ADVANCED LEVEL GEOGRAPHY

NATURAL LANDSCAPE: Landform System

Topic 8: Drainage Basin System –

(ii) The Slope Sub-system (山坡次系統)

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(A) Definition:

There are few areas where the land surface is perfectly horizontal. The inclination of some

land surfaces with the horizontal is almost undetectable to the naked eye.

Slopes are vital part of our landscapes. Slope is an inclined being, and is the most

fundamental unit of the surface landscape. A remarkable feature of the land surface of the

earth is that it is largely made up of slopes. This is not only true of mountainous and hilly

regions but also of many plains , e.g. the Great Plains of the America West are not flat but

undulating' consisting of a series of gentle slope and valley floors.

However the slight the gradient the slope is, it is sometimes of a very complex nature.

Relatively few slopes are formed of bare rock. Most have a cover of weathered materials,

called regolith. The regolith includes both the soil and the weathered rock beneath it. When

rock is weathered, some is lost in solution, but much material passes into the regolith. So

the regolith is not static, its composition is being changed by continued weathering, and at

the same time, it is being carried down the slope by processes of transport.

Importance of slope study

It is the primary concern of geomorphologist but is also the least studied. The reason for unconcern of slope are :

a. Many research worker prefer to study unique or well defined feature instead of having to face the massive sampling which may involved in the study of slope. However, with the progress made recently in the sampling techniques, the above difficulties are solved.

b. Some of the hill slopes present difficult research problem because

their forms change either too slowly or too rapidly particularly as compared to the work of the streams and rivers. For instance, in a rainstorm, landslides on the whole slope can be completed in a second. Therefore, it is difficult to make direct measurement of slope.

Importance:

It is a mean for controlling some hazardous slope processes (e.g.landslides) when man modify the earth landscape.

For instance, landslides and rockfalls may creates risky site adjacent to

building. Therefore, the study of slopes is giving much attention.

(B) The concept of slope form, slope processes, and evolution.

(1) FORM "refers to the shape of the land surface which make up the slope, i.e. slope profile.

Slope Profile:

= It is the shape of the slope viewed as a cross-section at right angle to the hillside.

= For analysis, it is divided into slope units. These are straight and curved parts.

= Straight parts are called rectilinear segments. It is characterized by a constant slope angle.

= Curved parts are either Concave or Convex elements

= The maximum segment is the part which is steeper than the slope units above or below it. Below the maximum segment is the concave slope and above it is convex slope.

【Diagram Drawing – Convex segment—maximum segment—convex slope】

= On simple slopes there is one sequence of convexity-maximum segment-

concavity. More complex slopes have more than one sequence.

= Free face : It is a steep or part of slope formed of bare rock.

Nearly, all slopes consist of a number of distinct parts : > V

- Each part called a segment

- Facet - a rectilinear segment

- Elements - those have a 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.........

(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.

(C). Time-dependence and Time-independence of Form.

a. Slopes are said to have time-dependence form because their forms depends on the stage of evolution which they have reached. If a slope retains the same form at successive stages which they have reached, it is said to have time-independence form during the period of its evolution. Its length, steepness and shape remain the same.

b. The extent to which slopes are time dependent or time independent is an important question in geomorphology. The Davisian cycle of erosion emphasized the time-dependent approach. Slopes were assumed to become progressively gentler from youth through maturity to old stage.

c. On the other hand, it is a common fact of observations that particular characteristics , neither the time-dependent nor time-independent approach offers a complete a complete explanation of slope from. Each has a varying measure of truth according to local circumstances.

(D). Factors affecting the stability of slope

1. Explanation:

The weathering process render particles of soil or rock ready for downslope

transportation. Whether or not these particles are actually moved implies the stability of

these particles.

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.

Therefore, state the nature of the slope under the following conditions:

a. Stress > Strength : ________________

b. Stress < Strength : ________________

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. As well as the direct action of gravity, there may also be an indirect

effect which operates through a transporting medium such as flowing water, raindrop

impact, wind or moving ice.” Strength: “ Weathered material on a slope would immediately slide down to the river

were it not for the fact that it posses a certain strength. This strength is due to a number

of material properties.

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.”

As a summery,

Stresses: ________________ Strength : _______________________

________________

_______________________

________________

_______________________

________________

_______________________

Slope instability, therefore, is caused by increase in stress / strength or decrease in

stress / strength of the slope-forming materials.

