Erosion is a natural process by which soil and rock materials are broken down and moved. Erosion by the action of water, wind and ice has some of the most spectacular landscapes we know. Natural erosion occurs primarily on geological time scale and is something that we are not that concerned with.

Man's activities alter the landscape, and the erosion processes may be greatly accelerated. This is the form of erosion that we are most concerned with.

  1. Erosion by Water

    1. Accelerated Erosion

      Two steps are recognized in accelerated erosion:

      1. detachment

        - freeze/thaw, wet/dry, flowing water, wind, raindrops

      2. transportation

        - floating, rolling, dragging, splashing, etc.

    2. Power of Raindrops

      Raindrops impact has 3 impostant effects

      1. It detaches soil - force of falling water.

      2. Beatings tend to destroy granulation at surface

        - crusting leads to more runoff

      3. May transport soil

        - discuss mud on walls, movement of soil 5 to 6ft at a time
        - add a bit of wind and you may move 100's of lps of soil

  2. Types of Water Erosion

    There are 4 types of water erosion that may occur on bare terrain.

    1. Splash Erosion

      Splash Erosion results with the force of raindrops falling on bare or sparsely vegetated surfaces. The particles are detached by the force of falling raindrops. The soil rises into the air and maybe moved by gravity or wind or both downslope.

      - examples are sides of buildings etc..

    2. Sheet Erosion

      Soil particles are easily transported in a thin layer or sheet by flowing wate. This may be one of the major forms of erosion.

    3. Rill and Gully Erosion

      If the sheet runoff is allowed to concentrate and gain velocit or energy, it will cut rills and gulles as it detaches more soil particles. As the erosive force of flowing water increases with slope length and gradient, gullies become deep channels and gorges. the greater the distanceand slope, the more difficult it is to control the increasing volume and velocity of runoff and the greater the resultant damage.

    4. Stream and Channel Erosion

      Erosion that occurs along the banks of streams and channels. Increased erosion occurs when level of runoff is increased to the river or stream. This may occur when a large parking lot or subdivision is put in any inriltration of the soil is reduced.

  3. Factors that Influence Erosion

    Soil erosion is the result of a number of soil characteristics. 4 of these are most important.

    1. Soil texture
    2. Organic Matter
    3. Soil Structure
    4. Soil Permeability

    1. Soil Texture

      1. Sandy textures have good infiltration and reduced runoff. The particles are large and require much energy to move them. These types of textures have the lowest potential for erosion.

      2. Clay soils are very fine textured (very small particle sizes) but the clay particles tend to bind together and act as large particles. This tends to limit their erosion.

      3. Silt and very fine sand textures are the most erodible. They are small in size compared to sands and do not bind together like clays and are subject to the most erosion.

        - Discuss the high erosion rates in Tenn etc. where the silt is dominant.

    2. Organic Matter

      Plant material such as humus, manures, composts, etc.. help improve soil structure and increase infiltration. The greater the infiltration the lower the amount of runoff and the less erosion.

    3. Soil Structure

      A stable granular structure absorbs water readily (greater infiltration). This is also important from the water management standpoint.

    4. Soil Permeability

      Soils ability to transmit air and water. Soils that are least subject to erosion from rainfall and surface runoff are those with high permeability rates.

    5. Surface Cover

      Vegetation the most effective means of stabilising soils and controlling erosion. It shileds the soil surface from the impact of falling rain, reduces flow velocity and disperses flow. Vegetation provides a rough surface that slows the runoff velocity and promotes infiltration and deposions of sediment. Plants remove water from the soil and thus increase the soils capacity to absorb water. plants leaves and stems protect the soil surface from the impact of raindrops, ans roots help maintain the soil structure. Non-vegetative covers such as mulches, paving and stone aggregates also protect soils from erosion.

    6. Topography

      Topographic features distinctly influence erosion potential. Watershed size and shape affect runoff rates and volume. Long, steep slopes increase runoff velocity. Swales and channels concentrate surface flow. Exposed south-facing soils are hotter and drier, which makes vegetation more difficult to establish.

