1-10

22 Two, expose the smallest practical area of land for the shortest possible time, by scheduling and staging project activities. This means that the soil surfaces exposed during the first phase of the project are stabilized before beginning construction on the next phase. Daily seeding and mulching with permanent or temporary seeding mixtures is recommended. Three, apply soil erosion prevention practices as a first line of defense against onsite damage. Use practices that minimize erosion on a site to prevent sediment from being produced and the need for costly controls to trap and control sediment. Examples of erosion control practices include: Special grading methods Diversions Runoff control structures Temporary and permanent vegetation (Figure 1-22) Mulching Figure 1-22 Four, apply sediment control practices as a perimeter protection to prevent sediment from leaving the site. Use practices that control sediment once it is produced, and prevent it from getting off-site. Examples of sediment control are: Silt fences (Figure 1-23) Interceptor dikes and ditches Sediment traps Vegetative filters Sedimentation basins Figure 1-23 Five, implement a thorough inspection, maintenance, and follow-up program. Erosion and sedimentation cannot be effectively controlled without a thorough, periodic check of the site and continued maintenance of the control measures. An example of applying this principle would be a routine end-of-day check to be sure all control practices are working properly. 1-11

23 SUMMARY In summary, erosion and sedimentation includes the entire process whereby soil particles are detached from the ground surface, carried away, and eventually deposited. Erosion is the process by which the land surface is worn away by the action of wind, water, ice, or gravity. In simple terms, it is the process where soil particles are dislodged or detached and put in motion. The most effective way to prevent erosion is to keep the soil covered with vegetation, which: Shields soil from the impact of raindrops and the force of wind. Binds soil particles together to protect against the eroding force of runoff Provides a continuing supply of organic matter for the soil. Slows runoff to non-erosive velocities. Filters sediment Apply soil erosion prevention practices as a first line of defense against on-site damage. The second line of defense is to control sedimentation. Sedimentation is the process whereby detached soil particles are deposited. Sediment, the product of erosion, is by volume the greatest pollutant entering our lakes and streams. An effective soil erosion and sedimentation control plan addresses both erosion prevention and sediment control. Remember that it is much more effective to prevent erosion than to control or remove the sediment generated from erosion. 1-12

24 UNIT ONE REVIEW 1. Erosion and sedimentation result in: a. loss of b. ditches and storm sewers c. muddy or lakes and streams d. damage to and life e. damage to roads and buildings 2. Erosion is the process where soil particles are or and put in motion. 3. Sedimentation is the process whereby detached particles are elsewhere. 4. Sediment is the product of. 5. The two types of erosion are and. 6. Accelerated erosion is a speeding up of erosion due to activity. 7. Major erosive forces are water and. 8. In some areas, more soil is lost from erosion than from water erosion. 9. The effects of erosion are usually more visible than erosion. 10. The three categories of overland erosion are: a. b. c. 11. erosion is the removal of a uniform layer of soil that generally goes unnoticed. 12. The of erosion defines the difference between rills and gullies. 13. The physical factors affecting erosion are: a. b. c. d. 1-13

25 14. The climatic factors that influence erosion are amount,, and. 15. Organic matter reduces erosion and off-site sedimentation by: a. shielding the soil from the impact of b. up the rain water c. providing essential for plant growth. 16. Soil properties influencing erodibility include: a. b. c. 17. Texture refers to the of soil particles. 18. Structure refers to how soil particles are together. 19. Slope,, and affect erodibility. 20. Sediment deposition occurs when soil-laden water enough to allow the soil particles to settle out. 21. The,, and of the transported particles influence the rate at which they settle out. 22. Smaller, lighter particles such as are easily transported and are to settle out. 23. The five principles of erosion and sedimentation control are: a. the development to fit the natural site conditions b. expose the practical area for the possible time c. apply prevention practices as first line of defense d. Apply control practices to prevent off-site sedimentation e. implement a thorough and program 24. The most effective way to prevent erosion is keep the soil covered with. 25. It is much more effective to prevent than to control the generated from erosion. 1-14

