U.S. Department of the Interior

U.S. Geological Survey

Circular 1325 

The Landslide Handbook—

A Guide to Understanding Landslides
Cover: The threat of landslides like this have made the long drive to Wenchuan a risky affair for rescuers 

and aid delivery teams as they tried to reach the epicenter of the Wenchuan, China, earthquake of 

May 12, 2008. This photograph shows a landslide that sealed the entrance and exit to Wenchuan on 

May 23, 2008. (Photograph by Chua Chin Hon, Straits Times, Singapore.)
The Landslide Handbook—

A Guide to Understanding Landslides

By Lynn M. Highland, United States Geological Survey, and

Peter Bobrowsky, Geological Survey of Canada

Circular 1325

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS--the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Highland, L.M., and Bobrowsky, Peter, 2008, The landslide handbook—A guide to understanding landslides: Reston, 

Virginia, U.S. Geological Survey Circular 1325, 129 p.
iii

Acknowledgments 

The authors thank the International Consortium on Landslides for sponsoring this project 

and the many reviewers who spent much of their time and effort on the careful review of 

this book. Thanks also to those who gave permission to use their previously published text,

photographs and graphics and to those authors of landslide research and information whose 

painstaking work was crucial to its completion. We thank the Geological Survey of Canada, and 

especially Jan Aylsworth for her valuable review and suggestions. In addition, we acknowledge 

the U.S. Geological Survey, especially Paula Gori who supported the entire handbook process 

and provided advice and encouragement. This is Geological Survey of Canada Contribution 

Number 20080377.
iv

Earthquake-induced landslides,

Sichuan Province, China, May 12, 2008.

Photograph courtesy of Dr. Yin Yueping,

China Geological Survey,

Ministry of Land and Resources, China.
v

Contents

Acknowledgments........................................................................................................................................... iii

How to read this guide.................................................................................................................................. xiii

Please note............................................................................................................................................ xiii

Introduction ....................................................................................................................................................... 1

For more information........................................................................................................................................ 2

Section I. Basic Information About Landslides...................................................................................... 3

Part A. What is a Landslide?....................................................................................................................... 4

Part B. Basic Landslide Types ................................................................................................................... 5

Falls............................................................................................................................................................ 6

Rockfall ............................................................................................................................................ 6

Occurrence and relative size/range.................................................................................. 6

Velocity of travel.................................................................................................................... 6

Triggering mechanism.......................................................................................................... 6

Effects (direct/indirect)........................................................................................................ 6

Corrective measures/mitigation......................................................................................... 6

Predictability.......................................................................................................................... 7

Topple ........................................................................................................................................................ 8

Occurrence............................................................................................................................ 8

Velocity of travel.................................................................................................................... 8

Triggering mechanism.......................................................................................................... 8

Effects (direct/indirect)........................................................................................................ 8

Corrective measures/mitigation......................................................................................... 8

Predictability.......................................................................................................................... 8

Slides ....................................................................................................................................................... 10

Rotational Landslide .................................................................................................................... 10

Occurrence.......................................................................................................................... 10

Relative size/range ............................................................................................................. 10

Velocity of travel (rate of movement).............................................................................. 10

Triggering mechanism........................................................................................................ 10

Effects (direct/indirect)...................................................................................................... 10

Mitigation measures .......................................................................................................... 10

Predictability........................................................................................................................ 11

Translational Landslide ............................................................................................................... 12

Occurrence.......................................................................................................................... 12

Relative size/range ............................................................................................................. 12

Velocity of travel.................................................................................................................. 12

Triggering mechanism........................................................................................................ 12

Effects (direct/indirect)...................................................................................................... 12

Mitigation measures .......................................................................................................... 12

Predictability........................................................................................................................ 13

Spreads ................................................................................................................................................... 14

Lateral Spreads................................................................................................................... 14

Occurrence.......................................................................................................................... 14

Relative size/range ............................................................................................................. 14
vi

Velocity of travel.................................................................................................................. 14

Triggering mechanism........................................................................................................ 14

Effects (direct/indirect)...................................................................................................... 14

Mitigation measures .......................................................................................................... 15

Predictability........................................................................................................................ 15

Flows.....................................................................................................................................................16

Debris Flows.................................................................................................................................. 16

Occurrence.......................................................................................................................... 16

Relative size/range ............................................................................................................. 16

Velocity of travel.................................................................................................................. 16

Triggering mechanisms ..................................................................................................... 16

Effects (direct/indirect)...................................................................................................... 16

Mitigation measures .......................................................................................................... 17

Predictability........................................................................................................................ 17

Lahars (Volcanic Debris Flows)................................................................................................. 18

Occurrence.......................................................................................................................... 18

Relative size/range ............................................................................................................. 18

Velocity of travel.................................................................................................................. 18

Triggering mechanism........................................................................................................ 18

Effects (direct/indirect)...................................................................................................... 18

Mitigation measures .......................................................................................................... 18

Predictability........................................................................................................................ 19

Debris Avalanche......................................................................................................................... 20

Occurrence.......................................................................................................................... 20

Relative size/range ............................................................................................................. 20

Velocity of travel.................................................................................................................. 20

Triggering mechanism........................................................................................................ 20

Effects (direct/indirect)...................................................................................................... 20

Corrective measures/mitigation....................................................................................... 20

Predictability........................................................................................................................ 21

Earthflow........................................................................................................................................ 22

Occurrence.......................................................................................................................... 22

Relative (size/range)........................................................................................................... 22

Velocity of travel.................................................................................................................. 22

Triggering mechanisms ..................................................................................................... 22

Effects (direct/indirect)...................................................................................................... 22

Corrective measures/mitigation....................................................................................... 22

Predictability........................................................................................................................ 23

Slow Earthflow (Creep)............................................................................................................... 24

Occurrence.......................................................................................................................... 24

Relative size/range ............................................................................................................. 24

Velocity of travel.................................................................................................................. 24

Triggering mechanism........................................................................................................ 24

Effects.................................................................................................................................24

Corrective measures/mitigation....................................................................................... 24

Predictability........................................................................................................................ 25
vii

Flows in Permafrost..................................................................................................................... 26

Occurrence.......................................................................................................................... 26

Relative size/range ............................................................................................................. 26

Velocity of travel.................................................................................................................. 26

Triggering mechanisms ..................................................................................................... 26

Effects (direct/indirect)...................................................................................................... 26

Corrective measures/mitigation....................................................................................... 26

Predictability........................................................................................................................ 26

Part C. Where Do Landslides Occur?...................................................................................................... 29

Part D. What Causes Landslides?............................................................................................................ 30

Natural Occurrences............................................................................................................................ 30

Landslides and Water.................................................................................................................. 30

Landslides and Seismic Activity................................................................................................ 32

Landslides and Volcanic Activity .............................................................................................. 33

Human Activities ................................................................................................................................... 33

Part E. What are the Effects and Consequences of Landslides?....................................................... 34

Effects of Landslides on the Built Environment............................................................................... 34

Effects of Landslides on the Natural Environment.......................................................................... 36

Part F. Interrelationship of Landslides with Other Natural Hazards—

The Multiple Hazard Effect ............................................................................................................. 40

Section II. Evaluating and Communicating Landslide Hazard .......................................................... 43

Part A. Evaluating Landslide Hazards ..................................................................................................... 44

Observation and (or) inspection by local experts and (or) municipal officials, 

and property owners............................................................................................................... 44

Features that might indicate landslide movement................................................................. 44

Technological Tools for Evaluation of Landslides—Mapping, Remote Sensing, 

and Monitoring......................................................................................................................... 46

Map Analysis ................................................................................................................................ 46

Aerial Reconnaissance............................................................................................................... 46

Field Reconnaissance................................................................................................................. 46

Drilling..........................................................................................................................................47

Instrumentation ............................................................................................................................ 47

Geophysical Studies.................................................................................................................... 47

Acoustic Imagery and Profiles .................................................................................................. 47

Computerized Landslide Terrain Analysis ............................................................................... 47

Part B. Communicating Landslide Hazard.............................................................................................. 48

Safety Information................................................................................................................................. 48

Building and Construction Information ............................................................................................. 48

Suggested Local Government Outreach for Landslide Hazard .................................................... 49

Examples of Hazard Warning Signs................................................................................................... 50

Section III. Mitigation Concepts and Approaches .............................................................................. 51

Part A. Overview of Mitigation Methods for Various Types of Landslide Hazards......................... 52

Soil Slope Stabilization......................................................................................................................... 52

Rockfall Hazard Mitigation .................................................................................................................. 53

Debris-Flow Hazard Mitigation........................................................................................................... 53

Landslide Dam Mitigation.................................................................................................................... 53

Methods of Biotechnical Landslide Mitigation................................................................................ 55
viii

Part B. Simple Mitigation Techniques for Home and Businesses, Managers, and Citizens ........ 56

Part C. List of Works Consulted/Cited/Quoted and for Further Reading........................................... 57

Appendix A. Basic Information about Landslides ............................................................................... 59

Part 1. Glossary of Landslide Terms ........................................................................................................ 60

References for Glossary ............................................................................................................. 62

Sources of information on nomenclature................................................................................ 63

Part 2. Parts of a Landslide—Description of Features/Glossary ....................................................... 63

Part 3. Landslide Causes and Triggering Mechanisms........................................................................ 64

Natural Causes ...................................................................................................................................... 64

Geological causes ....................................................................................................................... 64

Morphological causes ................................................................................................................ 64

Physical Causes—Triggers................................................................................................................. 64

Human Causes....................................................................................................................................... 64

Appendix B. Introduction to Landslide Evaluation Tools—Mapping, Remote Sensing, and 

Monitoring of Landslides................................................................................................................ 65

Part 1. Mapping........................................................................................................................................... 66

Regional mapping.................................................................................................................................. 66

Community-level mapping ................................................................................................................... 66

Site-specific mapping........................................................................................................................... 66

Three Important Criteria for Landslide Maps................................................................................... 67

Landslide inventory maps........................................................................................................... 67

Landslide susceptibility maps.................................................................................................... 68

Landslide hazard maps ............................................................................................................... 69

Part 2. Remote Sensing and Other Tools that Show Features of Landslide Activity ...................... 70

Part 3. Real-Time Monitoring of Landslides and Landslide Instrumentation................................... 74

Appendix C. Introduction to Landslide Stabilization and Mitigation .............................................. 75

Part 1. Earth Slope Stabilization/Mitigation ........................................................................................... 76

Excavation .............................................................................................................................................. 76

Removal of soil from the head of a slide.................................................................................. 76

Reducing the height of the slope .............................................................................................. 76

Backfilling with lightweight material........................................................................................ 78

Benches......................................................................................................................................... 79

Flattening or reducing slope angle, or other slope modification ........................................ 79

When not to excavate a slide mass.......................................................................................... 79

Strengthening Slopes ........................................................................................................................... 80

Plastic mesh reinforcement....................................................................................................... 80

Rock-fill buttresses...................................................................................................................... 80

Stream channel linings ............................................................................................................... 82

Check Dams ......................................................................................................................... 83

To prevent check dam failure ........................................................................................... 83

Drainage Techniques............................................................................................................................ 86

Site leveling................................................................................................................................... 86

Ditches and drains....................................................................................................................... 86

Drainpipes ..................................................................................................................................... 88

Straw wattles and straw bales.................................................................................................. 89

Retaining Walls............................................................................................................................. 90
ix

Timber crib ........................................................................................................................... 90

Steel bin wall ....................................................................................................................... 91

Reinforced earth wall ......................................................................................................... 92

Gabion walls ........................................................................................................................ 93

Piles ....................................................................................................................................... 94

Slope Stabilization Using Vegetation........................................................................................ 95

Types of seeds ..................................................................................................................... 95

Mulching ............................................................................................................................... 95

Biotechnical Slope Protection ................................................................................................... 96

Part 2. Rock Slope Stabilization/Mitigation Techniques ...................................................................... 99

Safe Catching Techniques ................................................................................................................. 100

Catch Ditches .............................................................................................................................. 100

Cable, Mesh, Fencing, and Rock Curtains ............................................................................ 100

Retaining Walls ........................................................................................................................... 102

Rock Sheds/Shelters ................................................................................................................. 102

Rock Ledge Reinforcement ...................................................................................................... 102

Excavation of Rock .............................................................................................................................. 104

Benches ....................................................................................................................................... 104

Scaling and Trimming ................................................................................................................ 105

Reinforcing Potential Rockfall Areas ............................................................................................... 107

Shotcrete and Gunite ................................................................................................................ 107

Anchors, Bolts, and Dowels ..................................................................................................... 108

Part 3. Debris-Flow Mitigation ................................................................................................................ 109

Strengthening Slopes for Erosion/Debris Flows ............................................................................ 109

Strengthening the soil to resist erosion ................................................................................. 110

Proper planting of vegetation on slopes can prevent erosion .......................................... 110

Keeping slopes free from fuel for wildfires ........................................................................... 110

Structures for Mitigating Debris Flows ........................................................................................... 111

Debris-flow basins ..................................................................................................................... 111

Check dams ................................................................................................................................. 111

Debris-flow retaining walls ...................................................................................................... 112

Box Debris-Flow Mitigation for the Homeowner .................................................................. 113

 Box Basic Things to Remember Concerning Mitigation and Response to

 Debris-Flow and Other Landslide Hazards/Emergency Response ............. 122

Landslide Dam Mitigation ......................................................................................................... 123

Diversion of inflow water before it reaches the lake formed by 

the landslide dam ................................................................................................ 123

Temporary drainage from the impoundment by pumps or siphons ......................... 123

Construction of an erosion-resistant spillway............................................................. 124

Drainage tunnel through an abutment .......................................................................... 124

Appendix D. Sample Safety Information for Landslides/Debris Flows ......................................... 127

What Can You Do If You Live Near Steep Hills? ...................................................................................... 128

Prior to Intense Storms ...................................................................................................................... 128

During Intense Storms ........................................................................................................................ 128

What to do if you suspect imminent landslide danger ................................................................. 128

After landslides occur ........................................................................................................................ 128

Index ............................................................................................................................................................129
x

Figures

1. This landslide occurred at La Conchita, California, USA, in 2005.........................................4

2. A simple illustration of a rotational landslide that has evolved into an earthflow ............5

3. Schematic of a rockfall................................................................................................................7

4. A rockfall/slide that occurred in Clear Creek Canyon, Colorado, USA, in 2005 .................7

5. Schematic of a topple..................................................................................................................9

6. Photograph of block toppling at Fort St. John, British Columbia, Canada..........................9

7. Schematic of a rotational landslide.........................................................................................11

8. Photograph of a rotational landslide which occurred in New Zealand ............................11

9. Schematic of a translational landslide....................................................................................13

10. A translational landslide that occurred in 2001 in the Beatton River Valley, British 

Columbia, Canada.......................................................................................................................13

11. Schematic of a lateral spread ..................................................................................................15

12. Photograph of lateral spread damage to a roadway as a result of the 1989 Loma 

Prieta, California, USA, earthquake.........................................................................................15

13. Schematic of a debris flow .......................................................................................................17

14. Debris-flow damage to the city of Caraballeda.....................................................................17

15. Schematic of a lahar..................................................................................................................19

16. Photograph of a lahar................................................................................................................19

17. Schematic of a debris avalanche ............................................................................................21

18. A debris avalanche that buried the village of Guinsaugon, Southern Leyte,

Philippines....................................................................................................................................21

19. Schematic of an earthflow........................................................................................................23

20. The 1993 Lemieux landslide ......................................................................................................23

21. Schematic of a slow earthflow.................................................................................................25

