drainage basin system – (ii)  the slope sub-system ( 山坡次系統 )

Drainage Basin System – (ii)  The Slope Sub-system

(山坡次系統 )

(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

(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 .

• (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.

(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.

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

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.”

• 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.”

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

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.

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. .

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.

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.

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.

SLOW MOVEMENT

• 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 t

here is a vegetation cover.

• 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.

• 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..

• 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

• 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.

FLOW MOVEMENT

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

• 2. Mud flow: (泥流 )

~Mud flow are more rapid (10 km per hr.)

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.

• 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.

However, Solifluction is not confined to the tundra’s permafrost zone.

It can occur wherever there is sever winter freezing, including the temperate zone.

4. Surface Wash:

• 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

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.

Very Rapid Movements

• 6. Rockfalls:

• ~ These are rapid debris movements on steepest bare rock slope which exceeds 40.

• ~ They may result from extreme physical a

nd chemical weathering in mountains or earthquakes.

Factors affecting the development of slope

1. Climate 2. Gravity3. Changes in the base level – uplift4. Presence of lines of weakness on a slope5. Degree of saturation of regolith with wa

ter6. Vegetation /Animals / Human activities

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.

(H) Classification of Slope Segments

Slope segments can broadly be classified by 2 ways :

A. According to slope form1.Waxing slope (also called convex slope, upper wash slop

e)

2.Mid /backslope (also termed free face)

3.Waning slope ( concave slope ,lower wash slope )

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.

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)

• 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.

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

• - 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.

• - 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.

- Lower part of the slope - with the accumulation of the transported regolith, lowering is less.

- Davis, the original proponent of this theory, based his arguments on visual assessment of slopes in humid temperate areas.

figure . Slope decline .1-5' are successive stages of evolution

2.    Slope Replacement (By Walter Penck, 1924)

• Slope replacement means original steep slopes being replaced by lower angle slopes which extends upwards from the base at a constant angle.

• A free face slope is slowly buried by a scree which accumulates at the base of cliff. (It means the replacement of a cliff by a scree)

• All parts of the cliffs face are exposed to weathering.

• The scree accumulating at the base increase in height and if it is not removed, it will eventually replace the entire cliff by a slope of about 35° , the angle of rest.

• Continued weathering and removal leads to an upward extension of this gentler slope. This continues until the whole of the 35° slope has been replaced from below by the gentler slope.

3.   Parallel Retreat (By King, 1948)

Concave-convex slopes are ubiquitous. In many semi-arid areas, more complex profile are common, comprising an upper convexity, a free-face, a rectilinear debris slope, and a gently concave pediments, only 3°-5°.

Each of the upper parts of the slope retreats by the same amount and maintain the same angle .

 Therefore, the convexity, free face and debris slope all retain the same length.

The concavity extends in length and becomes slightly gentler in angle. This is called pediment.

Pediment : The pediment is the name given to the gently concave area which extends from the foot of the debris slope and becomes wider and wider as the slopes retreat.

• The pediment is generally slightly concave but the slopes are very gentle, only 3°-5°, and often they appear to be completely flat.

• pediment is only covered by a thin sheet of regolith.

In the arid area, where chemical weathering is greatly impeded. The supply of regolith is limited, the input of transported materials at the bottom of debris slope is little, with respect to the rate of output.

As a result, the debris slope is able to maintain a constant angle, with an continual extension of pediments primarily by the process of sheetwash.

• Late in the erosion cycle, the hills are left as isolated, steep-sided relicts, called inselbergs in Africa, or buttes and mesas in North America.

• (a) Of a slope containing a free face and debris slope, the angle of the free face is determined by the strength of the rock. A strong will often form a vertical free face.

• (b)  Of a slope without a free face • ~ In less resistant rocks, the free face may not be

present. Weathering reduces the slope to a continuous debris-covered slope.

•

• Conclusion : There are actually no universal pattern of slope evolution, slopes are related to rock type, vegetation and various weathering and sediment transport processes.

The theories of slope decline and replacement are time-dependent approaches to slope development; slopes were assumed to become progressively with time.

• On the other hand, the theory of parallel retreat adopts a time-independent approach; slopes retain a similar form as they evolve.

• Neither the time-dependent nor the time-independent approach offers a complete explanation of the slope form. each has a varying measure of truth according to local circumstances.

(J) Slope as an open system