Discussion:

Factors leading to a decrease in strength (resistance) ?

1. Material factor:

~ it includes a host of bedrock features : in sedimentary rocks joints and bedding plane, in

schists foilation, cleavage, faults.

~ Materials may have low internal cohesion – it doesn’t hold together to well. Sand is an obvious example but clays and organic matter exhibit the same properties.

~ Bedding planes may decrease in strength if the water content increases.

2. Weathering changes:

~ the reduction in effective cohesion and shearing resistance. As a result of weathering,

material may be more susceptible to movements on steep slope.

3. Increase in pore-water pressure:

~ The regolith, consists of rock particles, minerals and organic matter, contains pores.

After heavy rainfall, or as a result of human interference with drainage, the pores can be

filled with water and the 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.

~ In some case, the seepage of water through the regolith may remove or wash out the

materials resulting in less underlying and lateral support for the remaining material.

~ The bulldozer might also be responsible when making hillside cuts for roads or

buildings.

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.

(E). Types of Slope Transport Processes:

Most landforms result from 2 groups of processes, action of rivers and the slope processes.

River erosion is confined to a relatively small area, the channel.

Surface processes include weathering and transport. Weathering breaks down the rock, converting it into regolith.

The process of slope transport carry the weathered material to the base of the slopes, it is usually carried away by rivers. For slope retreat to occur, there must be both weathering and transport.

There is an important distinction between the rate of weathering and the rate at which weathering can be removed by transport.

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

1. Soil Creep

Soil creep is the gradual downslope movement of the regolith.

It is a slow process, too slow to be seen by eye, but affects the whole of a slope. 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 piled up behind wall forcing it to bulge and break

curved outward.

Soil creep occurs mainly in humid climate where there 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.

When it contracts, cracks or other voids form and the gravitational force tend to cause a vertical downward movement. Thus each cycle of repeated expansion and contraction produces a net downslope movement.

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.

Factors: ~ If the regolith is deep, soil creep movement becomes very much faster.

~ If the soil is moist and dry during the whole year, soil creep is faster.

~ Soil creep occur in all climates. In many cases, however, it is relatively unimportant, as much more regolith material is transported by other processes.

FLOW MOVEMENT

Slope material with a high proportion of fine particles and water content are prone to

movements by flow. Flow was defined as movement of a mass by internal deformation

under its own weight.

2. Mud flow: (泥流)

~ Mud flow are more rapid (10 km per hr.), less viscous and flow on much

steeper slopes.

~ They occur most commonly in areas with very sparse vegetation cover and subject torrential downpour.

~ The exposed regolith rapidly becomes saturated and in effect becomes a viscous-river

~ Mudflows in desert wadis have been reported up to 2m thick and moving so fast that they had waves.

~ They have usually stopped on lower angle slopes where the water drains from the base of. the flow into permeable regolith.

3. Solifluction : (凍融泥流)

The word Solifluction literally means "soil flow" but the term is limited to 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 soil water content is very high because 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. In other word, where there is a permafrost layer, the regolith is particularly likely to become saturated because water cannot drain away downwards. Water content is thus important.

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 : - When raindrops hit bare regolith. Soil particles are detached. This produces

miniature craters.

- Raindrops in an intense storm have an immense amount of kinetic energy.

- Raindrops exert a considerable force when striking bare soil, detaching particles and throwing them downslope.

On the flat ground, these particles merely change positions, but on a slope there is a net downslope transfer since those particles splashed towards the lower part of the slope have slightly longer trajectories than those which move upslope from the point of impact.

- In sheltered areas, where the rain does not often fall at the angle as it is blown by the wind, large pebbles protect the soil beneath from the rain splash and an earth pillar forms when the unprotected soil around the pebbles is splashed away.

b. Surface flow :

- Surface flow can occur when:

(I) Rainfall intensity exceeds the rate of infiltration at' which the soil can absorb water. The ground is saturated and the rain no longer percolates into it.

(II)the water table rises (at the base of slopes) to the surface and thus the soil is having absorbed all the water that it can.

The rate of transport by surface wash varies with: = Slope angle - rate of surface flow increases with increasing angle.

= Distance from the crest - the volume of surface flow increases

downslope. Thus, for a given angle, wash becomes more powerful

towards the base of the slope.

= Rainfall- amount and intensity.