22. The effects of creep, in an area near East Sussex, United Kingdom ................................25

23. Schematic of a retrogressive thaw flow slide.......................................................................27

24. Photograph of a retrogressive thaw flow in the Northwest Territories, Canada.............28

25. Photograph showing lateral spreading damage...................................................................29

26. Photograph showing the Mameyes, Puerto Rico, landslide, 1985 .....................................31

27. Earthquake-induced landslide damage to a house built on artificial fill...........................32

28. Photograph showing the side of Casita Volcano in Nicaragua, Central America...........33

29. A landslide on the Pan American Highway in El Salvador, Central America ...................35

30. The active volcano, Mount Shasta in California, USA ........................................................37

31. View looking downstream at the confluence of the Río Malo............................................38

32. The Slumgullion landslide, Colorado, USA.............................................................................39

33. An example of a multi-hazard event........................................................................................41

34. The 1999 multi-hazard event in Tanaguarena, in coastal Venezuela, South America ....42

35. Photograph showing the aftereffects of a multi-hazard event...........................................42

36. Ground cracks.............................................................................................................................45

37. Sidewalk pulling away from house..........................................................................................45

38. Cracking of the foundation of a structure...............................................................................45

39. Example of a rockfall hazard sign............................................................................................50

41. Sign along a highway in Virginia, USA....................................................................................50

40. A notice for cliff hazards, city of Wanneroo, Australia ........................................................50
xi

42. The Thistle landslide in Utah, USA...........................................................................................54

43. Extensive damage in the mountainous terrain of Beichuan County..................................55

A1. Parts of a landslide.....................................................................................................................63

B1. Example of a landslide inventory map.....................................................................................67

B2. An example of a landslide susceptibility map........................................................................68

B3. Portion of shallow landslide hazard map................................................................................69

B4. An example of an aerial photograph.......................................................................................71

B5. Schematic showing satellite passes over an area of the Earth’s surface........................71

B6. Interferogram from InSAR imaging process..........................................................................72

B7. An oblique LiDAR image............................................................................................................73

B8. Measuring landslide movement using an extensometer.....................................................74

B9. Testing a solar-powered radiotelemetry system...................................................................74

B10. Example of a network for measurement and transmission of real-time 

landslide data ..............................................................................................................................74

C1. Illustration of the differences in stability resulting in excavation at 

the head and toe surfaces of a slope......................................................................................77

C2. Illustration of the difference in stability of loading either the head or 

the toe of a slope ........................................................................................................................77

C3. Illustration of the importance of water in the stability of a slope.......................................77

C4. Schematic and photograph of a lightweight backfill............................................................78

C5. Schematic and photograph of a Rockfill buttress.................................................................81

C6. Example of creek channel lining using rock ..........................................................................82

C7. Schematic and photograph of a crib wall check dam .........................................................84

C8. Upstream view of concrete crib-type check dam.................................................................85

C9. Schematic and photograph of a drain trench........................................................................87

C10. Schematic and photograph of drainpipes..............................................................................88

C11. Straw wattles on the side of a road.........................................................................................89

C12. Straw bales at a mine site.........................................................................................................89

C13. Schematic and photograph of a timber crib ..........................................................................90

C14. Schematic and photograph of a steel bin wall......................................................................91

C15. Schematic and photograph of a reinforced earth wall........................................................92

C16. Schematic and photograph of a gabion wall.........................................................................93

C17. A concrete-filled pile wall.........................................................................................................94

C18. A Vetiver grass system ..............................................................................................................97

C19. Worldwide distribution of active Vetiver grass programs ...................................................98

C20. Rockfall countermeasures........................................................................................................99

C21. Example of wire mesh placed over a rocky slope...............................................................100

C22. Protective rock barrier fencing..............................................................................................101

C23. Example of a “rock curtain”....................................................................................................101

C24. Pitquah rock sheds...................................................................................................................102

C25. An open rock shed....................................................................................................................103

C26. A rock avalanche shelter........................................................................................................103

C27. A reinforced rock ledge...........................................................................................................103

C28. Schematic and photograph of rock benches.......................................................................104

C29. Rocks dislodged by scaling and blasting..............................................................................105

C30. A hydraulic rock hammer in action........................................................................................106
xii

C31. Shotcrete operation .................................................................................................................107

C32. Closeup photograph of a rock anchor...................................................................................108

C33. A rock bolting operation..........................................................................................................108

C34. Wildfire-burned slopes and debris flow ...............................................................................109

C35. Aerial photograph of a debris-flow basin.............................................................................111

C36. A steel-and-timber retaining wall..........................................................................................112

C37. A debris-flow retaining wall....................................................................................................112

C38. Schematic of an unprotected home......................................................................................113

C39. Schematic example of a house with protective structures in place ...............................113

C40. Schematic of typical materials for helping to reduce damage from flood/

debris-flow events....................................................................................................................114

C41. Sandbags are basically for low-flow protection.................................................................115

C42. Schematic of a typical placement of sandbags for home protection..............................116

C43. Sandbags help in directing debris away from buildings....................................................116

C44. Controlling debris or stormflows in streets with sandbags...............................................117

C45. Directing flows between buildings by using sandbags......................................................117

C46. Sliding glass door sealing .......................................................................................................118

C47. Typical window and door protection.....................................................................................119

C48. Nailing up plywood or lumber for window and door protection.......................................119

C49. A timber deflector.....................................................................................................................120

C50. Closeup schematic of a timber deflector installation technique......................................120

C51. An alternative to timber deflectors........................................................................................121

C52. Removable driveway barrier...................................................................................................121

C53. The Thistle landslide, in Utah, USA, 1983..............................................................................125

C54. Closeup view of mitigation measures taken to reduce the impact of the 

Thistle landslide dam ...............................................................................................................126

C55. Photograph with annotations showing the reactivation and enlargement 

of the Thistle landslide dam....................................................................................................126
xiii

How to read this guide

This guide consists of three primary sections with a series of extensive appendixes and 

an index. This style provides the most flexibility for users with considerable differences of 

interest and level of detail. Much of the detailed and expanded explanatory information can 

be found in the appendixes, including a Glossary of Landslide Terms. References are provided for more information. 

Please note

•	 For ease of reading, references are numbered at the end of sections and are not 

embedded within the text. The user may also contact either the United States Geological Survey or the Geological Survey of Canada for further guidance and assistance.

•	 Web sites are used as references for this book; however, Web site addresses (URLs) 

can change over time, and the Internet links given in this publication may become 

inactive or erroneous. It is suggested that users consult a Web-based keyword search 

engine if links are no longer accessible.
xiv

The Donghekou landslide, caused by the May 2008 Wenchuan Earthquake 

in Sichuan County, China. This extremely large landslide buried hundreds of 

people and caused a landslide dam to form in the Dong He River. Photograph 

by Lynn Highland, U.S. Geological Survey. 
Introduction

This handbook is intended to be a resource for people affected by landslides to 

acquire further knowledge, especially about the conditions that are unique to their 

neighborhoods and communities. Considerable literature and research are available 

concerning landslides, but unfortunately little of it is synthesized and integrated 

to address the geographically unique geologic and climatic conditions around the 

globe. Landslides occur throughout the world, under all climatic conditions and 

terrains, cost billions in monetary losses, and are responsible for thousands of deaths 

and injuries each year. Often, they cause long-term economic disruption, population 

displacement, and negative effects on the natural environment.

Outdated land-use policies may not always reflect the best planning for use of 

land that is vulnerable to landslides. The reasons for poor or nonexistent land-use 

policies that minimize the perceived or actual danger and damage potential from 

geologic hazards are many and encompass the political, cultural, and financial complexities and intricacies of communities. Landslides often are characterized as local 

problems, but their effects and costs frequently cross local jurisdictions and may 

become State or Provincial or national problems.

Growing populations may be limited in their geographic expansion, except to 

occupy unstable, steep, or remote areas. Often, stabilizing landslide-scarred areas 

is too costly, and some inhabitants have no other places to relocate. Fortunately, 

simple, “low-tech” precautions and actions can be adopted to at least ensure an 

individual’s immediate safety, and this handbook gives a brief overview of many of 

these options. We strongly suggest that, where possible, the assistance of professional engineers/geologists or those experienced in the successful mitigation of 

unstable slopes be consulted before actions are taken. This handbook helps homeowners, community and emergency managers, and decisionmakers to take the 

positive step of encouraging awareness of available options and recourse in regard to 

landslide hazard.

The Landslide Handbook—

A Guide to Understanding Landslides

By Lynn M. Highland, United States Geological Survey, and

Peter Bobrowsky, Geological Survey of Canada
2 The Landslide Handbook—A Guide to Understanding Landslides

We provide a list of references, available in print or on the World Wide Web 

(Internet), that can be used for further knowledge about landslides. We recommend 

this handbook to managers and decisionmakers in communities in the hope that 

the information will be disseminated by such officials to other members of those 

communities. In response to the differing levels of literacy around the globe, we 

have emphasized visual information through the use of photographs and graphics. 

We plan to translate the handbook into additional languages as funding permits to 

further facilitate its use.

We welcome comments and critiques and have provided our contact information and the names and addresses of our respective agencies.

For more information 

For questions on the content of this book or other inquiries regarding landslide 

issues, please be aware that the U.S. Geological Survey (USGS) National Landslide 

Information Center (NLIC), in Golden, Colorado, USA, is available as a resource to 

answer questions, help with interpretations, or otherwise support users of this book 

in providing additional information. Please contact the center by telephone, email, 

or written inquiry.

United States Geological Survey

Landslide Program and National Landslide Information Center

Mail Stop 966, Box 25046, Denver Federal Center

Denver, Colorado, 80225 USA

Web address: http://landslides.usgs.gov/

Telephone: 1-800-654-4966, or 1-303-273-8586

Highland@usgs.gov

Geological Survey of Canada

Landslides and Geotechnic Section

601 Booth Street

Ottawa, Ontario, Canada KIA 0E8

Web address: http://gsc.nrcan.gc.ca/landslides/index_e.php

Telephone: 1-613-947-0333

pbobrows@nrcan-rncan.gc.ca
Section I.

Basic Information About Landslides
4 The Landslide Handbook—A Guide to Understanding Landslides

Figure 1. This landslide occurred at La Conchita, California, USA, in 2005. Ten people 

were killed. (Photograph by Mark Reid, U.S. Geological Survey.)

Part A. What is a Landslide?

Geologists, engineers, and other professionals often rely on unique and slightly 

differing definitions of landslides. This diversity in definitions reflects the complex 

nature of the many disciplines associated with studying landslide phenomena. For 

our purposes, landslide is a general term used to describe the downslope movement 

of soil, rock, and organic materials under the effects of gravity and also the landform 

that results from such movement (please see figure 1 for an example of one type of 

landslide).

Varying classifications of landslides are associated with specific mechanics 

of slope failure and the properties and characteristics of failure types; these will be 

discussed briefly herein.

There are a number of other phrases/terms that are used interchangeably with 

the term “landslide” including mass movement, slope failure, and so on. One commonly hears such terms applied to all types and sizes of landslides. 

Regardless of the exact definition used or the type of landslide under discussion, understanding the basic parts of a typical landslide is helpful. Figure 2 shows 

the position and the most common terms used to describe the unique parts of a landslide. These terms and other relevant words are defined in the Glossary of Landslide 

Terms included in Appendix A.
Part B. Basic Landslide Types 5

Part B. Basic Landslide Types

A landslide is a downslope movement of rock or soil, or both, occurring on 

the surface of rupture—either curved (rotational slide) or planar (translational slide) 

rupture—in which much of the material often moves as a coherent or semicoherent 

mass with little internal deformation. It should be noted that, in some cases, landslides may also involve other types of movement, either at the inception of the failure 

or later, if properties change as the displaced material moves downslope. 

This section provides descriptions and illustrations of the various types of landslides. Understanding the characteristics of the specific type of landslide hazard in 

your area is vitally important to consider when planning or adopting appropriate mitigative action to lessen the risk of loss and damage. The type of landslide will determine the potential speed of movement, likely volume of displacement, distance of 

run-out, as well as the possible effects of the landslide and the appropriate mitigative 

measures to be considered.

Landslides can be classified into different types on the basis of the type of movement and the type of material involved (please see References 9 and 39). In brief, 

material in a landslide mass is either rock or soil (or both); the latter is described as 

earth if mainly composed of sand-sized or finer particles and debris if composed of 

coarser fragments. The type of movement describes the actual internal mechanics of 

how the landslide mass is displaced: fall, topple, slide, spread, or flow. Thus, landslides are described using two terms that refer respectively to material and movement 

(that is, rockfall, debris flow, and so forth). Landslides may also form a complex failure encompassing more than one type of movement (that is, rock slide—debris flow). 

For the purposes of this handbook we treat “type of movement” as synonymous 

with “landslide type.” Each type of movement can be further subdivided according 

to specific properties and characteristics, and the main subcategories of each type are 

described elsewhere. Less common subcategories are not discussed in this handbook 

but are referred to in the source reference.

Direct citations and identification of sources and references for text are avoided 

in the body of this handbook, but all source materials are duly recognized and given 

in the accompanying reference lists.

Figure 2. A simple illustration of a rotational landslide that has evolved into an earthflow. 

Image illustrates commonly used labels for the parts of a landslide (from Varnes, 1978, 

Reference 43). 

Transverse cracks

Minor scarp

Head

Main scarp

Crown cracks

Crown

Surface of rupture

Main body

Toe of surface of rupture Foot

Surface of separation

Toe

Radial

cracks

Transverse ridges

Right flank

Original 

ground 

surface
For further reading: 

References 9, 39, 43, and 45

6 The Landslide Handbook—A Guide to Understanding Landslides

Falls

A fall begins with the detachment of soil or rock, or both, from a steep slope 

along a surface on which little or no shear displacement has occurred. The material 

subsequently descends mainly by falling, bouncing, or rolling. 

Rockfall

Falls are abrupt, downward movements of rock or earth, or both, that detach 

from steep slopes or cliffs. The falling material usually strikes the lower slope at 

angles less than the angle of fall, causing bouncing. The falling mass may break 

on impact, may begin rolling on steeper slopes, and may continue until the terrain 

flattens.

Occurrence and relative size/range

Common worldwide on steep or vertical slopes—also in coastal areas, 

and along rocky banks of rivers and streams. The volume of material in 

a fall can vary substantially, from individual rocks or clumps of soil to 

massive blocks thousands of cubic meters in size.

Velocity of travel

Very rapid to extremely rapid, free-fall; bouncing and rolling of detached 

soil, rock, and boulders. The rolling velocity depends on slope steepness.

Triggering mechanism

Undercutting of slope by natural processes such as streams and rivers or 

differential weathering (such as the freeze/thaw cycle), human activities 

such as excavation during road building and (or) maintenance, and earthquake shaking or other intense vibration.

Effects (direct/indirect)

Falling material can be life-threatening. Falls can damage property 

beneath the fall-line of large rocks. Boulders can bounce or roll great 

distances and damage structures or kill people. Damage to roads and 

railroads is particularly high: rockfalls can cause deaths in vehicles hit 

by rocks and can block highways and railroads.

Corrective measures/mitigation

Rock curtains or other slope covers, protective covers over roadways, 

retaining walls to prevent rolling or bouncing, explosive blasting of 

hazardous target areas to remove the source, removal of rocks or other 

materials from highways and railroads can be used. Rock bolts or other 

similar types of anchoring used to stabilize cliffs, as well as scaling, can 

lessen the hazard. Warning signs are recommended in hazardous areas 

for awareness. Stopping or parking under hazardous cliffs should be 

warned against.
Part B. Basic Landslide Types 7

Figure 4. A rockfall/slide that occurred in Clear Creek Canyon, Colorado, USA, 

in 2005, closing the canyon to traffic for a number of weeks. The photograph 

also shows an example of a rock curtain, a barrier commonly applied over 

hazardous rock faces (right center of photograph). (Photograph by Colorado 

Geological Survey.) 

