UNIVERSITY OF DAR ES SALAAM.

COLLEGE OF NATURAL AND APPLIED SCIENCES

(CoNAS)

DEPARTMENT OF GEOLOGY,

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GY 252: FUNDAMEMTALS TO ENGINEERING

GEOLOGY.

HOMEWORK # 2:A GEOLOGICAL PAPER.

TOPIC; LINKING LANDSLIDES AND SURFACE

EROSION.

COURSE INSTRUCTOR: DR. CHARLES LUCIAN.

STUDENT’S NAME: MGAYA, FRANCIS.

REGISTRATION #: 2011-04-02327

DEGREE PROGRAM: B.Sc.in Eng. Geology.

A GEOLOGICAL PAPER ON LINKING LANDSLIDES AND SURFACE EROSION.

INTRODUCTION

Landslides, used here as a term to represent mass movement on the land surface that consist primarily of rock and/or soil and thus including rock falls and debris flows are an important landscape forming process, providing the main mechanism for sediment release from slope to permit transportation through fluvial system.

This paper examines the link between landslides and surface erosion.Surface erosion and landslides are intimately related;involving erosion of rock, generation of sediment through various Earth surface environment.These processes and the deposits, and landforms that they generate have a fundamental importance on engineering, environmental and public safety issues on recovery of economic resources,and our understanding of Earth history.Therefore the paper focuses on the nature of erosion on landslides by examining different types of landslides and how they cause surface erosion ,although it begins describing the causes of landslides in general and the meaning of surface erosion.

Surface erosionErosion is the process by which natural forces move weathered rock and soil from one

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place to another.Gravity, Running water, Glaciers, Waves, and Wind all cause erosion.The material moved by erosion is called a sediment,When agents of erosion lay down sediment, deposition occurs.Deposition changes the shape of the land.Weathering, erosion and deposition act together in a cycle, called the Geologic Cycle.

Gravity pulls everything toward the center of the earth, gravity is the force that moves rock and other materials downhill.Gravity causes mass movement, mass movements include landslides(mudflows,slump, creep).Mass movement can be rapid or slow.

Surface erosion can be classified in three major forms, 1-Water erosion ie. Rainfall and rivers and streams,Coastal erosion, Floods,Glaciers,Freezing and thawing, 2-Wind erosion, 3-Gravitational erosion. (as discussed by the British Society for Geomorphology (BSG). 2012.)

Causes of Landslides

The main causes of landslides consist of Geological factors, Morphological factors, Physical factors and factors associated with human activity, and one landslide trigger.Causes may be considered to be factors that made the slope vulnerable to failure, that predispose the slope to become unstable, but the trigger is the single event that finally initiated the landslide.Thus causes combine to make a slope vulnerable to failure and the trigger finally initiates the movement, landslides can have many causes but only have one trigger.A good example of a trigger can be an Earthquake, or Prolonged rainfall.(as reviewed by J.Grotzinger, Thomas H. J, F.Press and R. Siever. 2006.Understanding Earth 5th edition.)

Examples of causes of landslides include;

Geological causes;

Weathered Materials

Sheared materials

Permeability contrasts

Material contrasts

Rainfall and snow fall

Earthquakes

Morphological causes;

Slope angle

Uplift

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Rebound

Fluvial erosion,Wave erosion,Glacial erosion

Slope loading

Vegetation change

Human causes;

Excavation

Loading

Water management

Mining

Quarrying

Deforestation

Pollution

In the majority of cases the main trigger of landslides is heavy or prolonged rainfall. Generally this takes the form of either an exceptional short lived event, such as the passage of a tropical storm or even the rainfall associated with a very intense thunderstorm or of a long duration rainfall event with lower intensity. The importance of rainfall as a trigger for landslides cannot be underestimated a good example is the 1997-1998 El Nino event that triggered landslides along the west coast of North, Central and South America(as seen in National Geographic Magazine.1998).So one can ask the question ,why does rainfall trigger so many landslides? Principally this is because the rainfall drives an increase in pore water pressures within the soil.

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Types of Landslides and how they cause Surface erosion

Debris Flows;Debris flows are fast moving flows of mud and rock and they are the most numerous and dangerous of all the landslides.Debris flows generally occur during periods of intense rainfall or snow melt.They usually begin on the top of steep hills with saturated soil, they are so dangerous because they move quickly, destroy without warning, and wipe out everything in their path.They can destroy homes, knock down trees, and obstruct streets and roadways. Their average speed is 10 miles an hour, but some have been known to exceed 35 miles per hour.Their viscosity ranges from thick, rocky mud to water mud. The following are several types of debris flows.

Earthflow: The wet ground breaks up and falls down the hillside in a rounded shape. It usually occurs on clay or sand and it is the slowest and driest type of flow.(as reviewed by John Grotzinger, Thomas H. J, Frank Press and R. Siever. 2006.)