= Vegetation cover - as a protection,

the effect of sheet flow is diminished drastically by the presence

of vegetation of the slope. It reduces the velocity of flow by

forming miniature dams and diversions. The soil surface is bound

together mechanically by roots and by the formation of humus

which, together with clay minerals, forms cohesive units of soil

particles.

The vegetation cover depends on climate:

e.g. humid temperate zone - wash is very ineffective because of

the low rainfall intensity and dense vegetation cover. Rates of

ground lowering as low as 0.01mm a year have been recorded.

e.g. Savannas - wash is more effective because grasses grow in

tufts leaving bare ground between them, so enhance surface wash

e.g. Semi-arid desert - wash reaches a high intensity (effective)

where rates of erosion as high as 5mm a year have been observed.

- though rainstorms are infrequent they have a powerful erosive effect owning to the sparse vegetation and high rainfall intensity.

-

Rapid Movements 5. landslides:

There are two kinds of landslides: slides and slump.

Landslides may be planar landslides (slides) or rotational landslides (slumping).

Slides:

- Slides move "en masse" and are not affected by internal derangement.

- Rocks which are jointed or having inclined bedding planes roughly parallel to the angle of slope particular susceptible to slides.

- In a planar slide, the weathered rock moves downhill leaving

behind it a flat rupture surface.

Slides affects both hard rocks and unconsolidated regolith.

Slumping: - In a slumping (rotational landslides), a curved slip plane

(rotational rupture surface) is produced. It occurs more likely in areas with softer materials (clay or sand).

- It occurs in weaker rocks than slides.

Characteristics :

1. Conditions favouring landslides occurrence are:

- Lithological — weaker or unconsolidated rock, e.g.clay

- Structural –- permeable beds overlying impermeable, dips

towards a slope.

- Topographical – cliffs or steep slopes formed by basal

erosion.

- Hydrological -- conditions causing surface or subsurface

concentration of water, there is water

seepage within the rock or regolith.

- Climatic – liability to high-intensity rainstorms

after the regolith has been saturated by exceptionally

heavy rainfall,

2. fast movement, localized and sporadic in their occurrence.

3. may occur at long intervals.

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.

~ Falls usually occur on the steepest of 70-90, where the angle of

friction is greater exceeded, e.g. mountain slope and coastal cliff are

most common sites for rockfalls.

~ Mechanism: A number of processes are thought to be responsible for

actually triggering rock falls, including thermal or freezing expansion,

water pressure in pores or joints and chemical activity.

~ When the particles becomes detached, it moves down the slope

by a combination of falling. Rolling and bouncing until it reaches a point where the

slope angle is low enough to allow the particle to come to rest.

~ As a result, scree / talus is formed. Scree is an accumulation of rock

fragments at the foot of a slope. The rock fragments detached are usually

pebble sized or larger.

(G). FACTORS AFFECTING THE DEVELOPMENT OF SLOPE

Slope processes / development is the result of the interaction of

several factors.

(A) EXOGENETIC FACTORS:

Theses are those factors operating at or near the earth's surface. .

1. Effects of climate:

- determines the types of slope processes e.g. soil creep, solifluction , landslides. - Temperature determines the rate and types of weathering, which provide

weathered materials for the slope processes.

e.g. increasing temperature increase chemical weathering,

increasing temperature range increase physical weathering

- Rainfall determines the rates and types of weathering,

e.g. ~ increasing rainfall increase chemical weathering

~ affects the amount of surface runoff, freezing / frost

shattering, and so the nature of surface processes.

~ the soil moisture acts as lubrication effects within soil particles and also weight-adding effects.

- Climate (temperature and rainfall) also indirectly affect the extent of

vegetation cover and consequently the nature of weathering and surface

process.

2. Gravity:

Gravity is the principal force affecting slope processes.

We may consider it as acting vertically downslope from the

center of mass of any particles. It has two effects: (i) to slide the particles down the slope, (ii)to stick the particles to the slope

3. Changes in the base lever -- uplift

The land uplift (changes in the base level) can affect the slope form.

(B) ENDOGENETIC FACTORES:

These are factors reflecting conditions within the earth.

4. Nature of the underlying rock structure and lithology:

a. Slopes of homogeneous lithology:

~ tend to be rectilinear (quite flat and straight)

b. Slopes of heterogeneous lithology:

-- more resistant rock overlying a less resistant, the underlying less resistant is more rapidly weathered and the regolith removed by surface wash or landslides. This undercuts the more resistant rock above, producing a free face. (because. steep slope cannot hold the regolith)

- therefore, there are frequent marked breaks of slopes. The steeper slopes being associated with the more resistant rocks and gentler with the less resistant rock.

