Water Erosion Types and Factors

Water Erosion

• Factors affecting erosion by water includes climate, soil, vegetation, and topography.
• Raindrop Erosion is soil detachment and transport resulting from the impact of water drops directly on soil particles or thin water surfaces.
• E = kinetic energy in MJ/ha-mm
• i = intensity of rainfall in mm/h
• E= 0.119 + 0.0873log I

Sheet and Interill Erosion

• Sheet Erosion is the uniform removal of soil layers from sloping land, resulting from sheet or overland flow.
• Interill Erosion is splash and sheet erosion combined.
• D = interill erosion rate in kg/m-s
• Ķ = interill erodibility in kg-s/m
• i = rainfall intensity in m/s
• S = slope factor = 1.05 - 0.485^A
• where A = slope in degrees
• D = KiS

Rill Erosion

• Rill Erosion is the detachment and transport of soil by a concentrated flow of water.
• Dr = rill detachment rate in kg/m-s
• Kr = rill erodibility resulting from shear in s/m
• τ’ = hydraulic shear of flowing water with units in Pa, pgrs
• τc = critical shear below which no erosion occurs with units in Pa
• Qs = rate of sediment flow in the rill with units in kg/m-s
• Tc = sediment transport capacity of rill in kg/m-s
• p = density of water in kg/m
• g = acceleration due to gravity in m/s
• r = hydraulic radius of rill in m
• s = hydraulic gradient of rill flow
• Dr = Κr(τ -τc)(1- Q/T)

Gully and Stream Channel Erosion

• Gully Erosion produces channels larger than rills, carrying water during and immediately after rains and cannot be obliterated by tillage.
• Stream Channel Erosion consists of soil removal from stream banks or soil movement in the channel.
• Sediment Transport:
  • T = transport capacity per unit width in kg/m-s
  • B = transport coefficient based on soil and water properties
  • τ = hydraulic shear of rill channel in Pa
  • Tc = Bτ^.5

Soil Loss Equation

• Universal Soil Loss Equation (USLE):
  • A = average annual soil loss in Mg/ha
  • R = rainfall and runoff erosivity index for geographic location
  • K = soil erodibility factor
  • L = slope length factor
  • S = slope steepness factor
  • C = cover management factor
  • P = conservation practice factor
  • A = RKLSCP
• R (Rainfall Factor) is the product of the kinetic energy of the storm and the maximum 30-minute intensity of that storm.

Calculating an Unknown K

• M = particle size parameter (%silt + %very fine sand) x (100 - %clay)
• a = percent organic matter
• b = soil structure code
  • 1 = very fine granular
  • 2 = fine granular
  • 3 = medium or coarse granular
  • 4 = blocky, platy, or massive
• c = profile permeability class
  • 1 = rapid
  • 2 = moderate to rapid
  • 3 = moderate
  • 4 = slow to moderate
  • 5 = slow
  • 6 = very slow
• K = 2.8x10^-5M {12-a) + 4.3x10^-3 (b-2) + 3.3x10^-3 (c-3)}

Determining Slope Characteristics

• Factors for slope length:
  • L = slope length factor
  • l = slope length in m
  • m = dimensionless exponent
  • L = (l/20)^m
• Recommended for conditions where rill erosion and interill erosion were about equal on a 9 percent, 22m long slope, the m can be determined by calculating:
  • m = [sinθ/(sinθ+0.269 (sin θ +0.05))]
  • θ = field slope steepness in degrees = tan (s/100)
  • s = field slope in percent
• Equation for when Slopes are Shorter than 4m:
  • S = 3.0(sinθ)^0.8 + 0.56
• Equation for when Slopes are Longer than 4m and s is less than 9 percent:
  • S = 10.8 sinθ + 0.03
• Equation for when Slopes are Longer than 4m and s is greater than 9 percent:
  • S = 16.8 sinθ + 0.50

Cover Management Factors

• Using vegetation cover to reduce soil loss.
• Follow/Bared land has a C factor of 1.0.
• Paddy field has a C factor of 0.34.
• Upland has a C factor of 0.31.
• Grassland (95~100% cover) Grass has a C factor of 0.003.
• Grassland Weeds has a C factor of 0.01.
• Grass (80% cover) Grass has a C factor of 0.01.
• Grass (80% cover) Weeds has a C factor of 0.04.
• Forest Weeds has a C factor of 0.09.
• Forest (75~100% cover) factor of 0.01.
• Forest (4075% cover) factor of 0.0020.004.
• Forest (2040% cover) factor of 0.0030.01.

