Hillslope Erosion

1
Hillslope Erosion
Driving Forces Resisting Force
Erosion
Tractive Force Material Strength
(?wDS) Material Strength
Erosion

?w Specific weight of water D Depth of
overland flow S Slope of the ground surface
2
Belt of No Erosion/Critical Distance

• Xc - critical distance. Taken as the upper
  boundary for the formation of rills.
• Works only for hortonian overland flow (semi-arid
  to arid areas).
• In humid-temperate regions, rills are generally
  associated with saturated overland flow and rills
  start closer to slope base and extend upward.

From Ritter et al., 1995
3
Channel Initiation
From Morisawa, 1985
4
Rills and Gullies
From Bloom, 1998
5
Basin Denudation and Erosion(Terms)

• Denudation amount of sediment eroded from the
  basin.
• Assumes sediment is completely removed from the
  basin.
• Assumes equal lowering of the surface over the
  entire basin area.
• Generally calculate on the bases of measurements
  made at a gaging station near basin mouth, or by
  determining the amount of sediment that was
  deposited in a reservoir in a specific period of
  time.
• Estimated as total volume of sediment divided by
  total basin area. Units L/T (e.g., cm/1000 yrs).

6
Sediment Yield

• Sediment yield amount of sediment eroded from
  the basin per unit basin area per unit time.
• Does not equal total amount of sediment eroded
  from upland areas. Some sediment is stored in
  valley bottom.
• Sediment delivery ratio - sediment yield as a
  proportion of upland erosion.

• Controlling Factors
• Climate and vegetation.
• Basin Size
• Elevation and Relief
• Rock Type
• Land-use and human activity

7
Langbein and Schumm Model
From Ritter et al., 1995 After Lanbein and
Schumm, 1958
8
Influence of Temperature on Runoff
From Ritter et al., 1995
9
Schumm and Langbein Model
Figure from Ritter et al., 1995 Model after
Langbein and Schumm, 1958
10
Role of Vegetation

• Vegetation and organic matter (litter) protects
  surface from erosion and raindrops.
• Preserves cohesion in aggregated-clay soil.
• Rootlets bind soil together
• Inhibits free flow of water and reduces overland
  flow velocities

11
Influence of Ground Coveron Upland Erosion
From Ritter et al., 1995
12
Influence of Basin Size on Sediment Yield

• Small basins have steep valley-side slopes
• Small, steep basins may exhibit significant mass
  movements
• High drainage densities in upland areas
• Increased storage along floodplain of large basins

From Ritter et al., 1995
13
Downstream Changes In Sediment Yield Per Unit
Area
From Ritter et al., 1995
14
Sediment Yield - Relief Relations
From Ritter et al., 1995
15
Land-Use Influences on Sediment Yield
After Wolman, 1967
Figure From Ritter et al., 1995