lecture 15 - 1 ers 482/682 (fall 2002) erosion and sediment transport ers 482/682 small watershed...
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ERS 482/682 (Fall 2002) Lecture 15 - 1
Erosion and sediment transport
ERS 482/682Small Watershed Hydrology
ERS 482/682 (Fall 2002) Lecture 15 - 2
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
Gully erosion
ERS 482/682 (Fall 2002) Lecture 15 - 7
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 EquationRKLSCPA
where A = soil loss (tons per acre)R = rainfall erosivity indexK = soil erodibility indexL = hillslope-length factorS = hillslope-gradient factorC = cropping-management factorP = erosion-control practice factor
ERS 482/682 (Fall 2002) Lecture 15 - 11
Universal Soil-Loss Equation
• Rainfall erosivity index, R– Depends on kinetic energy and rainfall
intensity
RKLSCPA
3010log331916 IE 100
130
n
iii IE
R
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
ERS 482/682 (Fall 2002) Lecture 15 - 12
Universal Soil-Loss Equation
• Rainfall erosivity index, R– Depends on kinetic energy and rainfall
intensity
RKLSCPA
Figure 15-16 (Dunne & Leopold 1978)
ERS 482/682 (Fall 2002) Lecture 15 - 13
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
RKLSCPA
ERS 482/682 (Fall 2002) Lecture 15 - 14
Universal Soil-Loss Equation
• Soil erodibility factor, K
RKLSCPA
Figure 7.4 (Brooks et al. 1991)
ERS 482/682 (Fall 2002) Lecture 15 - 15
Universal Soil-Loss Equation
• Length and slope factors, LS
RKLSCPA
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)
RKLSCPA
ERS 482/682 (Fall 2002) Lecture 15 - 17
Universal Soil-Loss Equation
• Erosion control practice factor, P– Varies with technique
RKLSCPA
Table 15-5: Dunne and Leopold (1978)
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Modified USLE VMRKLSA
where VM = vegetation management factor
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Figure 7.5 (Brooks et al. 1991)
How high canopy isand how muchcanopy cover
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
Rockfall
Slump
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Figure 15-40: Dunne and Leopold (1978)
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Figure 15-41 (Dunne and Leopold 1978)
Figure 8.5 (Brooks et al. 1991)
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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
tonne = 1000 kg
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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)
Particles beingpicked up
Particles beingdeposited
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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)
Discharge
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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