Predictability

Mapping of hazardous rockfall areas has been completed in a few areas 

around the world. Rock-bounce calculations and estimation methods for 

delineating the perimeter of rockall zones have also been determined 

and the information widely published. Indicators of imminent rockfall 

include terrain with overhanging rock or fractured or jointed rock 

along steep slopes, particularly in areas subject to frequent freeze-thaw 

cycles. Also, cut faces in gravel pits may be particularly subject to falls. 

Figures 3 and 4 show a schematic and an image of rockfall.

Figure 3. Schematic of a rockfall. (Schematic modified from Reference 9.)
For further reading: 

References 9, 39, 43, and 45

8 The Landslide Handbook—A Guide to Understanding Landslides

Topple

A topple is recognized as the forward rotation out of a slope of a mass of soil 

or rock around a point or axis below the center of gravity of the displaced mass. 

Toppling is sometimes driven by gravity exerted by the weight of material upslope 

from the displaced mass. Sometimes toppling is due to water or ice in cracks in the 

mass. Topples can consist of rock, debris (coarse material), or earth materials (finegrained material). Topples can be complex and composite.

Occurrence

Known to occur globally, often prevalent in columnar-jointed volcanic 

terrain, as well as along stream and river courses where the banks 

are steep.

Velocity of travel

Extremely slow to extremely rapid, sometimes accelerating throughout 

the movement depending on distance of travel.

Triggering mechanism

Sometimes driven by gravity exerted by material located upslope from 

the displaced mass and sometimes by water or ice occurring in cracks 

within the mass; also, vibration, undercutting, differential weathering, 

excavation, or stream erosion.

Effects (direct/indirect)

Can be extremely destructive, especially when failure is sudden and (or) 

the velocity is rapid.

Corrective measures/mitigation

In rock there are many options for the stabilization of topple-prone 

areas. Some examples for reinforcement of these slopes include rock 

bolts and mechanical and other types of anchors. Seepage is also a 

contributing factor to rock instability, and drainage should be considered 

and addressed as a corrective means.

Predictability

Not generally mapped for susceptibility; some inventory of occurrence 

exists for certain areas. Monitoring of topple-prone areas is useful; for 

example, the use of tiltmeters. Tiltmeters are used to record changes in 

slope inclination near cracks and areas of greatest vertical movements. 

Warning systems based on movement measured by tiltmeters could be 

effective. Figures 5 and 6 show a schematic and an image of topple.
Part B. Basic Landslide Types 9

Figure 5. Schematic of a topple. (Schematic from Reference 9.)

Figure 6. Photograph of block toppling at Fort St. John, British Columbia, Canada. 

(Photograph by G. Bianchi Fasani.)
10 The Landslide Handbook—A Guide to Understanding Landslides

Slides

A slide is a downslope movement of a soil or rock mass occurring on surfaces 

of rupture or on relatively thin zones of intense shear strain. Movement does not initially occur simultaneously over the whole of what eventually becomes the surface 

of rupture; the volume of displacing material enlarges from an area of local failure.

Rotational Landslide

A landslide on which the surface of rupture is curved upward (spoon-shaped) 

and the slide movement is more or less rotational about an axis that is parallel to the 

contour of the slope. The displaced mass may, under certain circumstances, move as a 

relatively coherent mass along the rupture surface with little internal deformation. The 

head of the displaced material may move almost vertically downward, and the upper 

surface of the displaced material may tilt backwards toward the scarp. If the slide is 

rotational and has several parallel curved planes of movement, it is called a slump.

Occurrence

Because rotational slides occur most frequently in homogeneous materials, 

they are the most common landslide occurring in “fill” materials.

Relative size/range

Associated with slopes ranging from about 20 to 40 degrees. In soils, the 

surface of rupture generally has a depth-to-length ratio between 0.3 to 0.1.

Velocity of travel (rate of movement)

Extremely slow (less than 0.3 meter or 1 foot every 5 years) to moderately fast (1.5 meters or 5 feet per month) to rapid.

Triggering mechanism

Intense and (or) sustained rainfall or rapid snowmelt can lead to the 

saturation of slopes and increased groundwater levels within the mass; 

rapid drops in river level following floods, ground-water levels rising 

as a result of filling reservoirs, or the rise in level of streams, lakes, and 

rivers, which cause erosion at the base of slopes. These types of slides 

can also be earthquake-induced.

Effects (direct/indirect)

Can be extremely damaging to structures, roads, and lifelines but are 

not usually life-threatening if movement is slow. Structures situated on 

the moving mass also can be severely damaged as the mass tilts and 

deforms. The large volume of material that is displaced is difficult to 

permanently stabilize. Such failures can dam rivers, causing flooding.

Mitigation measures

Instrumental monitoring to detect movement and the rate of movement 

can be implemented. Disrupted drainage pathways should be restored or 

reengineered to prevent future water buildup in the slide mass. Proper 

grading and engineering of slopes, where possible, will reduce the 

hazard considerably. Construction of retaining walls at the toe may be 

effective to slow or deflect the moving soil; however, the slide may overtop such retaining structures despite good construction.
For further reading: 

References 9, 39, 43, and 45

Part B. Basic Landslide Types 11

Rotational landslide

Figure 7. Schematic of a rotational landslide. (Schematic modified from Reference 9.)

Figure 8. Photograph of a rotational landslide which occurred in New Zealand. The 

green curve at center left is the scarp (the area where the ground has failed). The 

hummocky ground at bottom right (in shadow) is the toe of the landslide (red line). This is 

called a rotational landslide as the earth has moved from left to right on a curved sliding 

surface. The direction and axis of rotation are also depicted. (Photograph by Michael J. 

Crozier, Encyclopedia of New Zealand, updated September 21, 2007.)

Predictability

Historical slides can be reactivated; cracks at tops (heads) of slopes 

are good indicators of the initiation of failure. Figures 7 and 8 show a 

schematic and an image of a rotational landslide.
12 The Landslide Handbook—A Guide to Understanding Landslides

Translational Landslide

The mass in a translational landslide moves out, or down and outward, along 

a relatively planar surface with little rotational movement or backward tilting. This 

type of slide may progress over considerable distances if the surface of rupture 

is sufficiently inclined, in contrast to rotational slides, which tend to restore the 

slide equilibrium. The material in the slide may range from loose, unconsolidated 

soils to extensive slabs of rock, or both. Translational slides commonly fail along 

geologic discontinuities such as faults, joints, bedding surfaces, or the contact 

between rock and soil. In northern environments the slide may also move along the 

permafrost layer.

Occurrence

One of the most common types of landslides, worldwide. They are found 

globally in all types of environments and conditions.

Relative size/range

Generally shallower than rotational slides. The surface of rupture has 

a distance-to-length ratio of less than 0.1 and can range from small 

(residential lot size) failures to very large, regional landslides that are 

kilometers wide.

Velocity of travel

Movement may initially be slow (5 feet per month or 1.5 meters per 

month) but many are moderate in velocity (5 feet per day or 1.5 meters 

per day) to extremely rapid. With increased velocity, the landslide mass 

of translational failures may disintegrate and develop into a debris flow.

Triggering mechanism

Primarily intense rainfall, rise in ground water within the slide due to 

rainfall, snowmelt, flooding, or other inundation of water resulting from 

irrigation, or leakage from pipes or human-related disturbances such as 

undercutting. These types of landslides can be earthquake-induced. 

Effects (direct/indirect)

Translational slides may initially be slow, damaging property and (or) 

lifelines; in some cases they can gain speed and become life-threatening. 

They also can dam rivers, causing flooding. 

Mitigation measures

Adequate drainage is necessary to prevent sliding or, in the case of an 

existing failure, to prevent a reactivation of the movement. Common 

corrective measures include leveling, proper grading and drainage, and 

retaining walls. More sophisticated remedies in rock include anchors, 

bolts, and dowels, which in all situations are best implemented by 

professionals. Translational slides on moderate to steep slopes are very 

difficult to stabilize permanently.
Part B. Basic Landslide Types 13

Surface

of 

rupture

Toe

Figure 9. Schematic of a translational landslide. (Schematic modified from Reference 9.)

Figure 10. A translational landslide that occurred in 2001 in the Beatton River Valley, 

British Columbia, Canada. (Photograph by Réjean Couture, Canada Geological Survey.)

Predictability

High probability of occurring repetitively in areas where they have 

occurred in the past, including areas subject to frequent strong earthquakes. Widening cracks at the head or toe bulge may be an indicator of 

imminent failure. Figures 9 and 10 show a schematic and an image of a 

translational landslide.

For further reading: 

References 9, 39, 43, and 45
14 The Landslide Handbook—A Guide to Understanding Landslides

Spreads

An extension of a cohesive soil or rock mass combined with the general subsidence of the fractured mass of cohesive material into softer underlying material. 

Spreads may result from liquefaction or flow (and extrusion) of the softer underlying material. Types of spreads include block spreads, liquefaction spreads, and 

lateral spreads.

Lateral Spreads

Lateral spreads usually occur on very gentle slopes or essentially flat terrain, 

especially where a stronger upper layer of rock or soil undergoes extension and 

moves above an underlying softer, weaker layer. Such failures commonly are accompanied by some general subsidence into the weaker underlying unit. In rock spreads, 

solid ground extends and fractures, pulling away slowly from stable ground and 

moving over the weaker layer without necessarily forming a recognizable surface of 

rupture. The softer, weaker unit may, under certain conditions, squeeze upward into 

fractures that divide the extending layer into blocks. In earth spreads, the upper stable 

layer extends along a weaker underlying unit that has flowed following liquefaction 

or plastic deformation. If the weaker unit is relatively thick, the overriding fractured 

blocks may subside into it, translate, rotate, disintegrate, liquefy, or even flow.

Occurrence

Worldwide and known to occur where there are liquefiable soils. 

Common, but not restricted, to areas of seismic activity.

Relative size/range

The area affected may start small in size and have a few cracks that may 

spread quickly, affecting areas of hundreds of meters in width.

Velocity of travel

May be slow to moderate and sometimes rapid after certain triggering 

mechanisms, such as an earthquake. Ground may then slowly spread 

over time from a few millimeters per day to tens of square meters 

per day.

Triggering mechanism

Triggers that destabilize the weak layer include:

•	 Liquefaction of lower weak layer by earthquake shaking

•	 Natural or anthropogenic overloading of the ground above an unstable slope

•	 Saturation of underlying weaker layer due to precipitation, snowmelt, and 

(or) ground-water changes

•	 Liquefaction of underlying sensitive marine clay following an erosional 

disturbance at base of a riverbank/slope

•	 Plastic deformation of unstable material at depth (for example, salt)

Effects (direct/indirect)

Can cause extensive property damage to buildings, roads, railroads, and 

lifelines. Can spread slowly or quickly, depending on the extent of water 

saturation of the various soil layers. Lateral spreads may be a precursor 

to earthflows.
Part B. Basic Landslide Types 15

Mitigation measures

Liquefaction-potential maps exist for some places but are not widely 

available. Areas with potentially liquefiable soils can be avoided as 

construction sites, particularly in regions that are known to experience 

frequent earthquakes. If high ground-water levels are involved, sites can 

be drained or other water-diversion efforts can be added.

Predictability

High probability of recurring in areas that have experienced previous 

problems. Most prevalent in areas that have an extreme earthquake 

hazard as well as liquefiable soils. Lateral spreads are also associated 

with susceptible marine clays and are a common problem throughout the 

St. Lawrence Lowlands of eastern Canada. Figures 11 and 12 show a 

schematic and an image of a lateral spread.

Firm clay

Bedrock

Soft clay with

water-bearing silt

and sand layers

Figure 11. Schematic of a lateral spread. A liquefiable layer underlies the surface layer. 

(Schematic modified from Reference 9.)

Figure 12. Photograph of lateral spread damage to a roadway as a result of the 1989 Loma 

Prieta, California, USA, earthquake. (Photograph by Steve Ellen, U.S. Geological Survey.)

For further reading: 

References 9, 39, 43, and 45
16 The Landslide Handbook—A Guide to Understanding Landslides

Flows

A flow is a spatially continuous movement in which the surfaces of shear are 

short-lived, closely spaced, and usually not preserved. The component velocities in 

the displacing mass of a flow resemble those in a viscous liquid. Often, there is a 

gradation of change from slides to flows, depending on the water content, mobility, 

and evolution of the movement.

Debris Flows

A form of rapid mass movement in which loose soil, rock and sometimes organic 

matter combine with water to form a slurry that flows downslope. They have been 

informally and inappropriately called “mudslides” due to the large quantity of fine 

material that may be present in the flow. Occasionally, as a rotational or translational 

slide gains velocity and the internal mass loses cohesion or gains water, it may evolve 

into a debris flow. Dry flows can sometimes occur in cohesionless sand (sand flows). 

Debris flows can be deadly as they can be extremely rapid and may occur without any 

warning.

Occurrence 

Debris flows occur around the world and are prevalent in steep gullies 

and canyons; they can be intensified when occurring on slopes or in 

gullies that have been denuded of vegetation due to wildfires or forest 

logging. They are common in volcanic areas with weak soil.

Relative size/range

These types of flows can be thin and watery or thick with sediment and 

debris and are usually confined to the dimensions of the steep gullies 

that facilitate their downward movement. Generally the movement is 

relatively shallow and the runout is both long and narrow, sometimes 

extending for kilometers in steep terrain. The debris and mud usually 

terminate at the base of the slopes and create fanlike, triangular deposits 

called debris fans, which may also be unstable.

Velocity of travel

Can be rapid to extremely rapid (35 miles per hour or 56 km per hour) 

depending on consistency and slope angle.

Triggering mechanisms

Debris flows are commonly caused by intense surface-water flow, due to 

heavy precipitation or rapid snowmelt, that erodes and mobilizes loose 

soil or rock on steep slopes. Debris flows also commonly mobilize from 

other types of landslides that occur on steep slopes, are nearly saturated, 

and consist of a large proportion of silt- and sand-sized material.

Effects (direct/indirect)

Debris flows can be lethal because of their rapid onset, high speed of 

movement, and the fact that they can incorporate large boulders and 

other pieces of debris. They can move objects as large as houses in 

their downslope flow or can fill structures with a rapid accumulation 

of sediment and organic matter. They can affect the quality of water by 

depositing large amounts of silt and debris.
Part B. Basic Landslide Types 17

Mitigation measures

Flows usually cannot be prevented; thus, homes should not be built in 

steep-walled gullies that have a history of debris flows or are otherwise 

susceptible due to wildfires, soil type, or other related factors. New flows 

can be directed away from structures by means of deflection, debris-flow 

basins can be built to contain flow, and warning systems can be put in 

place in areas where it is known at what rainfall thresholds debris flows 

are triggered. Evacuation, avoidance, and (or) relocation are the best 

methods to prevent injury and life loss.

Predictability

Maps of potential debris-flow hazards exist for some areas. Debris flows 

can be frequent in any area of steep slopes and heavy rainfall, either seasonally or intermittently, and especially in areas that have been recently 

burned or the vegetation removed by other means. Figures 13 and 14 

show a schematic and an image of a debris flow.

Figure 13. Schematic of a debris flow. (Schematic modified from Reference 9.)

