LANDSLIDE AND SOIL EROSION-Monika Ghimire

INTRODUCTION

LandslideThe term ‘landslide’ is generally used to denote a downslope movement of mass of earth, debris or rock down a slope due to the action of external forces such as rainfall, snowmelt, volcanic eruption, earthquakes, anthropogenic activities etc.

Landslide in Sindhupalchowk and Gorkha

In the context of Nepal…Landslides are more often in our country

because of it’s geographical landscape.The main reason is rainfall, beside that

snowmelt and earthquake may be the important factors for landslide in Nepal.

Human activities like construction, deforestation, agricultural practices in steep areas, overgrazing also creates a path for landslide to occur.

Landslides occur when the stability of the slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone.

Landslides cause loss of lives and properties from the area where landslides occur or from the downslope area due to the transport of the landslide materials (sediment).

CAUSES OF LANDSLIDE• Natural Factors • Concentrated precipitation • Seismic activity: The Himalayan Range lies in a

high seismicity belt. Several active faults have mapped.

• Rise in groundwater table in the monsoon. • Perched water table • Inadequate size and choking of roadside drains • Liquefaction • Swelling and Shrinking of Clay

Anthropogenic Factors

• Deforestation: intensive deforestation has taken place in most parts of the Himalaya excepting the higher Himalaya in the last decades

• Improper land-use: (a) Agricultural practices on steep slopes (b) Irrigation on steep and vulnerable slopes (c) Over-grazing (d) Quarrying for construction materials without considering the condition of the terrain• Construction activities

LANDSLIDE MECHANICS• Deep-seated Landslides • Landslides in which the sliding surface is mostly

deeply located below the maximum rooting depth of trees (typically a depth greater than 10 metres) are called deep-seated landslides.

• It is usually believed that the deep-seated landslides are triggered by moderate rainfall intensity distributed over long periods. Deep-seated landslides are generally slow moving in nature and rarely claim lives, but may cause high property damage. The failure modes in such cases are generally rotational or complex types.

Shallow Landslides • Landslides in which the sliding surface is located

within the soil mantle or weathered bedrock (typically to a depth from a few decimetres to several metres) are categorized as shallow landslides. The surface of the slope in steep hilly and mountainous regions is quite often underlain by a plane of weakness lying parallel to it and therefore, shallow landslides are predominant.

• In many cases, the shallow landslides are fast-moving and are extremely destructive, causing wide-spread damage and casualties. Shallow landslides can pose grave threat to life and property.

TYPES OF LANDSLIDES

Fall: A fall starts with detachment of soil or rock from a steep slope along a surface on which little or no shear displacement takes place. The material will then descend largely through air by falling or rolling

Topple: A toppling occurs as a result of overturning of blocks rather than sliding or falling. It is a forward rotation, out of the slope, of a mass of soil or rock about a point axis below the gravity of the displaced mass.

Slide: A slide is the downslope movement of a soil or rock mass occurring dominantly on the surface of rupture or relatively thin zones of intense shear strain.

Transitional Rotational

Flows: In flows, materials move as a coherent but constantly changing mass, involving internal shear or mixing of the mass and even sorting based on particle size and position in the flow. The distribution of velocities in displacing mass resembles that in a viscous fluid.

Earth flow Debris flow

• Spread: The term ‘spread’ refers to an extension of cohesive soil or rock mass combined with a general subsidence of the fractured mass of a cohesive material into softer underlying material. The spread may result in from liquefaction or flow of softer materials.

Creep: Creep is the imperceptibly slow, steady, downward movement of slope forming soil or rock. The movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure.

EFFECTS OF LANDSLIDES Physical damage- Debris may block roads,

supply lines (telecommunication, electricity, water etc.)

Loss of property. Casualties – death and injuries to people and

animal. Changes in the surface landscape.

MITIGATION MEASURES

Land use practices such as: Areas covered by degraded natural vegetation

in upper slopes are to be afforested with suitable species .Existing patches of natural vegetation should be preserved.

Any developmental activity initiated in the area should be taken up only after a detailed study and slope protection should be carried out if necessary.

In construction of roads, irrigation canals etc. proper care is to be taken to avoid blockage of natural drainage.

Total avoidance of settlement in the risk zone should be made mandatory.

Relocate settlements and infrastructure that fall in possible path of the landslide.

No construction of buildings in areas beyond a certain degree of slope.

Avoid building on or near steep, high risk slopes and avoid modifications that increases the risk of landslide.

Retaining walls can be built to stop land from slipping (these walls are commonly seen along roads in hill). It’s constructed to prevent smaller sized and secondary landslides that often occur along the toe portion of larger landslides.

Surface drainage control works: The surface drainage works should be implemented to control the movement of landslides accompanied by infiltration of rain water and spring flows.

Increasing vegetation cover is the cheapest and most effective way of arresting landslides. This helps to bind the top layer of soil with layers below, while preventing excessive runoff and soil erosion.

Gabion net, gabion wall and wire bolsters should be constructed in landslide occuringPlaces to prevent damages.

SOIL EROSION

Soil erosion is a naturally occurring process on all land.

Erosion occurs when soil is transported from one location by wind and water and moved to a new location, such as lakes and rivers.

The agents of soil erosion are water and wind, each contributing a significant amount of soil loss each year. 

Soil erosion may be a slow process that continues relatively unnoticed, or it may occur at an alarming rate causing serious loss of topsoil.

Types of Soil Erosion Rain drop or splash erosion: Erosion preceded by the

destruction of the crumb structure due to the impact of falling raindrop on the surface of soil is termed as splash erosion.

Sheet erosion: It is the fairly uniform removal of soil in thin layers from the land surface, often scarcely perceptible, especially when caused by wind. Areas where loose, shallow topsoil overlies compact soil are most susceptible to sheet erosion.

Rill erosion: A form of water erosion in which numerous very small and more or less straight channels are produced; the channels get obliterated by ordinary use. It can be removed by normal tillage operations.

Gully erosion: A form of water erosion in which gullies are produced by combination of unattended rills. 

Stream bank erosion: Stream banks are eroded by water either flowing over the sides of a stream or scouring at the base. It is aggravated by removal of vegetation, over grazing or cultivation near the stream banks.

Factors affecting soil erosion

Soil erosion

Soil structure and composition

Climate change

TopographyVegetative cover

Causes of soil erosion

PloughingPloughing with machinery compacts soil. This creates

channels for rapid water flow especially on slopes.

DeforestationWhen trees are deforested

the root holding soil are removed which then leaves bare top soil

Vulnerable to extreme wind and water damage.

FertilizersSoils are not given time to

recover naturally when soil is used intensively

OvergrazingWhen large number of animals are grazed in a

small area the vegetation is removed faster than it can regrow leaving the

topsoil exposed.

Effects of soil erosion - Loss of soil fertility. - Loss of ability to hold water. - Pollutes water, kills fish. - Damages crops through sandblasting of young seedlings - Desertification

Mitigation measures

Terrace farming-slopes are made into flat step areas or ‘terraces’ which means that water and soil are held by the walls thus minimizing the erosion.

Contour ploughing-this is when ploughing takes place across hillside. This reduces the flow of water(runoff) across the land surface thus preventing erosion.

Strip farming- Different crops are planted and harvested at different time that means the amount of bare soil is minimized.

Stone lines- These are kept in place following contour lines meaning runoff is reduced and soil can be collected and redistributed. This is a cheap And easy method.

THE END Thank you