Drainage Basin System – (ii) The Slope Sub-system ( 山坡次系統 )

1. Drainage Basin System – (ii)  The Slope Sub-system (山坡次系統)

2. (A)Definition: • The land surface of the earth is that it is largely made up of slopes. • Most have a cover of weathered materials, called regolith. It is being carried down the slope by processes of transportation

3. (1) FORMS: • Nearly, all slopes consist of a number of distinct parts : • Each part called a segment • Facet - a rectilinear segment • Elements - convex or concave profile. • Free face (fall face) - when a facet is at an angle of more than 45 .

5. (2)"PROCESS" refers to the agents that are bringing about changes in slope form, for example, the effect of rain,etc • (3)"EVOLUTION": As a result of the action of processes, slope form changes in the course of time. This changes, from past form to present form, is the "evolution" of the slope.

6. (D). Factors affecting the stability of slope 1.    Explanation: • It depends on the relative balance between the forces (stresses) which tends to induce movement and the resistance (strength) which tends to prevent it.

7. Look at the following diagrams, insert “stable slope” or “unstable slopein the brackets provided.

8. 2.    Where do the stresses and strength come from ? • Stresses: “Gravity exerts a constant pull on all earth materials. • Several other forces are at work on them: flowing water, raindrop impact, wind or moving ice.”

9. Strength: “ Weathered material would not slide down for the fact that it posses a certain strength. • First, all particles have rough surfaces, which results in friction between touching particles. Secondly, the shape of the particles may cause them to interlock rather as the pieces of a jigsaw puzzle interlock. • Finally, the clay particles of the weathered material attract each other by electric forces which produce cohesion between particles.”

10. slope instability, therefore, is caused by increase in stress or decrease in strength of the slope-forming materials.

11. Factors leading to a decrease in strength (resistance) ? 1.Material factor: Bedding planes may decrease in strength if the water content increases. 2. Weathering process The reduction in effective cohesion and shearing resistance. e.g. increase of water 3. Increase of pore water pressure increase in pore-water pressure can lead to a reduction in shearing resistance.

12. Factors leading to a increase in stress (forces) ? 1.Transitory earth stress: e.g. earthquakes, continual passing heavy traffic. 2.Increase disturbing force: e.g. accumulation of talus, snow and water, ~ man-made pressure through the construction of embankments, dams or buildings. .

13. 3.Removal of underlying or lateral support ~ This removal may be by natural erosional agents such as streams, waves or moving ice. ~It may be the result of weathering of weaker strata at the toe of the slope.

14. 4.   Water: Water can increase the actual weight of slope materials and thus increase stress. It is important to realize that water rarely acts as a simple lubricant, erasing the downslope movement as it were, in some situations it can actually increase friction between particles.

15. Types of Slopes Processes (Mass Movement) • The term Mass Movement: describes all downhill movements of weathered materials (regolith), include soil, loose stones and rock, in response to gravity. However, it excludes movements where material is carried by ice, river or wind. • When gravitational forces exceed forces of resistance, slope failure occurs and materials starts to move downwards.

19. SLOW MOVEMENT

20. Soil creep is the gradual downslope movement of the regolith. It is a slow process. • Evidence for the existence of soil creep is provided by • ~ telegraph pole tilted, • ~ tension gashes in road, • ~ fences broken • ~ base of tree turned downslope • ~ terracettes • Soil creep occurs mainly in humid climate where there is a vegetation cover.

22. The main cause of soil creep is the expansion and contraction of the soil, combined with the influence of gravity. This is called the Heave mechanisms. • When the soil increase in volume, the only direction in which it can expand is perpendicular to the ground (slope) surfaces.

23. Expansion and contraction of the soil is caused by freezing and thawing. When moist soil freezes, the water itself increase in volume. When the soil is wetted the clay minerals absorbs water and expand..