Figure 14. Debris-flow damage to the 

city of Caraballeda, located at the base of 

the Cordillera de la Costan, on the north 

coast of Venezuela. In December 1999, this 

area was hit by Venezuela’s worst natural 

disaster of the 20th century; several days 

of torrential rain triggered flows of mud, 

boulders, water, and trees that killed as 

many as 30,000 people. (Photograph by 

L.M. Smith, Waterways Experiment Station, 

U.S. Army Corps of Engineers.) 

For further reading: 

References 9, 39, 43, and 45
18 The Landslide Handbook—A Guide to Understanding Landslides

Lahars (Volcanic Debris Flows)

The word “lahar” is an Indonesian term. Lahars are also known as volcanic 

mudflows. These are flows that originate on the slopes of volcanoes and are a type 

of debris flow. A lahar mobilizes the loose accumulations of tephra (the airborne 

solids erupted from the volcano) and related debris.

Occurrence

Found in nearly all volcanic areas of the world.

Relative size/range

Lahars can be hundreds of square kilometers or miles in area and can 

become larger as they gain speed and accumulate debris as they travel 

downslope; or, they can be small in volume and affect limited areas of 

the volcano and then dissipate downslope.

Velocity of travel

Lahars can be very rapid (more than 35 miles per hour or 50 kilometers 

per hour) especially if they mix with a source of water such as melting 

snowfields or glaciers. If they are viscous and thick with debris and less 

water, the movement will be slow to moderately slow.

Triggering mechanism

Water is the primary triggering mechanism, and it can originate from 

crater lakes, condensation of erupted steam on volcano particles, or the 

melting of snow and ice at the top of high volcanoes. Some of the largest 

and most deadly lahars have originated from eruptions or volcanic venting which suddenly melts surrounding snow and ice and causes rapid 

liquefaction and flow down steep volcanic slopes at catastrophic speeds.

Effects (direct/indirect)

Effects can be extremely large and devastating, especially when triggered by a volcanic eruption and consequent rapid melting of any 

snow and ice—the flow can bury human settlements located on the 

volcano slopes. Some large flows can also dam rivers, causing flooding 

upstream. Subsequent breaching of these weakly cemented dams can 

cause catastrophic flooding downstream. This type of landslide often 

results in large numbers of human casualties.

Mitigation measures

No corrective measures are known that can be taken to prevent damage 

from lahars except for avoidance by not building or locating in their paths 

or on the slopes of volcanoes. Warning systems and subsequent evacuation work in some instances may save lives. However, warning systems 

require active monitoring, and a reliable evacuation method is essential.
Part B. Basic Landslide Types 19

Figure 15. Schematic of a lahar. (Graphic by U.S. Geological Survey.)

Figure 16. Photograph of a lahar caused by the 1982 eruption of Mount St. Helens in 

Washington, USA. (Photograph by Tom Casadevall, U.S. Geological Survey.)

Predictability

Susceptibility maps based on past occurrences of lahars can be constructed, as well as runout estimations of potential flows. Such maps are 

not readily available for most hazardous areas. Figures 15 and 16 show a 

schematic and an image of a lahar.

For further reading: 

References 9, 39, 43, and 45
20 The Landslide Handbook—A Guide to Understanding Landslides

Debris Avalanche

Debris avalanches are essentially large, extremely rapid, often open-slope flows 

formed when an unstable slope collapses and the resulting fragmented debris is rapidly transported away from the slope. In some cases, snow and ice will contribute to 

the movement if sufficient water is present, and the flow may become a debris flow 

and (or) a lahar.

Occurrence

Occur worldwide in steep terrain environments. Also common on very 

steep volcanoes where they may follow drainage courses. 

Relative size/range

Some large avalanches have been known to transport material blocks as 

large as 3 kilometers in size, several kilometers from their source. 

Velocity of travel

Rapid to extremely rapid; such debris avalanches can travel close to 

100 meters/sec.

Triggering mechanism

In general, the two types of debris avalanches are those that are “cold” 

and those that are “hot.” A cold debris avalanche usually results from a 

slope becoming unstable, such as during collapse of weathered slopes in 

steep terrain or through the disintegration of bedrock during a slide-type 

landslide as it moves downslope at high velocity. At that point, the mass 

can then transform into a debris avalanche. A hot debris avalanche is one 

that results from volcanic activity including volcanic earthquakes or the 

injection of magma, which causes slope instability.

Effects (direct/indirect)

Debris avalanches may travel several kilometers before stopping, or they 

may transform into more water-rich lahars or debris flows that travel 

many tens of kilometers farther downstream. Such failures may inundate towns and villages and impair stream quality. They move very fast 

and thus may prove deadly because there is little chance for warning 

and response.

Corrective measures/mitigation

Avoidance of construction in valleys on volcanoes or steep mountain 

slopes and real-time warning systems may lessen damages. However, 

warning systems may prove difficult due to the speed at which debris 

avalanches occur—there may not be enough time after the initiation of 

the event for people to evacuate. Debris avalanches cannot be stopped 

or prevented by engineering means because the associated triggering 

mechanisms are not preventable. 
Part B. Basic Landslide Types 21

Figure 17. Schematic of a debris avalanche. (Schematic modified from Reference 9.)

Figure 18. A debris avalanche that buried the village of Guinsaugon, Southern Leyte, 

Philippines, in February 2006. (Photograph by University of Tokyo Geotechnical Team.) 

Please see figure 30 for an image of another debris avalanche.

Predictability

If evidence of prior debris avalanches exists in an area, and if such 

evidence can be dated, a probabilistic recurrence period might be 

established. During volcanic eruptions, chances are greater for a debris 

avalanche to occur, so appropriate cautionary actions could be adopted. 

Figures 17 and 18 show a schematic and an image of a debris avalanche.

For further reading: 

References 9, 39, 43, and 45
22 The Landslide Handbook—A Guide to Understanding Landslides

Earthflow

Earthflows can occur on gentle to moderate slopes, generally in fine-grained 

soil, commonly clay or silt, but also in very weathered, clay-bearing bedrock. The 

mass in an earthflow moves as a plastic or viscous flow with strong internal deformation. Susceptible marine clay (quick clay) when disturbed is very vulnerable and 

may lose all shear strength with a change in its natural moisture content and suddenly liquefy, potentially destroying large areas and flowing for several kilometers. 

Size commonly increases through headscarp retrogression. Slides or lateral spreads 

may also evolve downslope into earthflows. Earthflows can range from very slow 

(creep) to rapid and catastrophic. Very slow flows and specialized forms of earthflow 

restricted to northern permafrost environments are discussed elsewhere.

Occurrence

Earthflows occur worldwide in regions underlain by fine-grained soil 

or very weathered bedrock. Catastrophic rapid earthflows are common 

in the susceptible marine clays of the St. Lawrence Lowlands of North 

America, coastal Alaska and British Columbia, and in Scandinavia.

Relative (size/range)

Flows can range from small events of 100 square meters in size to large 

events encompassing several square kilometers in area. Earthflows in 

susceptible marine clays may runout for several kilometers. Depth of the 

failure ranges from shallow to many tens of meters.

Velocity of travel

Slow to very rapid.

Triggering mechanisms

Triggers include saturation of soil due to prolonged or intense rainfall 

or snowmelt, sudden lowering of adjacent water surfaces causing rapid 

drawdown of the ground-water table, stream erosion at the bottom of 

a slope, excavation and construction activities, excessive loading on a 

slope, earthquakes, or human-induced vibration.

Effects (direct/indirect)

Rapid, retrogressive earthflows in susceptible marine clay may devastate 

large areas of flat land lying above the slope and also may runout for 

considerable distances, potentially resulting in human fatalities, destruction of buildings and linear infrastructure, and damming of rivers with 

resultant flooding upstream and water siltation problems downstream. 

Slower earthflows may damage properties and sever linear infrastructure.

Corrective measures/mitigation

Improved drainage is an important corrective measure, as is grading of 

slopes and protecting the base of the slope from erosion or excavation. 

Shear strength of clay can be measured, and potential pressure can be 

monitored in suspect slopes. However, the best mitigation is to avoid 

development activities near such slopes.
Part B. Basic Landslide Types 23

Predictability

Evidence of past earthflows is the best indication of vulnerability. Distribution of clay likely to liquefy can in some cases be mapped and has 

been mapped in many parts of eastern North America. Cracks opening 

near the top of the slope may indicate potential failure. Figures 19 and 

20 show a schematic and an image of an earthflow.

Original

position

Earthflow

Figure 19. Schematic of an earthflow. (Schematic from Geological Survey of Canada.)

Figure 20. The 1993 Lemieux landslide—a rapid earthflow in sensitive marine clay 

near Ottawa, Canada. The headscarp retrogressed 680 meters into level ground above 

the riverbank. About 2.8 million tons of clay and silt liquefied and flowed into the South 

Nation River valley, damming the river. (Photograph by G.R. Brooks, Geological Survey of 

Canada.)

For further reading: 

References 9, 39, 43, and 45
24 The Landslide Handbook—A Guide to Understanding Landslides

Slow Earthflow (Creep)

Creep is the informal name for a slow earthflow and consists of the imperceptibly slow, steady downward movement of slope-forming soil or rock. Movement 

is caused by internal shear stress sufficient to cause deformation but insufficient to 

cause failure. Generally, the three types of creep are: (1) seasonal, where movement 

is within the depth of soil affected by seasonal changes in soil moisture and temperature; (2) continuous, where shear stress continuously exceeds the strength of the 

material; and (3) progressive, where slopes are reaching the point of failure for other 

types of mass movements.

Occurrence

Creep is widespread around the world and is probably the most common 

type of landslide, often preceding more rapid and damaging types of 

landslides. Solifluction, a specialized form of creep common to permafrost environments, occurs in the upper layer of ice-rich, fine-grained 

soils during the annual thaw of this layer.

Relative size/range

Creep can be very regional in nature (tens of square kilometers) or 

simply confined to small areas. It is difficult to discern the boundaries of 

creep since the event itself is so slow and surface features representing 

perceptible deformation may be lacking.

Velocity of travel

Very slow to extremely slow. Usually less than 1 meter (0.3 foot) 

per decade.

Triggering mechanism

For seasonal creep, rainfall and snowmelt are typical triggers, whereas 

for other types of creep there could be numerous causes, such as chemical or physical weathering, leaking pipes, poor drainage, destabilizing 

types of construction, and so on.

Effects

Because it is hard to detect in some places because of the slowness of 

movement, creep is sometimes not recognized when assessing the suitability of a building site. Creep can slowly pull apart pipelines, buildings, highways, fences, and so forth, and can lead to more drastic ground 

failures that are more destructive and faster moving.

Corrective measures/mitigation

The most common mitigation for creep is to ensure proper drainage 

of water, especially for the seasonal type of creep. Slope modification 

such as flattening or removing all or part of the landslide mass, can be 

attempted, as well as the construction of retaining walls.
Part B. Basic Landslide Types 25

Fence out of alignment

Soil ripples

Tilted pole

Curved tree trunks

Figure 21. Schematic of a slow earthflow, often called creep. (Schematic modified from 

Reference 9.)

Figure 22. This photograph shows the effects of creep, in an area near East Sussex, 

United Kingdom, called the Chalk Grasslands. Steep slopes of thin soil over marine 

chalk deposits, develop a ribbed pattern of grass-covered horizontal steps that are 

0.3 to 0.6 meter (1 to 2 feet) high. Although subsequently made more distinct by cattle 

and sheep walking along them, these terraces (commonly known as sheep tracks) 

were formed by the gradual, creeping movement of soil downhill. (Photograph by 

Ian Alexander.) 

Predictability

Indicated by curved tree trunks, bent fences and (or) retaining walls, 

tilted poles or fences, and small soil ripples or ridges on the surface. 

Rates of creep can be measured by inclinometers installed in boreholes 

or by detailed surface measurements. Figures 21 and 22 show a schematic and an image of creep.

For further reading: 

References 9, 39, 43, and 45
26 The Landslide Handbook—A Guide to Understanding Landslides

Flows in Permafrost

Failures in permafrost conditions involve the movement of fine-grained, previously ice-rich soil and can occur on gentle slopes. Seasonal thaw of the upper meter 

of frozen ground melts ground ice and results in oversaturation of the soil, which in 

turn loses shear strength and initiates flows. Solifluction, a form of cold environment 

creep, involves very slow deformation of the surface and forms shallow lobes elongated downslope. Active layer detachments, also known as skinflows, involve rapid 

flow of a shallow layer of saturated soil and vegetation, forming long, narrow flows 

moving on the surface but over the underlying permanently frozen soil. This type of 

movement may expose buried ice lenses, which when thawed may develop into retrogressive thaw flows or possibly debris flows. Retrogressive thaw flows are larger 

features with a bimodal shape of a steep headwall and low-angle tongue of saturated 

soil. This type of feature will continue to expand through headscarp retrogression 

until displaced vegetation buries and insulates the ice-rich scarp.

Occurrence

Flows are common in ice-rich permafrost soils in northern latitudes and 

high altitudes (cold environments).

Relative size/range

Flows are generally small but can increase in size through headscarp 

retrogression. They may evolve into a larger debris flow.

Velocity of travel

Very slow (solifluction); slow (retrogressive thaw flow); rapid (active 

layer detachment).

Triggering mechanisms

Above-average summer temperatures, frost wedges, wildfire, and 

anthropogenic disturbances to insulating peat layer. Such landslides are 

particularly likely in warming climates.

Effects (direct/indirect)

Damage to pipelines and roads and other structures can be severe.

Corrective measures/mitigation

Infrastructure designs that have minimal effect on the surface peat layer 

or temperature of the active layer and avoidance, when possible, of icerich soils when planning roads and other infrastructure, can reduce risk. 

Ice content of the upper soil can be readily tested.

Predictability

If ice-rich soil thaws, it will flow. In some areas, ice content has been 

mapped; in other areas, ice content can be estimated on the basis of 

specific mapped units shown on surficial geology maps. Figures 23 and 

24 show a schematic and an image of permafrost-related flow.

For further reading: 

References 2, 9, 39, 43, and 45
Part B. Basic Landslide Types 27

A. Original slope

B. Retrogressive thaw flow in progress

Tongue of

flowing mud

Possible retrogression

Original slope

Ice

Segregated ice bodies

Permanently frozen

icy sediment

Active layer

(seasonally thawed)

Retrogression

Figure 23. Schematic of a retrogressive thaw flow slide. (Schematic by Jan Aylsworth, 

Geological Survey of Canada.) 

A note about complex landslides:

These are landslides that feature components of two or more of the basic types 

of landslides and can occur either simultaneously or at different times during the 

onset of slope failure.
28 The Landslide Handbook—A Guide to Understanding Landslides

Figure 24. Photograph of a retrogressive thaw flow in the Northwest Territories, Canada. Wildfire has likely contributed to the size of 

the flow by means of damage to an insulating moss layer, resulting in the thickening of the active layer, which is thawing permafrost. 

(Photograph by Marten Geertsema, Ministry of Forests, British Columbia, Canada.)
Part C. Where Do Landslides Occur? 29

Part C. Where Do Landslides Occur?

A surprising fact to many people is that landslides can occur virtually anywhere 

in the world. The traditional viewpoint that landslides are restricted to extremely 

steep slopes and inhospitable terrain does not accurately reflect the real nature of the 

problem. Most countries in the world have been affected in some manner by landslides. The reason for such wide geographic coverage has much to do with the many 

different triggering mechanisms for landslides.