Conservation Practice Factor Equations

• Pc = contouring factor base on slope
• sP = strip cropping factor recommended
• tP = terrace sedimentation factor
• To determine the Conservation practice factor
• P= Pc x Ps x Pt
• For contour farming, land slope (percent) of "1 to 2" has factors of "0.6 Contour" and "0.3 Strip crop".
• For contour farming, land slope (percent) of "3 to 5" has factors of "0.5 Contour" and "0.25 Strip crop".
• For contour farming, land slope (percent) of "6 to 8" has factors of "0.5 Contour" and "0.25 Strip crop".
• For contour farming, land slope (percent) of "9 to 12" has factors of "0.6 Contour" and "0.3 Strip crop".
• For contour farming, land slope (percent) of "13 to 16" has factors of "0.7 Contour" and "0.35 Strip crop".
• For contour farming, land slope (percent) of "17 to 20" has factors of "0.8 Contour" and "0.4 Strip crop".

Wind Erosion and Control Practices

• Factors affecting erosion by wind:
  • Climate
  • Soil
  • Vegetation
  • Topography
• Areas most subject to damage includes sandy soils along streams, lakes, coastal plains, and organic soils.

Wind Distribution with Height

• u =*(u /k) In[(z-d)/z]
• u = wind velocity at z height, (L/T)
• µ = friction velocity, (L/T)=(τ /p)^1/2
• p = air density (M/L)
• k = von Karman's constant, usually taken as 0.4
• z = height above a reference surface (L)
• d = an effective surface roughness height (L)
• z = roughness parameter (L)
• Effective roughness height, d - d = 0.7h - h = crop height
• Effective roughness height, z - z = 0.13h - h = crop height

Wind Erosion Classification

• Wind erosion processes are divided into the three simple but distinct phases:
  • Initiation of movement is initiated as a result of turbulence and velocity of the wind.
  • Transportation is influenced by particle size, gradation of particles, wind velocity, and distance across the eroding area.
  • Deposition occurs when the gravitational force is greater than the forces holding the particles in the air.

Determining Wind Erosion

• E = the estimated average annual loss (Mg/ha-year) - I = soil erodibility index (Mg/ha-year) - K = ridge roughness factor - C = climate factor - L = unsheltered length of eroding field in meters - V = vegetative cover factor
  • E = f(I,K, C, LV)
• Determining the Soil erodibility index, I: - I = 525 * e^(-0.04F) - Where F = the percentage of dry soil fraction greater than 0.084mm

Soil Condition and Wind Erosion

• Very fine sand, fine sand, sand, or coarse sand is found in WEG 1.
  • Percent is 1 to 10.
  • Wind index is 310 to 134 T/Ac/Yr.
• For Loamy very fine sand, loamy fine sand, loamy sand, loamy coarse sand, or sapric (1) organic soil materials. sand is found in WEG 2.
  • Percent is 10.
  • Wind index is 134 T/Ac/Yr.
• For Very fine sandy loam, fine sandy loam, sandy loam, or coarse sandy loamy. sand is found in WEG 3. - Percent is 25.
  • Wind index is 86 T/Ac/Yr.
• For Clay, silty clay, noncalcareous clay loam, or silty clay loam with >35 percent clay content. It may also be for Calcareous (b) loam, silt loam, clay loam, or silty clay loam. This is found in WEG 4. - Percent is 25.
  • Wind index is 86 T/Ac/Yr.
• For Non calcareous loam and silt loam with <20 percent clay content, or sandy clay loam, sandy clay, and hemic (1) organic soil materials. is found in WEG 5. - Percent is 40. - Wind index is 56 T/Ac/Yr.
• For Noncalcareous loam and silt loam with >20 percent clay content, or noncalcareous clay loam with <35 percent clay content, is found in WEG 6. - Percent is 45.
  • Wind index is 48 T/Ac/Yr.
• Silt, noncalcareous silty clay loam with >35 percent clay content and fibric (1) organic soil material is found in WEG 7. - Percent is 50. - Wind index is 38 T/Ac/Yr.
• Soils not susceptible to wind erosion due to coarse fragments or wetness are found in WEG 8. - Wind index is 0 T/Ac/Yr.

Wind Erosion

• K = ridge roughness in mm
• h = ridge height in mm
• d = ridge spacing in mm