Chapter si xx1


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Chapter si xx1

  2. 2. Erosion Processes Types• Generally, there three erosion processes on upland watersheds: – Surface erosion – Gully erosion – Soil mass movementSurface Erosion: involves the detachment and subsequent removal of soil particles and small aggregates from land surfaces by wind or water• This type of erosion is cause by the action raindrops, thin film flows, concentrated overland flows, or by wind• While less serious in forested environments, surface erosion can be an important source of sediment from rangelands and cultivated agricultural lands
  3. 3. Erosion Processes TypesGully Erosion: is the detachment and movement of material, either individual soil particles or large aggregates, in a channel• This kind of erosion is a major form of geological erosion which can be accelerated greatly under poor land managementSoil Mass Movement: includes erosion in which cohesive masses of soil are displaced• Movement can be rapid, as with landslides, or it can be quite slow, as with soil creep and certain soil slumps• All of the above erosion processes can occur singly or in combination• Human activities, such as construction, road building, forest removal, intensive livestock grazing, and agriculture can accelerate these processes• However, it is difficult to distinguish or separate the basic types of erosion and whether they are naturally slow geologic processes or have been accelerated by poor land use practices
  4. 4. The Erosion Process• Soil erosion is the dislodgement and transport of soil particles from the surface by the two agents of erosion• The soil particles can be dislodged by the energies expended at the soil surface by raindrops or by the eddies in surface runoff and wind, and then transported by wind or water or by the force of gravity• Thus, erosion is a process in the physical sense that work requires the expenditure of energy• The energy is imparted by to the soil surface by the forces resulting from impulses produced by the momentum (mass * velocity) of falling raindrops or by the momentum of eddies in the turbulent flows of runoff or wind• I s it these forces that cause work to b done in both the dislodgement and transport phases of the process
  5. 5. Water Erosion• The dislodgement of soil particles at the soil surface by the energy imparted to the surface by falling rain drops is a primary agent of erosion, particularly on soils with sparse vegetative cover• The energy released at the surface during a large storm is sufficient splash over 200 metric tons of soil into the air on a single hectare of bare a nd loose soil• Individual soil particles can be splashed more than one-half meter in height and a half meters sideways
  6. 6. Water Erosion cont…• Surface runoff takes place when the rate of rainfall exceeds the infiltration rate on slopes• Just as with rainfall, the kinetic energy of surface runoff required to dislodge, terrain, and transport soil particles depends upon the mass (depth) of water and its velocity• Surface runoff is also turbulent, that is, eddies in the flow make up the turbulence• These eddies are random in size, orientation, and velocity and provide the impulses to dislodge and entrain soil particles – The intensity of the turbulence in surface runoff depends upon velocity and depth of runoff and the roughness of the surface over which water flows
  7. 7. Water Erosion cont…• Surface runoff combined with the beating action of raindrops causes rills to be formed in the soil surface• Rill erosion is the form of erosion that produces the greatest amount of soil loss world wide• Sheet erosion take place between rills, where it is termed as inter-rill erosion• Sheet erosion is the movement of a semisuspended layer of soil particles over the land surface• However, minute rills are formed silmutaneously with the first detachment and movement of particles
  8. 8. Water Erosion cont…• As, runoff becomes concentrated in rills and small channels and moves downslope, the velocity and mass of the suspension, as well as the intensity of the turbulence in the flow, increases• When the depth of runoff is shallow, raindrops striking the water surface can add to the turbulence• This increase of kinetic energy results in an even greater increase in the ability of the flow to dislodge and transport larger soil particles• If the flow carries a large load of sediment, the abrasive action of the load adds to the erosive power of the runoff• One steep, unobstructed slopes, and with heavy rains, soil lost in this manner can be dramatic;• it also is a common occurrence on dry lands, where the normally sparse vegetative cover has been disturbed by poor land use
  9. 9. Water Erosion cont…• The momentum can be gained by surface runoff on a sloping area and, consquently , the amount of soil that can be lost from the area depends opon both the inclination and the length of unobstructed slope• As the length of the slope increase, soil loss per unit length initially is accelerated, but then approaches a constant rate• However, as the inclination of the slope increase, soil loss increase• Slope angle and slope length that allow the build-up in momentum in flowing water are major factors in accelerating rill erosion;• The steeper the and the longer the slope, the greater become the problems of control• Once it becomes channelised, uncontrolled surface runoff is capable of creating the more spectacular gully erosion• Gullies are common features of nonvegetated lands, particularly those in dry regions that are subjected to poor management practices
  10. 10. Wind Erosion• In dry regions, erosion by both wind and water is a natural feature• Such erosion is an inevitable consequence of the environment, largely because is inadequate to support a protective cover of vegetation• Any use of drylands which further reduces the cover vegetation tends to accelerate beyond that which is a natural consequence of the environment• As a rule, watersheds that have natural; vegetative cover and that receive precipitation of over 400 mm/yr experience little wind erosion• When soils are exposed, excessive wind erosion can occur, even in regions with over 800mm/yr annual precipitation• In either case, wind erosion diminishes with increasing annual precipitation