Excessive precipitation, earthquakes, volcanoes, forest fires and other mechanisms, and more recently, certain dangerous human activities are just some of the 

key causes that can trigger landslides. See “Part D. What Causes Landslides?” for 

more information on triggering mechanisms. Figure 25 shows an example of lateral 

spreading, a type of ground failure often associated with earthquakes.

Similarly, landslides are known to occur both on land and under water; they can 

occur in bedrock or on soils; cultivated land, barren slopes and natural forests are all 

subject to landslides. Both extremely dry areas and very humid areas can be affected 

by slope failures, and most important, steep slopes are not a necessary prerequisite 

for landslides to occur. In some cases, gentle slopes as shallow as 1–2 degrees have 

been observed to fail.

Bearing in mind that landslides can happen virtually anywhere around the 

world, we do, however, recognize certain patterns in their occurrence. At the 

national scale in countries such as Canada and the United States, the association of 

hilly terrain such as the Rocky Mountains with certain types of landslides is clear. 

Other geographic trends in landslide distribution can be linked to natural patterns of 

climate and weather, wildfires, and stream/river courses or human patterns involving 

the clearing of vegetation, modification to slopes, and other urban and rural practices. In each of these cases, it is important to recognize that landslide types vary in 

relation to the local and regional conditions. 

Debris torrents require channels and ravines to occur, whereas rockfalls will 

happen only where steep, exposed faces of bedrock or boulder-rich deposits are 

present. Geology itself figures prominently in the occurrence of many landslides. 

The correlation of seismic and volcanic activity to landslides is of paramount importance, so specialists often approach the evaluation of hazards from a multi-hazard 

perspective, which by definition takes into account most of the factors discussed 

previously.

For further reading: 

Reference 2

For further reading: 

References 8, 16, 19, 25, 30, and 45

Figure 25. Lateral spreading damage. 

Photograph shows the Puget Sound 

area in Washington, USA, after the 2001 

Nisqually earthquake. (Photograph 

courtesy of the Seattle Times.)
30 The Landslide Handbook—A Guide to Understanding Landslides

Part D. What Causes Landslides?

There are two primary categories of causes of landslides: natural and humancaused. Sometimes, landslides are caused, or made worse, by a combination of the 

two factors.

Natural Occurrences 

This category has three major triggering mechanisms that can occur either singly 

or in combination —(1) water, (2) seismic activity, and (3) volcanic activity. Effects 

of all of these causes vary widely and depend on factors such as steepness of slope, 

morphology or shape of terrain, soil type, underlying geology, and whether there are 

people or structures on the affected areas. Effects of landslides will be discussed in 

more detail in Part E.

Landslides and Water 

Slope saturation by water is a primary cause of landslides. Saturation can occur 

in the form of intense rainfall, snowmelt, changes in ground-water levels, and surfacewater level changes along coastlines, earth dams, and in the banks of lakes, reservoirs, canals, and rivers. Landslides and flooding are closely associated because both 

are related to precipitation, runoff, and the saturation of ground by water. Flooding 

may cause landslides by undercutting banks of streams and rivers and by saturation 

of slopes by surface water (overland flow). In addition, debris flows and mudflows 

usually occur in small, steep stream channels and commonly are mistaken for floods; 

in fact, these two events often occur simultaneously in the same area. Conversely, 

landslides also can cause flooding when sliding rock and debris block stream channels and other waterways, allowing large volumes of water to back up behind such 

dams. This causes backwater flooding and, if the dam fails, subsequent downstream 

flooding. Moreover, solid landslide debris can “bulk” or add volume and density to 

otherwise normal streamflow or cause channel blockages and diversions, creating 

flood conditions or localized erosion. Landslides also can cause tsunamis (seiches), 

overtopping of reservoirs, and (or) reduced capacity of reservoirs to store water. Steep 

wildfire-burned slopes often are landslide-prone due to a combination of the burning 

and resultant denudation of vegetation on slopes, a change in soil chemistry due to 

burning, and a subsequent saturation of slopes by water from various sources, such 

as rainfall. Debris flows are the most common type of landslide on burned slopes 

(for a description and images of a debris flow, see “Part B. Basic Landslide Types” 

in Section I). Wildfires, of course, may be the result of natural or human causes. 

Figure 26 shows a devastating landslide caused by rainfall, and possibly made worse 

by a leaking water pipe, which added even more water to the soil.
Part D. What Causes Landslides? 31

Figure 26. The Mameyes, Puerto Rico, landslide, 1985. This landslide destroyed 120 houses and killed at least 129 people. The 

catastrophic slide was triggered by a tropical storm that produced extremely heavy rainfall. Contributing factors could also have 

included sewage saturating the ground in the densely populated area, and a leaking water pipe at the top of the landslide. (Photograph 

by Randall Jibson, U.S. Geological Survey.)
32 The Landslide Handbook—A Guide to Understanding Landslides

Landslides and Seismic Activity

Many mountainous areas that are vulnerable to landslides have also experienced 

at least moderate rates of earthquake activity in recorded times. Earthquakes in steep 

landslide-prone areas greatly increase the likelihood that landslides will occur, due 

to ground shaking alone, liquefaction of susceptible sediments, or shaking-caused 

dilation of soil materials, which allows rapid infiltration of water. For instance, the 

1964 Great Alaska earthquake in the United States caused widespread landsliding 

and other ground failure, which led to most of the monetary loss attributed to the 

earthquake. Other areas in North America, such as the State of California, the Puget 

Sound region in Washington, and the St. Lawrence lowlands of eastern Canada, have 

experienced landslides, lateral spreading, and other types of ground failure classified 

as landslides, due to moderate to large earthquakes. Rockfalls and rock topples can 

also be caused by loosening of rocks or rocky formations as a result of earthquake 

ground shaking. Figure 27 shows damage from a landslide that was triggered by 

an earthquake. There is also a great danger of landslide dams forming in streams 

and rivers below steep slopes, a result of rock and earth being shaken down by the 

earthquake. These landslide dams often completely or partially block the flow of 

water, causing water to back up behind the landslide dam, flooding areas upriver. As 

these dams are often unstable, they may erode either quickly or over a period of time 

and fail catastrophically, unleashing the backed up water as a rapid deluge below the 

dam. This deluge is capable of causing a great deal of damage downriver. 

Figure 27. Earthquake-induced landslide damage to a house built on artificial fill, after 

the 2004 Niigata Prefecture earthquake in Japan. (Photograph by Professor Kamai, 

Kyoto University, Japan.)

Figures 32, 42, C53, C54, and C55 

show examples of large landslide 

dams that still exist
Part D. What Causes Landslides? 33

Landslides and Volcanic Activity 

Landslides due to volcanic activity represent some of the most devastating 

types of failures. Volcanic lava may melt snow rapidly, which can form a deluge 

of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path. These volcanic debris flows (also known as 

lahars, an Indonesian term) can reach great distances after they leave the flanks 

of the volcano and can damage structures in flat areas surrounding the volcanoes. 

Volcanic edifices are young, unconsolidated, and geologically weak structures that 

in many cases can collapse and cause rockslides, landslides, and debris avalanches. 

Many islands of volcanic origin experience periodic failure of their perimeter areas 

(due to the weak volcanic surface deposits), and masses of soil and rock slide into 

the ocean or other water bodies, such as inlets. Such collapses may create massive 

sub-marine landslides that may also rapidly displace water, subsequently creating 

deadly tsunamis that can travel and do damage at great distances, as well as locally. 

Figure 28 shows a collapse of the side of a volcano and the resulting devastation.

Human Activities

Populations expanding onto new land and creating neighborhoods, towns, and 

cities is the primary means by which humans contribute to the occurrence of landslides. Disturbing or changing drainage patterns, destabilizing slopes, and removing 

vegetation are common human-induced factors that may initiate landslides. Other 

examples include oversteepening of slopes by undercutting the bottom and loading 

the top of a slope to exceed the bearing strength of the soil or other component material. However, landslides may also occur in once-stable areas due to other human 

activities such as irrigation, lawn watering, draining of reservoirs (or creating them), 

leaking pipes, and improper excavating or grading on slopes. New construction on 

landslide-prone land can be improved through proper engineering (for example, 

grading, excavating) by first identifying the site’s susceptibility to slope failures and 

by creating appropriate landslide zoning.

See Appendix A for an expanded, detailed list of causes/triggering mechanisms 

of landslides.

Figure 28. The side of Casita Volcano in Nicaragua, Central America, collapsed on 

October 30, 1998, the day of peak rainfall as Hurricane Mitch moved across Central 

America. This lahar killed more than 2,000 people as it swept over the towns of 

El Porvenir and Rolando Rodriguez. (Photograph by K.M. Smith, U.S. Geological Survey.)

For further reading: 

References 16, 19, 32, 38, 39, 43,

and 45
34 The Landslide Handbook—A Guide to Understanding Landslides

Part E. What are the Effects and Consequences of 

Landslides?

Landslide effects occur in two basic environments: the built environment 

and the natural environment. Sometimes there is intersection between the two; for 

example agricultural lands and forest lands that are logged. 

Effects of Landslides on the Built Environment

Landslides affect manmade structures whether they are directly on or near a 

landslide. Residential dwellings built on unstable slopes may experience partial 

damage to complete destruction as landslides destabilize or destroy foundations, 

walls, surrounding property, and above-ground and underground utilities. Landslides 

can affect residential areas either on a large regional basis (in which many dwellings are affected) or on an individual site basis (where only one structure or part of 

a structure is affected). Also, landslide damage to one individual property’s lifelines 

(such as trunk sewer, water, or electrical lines and common-use roads) can affect the 

lifelines and access routes of other surrounding properties. Commercial structures 

are affected by landslides in much the same way residential structures are affected. 

In such a case, consequences may be great if the commercial structure is a commonuse structure, such as a food market, which may experience an interruption in business due to landslide damage to the actual structure and (or) damage to its access 

roadways. 

Fast-moving landslides such as debris flows are the most destructive type of 

landslide to structures, as they often occur without precursors or warnings, move too 

quickly for any mitigation measures to be enacted, and due to velocity and material are often very powerful and destructive. Fast-moving landslides can completely 

destroy a structure, whereas a slower moving landslide may only slightly damage 

it, and its slow pace may allow mitigation measures to be enacted. However, left 

unchecked, even slow landslides can completely destroy structures over time. Debris 

avalanches and lahars in steep areas can quickly destroy or damage the structures 

and lifelines of cities, towns, and (or) neighborhoods due to the fact that they are an 

extremely fast-moving, powerful force. The nature of landslide movement and the 

fact that they may continue moving after days, weeks, or months preclude rebuilding 

on the affected area, unless mitigative measures are taken; even then, such efforts are 

not always a guarantee of stability.

One of the greatest potential consequences from landslides is to the transportation industry, and this commonly affects large numbers of people around the world. 

Cut and fill failures along roadways and railways, as well as collapse of roads from 

underlying weak and slide-prone soils and fill, are common problems. Rockfalls may 

injure or kill motorists and pedestrians and damage structures. All types of landslides 

can lead to temporary or long-term closing of crucial routes for commerce, tourism, and emergency activities due to road or rail blockage by dirt, debris, and (or) 

rocks (fig. 29). Even slow creep can affect linear infrastructure, creating maintenance 

problems. Figure 29 shows a landslide blocking a major highway. Blockages of 

highways by landslides occur very commonly around the world, and many can simply be bulldozed or shoveled away. Others, such as the one shown in figure 29, will 

require major excavation and at least temporary diversion of traffic or even closure of 

the road.

As world populations continue to expand, they are increasingly vulnerable 

to landslide hazards. People tend to move on to new lands that might have been 

deemed too hazardous in the past but are now the only areas that remain for a 

growing population. Poor or nonexistent land-use policies allow building and other 

construction to take place on land that might better be left to agriculture, open-space 

parks, or uses other than for dwellings or other buildings and structures. Communities often are not prepared to regulate unsafe building practices and may not have the 

legitimate political means or the expertise to do so.

Note: In many areas of the world that 

provide private disaster insurance, 

damage from landslides is not covered 

in these insurance policies, and the 

costs of damages must be borne by the 

individual homeowner.
Part E. What are the Effects and Consequences of Landslides? 35

Figure 29. A landslide on the Pan American Highway in El Salvador, Central America, near the town of San Vicente, in 2001. 

(Photograph by Ed Harp, U.S. Geological Survey.)
36 The Landslide Handbook—A Guide to Understanding Landslides

Effects of Landslides on the Natural Environment

Landslides have effects on the natural environment: 

•	 The morphology of the Earth’s surface—mountain and valley systems, both 

on the continents and beneath the oceans; mountain and valley morphologies 

are most significantly affected by downslope movement of large landslide 

masses; 

•	 The forests and grasslands that cover much of the continents; and 

•	 The native wildlife that exists on the Earth’s surface and in its rivers, lakes, 

and seas.

Figures 30, 31, and 32 show the very large areal extent of some landslides and 

how they may change the face of the terrain, affecting rivers, farmland, and forests.

Forest, grasslands, and wildlife often are negatively affected by landslides, 

with forest and fish habitats being most easily damaged, temporarily or even rarely, 

destroyed. However, because landslides are relatively local events, flora and fauna 

can recover with time. In addition, recent ecological studies have shown that, under 

certain conditions, in the medium-to-long term, landslides can actually benefit fish 

and wildlife habitats, either directly or by improving the habitat for organisms that 

the fish and wildlife rely on for food. 

The following list identifies some examples of landslides that commonly occur 

in the natural environment:

•	 Submarine landslide is a general term used to describe the downslope mass 

movement of geologic materials from shallower to deeper regions of the 

ocean. Such events may produce major effects to the depth of shorelines, 

ultimately affecting boat dockings and navigation. These types of landslides 

can occur in rivers, lakes, and oceans. Large submarine landslides triggered 

by earthquakes have caused deadly tsunamis, such as the 1929 Grand Banks 

(off the coast of Newfoundland, Canada) tsunamis.

•	 Coastal cliff retreat, or cliff erosion, is another common effect of landslides 

on the natural environment. Rock-and-soil falls, slides, and avalanches are 

the common types of landslides affecting coastal areas; however, topples 

and flows also are known to occur. Falling rocks from eroding cliffs can be 

especially dangerous to anyone occupying areas at the base of cliffs, or on 

the beaches near the cliffs. Large amounts of landslide material can also be 

destructive to aquatic life, such as fish and kelp, and the rapid deposition of 

sediments in water bodies often changes the water quality around vulnerable 

shorelines. 

•	 Landslide dams can naturally occur when a large landslide blocks the flow 

of a river, causing a lake to form behind the blockage. Most of these dams 

are short-lived as the water will eventually erode the dam. If the landslide 

dam is not destroyed by natural erosional processes or modified by humans, 

it creates a new landform—a lake. Lakes created by landslide dams can last 

a long time, or they may suddenly be released and cause massive flooding 

downstream. There are many ways that people can lessen the potential dangers of landslide dams, and some of these methods are discussed in the safety 

and mitigation sections of this volume. Figure 32 shows the Slumgullion 

landslide one of the largest landslides in the world—the landslide dam it has 

formed is so large and wide, that it has lasted 700 years. Figures C53, C54, 

and C55 (in Appendix C) also show aspects of another large landslide dam.

See Appendix C for more information on mitigating the effects of landslide dams.

