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

(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)

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.

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

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

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.

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).

Hillslope profiles….

The soil factor Physical properties Human use….

(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

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

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

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

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

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.

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.

Earth flows in St-Lawrence River lowlands

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°)

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)

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

(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

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

Rates of denudation generally fall between 2 and 15 cm per 1000 years (regional scale estimate) High sediment yields (e.g. 0.5-1.0 m/1000 years) associated with excessive elevation and relief. Basin size and sediment yield:

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 10-20 t km-2 yr-1 In general, spatial variations in erosion rates reflect physiography, lithology, and quaternary history.

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)

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

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)