26 ANSWERS TO UNIT ONE REVIEW 1. Erosion and sedimentation result in: a. loss of fertile topsoil b. clogged ditches and storm sewers c. muddy or turbid lakes and streams d. damage to plant and animal life e. structural damage to roads and buildings 2. Erosion is the process where soil particles are dislodged or detached and put in motion. 3. Sedimentation is the process whereby detached particles are deposited elsewhere. 4. Sediment is the product of erosion. 5. The two types of erosion are geologic and accelerated. 6. Accelerated erosion is a speeding up of erosion due to human activity. 7. Major erosive forces are water and wind. 8. In some areas, more soil is lost from wind erosion than from water erosion. 9. The effects of water erosion are usually more visible than wind erosion. 10. The three categories of overland erosion are: a. sheet b. rill c. gully 11. Sheet erosion is the removal of a uniform layer of soil that generally goes unnoticed. 12. The depth of erosion defines the difference between rills and gullies. 13. The physical factors affecting erosion are: a. climate b. vegetative cover c. soil d. slope characteristics 1-15

27 14. The climatic factors that influence erosion are rainfall amount, intensity, and frequency. 15. Organic matter reduces erosion and off-site sedimentation by: a. shielding the soil from the impact of rain b. soaking up the rain water c. providing essential nutrients for plant growth. 16. Soil properties influencing erodibility include: a. texture b. structure c. cohesiveness 17. Texture refers to the size of soil particles. 18. Structure refers to how soil particles are clumped together. 19. Slope steepness, length, and roughness affect erodibility. 20. Sediment deposition occurs when soil-laden water slows enough to allow the soil particles to settle out. 21. The size, shape, and density of the transported particles influence the rate at which they settle out. 22. Smaller, lighter particles such as clay are easily transported and are slow to settle out. 23. The five principles of erosion and sedimentation control are: a. plan the development to fit the natural site conditions b. expose the smallest practical area for the shortest possible time c. apply soil erosion prevention practices as first line of defense d. Apply sediment control practices to prevent off-site sedimentation e. implement a thorough inspection, maintenance, and follow-up program 24. The most effective way to prevent erosion is keep the soil covered with vegetation. 25. It is much more effective to prevent erosion than to control the sediment generated from erosion. 1-16

28 Unit Two CONTROLLING RUNOFF AND EROSION ON CONSTRUCTION SITES INTRODUCTION This unit will present methods to control runoff and prevent or minimize erosion on construction sites. Storm water runoff is rain that does not infiltrate when it comes in contact with the soil. Runoff can carry several pollutants, including sediment, nutrients, oil, salt, and other toxic materials. The faster runoff travels, the more soil it erodes and carries (Figure 2-1). Figure 2-1 Without proper planning, construction activities can result in an increase in runoff (Figure 2-2). This increased runoff can cause erosion and flooding. One potentially damaging result of increased runoff is an increase in the amount of sediment (Figure 2-3). Figure 2-2 Figure

29 There are three primary reasons why runoff increases during and after construction. The first one is that grading removes vegetation (Figure 2-4). Vegetation is nature's greatest runoff protector. The second reason is that grading compacts the soil, thus reducing the amount of infiltration. The third reason is that construction generally results in covering large portions of the soil surface with concrete, asphalt, roofs, and other impervious surfaces (Figure 2-5). A small increase in impervious area can cause a disproportionate increase in runoff during a rainfall. Figure 2-4 Figure 2-5 CONSTRUCTION PRACTICES TO CONTROL RUNOFF There are several construction practices or control measures that will minimize runoff and thus control erosion. Detailed information for commonly used practices can be found in the Natural Resources Conservation Service s (NRCS) Standards and Specifications, the Michigan Department of Transportation s Soil Erosion and Sedimentation Control Manual, and the Michigan Department of Environmental Quality s Guidebook of Best Management Practices for Michigan Watersheds. Scheduling The first construction practice is scheduling. Scheduling is a planning process that provides a basis for implementing all control measures in a timely and logical fashion during construction. It may be necessary to implement control measures sequentially instead of all at one time (Figure 2-6). Staging of construction is part of scheduling. Staging is sometimes called phasing. With staging, grading and stabilization are finished in one area before proceeding to the next (Figure 2-7). Staging allows you to take advantage of the existing vegetation on the site. Plan the stages or phases of development so that only areas which are actively under construction are exposed. All other areas should have a good cover of vegetation or mulch. 2-2