- where there are several rock types of different resistance outcropping on a slope, the resulting form is a repeated succession of convexity-free-face-concavity slope form.

c. Presence of lines of weakness on a slope

- faults, thrust planes, it means increased permeability leads to increased seepage of water and hence deeper weathering front.

- = = > Rock resistance is reduced and hence the slope there are likely to be gentle.

- horizontally bedded sedimentary rocks tend to maintain steep slopes because of their resistance and their lines of weakness lie horizontally and hence not affected by erosion. On the contrary, tilted sedimentary rock strata give uneven profiles at the outcropping sites.

d. Strength of rock:

e.g. Clay : - low strength

- subject to landslides at angles steeper than 10 . - therefore, rock made of clay has predominantly gentle slopes - sand – can be stable at up to 35.

e. Permeability

- for permeable rock, surface wash is less important.

4. Slope Stability:

(Regolith is able to resist downpull movement of gravity)

Slope stability depends on :

a. friction among the materials making up the weathered regolith. this can be reduced by the presence of water which acts as a lubricant b. State of interlocking of the regolith materials :

- coarse materials tend to be well interlocked among one another

because they have more surface area of contact, so they would give

steeper slopes.

- accordingly, gravel, sand, silt and clay particles tend to show

increasingly smaller angles of rest.

- The closest state of interlocking is found in materials of a variety of sizes.

- Effects of vegetation cover

positive - binding effect of plant roots -e.g. in TRF, a close canopy can thus increasing soil permeability and hence reduce runoff, reduce raindrop effect, and promote regolith stability. It can check minor and sloe mass movement.

5. Degree of saturation of regolith with water

a. dry regolith :

unstable, because the materials are loose and may be subject to

wind deflation, falling by gravity.

b. unsaturated - stable, because a cohesive regolith has been

built up with the filling up of pores spaces by the swelling of

clay particles.

c. saturated - very unstable, because of the heavy weight of water and lubrication by water.

(C) Vegetation /Animals / Human activities.

Vegetation:

The main effect of climate is achieved indirectly through its influence on vegetation. The main effects of vegetation is the protection it gives the soil from surfaces wash and rainsplash. Other effects are : the action of rocks in holding the regolith on the slope, the contribution to chemical weathering by the products of organic acids and the supply of organic matters which improves soil structures.

Animals:

- a very powerful source of downslope movement

- e.g. = trampling of animals

- grazing on a grassy slope, cattle or sheep disturb surface particles of

soil which are pushed downslope by their feet.

- large burrowing animals like rabbits may dig out earth and much

smaller one like mice, moles, worms and termites must also make a

big contribution to surface movement by construction their nest.

Therefore, large quantities of soil are transported to the

surface. The nest collapse after the abandonment and hollow will

then be filled with earth until the earth is once again leveled. –

Human activities:

Increased slope stability by :

~ improving drainage,

~ construction of retaining walls,

~ afforestation,

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 form

1. Waxing slope

- also called convex slope, upper wash slope

- on the highest segment or the top of a hill / slope where the

slope curves over to meet the vertical face below.

- here the main processes: surface wash, creep, subsurface soil

water

- the gradient is so slight that nearly all movement is through

the downward action of soil water. As the angle begins to

steepen on the edge, there is a large component of downslope

movement. With growing steepness, the speed of movement

increase and the slope becomes potentially unstable.

2. Mid / Backslope

- free face, also termed as gravity/derivation slope, since any

rock waste littering the lower slopes comes from this face.

- it is any outcrop of bare rock, e.g. scarp, bluff of cliff

- because of the steep gradient, rock-waste falls or rolls rapidly

under gravity. Therefore, it is too steep to allow any

accumulation of weathered materials. - Frequently the slope is

rectilinear in form. "

3. Waning slope ,

- concave slope, lower wash slope

- stretches to the valley floor or other local base level .

- built of alluvial deposits of finer materials brought by

sheetflow, usually dissected by streams.

According to local variations, the slope segments may come in perfect order, repeating some segments or absent in some.

B. According to Slope Processes:

Slope segments can also be classified according to the different processes working. 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.