For further reading: 

References 4, 11, 14, 16, 19, 31, 35, 

36, 39, and 43
Part E. What are the Effects and Consequences of Landslides? 37

Figure 30. The active volcano, Mount Shasta in California, USA. Note the landforms in the foreground, caused by a debris avalanche 

that occurred about 300,000 years ago. The debris avalanche traveled great distances from the volcano and produced lasting landform 

effects that can still be seen today. (Photograph by R. Crandall, U.S. Geological Survey.)
38 The Landslide Handbook—A Guide to Understanding Landslides

Figure 31. View looking downstream at the confluence of the Río Malo (flowing from lower left) and the Río Coca, northeastern 

Ecuador, in South America. Both river channels have been filled with sediment left behind by debris flows triggered by the 1987 

Reventador earthquakes. Slopes in the area had been saturated by heavy rains in recent days before the earthquake. Debris/earth 

slides, debris avalanches, debris/mudflows, and resulting floods destroyed about 40 kilometers of the Trans-Ecuadorian oil pipeline and 

the only highway from Quito. (Photograph by R.L. Schuster, U.S. Geological Survey; information from Reference 32.)
Part E. What are the Effects and Consequences of Landslides? 39

Figure 32. The Slumgullion landslide, Colorado, USA. This landslide (formally referred to also as an earthflow) dammed the Lake Fork 

of the Gunnison River, which flooded the valley and formed Lake Cristobal. (Photograph by Jeff Coe, U.S. Geological Survey.)
40 The Landslide Handbook—A Guide to Understanding Landslides

Part F. Interrelationship of Landslides with Other 

Natural Hazards—The Multiple Hazard Effect

Natural hazards such as floods, earthquakes, volcanic eruptions, and landslides 

can occur simultaneously, or one or more of these hazards can trigger one or more of 

the others. Landslides are often the result of earthquakes, floods, and volcanic activity and may in turn cause subsequent hazards; for example, an earthquake-induced 

landslide can cause a deadly tsunami if sufficient landslide material slides into a 

body of water to displace a large volume of water. Another example would be a 

volcanic eruption-induced or earthquake-induced landslide that blocks a river, causing water to back up behind the mass and flood the upstream area. Should the dam 

fail, the impounded water will be suddenly unleashed to cause flooding downstream. 

This flooding can then add to riverbank and coastal erosion and destabilization 

through rapid saturation of slopes and undercutting of cliffs and banks. It is therefore imperative, when evaluating an area’s vulnerability to landslides, to examine 

all other possible natural hazards. At this time, few maps exist that show multiplehazard susceptibilities; for most areas, if hazards are mapped at all, only a single 

hazard is mapped.

Figures 33–35 show multi-hazard events involving landslides.

For further reading: 

References 17, 20, 35, 39, 43, and 45
Part F. Interrelationship of Landslides with Other Natural Hazards—The Multiple Hazard Effect 41

Figure 33. An example of a multi-hazard event. Photograph shows an aerial view of Lituya Bay, Alaska, USA. On July 9, 1958, an 

earthquake occurred which caused a landslide to enter the bay. The landslide in turn, caused a tsunami wave that had a run-up of 

174 meters on the opposite shore, and a 30-meter wave passed beyond Lituya Bay. It is the largest landslide-generated wave ever 

documented. Note the extent of the nonforested areas of land lining the shore of the bay, which marks the approximate reach of the 

tsunami wave. (Photograph by D.J. Miller, U.S. Geological Survey.)
42 The Landslide Handbook—A Guide to Understanding Landslides

Rock

avalanche

deposit

Debris

flow

deposit

Ridge

Figure 35. This is a photograph showing the aftereffects of a multi-hazard event. It is 

an aerial view showing part of the Andes Mountains and Nevado Huascaran, the highest 

peak in Peru, South America. A massive avalanche of ice and rock debris, triggered by 

the May 31, 1970, earthquake, buried the towns of Yungay and Ranrahirca, killing more 

than 20,000 people, about 40 percent of the total death toll of 67,000. The avalanche 

started with a sliding mass of glacial ice and rock about 1,000 meters (3,000 feet) wide 

and 1.6 kilometers (one mile) long that swept downslope about 5.4 kilometers (3.3 miles) to 

Yungay at average speed of more than 160 kilometers per hour. The ice picked up morainal 

material of water, mud, and rocks. (Photograph by Servicio Aerofotográfico National, 

graphics by George Plafker, U.S. Geological Survey.) Photograph and information from the 

U.S. Geological Survey Photographic Archives: http://libraryphoto.cr.usgs.gov/

Figure 34. The 1999 multi-hazard event in Tanaguarena, in coastal Venezuela, South 

America. The floods and landslides were triggered by heavy rains. (Photograph by 

Matthew Larsen, U.S. Geological Survey.)
Section II.

Evaluating and Communicating Landslide Hazard

Worldwide, information about landslides varies in its quantity and complexity 

and ranges in quality from detailed inventories of past landslides and resultant 

susceptibility and hazard maps to no information at all. People in some regions of 

the world have collective memories of past landslide events and know where slopes 

are unstable and (or) dangerous, and as a result, intuitively know where it is advantageous to build or not build. However, many areas are not readily obvious as to 

potential landslide hazard, and ground failure does not occur on any kind of regular 

basis. Also, some triggering mechanisms occur sporadically and have a gradual 

and cumulative effect and are not readily obvious. 
44 The Landslide Handbook—A Guide to Understanding Landslides

Part A. Evaluating Landslide Hazards 

There are many different means of assessing landslide hazard for an area; it 

is always advisable to consult with an expert for the most accurate assessment, 

although this is not always possible. Two types of landslide hazard evaluation, direct 

observation and the use of technological tools, are discussed here.

Observation and (or) inspection by local experts and (or) 

municipal officials, and property owners.

The following simple guide may assist individuals in observing and 

assessing a potential landslide hazard. It is important to note that some 

of these features can also be due to causes other than landslides, such 

as swelling soils.

Features that might indicate landslide movement:

•	 Springs, seeps, and wet or saturated ground in previously dry areas on or 

below slopes.

•	 Ground cracks—cracks in snow, ice, soil, or rock on or at the head of slopes.

•	 Sidewalks or slabs pulling away from structures if near a slope; soil pulling 

away from foundations.

•	 Offset fence lines, which were once straight or configured differently (see 

photograph in figure 22).

•	 Unusual bulges or elevation changes in the ground, pavements, paths, 

or sidewalks.

•	 Tilting telephone poles, trees, retaining walls, fences.

•	 Excessive tilting or cracking of concrete floors and foundations.

•	 Broken water lines and other underground utilities.

•	 Rapid increase or decrease in stream-water levels, possibly accompanied by 

increased turbidity (soil content clouding the water).

•	 Sticking doors and windows and visible open spaces, indicating walls and 

frames are shifting and deforming.

•	 Creaking, snapping, or popping noises from a house, building, or grove of 

trees (for example, roots snapping or breaking).

•	 Sunken or down-dropped roads or paths.

For further reading: 

References 1, 3, 4, 19, 20, 21, 25, 26, 

36, 39, 42, and 44

Note to Managers: It is important that governing bodies provide a means of 

keeping records, preferably in written format, about the occurrences of landslides, with photographs and (or) diagrams where possible. For areas of the world 

that do not already have laws or regulations that require disclosure of landslide 

hazards to property owners, it is important for villages, municipalities, or cities 

to establish some source of authority for hazard information. This need not be 

sophisticated or expensive but will provide a means to sustain landslide knowledge through time. Although some information may be politically sensitive, such 

as landowner rights, it is important that landslide information be made available, 

in some manner, to the general population. 
Part A. Evaluating Landslide Hazards 45

Figure 36. Ground cracks. (Phtograph courtesy of Utah Geological Survey.) 

Figure 37. Sidewalk pulling away from house. (Photograph courtesy of Utah 

Geological Survey.)

Figure 38. Cracking of the foundation of a structure. (Photograph courtesy of Utah 

Geological Survey.)
46 The Landslide Handbook—A Guide to Understanding Landslides

Technological Tools for Evaluation of Landslides—

Mapping, Remote Sensing, and Monitoring

One of the guiding principles of geology is that the past is the key to the 

future. In evaluating landslide hazards this means that future slope failures could 

occur as a result of the same geologic, geomorphic, and hydrologic situations that 

led to past and present failures. Based on this assumption, it is possible to estimate 

the types, frequency of occurrence, extent, and consequences of slope failures that 

may occur in the future. However, the absence of past events in a specific area does 

not preclude future failures. Human-induced conditions, such as changes in the natural topography or hydrologic conditions, can create or increase an area’s susceptibility to slope failure.

In order to predict landslide hazards in an area, the conditions and processes 

that promote instability must be identified and their relative contributions to slope 

failure estimated, if possible. Useful conclusions concerning increased probability 

of landsliding can be drawn by combining geological analyses with knowledge of 

short- and long-term meteorological conditions. Current technology enables persons 

monitoring earth movements to define those areas most susceptible to landsliding 

and to issue warnings and “alerts” covering time spans of hours to days when 

meteorological conditions or thresholds known to increase or initiate certain types of 

landslides are met. 

Map Analysis

Map analysis is usually one of the first steps in a landslide investigation. Necessary maps include bedrock and surficial geology, topography, soils, and if available, 

geomorphology maps. Using knowledge of geologic materials and processes, a 

trained person can obtain a general idea of landslide susceptibility from such maps. 

Appendix B at the end of this report contains a section on the various types of maps 

used in landslide analysis.

Aerial Reconnaissance

Analysis of aerial photography is a quick and valuable technique for identifying landslides, because it provides a three-dimensional overview of the terrain and 

indicates human activities as well as much geologic information to a trained person. 

In addition, the availability of many types of aerial imagery (satellite, infrared, radar, 

and so forth) makes aerial reconnaissance very versatile although cost-prohibitive in 

some cases.

Field Reconnaissance 

Many of the more subtle signs of slope movement cannot be identified on maps 

or photographs. Indeed, if an area is heavily forested or has been urbanized, even 

major features may not be evident. Furthermore, landslide features change over time 

on an active slide. Thus, field reconnaissance is always mandatory to verify or detect 

landslide features, and to critically evaluate the potential instability of vulnerable 

slopes. It identifies areas with past landslides (which could indicate future likelihood 

of landslides) by using field mapping and laboratory testing of terrain through the 

sampling of soil and rock. Mapping and laboratory testing for example, may identify 

vulnerable clays or other susceptible soils and show where they exist and their size 

and extent.
Part A. Evaluating Landslide Hazards 47

Drilling 

At most sites, drilling is necessary to determine the type of earth materials involved in the slide, the depth to the slip surface, and thus the thickness and 

geometry of the landslide mass, the water-table level, and the degree of disruption 

of the landslide materials. It also can provide suitable samples for age-dating and 

testing the engineering properties of landslide materials. Finally, drilling is needed 

for installation of some monitoring instruments and hydrologic observation wells. 

Note that drilling for information on stratigraphy, geology, water-table levels and for 

installation of instruments, for example, is also done for areas that have never had a 

landslide but for which the possibility exists.

Instrumentation 

Sophisticated methods such as electronic distance measurement (EDM), instruments such as inclinometers, extensometers, strain meters, and piezometers (see 

Glossary for definitions of these instruments), and simple techniques, such as establishing control points by using stakes can all be used to determine the mechanics of 

landslide movement and to monitor and warn against impending slope failure. 

Geophysical Studies 

Geophysical techniques (measurement of soil’s electrical conductivity/

resistivity, or measurement of induced seismic behavior) can be used to determine 

some subsurface characteristics such as the depth to bedrock, stratigraphic layers, 

zones of saturation, and sometimes the ground-water table. It can also be used to 

determine texture, porosity, and degree of consolidation of subsurface materials and 

the geometry of the units involved. In most instances, such surface survey methods can best be used to supplement drilling information, spatially extending and 

interpolating data between boreholes. They can also offer an alternative if drilling 

is impossible. Downhole geophysical methods (nuclear, electrical, thermal, seismic) also can be applied to derive detailed measurements in a borehole. Monitoring of natural acoustic emissions from moving soil or rock has also been used in 

landslide studies.

Acoustic Imagery and Profiles 

Profiles of lakebeds, river bottoms, and the sea floor can be obtained using 

acoustic techniques such as side-scan sonar and subbottom seismic profiling. 

Surveying of controlled grids, with accurate navigation, can yield three-dimensional perspectives of subaqueous geologic phenomena. Modern, high-resolution 

techniques are used routinely in offshore shelf areas to map geologic hazards for 

offshore engineering. 

Computerized Landslide Terrain Analysis 

In recent years, computer modeling of landslides has been used to determine 

the volume of landslide masses and changes in surface expression and cross section 

over time. This information is useful in calculating the potential for stream blockage, 

cost of landslide removal (based on volume), and type and mechanism of movement. Very promising methods are being developed that use digital elevation models 

(DEMs) to evaluate areas quickly for their susceptibility to landslide/debris-flow 

events. Computers also are being used to perform complex stability analyses. Software programs for these studies are readily available for personal computers.

See Appendix B for more information and images of map types.

For further reading: 

References 4, 15, 18, 24, 25, 39, 

and 46
48 The Landslide Handbook—A Guide to Understanding Landslides

Part B. Communicating Landslide Hazard 

The successful translation of natural hazard information into a form useful for 

nontechnical users conveys the following three elements in one form or another:

•	 Likelihood of the occurrence of an event of a size and in a location that 

would cause casualties, damage, or disruption to an existing standard 

of safety.

•	 Expected location and extent of the effects of the event on the ground, 

structures or socioeconomic activity.

•	 Estimated severity of the effects on the ground, structures, or socioeconomic 

activity.

These elements are needed because engineers, planners, and decisionmakers 

usually will not be concerned with a potential hazard if its likelihood is rare, its location is unknown, or its severity is slight.

Unfortunately, these three pieces of information can come in different forms 

with many different names, some quantitative and precise, others qualitative and 

general. For a product to qualify as useful hazard information, the nontechnical user 

must be able to perceive likelihood, location, and severity of the hazard so that they 

become aware of the danger, can convey the risk to others, and can use the translated 

information directly to reduce a threat.

Safety Information 

Safety is no doubt the first order of business for managers and municipal 

officials. People living in areas prone to fast-moving, deadly debris flows need 

information on the likelihood of the hazard; for example, when it is most dangerous 

to be in the path of potential debris flows (such as during heavy rainstorms) and at 

what point to evacuate and (or) cease walking or driving in a hazardous area. Safety 

information about slow-moving landslides is equally important as these types of 

landslides can damage and (or) break electrical and gas lines, creating an additional 

hazard of fire, electrocution, and gas fumes.

Building and Construction Information

This information is also valuable to communities, so that some of the causes 

of landslides might be avoided. This information is discussed in Section III, 

“Mitigation Concepts and Approaches.”

Safety, education, and building information can be made available to community residents in various ways. A list of building codes, building inspection 

processes, and potential areas where destabilization may cause landslide problems 

can be made available through the following:
Part B. Communicating Landslide Hazard 49

Suggested Local Government Outreach for Landslide Hazard

•	 Newspaper bulletins/advertisements. 

•	 Public-service brochures distributed door-to-door or displayed in public places.

•	 Community meeting discussions.

•	 Posters in public buildings and (or) marketplaces with as much visual information as possible.

•	 Media announcements through radio, television, loudspeakers, or other means.

•	 Public lectures by experts or other officials.

•	 Signs posted in immediate areas of hazards, informing people of the kind of hazard and warning them to 

be cautious. An example of this would be a sign warning of rockfall hazards along well-used footpaths. 

•	 In areas where literacy rates are low, oral communications with graphics, photographs, and illustrations 

of hazards can be extremely effective. Pictures may take the place of a thousand words!

•	 Telephone book—In areas where phone service is widespread or accessible, municipal listings for 

engineering, emergency planning, and police or fire departments.

•	 Where possible a municipal Internet Web site is a useful source of safety information and contact phone 

numbers and emails for emergency personnel and engineering and (or) planning departments.