30 Seeding and Mulching Figure 2-6 Figure 2-7 Source: John Warbach, Planning and Zoning Center, Inc. The second practice is to seed and mulch all areas that have no vegetative cover (Figure 2-8). If it is not feasible to permanently seed, establish a quick-growing temporary grass cover. Mulch should always be placed on bare soil to protect it from rain or wind, whether or not it has been seeded. Preserve Vegetative Buffers Third, preserve vegetated buffer areas above and below the graded area. The buffer above will slow the runoff before it has a chance to erode. The buffer below slows runoff and will filter some of the sediment before the runoff leaves the site (Figure 2-9). Surface Roughening Figure 2-8 Figure 2-9 Fourth, decrease the rate of runoff by surface roughening. It is an easy and economical method that simply creates an uneven or bumpy condition on the soil surface. Horizontal grooves tend to spread runoff over the slope, slowing it down and allowing more of it to infiltrate into the soil (Figure 2-10). Scarification is one way to roughen the soil surface. It can be easily accomplished with a drag, cultivator, or by back blading perpendicular to the slope (Figure 2-11). Roughening 2-3

31 also produces a soil surface more suitable for the growth of vegetation because it will hold the seed and retain moisture. Diversions Figure 2-10 Figure 2-11 Fifth, use diversions to intercept runoff that would otherwise flow across the exposed soil (Figure 2-12). Care must be taken to divert runoff to an area where it can infiltrate or be safely discharged. A diversion is generally constructed as a channel with a ridge on the lower side. Often the excavated material from the channel is used to construct the ridge. The channel and ridge can be bare compacted soil or vegetated. When the anticipated runoff velocities exceed 1.5 to 2.0 feet per second, diversions should be vegetated. Soil reinforcement measures, such as erosion control blankets, may be necessary while establishing vegetation in the channel or on the ridge (Figure 2-13). Figure 2-12 Source: John Warbach, Planning and Zoning Center, Inc. Figure 2-13 Source: John Warbach, Planning and Zoning Center, Inc. All diversions should be constructed in accordance with NRCS specifications to ensure adequate flow capacity and to keep velocities within acceptable ranges. Specifications for permanent diversions are more rigorous than for temporary ones. The slope of the channel should be sufficient to generate an adequate runoff velocity to create good positive drainage. Care must be taken not to exceed velocities that will erode the diversion channel (Figure 2-14). 2-4

32 Care must be exercised when using diversions above steep slopes; a slide could occur (Figure 2-15). The major cause of sliding is the saturation of the soil by water concentrated behind and within the diversion structure. Soil saturation can be prevented by increasing the channel grade or lining the diversion channel with impervious materials such as concrete or asphalt. Figure 2-14 Figure 2-15 Overland flow that is diverted and concentrated must be disposed of without causing erosion. This can be done by diverting the flow onto vegetation, into a basin or grade stabilization structure, or by minimizing flow velocities within the channel or at the discharge point with energy dissipaters (Figure 2-16). The easiest way to dispose of diverted water is directly onto well-established vegetation. Vegetation has limits, however, and will erode if runoff velocities become too high or are subjected to continuous runoff for extended periods of time. Newly planted grass cannot withstand concentrated flows (Figure 2-17). It may be necessary to temporarily divert the runoff until the seeded areas become permanently stabilized. Figure 2-16 Figure 2-17 Grade Stabilization Structures Grade stabilization structures are used to carry runoff from one level to another (Figure 2-18). All grade stabilization structures must be designed to carry the anticipated runoff from the site and constructed in such a manner to prevent "piping." Piping occurs when water erodes small channels under or along the side 2-5