1. Convex slope (Unit 1 to 3)

- main processes : ____________________

a. Unit 1: (interfluve)

- water divide (watershed), 0-1

- nearly horizontal to slightly inclined convex surface

- gentle gradient, therefore subsurface water movement is

mainly vertical rather lateral.

- has grass or scrub cover

b. Unit 2: (seepage slope)

- seepage slope, 2-4

- relatively potentially unstable due to increase gradient

- water movement - mainly percolation subsurface water flow

c. Unit 3: (Convex creep slope)

- convex creep slope

- more potentially unstable due to even more steep gradient

- soil creeps takes place and terracettes are marked resulting

in a stepped outline.

2. Convex slope (Unit 1 to 3)

- main processes : _________________________

a. Unit 4: (free face)

- fall face greater than 45 and normally > 65.

- more or less cliffs and rocky scarps

- uneven surface of the slope due to slumping of

materials

- it is the slope element where rapid mass movement occurs.

b. Unit 5: (transportation midslope)

- 26 - 35.

- it is located at the base of free face, constant slope

(rectilinear slope)

- coarse regolith covered with short grass

- bare soil or rock surfaces in isolated patches can be

found

- characterized with active surface and subsurface water

action, rills or gullies can also be identified.

3. Footslope (Unit 6 & 7) (Colluvial footslope & Alluvial toe-slope)

- Main processes: slow downslope processes because of

increasing slope stability, transportation of materials

downslope, solution of minerals in soil particles over

the whole slope.

a. Unit 6 - colluvial footslope

- it is located at the base of the transportational mid-

slope and is characterized with collection of rock debris

originated from the upper slope.

b. Unit 7 - alluvial toe-slope,

- 0-4

- concave slope segment

- dissected by gullies and streams with alluvial

deposits

4. Channel Wall (Unit 8) and Channel Bed (Unit 9)

- main processes : _______________________________________

a. Unit 8 - channel wall, close to river valley - deep rocky cliffs in many mountainous regions without vegetation cover, where active lateral fluvial erosion takes place.

- It is subject to slumping, falls and river corrasion

c. Unit 9 - Channel bed

- right at the valley where fluvial processes are very

active,

- often completely covered by river water

(depend on the volume of flow)

- in the upper course of river channel beds are

usually uneven, asymmetrical and gentle gradient

slopes.

(I). Slope Evolution (Slope Development)

As a result of various processes, slope form may change in the course of time. This change from past to the present form, is the evolution of the slope.

If the weathering is faster, a regolith will form. It is then the rate at which regolith material can be removed that limits the rate of slope retreat, and thus the slope is usually referred to as transport-controlled slope.

On the other hand, on some part of the slope transport processes are potentially able to remove material faster than it is produced by weathering, a free face will be formed. The rate of weathering is then the limiting factor for the rate of slope retreat and thus the slope is weathering-controlled slope.

The slope processes examined may be thought of as conveyor belt, transporting the products of weathering on the initial part of their journey to the sea. The lower end of the conveyor usually dumps its load into a stream channel and it is rapidly taken away. If this does not happen, the conveyor grinds to a halt, debris accumulates and weathering slows down as a protective mantle builds up.

Different processes should give rise to different slope forms. Over time, slopes should develop towards predictable, equilibrium forms.

The volume of regolith passing through a slope segment in a given time interval increases towards the base of the slope. Each successive downslope segment must

accommodate the input of the previous segment plus the products of-weathering from that segment in order to keep angle and regolith thickness constant. In order to achieve this, the transport rate on the slope must increase. This steady state with a constant regolith thickness is known as a graded slope.

If the input exceeds the output, the volume of materials in store increase, and regolith at the top of the segment thickens. The slope of surface then becomes steeper than the bedrock slope. The causes an increase in the velocity of slope movement which tends to lower the slope angle by transporting more materials to the bottom of the slop and effectively restores the balance between input and output

When output is greater than the input, the volume in store falls, the regolith thins as debris is moved away downslope and. if the process continues, eventually bedrocks will be exposed.

The base of the slope usually 'output' debris into a stream channel or into the sea. Both these agents are very effective in removing the debris faster than the slope can supply it. This is termed unimpeded basal removal and It tends to accelerate slope movements by removing resistance to downslope movement from the last segment The effect is feed back up the slope from segment to segment eventually affecting the whole profile.