(image by the United States Geological Survey) Mudflow: Sometimes referred to as a mudslide, a mudflow is when the soil becomes so saturated with water that it speeds down the hill in a muddy river carrying debris. It is the fastest and wettest type of flow.

Debris flow mainly cause erosion by acting as a transport agent for mainly sand/clay and mud.

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FallsFalls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become disconnected from steep slopes or cliffs.Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling. Falls are strongly influenced by gravity,mechanical weathering and the presence of interstitial water.Falls mainly contribute to surface erosion by transporting mainly rocks and boulders from high slopes to the low land areas.(Frederick K. Lutgens and Edward J. Tarbuck. 2012. Essentials of Geology, 11th Edition)

CreepCreep is the impossibly slow, steady, downward movement of slope-forming soil or rock. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. There are generally three types of creep: (1) seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature; (2) continuous, where shear stress continuously exceeds the strength of the material; and (3) progressive, where slopes are reaching the point of failure as other types of mass movements. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges.

Creep is not the main cause of erosion mainly due to the fact that it is a slow movement,very slow usually much less than one inch per year.

(.image by the United States Geological Survey)

Lateral spreads: Lateral spreads are characteristic because they usually occur on very gentle slopes or flat terrain . The dominant mode of movement is lateral extension accompanied

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by shear or tensile fractures. The failure is caused by liquefaction, the process whereby saturated, loose, cohesionless sediments (usually sands and silts) are transformed from a solid into a liquefied state. Failure is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced. When coherent material, either bedrock or soil, rests on materials that liquefy, the upper units may undergo fracturing and extension and may then subside, translate, disintegrate, or liquefy and flow. Lateral spreading in fine-grained materials on shallow slopes is usually progressive. The failure starts suddenly in a small area and spreads rapidly. Often the initial failure is a slump, but in some materials movement occurs for no apparent reason. Combination of two or more of the above types is known as a complex landslide.

(image by the United States Geological Survey)Lateral spreading are mainly evident in coastal areas and are among the major contributors to coastal erosion by transporting and depositing materials to the shoreline they expose these materials to coastal erosion.

How surface erosion causes landslides

As we have previously stated in this paper, surface erosion is a process by which soil and rock are removed from the earth's surface by natural forces and then transported and deposited in other locations.Below we examine the contributions of surface erosion to the occurrence of landslides.

i. Water erosion is the main contributor to landslides.Specifically rainfall, high intensity rainfall along with erosivity of the soil are the causes for the loosening of the soil, weakening of slopes and ultimately leading to mass movements of solids and semisolid materials such as creeps, landslips, and landslides.Therefore areas with high susceptibility of rainfall are the most prone to landslides events.

ii. Deforestation is a human activity that triggers soil erosion, deforestation carried out in hilly areas especially steep slopes can be a catalyst to erosion and hence forth lead to landslides by weakening the slopes of the hills and making them vulnerable to landsides.Controlled deforestation with immediate replanting can minimize erosion

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but when it is carried out rapidly at a non-sustainable rate it brings about landslides.

iii. Highway construction through dense forest is another human activity that triggers soil erosion.These activities destroy the natural forest cover and expose steep slopes to erosion.The eroded slopes now become vulnerable and landslides will inevitably occur.

Surface erosion is among the main causes of landsliding,its control becomes essential in order to prevent fatal landslides.By adjusting the factors which are responsible for soil erosion like land use/cover etc, the rate of soil erosion can be minimized.

Conclusion

Landslides and surface erosion are interconnected processes.Landslides can be categorized as gravitational erosion because their mass movements depend on the force of gravity.The contribution of landslides to surface erosion can be seen by examining different types of landslides namely, debris flows, falls, and lateral spreads.

Surface erosion causes landslides in the form of water erosion(ie rainfall), Deforestation in hilly areas, and Highway construction through dense forest areas just to name a few.

The link between landslides and surface erosion is clearly obvious, by transporting and depositing soils and rocks to new location we see how landslide contribute to surface erosion, and by acting as a trigger to the action of landslides we also see how surface erosion cause landslides.

References

British Society for Geomorphology (BSG). 2012. Earth Surface Processes and Landforms journal, Southampton.

Curt Suplee. 1998. National Geographic Magazine, Washington.

Frederick K. Lutgens and Edward J. Tarbuck. 2012. Essentials of Geology, 11th Edition.

John Bridge and Robert Demicco. 2008. Earth Surface Processes, Landforms and Sediment Deposits. John Grotzinger, Thomas H. J, Frank Press and R. Siever. 2006.Understanding Earth 5th edition.

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United States Government.2012.United States Geological Survey.

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