    7. Climate

      The frequency, intensity and duration of rainfall and the size of the area on which the rain falls are fundamental factors in determining the amount of runoff produced. Seasonal temperature changes also define periods of high erosion risk. For example, precipitation as snow creates no erosion,but repeated freezing and thawing breaks up soil aggregates.

  4. NC Sedimentation Control Law

    The law governs all land - disturbing activities except agriculture, forestry and mining. Erosion and sedimentation control are required regardless of the size of the disturbance. the law requires land developers to plan and implement effective temporary and permanent control measures to prevent accelerated erosion off-site sedimentation.


    The law requires instalation and maintenance of sufficient erosion control pracrices to retain sediment within the boundaries of the site.It also requires that surfaces be non-erosive and stable within 30 working days or 120 calendar dats after completion of the activity, whichever period is shorter.

    Main ideas:

    Sufficicent buffer zone must be retained or established along any natural watercourse to contain all visible sediment to the first 25% of the buffer strip nearest the disrtributed land.

    Principles of Erosion and Sedimentation Control:

    Effective erosion and sedimentation control requires that first the soil surface be protected from the erosive forces of wind, rain and runoff and second that eroded soil be captured on-site. The following principles are not complex but are effective:

    1. Fit the development to existing site conditions

      Review and consider all existing conditions in the initial site selection for the project. Select a site that is suitable rather than force the terrain to conform to development needs. The natural landscape exists for a certain purpose. Ensure that development features follow natural contours. Steep slopes, areas subject to flooding and highly erosive soils severely limit a sites use.

      Any modification of a site's drainage features or topography requires protection from erosion and sedimentation.

      Briefly discuss common sense vs. Engineering

    2. Minimize the extent and duration of exposure

      Scheduling can be a very effective means of reducing the hazards of erosion. Take into account the season and the weather forecast. Stabilize distributed areas as quickly as possible.

    3. Protect areas to be disturbed from stormwater runoff.

      Use dikes, diversions and waterways to intercept runoff and divert it away from slopes or other disturbed areas. To reduce on-site erosion, install these measures before cleaning and grading.

    4. Stabilize disturbed areas.

      Plan and implement temporary or permanent vegetation, mulches, or other protection practices to correspond with construction activities. Protect channels from erosive forces by using linings and the appropriate channel design.

    5. Keep runoff velocities low

      Clearing existing vegetation reduces the surface roughness and infiltration rate and thereby increases runoff velocities. Use measures that break the slopes to reduce the problems associated with concentrated flow volumes. Keep water away from steep slopes. Mulch, stabilize outlets, and re-vegetating as soon as possible.

    6. Retain sediment on the site

      Even with careful planning some erosion is unavoidable. The resulting sediment must be trapped on the site. Plan the location where sediment deposition will occur and maintain access for cleanout. Protect low points below disturbed areas by building barriers to reduce sediment loss. Whenever possible, plan and construct sediment traps and basins before other land-disturbing activities.

    7. Inspect and maintain control measures

  5. Wind Erosion

    Wind erosion is most common in the semi-arid and arid refions, but some wind occurs everywhere. It is mainly a dry weather concern, and is set up by soil moisture. 1934 great dust storm days.

    - remember loess and geologic time idea

    Most of the wind erosion on agricultural lands I caused by mismanagement of plowed lands and the lowered holding power of the range grasses due to overgrazing.

    Sandy soils and peat soils cultivated for many years are often affected by the wind. Drying out of the finely divided surface layer makes then ripe for erosion. Over-plowing is of great concern.

    1. Mechanics of Soil Erosion

      1. Detachment

      2. Transportation

    2. Types of Wind Erosion

      1. Saltation

        Movement of the soil by a series of short bounces. Wroks to detach and transport soil. may acount for 70% of the total soil movement.

      2. Soil Creep

        Rolling movement of larger particles. Saltation May help move these.

      3. Suspension

        Very fine particles are moved parallel to the ground surface. Some of these particles may get carried upward into the atmosphere.

    3. Factors Affecting Wind Erosion

      1. Wind velocity and turbulence

      2. Soil surface characteristics

        - gravelly, very rough, etc.

      3. Soil characteristics

        - texture, structure, etc.

      4. Nature and orientation of vegetation

    4. Control

      Use of surface covers and windbreaks.