Should the removal of debris from the slope be inadequate to keep up with the rate of supply from the slope, the feedback will be to slow down the rate of movement as the regolith accumulates at the bottom of the slope. This is the situation of impeded basal removal.

Evolution is the change of form over time as brought about by the action of processes. The evolution of slopes provides clues as to how the landscapes as a whole has evolved.

The slope processes may be thought as transporting the weathering products on their initial part to the sea. It usually dumps its load into a stream channel and it is rapidly taken away. If this does not happen, the debris accumulates and the weathering slows down as a protective mantle builds up.

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. It becomes less steep and a concavity develops at the base, the convexity extends in length and becomes more gently curved, i.e. smoothly convex-concave forms and in particular long and gently-curved convexities associated with decline.

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 and more output than input, whereas the lower concavity receives more input than what it can 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 17 Slope decline .1-5' are successive stages of evolution Stage 1 is an ungraded slope with rock outcrop. The slope becomes graded from stage 2 onwards

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, by no means 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. The 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.

Figure 20 Parallel retreat (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

Slope can be viewed as a “natural system”, since without man’s modification, there

are numerous and complex linkages between factors, processes and forms within it.

2 main types of system can be identified : closed and open. Open systems occur widely in geomorphology; their prime attribute is that they exchange energy and materials with their surroundings.

With the help of above diagram, complete the following table:

INPUT OUTPUT

Types Sources ENERGY

MATERIALS

Identify the important processes on the slopes leading to the transport of materials (i.e. water and soil)

Materials to be transported Processes involved

1. Water - Runoff - Infiltration - Underground flow

2. Soil / weathered debris - River transport / surface wash - Mass Movement, e.g. -

- Open System: - an exchange of energy and mass across the system boundary

- An individual slope (or part of one) may be regarded as an open system;

in which there are inputs of energy in the form of solar heat, rain, wind

and past uplift and mass from above slope.

- The slope itself may be considered as bedrock with a mantle of regolith

- continually moving downwards as a result of past and present energy

inputs.

- The streams acts as base level for the slope and provides the means of

transporting water and soil brought down the slope across the boundary

out of the system.

- Inputs: - Energy -- (solar heat), winds, initial and subsequent uplift from earth movement, mountain building and vulcanicity.

- Mass : wind-borne deposits and materials from upslope; Water in the form of rainfall and runoff. rain, snowfall

- Outputs: - Energy

loss of heat to the atmosphere (terrestrial radiation)

- Mass :

- loose regolith being eroded off from the slope

surface by a slope process, wind loads or river loads

- water is lost by ______, ______ and ;_________.

Therefore, slope is regarded as a dynamic moving natural mechanisms with constant complex interactions between the force of gravity, rain, heat, wind and vegetation and rock of which it is made.

(K). Slope systems in the Sample Landscapes:

(1) Tropical Rain Forest :

Read the following extract:

“Chemical weathering reaches its maximum intensity in the rainforest zone, and even quartz grains are slowly attacked. Consequently, solution loss by the surface wash is an important processes of removal.

Both creep and surface wash occur.

The frequency of landslides scars has often been noted in slopes of 35 – 50.

Regolith – covered slopes of up to 70 have been reported, th soil being held in place by tree, but such cases are exceptional and slopes do not normally exceed 50.:”

Q1. State the important slope processes in the TRF and say why it is favoured.

Slope processes are not very active in tropical rain forest because runoff is checked by

plant interception and infiltration, and infiltrating water is readily taken up by the large

number of plant roots.

All the slopes processes except solifluction have a similar chance of occurrence if they do

occur. Solifluction is non-existent because the subsoil is never frozen.

1. landslides (slides / slump)

2. Solution loss by surface wash

3. Soil creep

Q2. What is the main slope stabilizing factor in the TRF ?

~ very dense tree roots.

Q3. What kinds of human activities will likely lead to the removal of such stabilizing

factor ?

~ deforestation

~ mining

~ clear for cultivation

~ lumbering………

As a slope form, we put the following conclusion:

Upper slope: ~ convex in shape (with rapid chemical weathering and removal in the

upper slope)

.

Mid slope: ~ small and short

Base slope: ~ Concave in shape (large supply of sediment from above cause ,

cause rapid deposition of sediment at foothill).

The entire slope is smooth because the whole slope is cover by a thick layer of weathered

materials.

Explanation:

In the rainforest region the ground is warm and moist throughout the year and

consequently chemical weathering reaches its maximum effectiveness. Weathering tend to

be faster than transport and so the regolith is deep.