24. Besides the heave mechanisms, another cause of soil creep is the movement of individual soil particles. These are caused by the growth of roots, trampling of animals and other disturbances. In every cases, gravity acts as a constant force tending to give a net movement downslope

25. Measurements of the rate of soil creep have shown that: • In humid temperate climates there is a downslope movement of about 1-2 mm a year. • In rainforest zone, values up to 5 mm have been recorded.

26. FLOW MOVEMENT • Slope material with a high proportion of fine particles and water content are prone to movements by flow.

27. 2. Mud flow: (泥流) ~Mud flow are more rapid (10 km per hr.)

28. 3. Solifluction : (凍融泥流) • Soil movement that takes place in cold climates (periglacial areas) where vegetation cover is limited. • In summer, the layer above the permafrost melts and becomes mobile. This is called active layer. • The permafrost prevents downward drainage of melt-water and also temperature are too low for effective evaporation. Top soil will soon become saturated.

29. Soil flow may then occur in the active layer. These process produces Solifluction sheet or lobes which can be 10-50m wide and up to 2m high in a series of step-like terraces down the slope.

30. However, Solifluction is not confined to the tundra’s permafrost zone. It can occur wherever there is sever winter freezing, including the temperate zone.

31. 4. Surface Wash:

32. It means the transport of soil by water flowing across the ground surface. There are two processes : a. Rain splash b. Surface flow : (i) The water can’t infiltrate into the soil (ii) The water table rises

35. Rapid Movements 5.landslides: There are two kinds of landslides: slides and slump. • Landslides may be planar landslides (slides) or rotational landslides (slumping). • In a slumping (rotational landslides), a curved slip plane (rotational rupture surface) is produced • It occurs in weaker rocks than slides.

36. Very Rapid Movements • 6. Rockfalls: • ~ These are rapid debris movements on steepest bare rock slope which exceeds 40. • ~ They may result from extreme physical and chemical weathering in mountains or earthquakes.

40. Factors affecting the development of slope 1. Climate 2. Gravity 3. Changes in the base level – uplift 4. Presence of lines of weakness on a slope 5. Degree of saturation of regolith with water 6. Vegetation /Animals / Human activities

41. Human increased slope instability by : ~ road cutting or shaking action of heavy traffic. ~ removal of vegetation/ deforestation increase the rate of slope movement. ~ construction of roads, quarrying, building and tunnels at the foot of slopes upset the equilibrium. ~ the grazing of animals and ploughing lossen soil and remove protective vegetation cover.

42. (H) Classification of Slope Segments Slope segments can broadly be classified by 2 ways : A. According to slope form 1.Waxing slope (also called convex slope, upper wash slope) 2.Mid /backslope (also termed free face) 3.Waning slope ( concave slope ,lower wash slope )

43. B. According to Slope Processes: • Such classification has been done in humid temperate slope in New Zealand, suggested by Dalrymple in 1968. • The classification divides a slope into 9 units. Like slope form. some segments may repeat or be absent, or even there may identify more units.

46. 3 models of slope evolution have been proposed and widely accepted. 1.Slope decline (W.M. Davis, 1899) 2. Slope replacement (W. Penck.1924) 3. Slope retreat (L.C.King, 1948)

47. No single one of these is universally correct, not one of them is completely appropriate to all slopes. The extent to which the retreat of actual slopes Corresponds to any one of them depends mainly upon structure and climate.

48. 1. Slope Decline (By W.M. Davis, 1899) • The slope becomes progressively decrease in the angle of slope in each phase of their development. • a concavity develops at the base, • the convexity extends in length and becomes more gently curved • Cause: it is equilibrium between the rates of weathering and transport

49. - In stage 1, the elimination of the free faces is done by the processes of fall and slump of the bedrock until the slope is gentle enough to develop a cover of regolith. - Stage 2 shows this phase which is called the graded slope. The regolith maintains a constant thickness over the slope and all the weathered materials is transported by mass movements and wash. The form of the slope is concave-convex.

50. - Stage 3 & 4, the length of the straight segment increased. • The curvature of the elements decreases as the slope continues to decline, and the length of straight segment diminishes. • The upper convexity experiences more output than input, whereas the lower concavity receives more input than output.