Lecture 15

```Erosion and sediment transport
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Small Watershed Hydrology
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Figure 7.1 (Brooks et al. 1991)
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Figure 15-1: Dunne &amp; 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 
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Kinetic energy 
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Water erosion
• Surface runoff
– Transports soil particles
– Closes soil surface  increase surface runoff
• Rill erosion
Gully 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
A  RKLSCP
where A = soil loss (tons per acre)
R = rainfall erosivity index
K = soil erodibility index
L = hillslope-length factor
C = cropping-management factor
P = erosion-control practice factor
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Universal Soil-Loss Equation
A  RKLSCP
• Rainfall erosivity index, R
– Depends on kinetic energy and rainfall intensity
n
 Ei I 30i
E  916  331log10 I 30
R  i 1
100
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
A  RKLSCP
• Rainfall erosivity index, R
– Depends on kinetic energy and rainfall intensity
Figure 15-16 (Dunne &amp; Leopold 1978)
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Universal Soil-Loss Equation
A  RKLSCP
• 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
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Universal Soil-Loss Equation
A  RKLSCP
• Soil erodibility factor, K
Figure 7.4 (Brooks et al. 1991)
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Universal Soil-Loss Equation
A  RKLSCP
• Length and slope factors, LS
Figure 15-19 (Dunne &amp; Leopold 1978)
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Universal Soil-Loss Equation
A  RKLSCP
• 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
A  RKLSCP
• Erosion control practice factor, P
– Varies with technique
Table 15-5: Dunne and Leopold (1978)
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Modified USLE
A  RKLS VM 
where VM = vegetation management factor
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How high canopy is
and how much
canopy cover
Figure 7.5 (Brooks et al. 1991)
How much
ground cover
% of fine
roots 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
tonne = 1000 kg
Expressed as:
•Weight per area per time or
•Volume per area per time
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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
Particles being
deposited
Particles being
picked up
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
Discharge 
Figure 3.8A: Knighton (1998)
SS 
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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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