Although the existence of deep weathering profile indicate that in many areas the removal

of material from slopes is slower that the rate of production of weathered material, this

does not mean that mass movements are ineffective.

The high intensity of rainfall, the high amounts of rainfall, the lack of much vegetation or

humus on the forest floor and the impermeable nature of many clay rich soils mean that

there are probably considerable quantities of surface wash over tropical forest floor caused

by rain splash. It was formerly thought that the forest protected the ground from surface

wash but measurement show that this is not so. Although much rainfall is intercepted by

leaves, it then drips from canopy with almost as much velocities the original rain; the leaf

litter on the ground surface is itself subject to wash during storms. There is much loss of

material in solution , and soil creep also occurs.

Following intense storms, steep slopes are commonly affected by landslides, facilitated by

the combination of rotted rock, clay soils, and generally shallowly rooted trees. Such

process usually strips the regolith cover and so expose bedrock to renewed weathering.

This has proved a particular hazard when cities like Rio de \Janeiro, Hong Kong and Kuala

Lumpur have expanded, causing building to spread to steep slopes.

Pediments, which are also widely developed in humid tropics, are always deeply weathered

and often pass upwards into the hill slopes without the intervention of basal knick. In

Johore, Malaysia, the main slope elements are

(i) the hillslopes of 10 – 40 (ii) pediment at 1 – 9 (iii) the clay plain. The profile as a whole seems to be represent a modified

form of arid slope sequences.

The essential contrast between the arid and humid slopes may be stated as follows. In arid

areas there is active slope retreat, and extension of valley floors and plains via formation

of pediments. In humid areas the plain are formed by active weathering, which continually

leads to undercutting of hillslopes. Such contrast sees to suppose that this is primarily the

result of climatic control over weathering , mass transport and run-off.

(2) Tropical Desert :

Thing To Do:

Read the following extract:

“The general slow rate of denudation and lack of weathering mantle results in many steep bare rock surfaces upon which rockfall takes place. Scree slope transport most of the rockfall material, but there are also many slopes on which debris appears to be moving. On such slopes, debris is not moving by rolling and sliding. Movement is by downslope creeping following expansion and contraction with diurnal temperature ranges. Other mass movement such as landslides are less common in deserts.

In deserts, the main transporting process is sheetwash following rare storms.

Mountain areas are dissected by closely-spaced gullies with steep slopes.

Where mountains adjoin plains, the typical slope form is a steep-side above a very gentle pediment.

Most desert slopes are either steep or very gentle. The steep slopes include bare rock cliffs of up to 90, boulders-covered slopes of 20-35 (scree).”

Q1. What are the slope processes taking place ?

Slopes processes are not very active in TD because of the shortage of runoff water except

in the rare event of a heavy rainstorm.

~ sheetwash

~ soil creep – by expansion and contraction with diurnal range of temperature.

~ rockfall and earth flow are the only processes of importance in tropical deserts.

Q2. How and why the processes taking place ?

Q3. So what are the landform result ?

~ Landform produced by water-based processes (river erosion).

(Home revision exercise)

(You have leant desert landform in F.4. Label the desert landforms produced by

water in the diagram below. Briefly describe the formation and characteristics of

each.

As a slope form, we put the following conclusion:

Upper Slope: ~ small convex upper slope (limited chemical weathering)

Mid-Slope: ~ obvious and there are free-face.

Base Slope: ~ long concave slope (e.g., pediment)

~ There is little supply of material from upper slope so little

accumulation of material at the base slope and material

accumulated

in the base slopes can easily be eroded by sheet wash, and

flash

flood and wind.

Explanation:

The entire slope: there is a sudden change in slope gradient between upper, middle

and base slopes. The whole slope is covered by a very thin layer of weathered materials.

Landform features in deserts: rugged angular hills,

Cliffed canyons

Sand-covered plains,

Wadis,

Mesa and butte

Inselbergs, pediment

Mushroom rocks, sand

dunes

Alluvial fans.

Slope retreat takes place very slowly in tropical arid environment. Mechanical weathering

is more important than chemical weathering. The main transport process is sheetwash,

following occasional and torrential rainstorms. The effectiveness of sheetwash is enhanced

by the absence of vegetation. Most steep slopes are bare and chemical weathering is

limited..