•	 Determine local landslide hazard problems through a working committee and (or) secure professional 

advice. State/Provincial or Federal geological surveys, university geology or engineering departments, 

and private geotechnical companies are sources of advice. Provide for a mapping program where 

possible, either within local government or contracted with professionals.

•	 Conduct public education and information programs through community meetings, city council, or 

other councils.

•	 Adopt and enforce appropriate land-use policies—discuss with landowners, developers, buyers, and 

sellers. One option is to require disclosure of geological hazards during property sales to ensure that the 

new buyer is aware of any problems.

•	 Monitor changes in unstable slopes and take appropriate actions (see “Mitigation” section of this 

handbook).

•	 Construct street and drainage projects that meet local safety needs and ordinances.

•	 Pursue public grant programs, government programs for infrastructure, and public works 

improvement projects.

•	 Be informed about insurance programs available and liability issues and know where local government 

responsibility lies for public safety and well-being.

•	 Have an emergency response plan for the community. Consult with neighboring towns and (or) 

communities that have plans and have used them in an emergency. Evaluate their effectiveness for your 

own situation.
50 The Landslide Handbook—A Guide to Understanding Landslides

Examples of Hazard Warning Signs 

The following figures (figs. 39, 40, and 41) show examples of some simple 

warning signs that can be placed in hazardous areas. This information can also be 

used in emergency management policies for municipalities and (or) communities.

Please see Appendix B for samples of basic safety information for debris flow 

and other landslide hazards that are suitable for posting and distributing in public 

places.

Figure 40. A notice for cliff 

hazards, city of Wanneroo, 

Australia.

Figure 39. Example of a 

rockfall hazard sign.

Figure 41. Sign along a highway in Virginia, USA.

For further reading: 

References 1, 6, 19, 21, 23, 24, 26, 

36, and 41
Section III.

Mitigation Concepts and Approaches

Vulnerability to landslide hazards is a function of a site’s location (topography, geology, drainage), type of activity, and frequency of past landslides. The 

effects of landslides on people and structures can be lessened by total avoidance 

of landslide hazard areas or by restricting, prohibiting, or imposing conditions 

on hazard-zone activity. Local governments can accomplish this through landuse policies and regulations. Individuals can reduce their exposure to hazards by 

educating themselves on the past hazard history of a desired site and by making 

inquiries to planning and engineering departments of local governments. They 

could also hire the professional services of a geotechnical engineer, a civil engineer, or an engineering geologist who can properly evaluate the hazard potential of 

a site, built or unbuilt.
52 The Landslide Handbook—A Guide to Understanding Landslides

Part A. Overview of Mitigation Methods for Various 

Types of Landslide Hazards 

Seeking the advice of professionals is always advised where possible, but 

managers and homeowners should be educated about mitigation in order to make 

informed decisions concerning construction and land use. A few of these measures 

are discussed in this section. More detailed information on landslide mitigation is 

available in Appendix C and in Turner and Schuster (1996) ( Reference 39).

The simplest means of dealing with landslide hazards is to avoid construction 

on steep slopes and existing landslides; however, this is not always practical. Regulating land use and development to ensure that construction does not reduce slope 

stability is another approach. Avoidance and regulation rely on landslide maps and 

the underlying definitions of landslide areas to reduce hazard (Appendix B). In cases 

where landslides affect existing structures or cannot be avoided, physical controls 

can be used. In some cases, monitoring and warning systems (Appendix B) allow 

residents to evacuate temporarily during times when the probability of landslide 

activity is high.

Soil Slope Stabilization 

Stability increases when ground water is prevented from rising in the slide 

mass by

•	 directing surface water away from the landslide,

•	 draining ground water away from the landslide to reduce the potential for a 

rise in ground-water level,

•	 covering the landslide with an impermeable membrane, and (or)

•	 minimizing surface irrigation. Slope stability is also increased when weight 

or retaining structures are placed at the toe of the landslide or when mass 

(weight) is removed from the head of the slope.

Planting or encouraging natural growth of vegetation can also be an effective means 

of slope stabilization—this is further discussed in the section on biotechnical mitigation methods and Appendix C. 

An example of one means of slope stabilization is the use of retaining walls. 

Retaining walls are structures built to support a soil mass permanently. They also are 

used whenever space requirements make it impractical to slope the side of an excavation, or to prevent sloughing of loose hillslope soils onto roads or property. Retaining walls are also used to prevent or minimize toe erosion by river scour or to retard 

creep. They cannot, however, be used to stop landslides from occurring. Several 

basic types of wall are timber crib, steel bin, pile, cantilever, sheet pile, plastic mesh, 

and reinforced earth. Each of these types has advantages in certain situations, but 

cost is usually what determines which is type is adopted. More information about 

retaining walls is given in Appendix C. 

See Appendix C for more information on stabilization methods. 
Part A. Overview of Mitigation Methods for Various Types of Landslide Hazards 53

Rockfall Hazard Mitigation 

Rockfall is common in areas of the world with steep rock slopes and cliffs. 

Commonly, these are mountainous or plateau areas, whether in coastal areas or 

among isolated rock formations. Rockfall causes extraordinary amounts of monetary 

damage and death, the former mostly by impeded transportation and commerce due 

to blocked highways and waterways and the latter as direct casualties from falling 

rocks. Diverting paths and highways around rockfall areas is sometimes implemented but is not always practical. Many communities post danger signs around 

areas of high rockfall hazard. Some methods of rockfall hazard mitigation include 

catch ditches, benches, scaling and trimming, cable and mesh, shotcrete, anchors, 

bolts, dowels, and controlled blasting.

See Appendix C for more information on mitigation means for preventing and 

diverting rockfall.

Debris-Flow Hazard Mitigation 

Due to the speed and intensity of most debris flows, they are very hard to stop 

once they have started. However, methods are available to contain and deflect debris 

flows primarily through the use of retaining walls and debris-flow basins. Other mitigation methods include modifying slopes (preventing them from being vulnerable 

to debris-flow initiation through the use of erosion control), revegetation, and the 

prevention of wildfires, which are known to intensify debris flows on steep slopes.

See Appendix C for more information on methods for debris-flow hazard 

mitigation.

Landslide Dam Mitigation

Many problems arise when landslides dam waterways. Dams caused by 

landslides are a common problem in many areas of the world. Landslides can occur 

on the valley walls of streams and rivers. If enough displaced material (rock, soil, 

and (or) debris) fills the waterway, the landslide will act as a natural dam, blocking the flow of the river and creating flooding upstream. As these natural dams 

are frequently composed of loose, unconsolidated material, they commonly are 

inherently weak and are soon overtopped and fail due to erosion. When breaching happens quickly, the backed-up water rushes down the waterway, potentially 

causing catastrophic downstream flooding. An example of a landslide dam is the 

600-meter-high Usoi landslide dam in Tajikistan, one of the largest landslide dams in 

the world. A large earthquake-induced landslide dammed the Murghab River, creating Lake Sarez. The dam poses a hazard for people living downstream. Also, future 

seismic action may cause more landslides to slide into the dammed lake, causing a 

seiche (a tsunami-like wave in a closed water basin), which may weaken and (or) 

overtop the landslide dam. Figure 42 shows a landslide dam caused by the sliding of 

saturated slopes, and figure 43 shows a landslide dam caused by an earthquake.

See Appendix C for more details on mitigation methods for landslide dams. 
54 The Landslide Handbook—A Guide to Understanding Landslides

Figure 42. The Thistle landslide in Utah, USA. This 1983 landslide dammed the Spanish Fork River, backing up water that flooded the 

town of Thistle. Many landslide dams are much smaller than the one shown here and potentially can be overtopped by backed-up 

water, or eroded through. Some are much larger, and roads and railroad lines that are blocked or damaged must be diverted around 

the landslide mass. The concrete tunnel at the lower part of the bottom photograph shows where the rail line was rerouted around the 

Thistle slide and excavated through an adjacent mountain. 
Part A. Overview of Mitigation Methods for Various Types of Landslide Hazards 55

Methods of Biotechnical Landslide Mitigation 

This type of slope protection is used to reduce the adverse environmental consequences of landslide-mitigation measures. When used for landslide remediation or 

mitigation, conventional earth-retaining structures made of steel or concrete usually 

are not visually pleasing or environmentally friendly. These traditional “hard” remedial measures are increasingly being supplanted by vegetated composite soil/structure 

bodies that are environmentally more friendly; that is, a process that has come to be 

known as biotechnical slope protection. Common biotechnical systems include nets 

of various materials anchored by soil nails that hold in place soil seeded with grass. 

Research has been done on using plants to stabilize soil to prevent excessive erosion 

and also to mitigate the effect of landslides. One of the most promising types of plants 

is Vetiver, a type of grass that works very well to stabilize slopes against erosion in 

many different environments. See Appendix C for more information on Vetiver grass 

uses and its geographical suitability. 

See Appendix C for more information on mitigation techniques.

Figure 43. The great earthquake that 

struck China on May 12, 2008, caused 

extensive damage in the mountainous 

terrain of Beichuan County. In many 

cases, landslides in steep valleys formed 

landslide dams, creating new lakes 

in a period of hours. This pair of highresolution, photo-like images from Taiwan’s 

Formosat-2 satellite on May 14, 2006 (top), 

and May 14, 2008 (bottom), before and after 

the earthquake, show the large landslide 

that blocked the Jiangjian River, forming a 

dangerous landslide-dammed lake.
56 The Landslide Handbook—A Guide to Understanding Landslides

Part B. Simple Mitigation Techniques for Home and 

Businesses, Managers, and Citizens

There are simple and low-technology means for homeowners and others to 

implement methods and techniques that are effective and lessen the effects of 

landslides. First, it is always best to consult a professional, such as a geotechnical 

engineer or a civil engineer, as they have had the training and experience to solve 

instability problems; a local company or professional may be the best, as they may 

be familiar with the geology, soil types, and geography of the area in question. This 

is not always the case, but it is a basis for making inquiries. When there are local 

jurisdictions such as county and (or) city municipal offices, individuals within these 

institutions may be professional geologists, planners, and (or) building experts who 

can answer questions, provide maps, and explain building regulations and inspection procedures. Access to these types of officials varies widely around the world, 

and local situations may be handled differently. When consulting a professional is 

not possible, some steps can be taken in the meantime, as detailed in Appendixes C 

and D.

See Appendixes C and D for detailed information on mitigation techniques for 

property owners, citizens, and managers.

For further reading: 

References 4, 8, 11, 19, 20, 28, 30, 

31, 32, and 37
Part C. List of Works Consulted/Cited/Quoted and for Further Reading 57

1. Advisory Committee on the International Decade for Natural Hazard 

Reduction,Commission on Engineering 

and Technical Systems, 1987, Confronting natural disasters, an International 

Decade for Natural Hazard Reduction, National Research Council: U.S. 

National Academy of Sciences, Washington, D.C.

2. Aylsworth, J.M., Duk-Rodkin, A., 

Robertson, T., and Traynor, J.A., 2000, 

Landslides, in the physical environment 

of the Mackenzie Valley, Northwest 

Territories—A baseline for the assessment of environmental change, Dyke, 

L.D., and Brooks, G.R., eds.: Geological Survey of Canada, Bulletin no. 547, 

p. 41–48.

3. Barrows, Alan, and Smith, Ted, 

2004, Hazards from “mudslides,” 

debris avalanches and debris flows 

in hillside and wildfire areas: California Geological Survey Note 33. 

Online: http://www.consrv.ca.gov/cgs/

information/publications/cgs_notes/

note_33/index.htm

4. Blake, T.F., Holingsworth, R.A., and 

J.P. Stewart, eds., 2002, Recommended 

procedures for implementation of 

guidelines for analyzing and mitigating landslide hazards in California: 

Department of Mining and Geology 

special publication 117 American 

Society of Civil Engineers (ASCE), 

Los Angeles Section Geotechnical 

Group, published by Southern California 

Earthquake Center (SCEC). Online: 

http://www.scec.org/resources/catalog/

LandslideProceduresJune02.pdf

5. California Department of Conservation, Division of Mines and Geology, 

1997, Factors affecting landslides in 

forested terrain, Note 50. Online: http://

www.consrv.ca.gov/cgs/information/

publications/cgs_notes/note_50/ 

Documents/note50.pdf

6. Case, William F., (no date) Landslides—

What they are, why they occur: Utah 

Geological Survey, Utah Department of 

Natural Resources, Public Information 

Series 74. Online: http://geology.utah.

gov/online/pdf/pi-74.pdf

7. Case, William F., 2003, Debris-flow 

hazards: Utah Geological Survey, 

Public Information Series 70. Online: 

http://geology.utah.gov/online/pi-70/

debrisflow.htm

8. Case, William F., 2000, Rock-fall 

hazards: Utah Geological Survey, Public 

Information Series 69. Online: http://

geology.utah.gov/online/pdf/pi-69.pdf

9. Cruden, D.M, and Varnes, D.J., 1996, 

Landslide types and processes, in

Turner, A. Keith, and Schuster, Robert 

L. eds. Landslides—Investigation and 

mitigation: Transportation Research 

Board, Special report no. 247, National 

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10. Cruden, D.M., 1993, The multilingual 

landslide glossary: Richmond., British 

Columbia, Canada, Bitech Publishers,for 

the UNESCO Working Party on World 

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11. Chatwin, S.C., Howes, D.E., Schwab, 

J.W., and Swanston, D.N., 1994, A 

guide for management of landslideprone terrain in the Pacific Northwest, 

second edition: Ministry of Forests, 

31 Bastion Square, Victoria, British 

Columbia V8W3E7, 220 p. Online: 

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12. Creath, W.B., 1996. Homebuyers’ guide 

to geologic hazards—An AIPG issues 

and answers publication: Department 

of Natural Resources, Denver, Colorado Geological Survey, Miscellaneous 

Publication (MI) no. 58, 30 p.

13. Fleming, Robert W., and Taylor, Fred 

A., 1980, Estimating the costs of landslide damage in the United States: U.S. 

Geological Survey Circular 832, 21p. 

14. Gray, D.H., and Sotir, R.B., 1996, Biotechnical and soil bioengineering slope 

stabilization—A practical guide for 

erosion control: New York, John Wiley, 

378 p. 

15. Haugerud, Ralph A., Harding, David J., 

Johnson, Samuel Y., Harless, Jerry L., 

Weaver, Craig S., and Brian L. Sherrod, 

2003, High-resolution LiDAR topography of the Puget Lowland, Washington—A Bonanza for earth science: GSA 

Today, Geological Society of America, 

p. 4–10.

16. Highland, Lynn, 2004, Landslide types 

and processes: U.S. Geological Survey 

Fact Sheet FS–2004–3072. Online: 

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17. Jackson, Julia A., ed., 1997, Glossary of 

geology, fourth edition: American Geophysical Institute, Alexandria, Virginia, 

USA, 769 p.

18. Jibson, Randall W., Harp, Edwin L., and 

Michael, John A., 1998, A method for 

producing digital probabilistic seismic 

landslide hazard maps—An example 

from the Los Angeles, California, area: 

U.S. Geological Survey Open-File 

Report 98–113, 17 p. Online: http://

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19. Jochim, Candice, Rogers, William 

P., Truby, John O., Wold, Jr., Robert 

L., Weber, George, and Brown, Sally 

P., 1988, Colorado landslide hazard 

mitigation plan: Department of Natural 

Resources, Colorado Geological Survey, 

Denver, Colo., USA. 