33 of the water conveyance structure (Figure 2-19). The potential for piping can be minimized by using flared metal inlets and compacting the soil around the inlet section. Figure 2-18 Figure 2-19 Downdrains are one type of grade stabilization structure that is commonly used on construction sites. There are several types of downdrains: A pipe downdrain consists of a metal inlet and a rigid or collapsible drain tube made of metal or heavy-duty fabric (Figure 2-20). A chute or flume is a flat or round bottomed ditch usually lined with concrete or asphalt. Undercutting or flow around the side can be a problem unless a good bond is maintained between the diversion and the flume inlet (Figure 2-21). Figure 2-20 Figure 2-21 No matter what grade stabilization structure is used, care must be taken to prevent scouring or erosion at the outlet. Scouring can be prevented by using one or more of the following: place large rocks on geotextile material downstream of the outlet, use flared end sections, or place large rocks or concrete blocks in the flume channel (Figure 2-22). 2-6

34 Figure 2-22 Check Dams Check dams may be necessary to reduce the velocity of flow in roadside ditches or in other concentrated flow areas (Figure 2-23a). Check dams can reduce the potential for erosion and protect vegetation in early stages of growth. In some situations, vegetation may not become established without the help of check dams. The primary purpose of check dams is to reduce water flow to non-erosive velocities. In some situations, the water velocity will be slowed sufficiently to allow large-sized particles to settle out of the water and be deposited upstream of the check dam. The deposition of sediment can be increased by excavating sumps upstream of the check dams (Figure 2-23b). Figure 2-23a Figure 2-23b Check dams are generally constructed of rock. In low flow situations, pea-stone or gravel-filled bags may be used instead of rock. Sandbags should never be used in flowing water because water will not pass through the bags. When constructing check dams, place the rock in the ditch and up the sides to a level above that of the anticipated flow. The middle of the dam should be nine inches lower than the outer edges (Figure 2-24). This allows water to flow over the depression in the center of the check dam, as opposed to around the sides where it could erode the banks. 2-7

35 Figure 2-24 Source: John Warbach, Planning and Zoning Center, Inc. Check dams are usually used in a series (Figure 2-25). They should be located or spaced so that the toe of the upstream check dam is at the same elevation as the lowest point of the top of the downstream check dam (Figure 2-26). Therefore, the steeper the slope, the closer the check dams should be. L = The distance that points A and B are at the same elevation. Point B is the lowest point along the top of the check dam. Check Dam Spacing Figure 2-25 Figure 2-26 Riprap should be placed immediately below the check dam to help dissipate the energy of water flowing over the dam. Stone size should be increased with increased slope and velocity. The stone should be big enough to stay in place during anticipated high velocities. When larger sizes of stone are used, place smaller stones immediately upstream of the check dam to filter sediment (Figure 2-27). The size of stone used, as well as base preparation, must adhere to strict engineering standards or failure of the structure can occur. Figure 2-27 Source: John Warbach, Planning and Zoning Center, Inc 2-8