Large alluvial fans and wadis fills occur because storms are too short in duration to

transport debris far. Much weathered material is removed by traction, saltation and in

suspension.

Slope profiles in arid areas, being little obscured by vegetation, show angularity. Most

slopes in deserts are either steep or very gentle. In mountainous regions and on isolated

hills, slopes consist mainly of cliff and rectilinear slopes of 30 - 35. Frequently the layer of

boulders is thin and penetrated by outcrops o f rock in situ. Once a boulder-slope is formed,

it retreat parallel. Conical hills and narrow-crested ridges are formed by the intersection of

such slopes.

The activity of surface running water is extremely efficient, so much that the finer products

of weathering `are rapidly transported away, permitting optimum development of the

idealized 4 - unit slope model with 4 components: the waxing slope, a free face, a constant

slope and a pediment.

Fig.

the pediment are long gentle slopes, formed in solid rock, that occur below substantially

steeper slopes. It usually consists of a gently sloping rock-cut surface, generally at angle of

between 1 and 7.

(3) Tundra Landscapes: (苔原地區)

Periglacial regions are those areas in high latitudes where average air temperatures are

below freezing point but ice does not actually accumulate on the surface owning to

summer thawing when air temperatures are above freezing for several months,

Slope processes of mass movement occur with great power in the tundra environment.

There are various reasons for this.

First, the presence of the permafrost provides a barrier to the downslope infiltration of

water into the ground, thereby inducing high pressure in the soil pores in the surface layers.

Second, the layer above the permafrost, the active layer, is extremely moist and highly

unstable.

Third, the permafrost acts as a lubricated surface over which the materials of the active

layer can flow.

Fourth, the process of frost heave tends to cause a downhill movement of material under

the influence of gravity.

For this reason, hillside slopes tend to be characterized by instability, and slope failure is

frequent, posing problems for buildings and other engineering structures.

On exposed cliff faces blocks may be detached by frost weathering. Frost shattering is the

dominant process on steep slopes. On the hillslopes, weathering tends to be dominated by

freeze-thaw action, although chemical weathering is by no means entirely excluded.

Above 40 debris moves by rock avalanche. On slopes between 20-40, snow avalanches

transport frost-shattered debris.

Transport processes on hillslopes are debris flow, but solifluction is important.

Solifluction are the only important slope processes in tundra regions because the subsoil

is always frozen and the melting of ice, which supplies the bulk of water to slopes, is a

gradual and slow process. It is one of the most widespread processes of soil movement in

tundra regions. It forms terraces and lobes on slopes as gentle as 2.

The most characteristics feature of periglacial area, however, is the existence of permafrost.

This is the ground beneath the surface that is permanently frozen. In all the permafrost

areas, the top layers of soil thaw in summer. The depth of this active layers varies, being

up to 4m in the regions of 50-60 N in Siberia but decreasing northwards into cold

environments.

~ The effect of permafrost is most noticeable on slopes. When the surface layers thaw in

summer, the melt-water cannot percolate vertically into the underlying still-frozen ground

and the active layer becomes saturated. Under these condition, the active layer may move

downhill as an accelerated form of soil creep called solifluction (凍融泥流).

(L) Man – Slope Relationship

Read the following passages,

“ Slopes can affect man profoundly; and man, in turn, can cause important modifications to

slope systems.

Slope steepness particularly affect man’s activities in areas of high relief, where slopes are often too precipitous for cultivation or habitation, and soil are so thin that they have to be

gathered into artificial terraces. Steep slopes can give rise to serious hazards, such as

landslides of various kind which can destroy village.

Man can himself modify the natural slope system in a variety of ways. He can play a

significant role in initiating or reactivating landslides. Conversely, he can counter

instability by “grading” slope by drainage, and by the construction of retaining structures.”

Discussion:

1. What are the 2 relationship between man and slopes ?

2. What examples have been given to substantiate these relationship ?

Can you give examples too.

[END]

Reference materials:

1. Landscapes Processes: An Introduction to Geomorphology

Darrell and Valerie Weyman

George Allen and Unwin (1977)

2. Slopes

Anthony Toung

Longman (1972)

3. Process and Landform

Alan Clowes and Peter Comfort

Oliver and Boyd (1983)

4. Process and Pattern in Physical Geography

Keith Hilton

University Tutorial Press (1979)

5. Slopes and Weathering

Michael Clark and John Small

Cambridge University Press (1982)