  11. 11. Wind Erosion cont…• Wind can move larger soil particles by making them jump along the ground• The jumping particles also apply energy to the soil surface each time they hit the ground and, in doing so, dislodge other particles so that they too can be moved by the wind• This process is called saltation, which also a major process in the movement of bed load, the larger particles that move along the bottom of a stream channel
  12. 12. Wind Erosion cont…• The largest soil particles that can be moved to any extent by wind are about 1mm• Where the size of the soil particles extend over a wide range, wind has a sorting effect on the soil• Very fine clay and silt particles (less than 0.02mm) are lifted into the air and carried away as wind blown dust• Sand size particles are carried along in the air layer near the ground by saltation until they reach an obstruction, where they can pile up into drifts and (under extreme conditions)into dunes• Just as gullies are advanced stages of water erorion, sand dunes are severe stages of wind erorion
  13. 13. Wind Erosion cont…• The erosive power of wind, as that of water, increase exponentially with velocity; unlike water, it is not affected by the force of gravity• Therefore, slope inclination is not a factor in wind erosion, except where sloping or hilly terrain forms barriers or influence wind direction• However, similar the effect on the erosive power of water, the length of unobstructed terrain (fetch) over which the wind flows is important in allowing the wind to gain momentum and to increase its erosive power• Winds with velocities less than about 12 to 19 km/hr at one meter above the ground seldom impart sufficient energy at the soil surface to dislodge and put into motion sand-size
  14. 14. Preventing Soil Erosion• Avoiding erosion-susceptible situations and inappropriate land uses is the most economical and effective means to combat soil and to maintain the productivity of watersheds• The guiding rule is that the land user should consider carefully the principles of water and wind action in relation to each management decision, whether this concerns soil conservation techniques, water resource development, range management, fuelwood management, or agriculture
  15. 15. Preventing Soil Erosion cont…Situations that are particularly susceptible to soil loss include;1.Sloping ground, particularly hills with shallow soils2.Soils with inherently low permeabilities, and3.Sites where denudation of vegetation is likely
  16. 16. Guidelines for Preventing Water and Wind erosion• In preventing water erosion, the key is to maintain the surface soil in a condition that readily accepts water• The more water infiltrates, the better the chance of sustaining plant growth and reducing the erosive effects of surface runoff
  17. 17. Guidelines for Preventing Water and Wind erosion cont..Water Erosion• Avoid land use practices that reduce infiltration capacity and soil permeability• Encourage grass and herbaceous cover of the soil for as long as possible each year• Locate livestock watering facilities to minimize runoff production to water bodies• Avoid logging and heavy grazing on steep slopes• Conduct any skidding of logs on steep slopes in upward directions to counteract drainage concentrations patterns• Lay out roads and trails so that runoff is not channelized on steep, susceptible areas• Apply erosion control techniques on agricultural fields, and promote infiltration• Remember that the more water that goes into the soil, the better is the chance of sustaining plant growth and reducing the erosive effect of surface
  18. 18. Guidelines for Preventing Water and Wind erosion cont..Wind Erosion• Avoid uses which will lead to the elimination, over large areas, of shrubs and trees• Avoid locating livestock watering facilities on erodible soils• Protect agricultural fields and heavy use areas with shelterbelts• Manage animals and plants in your area to maintain a good balance between range plants, woody trees, and shrubs• When planting shrubs and trees on grazing lands locate and space them to reduce wind velocity
  19. 19. Controlling Soil Erosion• Actions that protect the soil surface against the energy of rainfall impact and increase the roughness of the surface• Thus the tortuosity of the flow path, reduce the energy of rainfall and surface runoff• Mechanical treatments that shorten the length os slopes and reduce inclination of slopes lessen the energy of overland flow and can reduce the quantity and velocity of surface runoff• Any actions that prevent the channelization of surface runoff will reduce the opportunity for gully formation• Often, strips of vegetation perpendicular to the slope can slow surface runoff and reduce surface runofff
  20. 20. Controlling Soil Erosion cont..• In the case of controlling wind erosion, actions should be taken that reduce the length of fetch to reduce the momentum of wind and that increase soil cohesiveness or armor the soil surface to prevent the lifting of soil particles by wind• The key is to reduce velocity of wind near the ground and to deflect the direction of the wind• The most effective techniques are those that combine several of these actions
  21. 21. Surface Erosion Control on Forest Lands• A minimal amount of surface erosion is expected under natural forest• Undistributed forest rarely experience erosion rates in excess of 0.04 t/ha/yr• Activities that remove vegetative cover and, most importantly, expose mineral soil lead to high rates of surface erosion• Silvicultural treatments in which the destruction of lesser vegetation and litter accumulations is minimized will help to control erosion by reducing the raindrop impact to a soil surface and by maintaining high infiltration rates• Residual strips of vegetation alternated with clearcuts and aligned perpendicularly to the lope can function as barriers to flowing water and the downslope movement of soil particles• Retaining strips of vegetation also can be employed to protect channel banks and streambeds during timber harvesting operations• However, leaving residual strips of vegetation is usually of little value in controlling erosion in mountainous watersheds because of rapid channeling of sheetflow