Erosion and sediment transport

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Erosion and sediment transport

• ERS 482/682
• Small Watershed Hydrology

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Figure 7.1 (Brooks et al. 1991)
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Figure 15-1 Dunne Leopold (1978)
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Figure 15-3 Dunne and Leopold (1978)
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Water erosion
Figure 7.2 (Brooks et al. 1991)
Rainfall intensity
?
Kinetic energy
?
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Water erosion

• Surface runoff
• Transports soil particles
• Closes soil surface ? increase surface runoff
• Rill erosion
• Microchannels (50-300 mm wide up to 300 mm deep)
• Sheet erosion (inter-rill erosion)
• Movement of semi-suspended particles over land
  surface

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Gully erosion
Figure 8.1 (Brooks et al. 1991)
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Pawnee Buttes, CO
Knickpoint
Gully erosion
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Figure 15-15 Dunne and Leopold (1978)
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Universal Soil-Loss Equation
where A soil loss (tons per acre) R rainfall
erosivity index K soil erodibility index L
hillslope-length factor S hillslope-gradient
factor C cropping-management factor P
erosion-control practice factor
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Universal Soil-Loss Equation

• Rainfall erosivity index, R
• Depends on kinetic energy and rainfall intensity

where E kinetic energy (ft ton ac-1 in-1) I30
maximum 30-minute intensity (in hr-1) n
total number of storms in period of interest
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Universal Soil-Loss Equation

• Rainfall erosivity index, R
• Depends on kinetic energy and rainfall intensity

Figure 15-16 (Dunne Leopold 1978)
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Universal Soil-Loss Equation

• Soil erodibility factor, K
• Average soil loss (per rainfall erosivity) when
  the soil is exposed as cultivated bare fallow
  under specified conditions of hillslope length
  and gradient

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Universal Soil-Loss Equation

• Soil erodibility factor, K

Figure 7.4 (Brooks et al. 1991)
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Universal Soil-Loss Equation

• Length and slope factors, LS

Figure 15-19 (Dunne Leopold 1978)
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Universal Soil-Loss Equation

• Cropping-management factor, C
• Examples from Dunne and Leopold (1978)
• Agricultural land (Table 15-2)
• Woodland (Table 15-3)
• Pasture, rangeland, and idle land (Table 15-4)

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Universal Soil-Loss Equation

• Erosion control practice factor, P
• Varies with technique

Table 15-5 Dunne and Leopold (1978)
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Modified USLE
where VM vegetation management factor
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How high canopy isand how muchcanopy cover
Figure 7.5 (Brooks et al. 1991)
How muchground cover
of fineroots in ground
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Soil mass movement

• Downslope movement of finite masses of soil, rock
  and debris
• Driven by gravity

Figure 8.5 (Brooks et al. 1991)
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Figure 15-29 (Dunne and Leopold 1978)
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Pawnee Buttes, CO
Slump
Rockfall
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Figure 15-40 Dunne and Leopold (1978)
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Figure 8.5 (Brooks et al. 1991)
Figure 15-41 (Dunne and Leopold 1978)
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Sediment yield

• Total sediment outflow from a watershed for a
  specific period of time at a defined point in the
  channel

• Expressed as
• Weight per area per time or
• Volume per area per time

kg ha-1 yr-1
m3 ha-1 yr-1
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Sediment transport
Figure 9.1 (Brooks et al. 1991)
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Sediment transport
Figure 9.2 (Brooks et al. 1991)
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Estimating sediment yield

• USLE
• Measuring suspended sediment concentrations

Figure 7.1 (Stednick 1991)
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Estimating sediment yield

• USLE
• Measuring suspended sediment concentrations

Figure 3.8A Knighton (1998)
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Estimating sediment yield

• USLE
• Measuring suspended sediment concentrations

• Regress with discharge or turbidity (Lewis 1996)
• Does not account for bedload

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Estimating sediment yield

• USLE
• Measuring suspended sediment concentrations

• Lake/reservoir surveys

Figure 3.8C and Figure 3.8D (Knighton 1998)
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Estimates of sediment yield
Table 3.1 and Table 3.2 (Knighton 1998)
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