1. 7. Hillslopes; surface erosion and mass movements
Erosion by rain drops
Surface erosion by running water
Mass movement processes
The heave mechanism
Sediment yield

2. (a) Erosion by raindrops
Controlling factors:
Raindrop effects:
Break down soil aggregates
Rain splash
Surface runoff
Compaction of soil surface by raindrops
Erosivity of raindrops is a function of the intensity of rainfall and kinematic energy of raindrops (= 0.5 x mass x velocity2)

3. Raindrops…
Raindrops detach soil particles while surface runoff transport them.
Raindrops compact the surface soil and seal the soil pores (e.g. by depositing fine particles in them). This, in turn, reduces infiltration and increases surface runoff.

4. (b) Surface erosion by running water
Four main processes:
Sheetwash
Rilling
Gullying
Piping
Sheetwash: overland flow
Shallow near crest; depth increasing downslope

5. Surface erosion and running water….
Rilling: positive feedback mechanism at work.
Sheetflow  concentrated into rills; development of a drainage pattern
Gullies: rills develop into gullies (width > 0.3 m); characterized by a vertical head scarp: knickpoint.
Piping: subsurface tunnels in the soil

6. Topographic factors…
There are two factors to consider here: slope length and slope angle.
e.g. the amount of erosion is proportional to the slope steepness and
E µ Sa
where E: erosion (soil loss)
S: slope angle
a: empirical constant ranging between 1 and 2.

7. Hillslope gradient and drainage basin
Increase valley-side gradient with increasing stream order
Increase in stream order associated with greater depth of incision –
both slope length and valley-side gradient tend to increase with stream order (maximum hillslope gradient in 4th or 5th order drainage basins but tends to decrease thereafter).

8. Hillslope profiles….

9. The soil factor
Physical properties
Human use….

10. (c) Mass movement processes
Emphasis on mass movements and form-process relations
Attempts have been made to classify mass movement processes in terms of:
Type and rate of movement
Type and water content of moving material

11. Mass movement - classification
One possible classification is that of Carson and Kirkby, in which mass movements are classified as a function of:
Water content
Rate of movement
type

12. Mass movement processes….
Differentiation between:
Slide
Flow
Creep (heave) processes
Fall mechanism

14. Factors that lead to an increase of “G” (force that tend to promote motion) tend to promote processes dominated by slide mechanisms
Factors associated with decrease of “R” more likely to correspond to processes dominated by flow (and fall) mechanisms

15. Mass movements…
Slide: Affect both hard rocks and unconsolidated debris
Moving mass slides down an inclined plane (more or less intact)
Failure frequently occurs along a slight weakness, at right angles to the slide plane.
2nd type characterized by having a rotational movement along a curved slip plane

16. Flow: Movement of a mass by internal deformation under its own weight
Fine material (e.g. clay) becomes saturated to a % greater than the liquid limit
Most originate on sloes greater than 10 degrees.

17. Flows… Flows can be divided into tow categories:
Earthflows
Mudflows
Mudflows are more rapid, less viscous, and flow usually on lower slopes than earthflows. Areas with sparse vegetation cover and subject to torrential rain.

18. Earth flows in St-Lawrence River lowlands

19. Fall mechanisms
Movement of rock debris on very steep slopes (e.g. 70°-90°), where the angle of friction is greatly exceeded
Occurs when internal strength of rock is overcome
Gravity alone incapable of causing rock to break or fail. Ice (freeze-thaw cycles) often the agent involved.
Detached rock fragments produce screes at the bottom of slopes (i.e. pile of unconsolidated rock fragments having surface slopes of between 34°-38°)

21. The Heave mechanism
Heave: displacement of hillslope material upward and normal to hillslope gradient
Main causes of heave:
Expansion due to wetting
Expansion due to freezing
Displacement by fauna
Thermal expansion
Displacement by plant roots
Heave followed by settling (downward, vertical movement)

22. Heave – creep process
Creep rate decreases with depth below surface (due to lower frequency of heaving at depth and greater difficulty of expansion)
Downslope creep rates extremely variable because of differences in slope angle, moisture content, etc.
Rates range form 01. to 15 mm/year when soil is vegetated
May increase to 0.5 m/yr or more on uncovered slopes where frost action is prevalent

23. (d) The sediment yield Sediment yields or loads Denudation rates:
e.g. t/year or t/km2/year
Denudation rates:
Volume of eroded material divided by drainage area (for a given period of time)
Estimates of denudation rates usually based on measurements of sediment loads made at gaging stations

24. Factors affecting sediment loads:
Precipitation and vegetation
Human activities
Basin size
Elevation and relief

25. Rates of denudation generally fall between 2 and 15 cm per 1000 years (regional scale estimate)
High sediment yields (e.g m/1000 years) associated with excessive elevation and relief.
Basin size and sediment yield:

26. Canadian landscape and sediment yield
Erosion rates (sediment yields) in Canada comparatively low (by global standards)
Unusual to find yields exceeding 50 t km-2 yr-1
Average of t km-2 yr-1
In general, spatial variations in erosion rates reflect physiography, lithology, and quaternary history.

27. Glacial legacy….
Positive relations between drainage basin area and specific sediment yield often observed in Canadian landscape
Trend clearly apparent within “undisturbed” basins
Dominance of Quaternary sediments as sources for current stream erosion (i.e. a significant component of present-day erosion is the continued removal of glacial debris)

28. Sediment yields and Canadian landscape
In many cases, therefore, contemporary sediment yields in Canada do not necessarily reflect present-day rates of primary denudation of the land surface

29. Sediment budget….
Quantitative analysis of a drainage basin that shows relations between:
Erosion of basin materials
Discharge of sediment
Changes in sediment storage
(in - out - storage)