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Construction Site Erosion and Sediment Controls. Section 3. Hydrology and Soil Erosion. Reason for Fundamentals. The erosion and sediment control plan included in the SWPPP is developed before the site is disturbed based on the best available information

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Construction Site Erosion and Sediment Controls

Section 3. Hydrology and Soil Erosion


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Reason for Fundamentals

  • The erosion and sediment control plan included in the SWPPP is developed before the site is disturbed based on the best available information

  • Construction site erosion and sediment control requires flexibility during construction projects; modifications are often necessary


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Reason for Fundamentals

  • Inspectors must have hydrology and soil erosion knowledge to critically examine erosion and sediment controls

  • Basic hydrology and soil erosion knowledge is also required to identify erosion prone areas and to know critical erosion time periods during the year


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Why is Hydrology Important?

  • Know drainage patterns and receiving water for the site

  • Land development disturbs the natural hydrologic cycle

  • Rainfall characteristics influence soil erosion

  • Runoff controls the rate of erosion from exposed soils


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Land Development

Land cover is changed from undisturbed soils and vegetation to compacted soils and impervious surfaces


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Land Development

Land development significantly alters the hydrologic cycle


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Land Development

Runoff volume and flow rate increases and time of concentration decreases




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Runoff

Runoff generation is dependent on many factors including:

  • Precipitation intensity, volume, and spatial and temporal distribution

  • Watershed area and topography

  • Ground cover and moisture conditions

  • Soil permeability characteristics


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Time of Concentration

Stream/Pipe Flow

Overland Flow

Shallow Concentrated Flow

Most Remote

Point

Hydraulically

Water Shed

Outlet


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Time of Concentration

Outlet

Outlet

Hydraulically most remote point

Water Shed

tc for top watershed is much longer than tc for the bottom watershed

Hydraulically most remote point


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Soil Erosion

  • Erosion is a natural process whereby soil particles are displaced and transported by wind, rain, or runoff

  • Erosion includes weathering, dissolution, abrasion, corrosion, and transportation, by which material is removed from the Earth’s surface


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Soil Erosion

Soil erosion occurs in three phases:

  • Particle detachment

  • Sediment transport

  • Sediment deposition


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Wind-Generated Soil Erosion

Three types of wind-generated erosion:

  • Saltation

  • Suspension

  • Surface creep

    Although wind-generated erosion is a major concern, we are focusing this course on water-generated erosion


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Water-Generated Soil Erosion

Erosion due to water action occurs in one of the following forms:

  • Splash erosion

  • Sheet-flow erosion

  • Rill erosion

  • Gully erosion

  • Stream erosion


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Types of Erosion

(Soil Conservation Service)


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Splash Erosion

The dislodging of soil particles by raindrop impact is a primary cause of surface erosion


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Raindrop Impact

(Seafriends 2001)


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Sheet-flow and Rill Erosion

The uniform removal of soil particles by sheet-flow runoff

Rills are long, narrow depressions or incisions caused by increased velocities


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Rill Erosion on Slopes

Unprotected slopes will develop rills,

which will eventually form gullies


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Gully Erosion

Gullies are larger and deeper depressions caused by the higher velocities associated with concentrated flows


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Gully Erosion

Gullies are very difficult to stop once they are started


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Gully Erosion

Long steep slopes are the primary place for rills and gullies to develop

Gullies can remove up to 10 times larger volumes of soil per unit area than sheet flows and rills


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Stream Erosion

Erosion of soil by increased stream flows


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Soil Erosion

Soil erosion is influenced by five primary factors:

  • Rainfall characteristics

  • Soil erodibility

  • Flow path length and slope

  • Land cover

  • Control measures


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Rainfall Characteristics

Characteristics of the region’s climate and rainfall have a significant influence over soil erosion:

  • Rainfall patterns

  • Rainfall intensity

  • Droplet size


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Soil Erodibility

The tendency of soil particles to become detached from the soil matrix is dependent on:

  • The soil texture, organic matter content, and structure

  • Soil permeability

  • Electrostatic charges



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Flow Path Length and Slope

  • The longer the flow path the more potential for soil erosion

  • The slope of the flow path has a significant influence over soil erosion: higher slopes increases runoff and erosion potential

  • High slopes and long flow paths should be reduced by creating contour diversions and benches


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Land Cover

  • Soil erosion rates are related to the type and amount of temporary and permanent cover

  • Land covers stabilize the soil matrix, reduce runoff velocities and volumes, and reduce the impact of rainfall droplets

  • Appropriate land cover should be established on disturbed areas as soon as possible after construction is completed in the area


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Control Measures

  • Control measures are activities performed on the disturbed land surface to mitigate the erosive forces of rainfall and runoff

  • Control measures can be directed toward land cover or can influence the flow path length and steepness


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Erosion Control Goals

Soil Erosion Rates:

  • Natural geologic rate (0.2 tons/ac/yr)

  • Managed forest (0.5 tons/ac/yr)

  • Agricultural lands (1.5 to 20 tons/ac/yr)

  • Construction activities (150 to 200 tons/ac/yr)

Goal - reduce erosion on construction sites to 1.5 tons/ac/yr


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Summary

  • Keep the principles of hydrology and soil erosion in mind as you inspect erosion and sediment controls

  • Verify that the controls are compatible with site hydrology and soil types



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