36 Channel and Slope Stabilization Check dams are not always capable of reducing water velocities to levels that will prevent erosion. When this occurs, additional measures must be used for stabilization. Anticipated velocities, and to lesser extent aesthetics, will dictate what stabilization measures will be used. For example, unvegetated bare channels can generally only sustain velocities up to 1.5 to 2 feet per second without eroding (Figure 2-28). Established grassed lined channels can accommodate velocities up to approximately 4 to 5 feet per second (Figure 2-29). Until grass is established, runoff may have to be diverted away from the exposed area to protect the seedlings and the channel itself from erosion. Under extreme conditions, channel velocities can reach 15 feet per second, and extreme measures will be needed for stabilization. Figure 2-28 Figure 2-29 Another option is to line the channels with erosion control blankets or turf reinforcement mats (Figure 2-30). Blankets and mats are manufactured by several companies, each of which has specific applications. Primary differences between blankets and mats are in the materials that are used and how they are constructed (Figure 2-31). Some are designed for low velocity situations while others are capable of accommodating higher velocities. Figure 2-30 Figure 2-31 Extremely high velocities in channels preclude the use of vegetation or blankets for stabilization. In these instances, it may be necessary to line the whole channel with riprap, rock gabions, or concrete (Figure 2-32). 2-9

37 Figure 2-32 Other products now available from the erosion control industry are cellular confinement systems (Figure 2-33) and pre-cast interlocking block systems (Figure-2-34). These provide good protection against high velocities in addition to providing a more natural appearance. For example, the cells can be filled with topsoil and seeded or filled with gravel to enhance stream habitats (Figure 2-35). Although much of this discussion has focused on channels, many of the products discussed are very effective in protecting slopes as well. For example, erosion control blankets and cellular confinement systems are routinely used on steep, difficult to stabilize, slopes (Figure 2-36). Figure 2-33 Figure 2-34 Figure 2-35 Figure

38 Contractors often feel as though some of the erosion prevention products are unnecessary and expensive. However, if the contractor has to return to the site several times to regrade and reseed, these products are very cost-effective (Figure 2-37). Additionally, it is often much cheaper to minimize erosion rather than constructing sedimentation basins to trap the sediment. If the resultant damage to the environment is included, the overall cost of stabilization becomes negligible. Figure 2-37 PRACTICES TO CONTROL WIND EROSION Up to this point, our discussion has focused on minimizing erosion by controlling runoff. We must not forget that wind can also be a major erosive force (Figure 2-38). Sandy and organic soils tend to be the most susceptible to wind erosion. Soil may start moving, or eroding, when wind speed exceeds 13 miles per hour measured at one foot off the ground. Similar to rainfall-induced erosion, the best way to protect against wind erosion is to keep the area covered with vegetation or with securely anchored mulch (Figure 2-39). Also, in areas subjected to strong winds, such as along the Great Lake shorelines, soil should never be placed in piles and left unprotected. Figure 2-38 Figure

39 Windbreaks Leave trees or other tall vegetation along the perimeter and intermittently across the site to serve as wind barriers (Figure 2-40). When trees or vegetation must be removed, snow fence can be used to form mini wind barriers. The snow fence must be placed perpendicular to the prevailing wind direction at evenly spaced intervals across the site. Most barriers will protect the soil downwind for a distance of about 10 times the height of the barrier. Therefore, place rows of snow fence about every 40 to 50 feet (Figure 2-41). Although the primary purpose of fencing or other barriers is to reduce the erosive velocity of wind, they also create barriers to stop wind-born soil. Thus they help keep wind-generated sediment on the site. Watering Figure 2-40 Figure 2-41 Source: John Warbach, Planning and Zoning Center, Inc. Another temporary measure for controlling wind erosion is to keep the bare soil moist by watering. A readily accessible water source is required. Water should be applied to the site whenever moderate to high winds are anticipated. Haul roads may have to be watered continuously (Figure 2-42). Figure

40 Chemical Binders In addition to watering, chemical binders can be sprayed on the soil surface. The chemical penetrates into the soil and bonds the individual soil particles, making them resistant to the forces of wind. MAINTENANCE To be effective, all control measures must be periodically inspected, maintained, and/or replaced when necessary. Correct all damaged areas immediately. If banks are severely eroded, consider installing slope stabilization. Sediment should be removed when it accumulates behind check dams or diversions (Figure 2-43). Figure