20. Los Angeles County Department of 

Public Works, Board of Supervisors, 

1993, Homeowner’s guide for flood, 

debris, and erosion control: Alhambra, 

California, in English and Spanish. 

Online: http://dpw.lacounty.gov/wmd/

Homeowners/index.cfm

21. McInnes, Robin, 2000, Managing 

ground instability in urban areas, a 

guide to best practice, Centre for the 

Coastal Environment, Isle of Wight 

Council: United Kingdom, Cross 

Publishing, Walpen Manor, Chale, 

Isle of Wight.

22. National Research Council, 1993, 

Vetiver grass—A thin green line against 

erosion: National Academy Press, 

Washington, D.C. Online: 

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23. Nichols, Donald R., and Catherine C. 

Campbell, eds., 1971, Environmental 

planning and geology: U.S. Department 

of Housing and Urban Development, 

U.S. Department of the Interior, U.S. 

Government Printing Office.

Part C. List of Works Consulted/Cited/Quoted and for Further Reading
58 The Landslide Handbook—A Guide to Understanding Landslides

24. Norheim, Robert A., Queija, Vivian R., 

and Haugerud, Ralph A., 2002, Comparison of LiDAR and InSAR DEMs 

with dense ground control: Proceedings, Environmental Systems Research 

Institute 2002 User Conference. Online: 

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proc02/pap0442/p0442.htm

25. Nuhfer, Edward B., Proctor, Richard J., 

and Moser, Paul H., 1993. The citizen’s 

guide to geologic hazards: The American Institute of Professional Geologists, 

134 p.

26. Olshansky, Robert B., 1996, Planning 

for hillside development: American 

Planning Association (APA), Planning 

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27. Pelletier, B.R., ed., 2000, Environmental atlas of the Beaufort coastlands, 

supplement to the Marine Science Atlas 

of the Beaufort Sea: Geological Survey 

of Canada, Natural Resources Canada. 

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index_e.php

28. Reid, Mark, and Ellis, William L., 

1999, Real-time monitoring of active 

landslides: U.S. Geological Survey 

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29. Rickenmann, Dieter, and Cheng-lung 

Chen, eds., 2003, Debris-flow hazards 

Mitigation—Mechanics, prediction, 

and assessment: Millpress, Rotterdam, 

The Netherlands.

30. Schuster, Robert L., and Highland, 

Lynn M., 2004, Impact of landslides and 

innovative landslide-mitigation measures 

on the natural environment: International 

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measures for landslide dams, in Security 

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June 8–13, p.170–176 [theme keynote 

paper].

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Lynn M., 2001, Socioeconomic 

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36. Solomon, Barry J., 2001, Using geologic 

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37. Swanston, D., ed., 1985, Proceedings of 

a workshop on slope stability—Problems and solutions in forest management: USDA Forest Service General 

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38. Swanston, D.N., 1983, Assessment of 

mass erosion risk from forest operations 

in steep terrain: International Association of Forestry Research Organizations 

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and Techniques, Munich, Germany, 

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L., 1996, Landslides— Investigation and 

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from space: U.S. Geological Survey 

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43. Varnes, D.J., 1978, Slope movement 

types and processes, in Schuster, R.L., 

and Krizek, R.J., eds., Landslides—

Analysis and control: Transportation 

Research Board Special Report 176, 

National Research Council, Washington, 

D.C., p. 11–23.

44. Weber, G., Von Schulez, W., and Czerniak, R., 1983, Flood hazard management plan for the Sheridan watershed 

area: Sheridan, Wyoming, Geographic 

Applications and Research Group, 

Boulder, Colorado.

45. Wieczorek, Gerald F., 1996, Landslide 

triggering mechanisms, in Turner, 

A. Keith, and Schuster, Robert L., 

eds., Landslides—Investigation and 

mitigation: Transportation Research 

Board, Special report no. 247, National 

Research Council, National Academy 

Press, Washington, D.C., p. 76–90.

46. Wold, Robert L., and Jochim, Candace 

L.,1989, Landslide loss reduction—A 

guide for state and local government 

planning: Special Publication 33, 

Department of Natural Resources, 

Colorado Geological Survey, Denver, 

Colo., 50 p. 

47. Yoon, P.K., 1994, Important biological 

considerations in use of Vetiver grass 

hedgerows (VGHR) for slope protection and stabilisation, in Vegetation and 

slopes— Stabilisation, protection and 

ecology: Proceedings, International 

Conference Institution of Civil Engineers, University Museum, Oxford, 

September 29–30, 1994, Thomas 

Telford, London, p. 212–221.
Appendix A.

Basic Information about Landslides
60 The Landslide Handbook—A Guide to Understanding Landslides

alluvial fan An outspread, gently sloping 

mass of alluvium deposited by a stream, 

especially in an arid or semiarid region 

where a stream issues from a narrow canyon 

onto a plain or valley floor. Viewed from 

above, it has the shape of an open fan, the 

apex being at the valley mouth. (Reference 3)

bedding surface/plane In sedimentary or 

stratified rocks, the division planes that separate each successive layer or bed from the 

one above or below. It is commonly marked 

by a visible change in lithology or color. 

(Reference 3)

bedrock The solid rock underlying gravel, 

sand, clay, and so forth; any solid rock 

exposed at the surface of the earth or overlain 

by unconsolidated superficial material. 

(Reference 3)

borehole A circular hole drilled into the 

earth, often to a great depth, as a prospective oil, gas, or water well or for exploratory 

purposes. (Reference 3)

check dams Check dams are small sediment storage dams built in the channels of 

steep gullies to stabilize the channel bed. A 

common use is to control channelized debrisflow frequency and volume. Check dams are 

expensive to construct and are therefore usually only built where important installations 

or natural habitat (such as a camp or unique 

spawning area) lies downslope. (Reference 2)

colluvium A general term applied to loose 

and incoherent deposits, usually at the foot of 

a slope or cliff and brought there chiefly by 

gravity. (Reference 2)

debris basin (sometimes called catch 

basins) A large excavated basin into which 

a debris flow runs or is directed and where it 

quickly dissipates its energy and deposits its 

load. Abandoned gravel pits or rock quarries 

are often used as debris basins. (Reference 3).

delta-front landsliding Delta fronts are 

where deposition in deltas is most active—

underwater landsliding along coastal and 

delta regions due to rapid sedimentation of 

loosely consolidated clay, which is low in 

strength and high in pore-water pressures.

Digital Elevation Model (DEM) A digital 

elevation model (DEM) is a digital file consisting of terrain elevations for ground positions at regularly spaced horizontal intervals. 

(A commercial definition – new technology)

Digital Terrain Model (DTM) The term used 

by United States Department of Defense and 

other organizations to describe digital elevation data. (Reference 3)

drawdown Lowering of water levels in rivers, lakes, wells, or underground aquifers due 

to withdrawal of water. Drawdown may leave 

unsupported banks or poorly packed earth 

that can cause landslides. (Reference 3)

electronic distance meter (EDM) A device 

that emits ultrasonic waves that bounce off 

solid objects and return to the meter. The 

meter’s microprocessor then converts the 

elapsed time into a distance measurement. 

Sound waves spread 1 foot wide for every 

10 feet measured. There are various types 

available.

epicenter The point on the Earth’s surface 

directly above the focus of an earthquake. 

(Reference 3)

expansive soils Types of soil that shrink 

or swell as the moisture content decreases or 

increases. Structures built on these soils may 

shift, crack, and break as soils shrink and 

subside or expand. Also known as swelling 

soils. (Reference 5)

extensometer An instrument for measuring small deformations, as in tests of stress. 

(Reference 3)

factor of safety The factor of safety, also 

known as Safety Factor, is used to provide 

a design margin over the theoretical design 

capacity to allow for uncertainty in the 

design process. The uncertainty could be any 

one of a number of the components of the 

design process including calculations and 

material strengths for example. Commonly, 

a factor of safety of less than 1, for instance, 

on an engineered slope indicates potential 

failure, where a factor of safety of greater 

than 1, indicates stability. (Reference 6)

geodesic/geodetic measurements The 

investigation of any scientific questions connected with the shape and dimensions of the 

Earth. (Reference 3)

fracture Brittle deformation due to a 

momentary loss of cohesion or loss of resistance to differential stress and a release of 

stored elastic energy. Both joints and faults 

are fractures. (Reference 3)

Geographic Information System (GIS) A 

computer program and associated data bases 

that permit cartographic information (including geologic information) to be queried 

by the geographic coordinates of features. 

Usually the data are organized in “layers” 

representing different geographic entities 

such as hydrology, culture, topography, and 

so forth. A geographic information system, 

or GIS, permits information from different 

layers to be easily integrated and analyzed. 

(Reference 3)

geologic hazard A geologic condition, 

either natural or manmade, that poses 

a potential danger to life and property. 

Examples: earthquake, landslides, flooding, 

faulting, beach erosion, land subsidence, 

pollution, waste disposal, and foundation and 

footing failures. (Reference 3)

geologic map A map on which is recorded 

the distribution, nature, and age relationships 

of rock units and the occurrence of structural 

features. (Reference 3)

geomorphology The science that treats the 

general configuration of the Earth’s surface; 

specifically, the study of the classification, 

description, nature, origin, and development of landforms and their relationships to 

underlying structures, and the history of geologic changes as recorded by these surface 

features. (Reference 3)

geophysical studies The science of the 

Earth, by quantitative physical methods, 

with respect to its structure, composition, 

and development. It includes the sciences of 

dynamical geology and physical geography 

and makes use of geodesy, geology, seismology, meteorology, oceanography, magnetism, 

and other Earth sciences in collecting and 

interpreting Earth data. (Reference 3)

hydraulic Of or pertaining to fluids in 

motion; conveying, or acting, by water; 

operated or moved by means of water, as 

hydraulic mining. (Reference 3)

hydrology The science that relates to the 

water of the Earth. (Reference 3)

inclinometer Instrument for measuring 

inclination to the horizontal. (Reference 3)

landslide dam An earthen dam created 

when a landslide blocks a stream or river. 

(Reference 3)

Part 1. Glossary of Landslide Terms

Full references citations for glossary are at the end of the list.
Part 1. Glossary of Landslide Terms 61

lahar Landslide, debris flow or mudflow, of 

pyroclastic material on the flank of a volcano; 

deposit produced by such a debris flow. 

Lahars are described as wet if they are mixed 

with water derived from heavy rains, escaping 

from a crater lake, or produced by melting 

snow. Dry lahars may result from tremors of 

a cone or by accumulating material becoming unstable on a steep slope. If the material 

retains much heat, it is termed a hot lahar. 

(Reference 3)

liquefaction The transformation of saturated, loosely packed, coarse-grained soils 

from a solid to a liquid state. The soil grains 

temporarily lose contact with each other, and 

the particle weight is transferred to the pore 

water. (Reference 4)

landslide inventory maps Inventories 

identify areas that appear to have failed by 

landslide processes, including debris flows 

and cut-and-fill failures. (Reference 4)

landslide susceptibility map This map goes 

beyond an inventory map and depicts areas 

that have the potential for landsliding. These 

areas are determined by correlating some of 

the principal factors that contribute to landsliding, such as steep slopes, weak geologic 

units that lose strength when saturated, and 

poorly drained rock or soil, with the past 

distribution of landslides. (Reference 5)

landslide hazard map Hazard maps show 

the areal extent of threatening processes: 

where landslide processes have occurred in 

the past, where they occur now, and the likelihood in various areas that a landslide will 

occur in the future. (Reference 5)

landslide risk map Landslide hazards and 

the probability that they will occur, expressed 

in statistical recurrence rates; risk maps may 

show cost/benefit relationships, loss potential 

and other potential socioeconomic effects on 

an area and (or) community. 

lithology The physical character of a rock, 

generally as determined at the microscopic 

level, or with the aid of a low-power magnifier; the microscopic study and description of 

rocks. (Reference 3)

loess A widespread, homogenous, commonly nonstratified, porous, friable, slightly 

coherent, usually highly calcareous, finegrained blanket deposit (generally less than 

30 m thick) consisting predominantly of silt, 

with subordinate grain sizes ranging from 

amounts of clay to fine sand. (Reference 3)

mitigation Activities that reduce or 

eliminate the probability of occurrence of a 

disaster and (or) activities that dissipate or 

lessen the effects of emergencies or disasters 

when they actually occur. (Reference 5)

mudslide An imprecise but popular term 

coined in California, USA, frequently used 

by the general public and the news media 

to describe a wide scope of events, ranging 

from debris-laden floods to landslides. Not 

technically correct. Please see “mudflow,” 

next Glossary entry. (Reference 5)

mudflow A general term for a mass-movement landform and process characterized by 

a flowing mass of predominately fine-grained 

earth material possessing a high degree of 

fluidity during movement. The water content 

may range up to 60 percent. (Reference 3)

perched ground water Unconfined ground 

water separated from an underlying main 

body of ground water by an unsaturated 

zone. (Reference 3)

piezometer An instrument for measuring 

pressure head in a conduit, tank, or soil—it is 

a small diameter water well used to measure the hydraulic head of ground water in 

aquifers. (Reference 3)

pore-water pressure A measure of the 

pressure produced by the head of water in 

a saturated soil and transferred to the base 

of the soil through the pore water. This is 

quantifiable in the field by the measurement of free water-surface level in the soil 

or by direct measurement of the pressure by 

means of piezometers. Pore-water pressure 

is a key factor in failure of a steep slope 

soil and operates primarily by reducing the 

weight component of soil shear strength. 

(Reference 2)

pore water, or interstitial water Subsurface 

water in an interstice, or pore. (Reference 3)

quick clay A clay that loses nearly all its 

shear strength after being disturbed; a clay 

that shows no appreciable gain in strength 

after remolding. (Reference 3)

reconnaissance geology/mapping A 

general, exploratory examination or survey 

of the main features of a region, usually 

preliminary to a more detailed survey. It 

may be made in the field or office, depending on the extent of information available. 

(Reference 2)

relief The difference in elevation between 

the high and low points of a land surface. 

(Reference 3)

risk The probability of occurrence or 

expected degree of loss, as a result of exposure to a hazard. (Reference 4)

rock mechanics The theoretical and 

applied science of the mechanical behavior 

of rocks, representing a “branch of mechanics concerned with the response of rock to 

the force fields of its physical environment.” 

(Reference 3)

sag pond A small body of water occupying 

an enclosed depression or sag formed where 

active or recent fault or landslide movement 

has impounded drainage. (Reference 3)

seepage Concentrated subsurface drainage indicated by springs, sag ponds, or moist 

areas on open slopes, and seepage sites along 

road cuts. The locations of these areas of concentrated subsurface flow should be noted on 

maps and profiles as potential sites of active, 

unstable ground. (Reference 2)

sea cliff retreat A cliff formed by wave 

action, causing the coastal cliff to erode and 

recede toward land. (Reference 3)

shear A deformation resulting from stresses 

that cause contiguous parts of a body to slide 

relative to each other in a direction parallel to 

their plane of contact. (Reference 3)

slurry A highly fluid mixture of water and 

finely divided material; for example, pulverized coal and water for movement by pipeline 

or of cement and water for use in grouting. 

(Reference 3)

soil mechanics The application of the 

principles of mechanics and hydraulics to 

engineering problems dealing with the behavior and nature of soils, sediments, and other 

unconsolidated accumulations; the study of 

the physical properties and utilization of soils, 

especially in relation to highway and foundation engineering. (Reference 3)

strainmeter A seismometer that is designed 

to detect deformation of the ground by measuring relative displacement of two points. 

(Reference 3)

stress In a solid, the force per unit area, 

acting on any surface within it, and variously 

expressed as pounds or tons per square inch, 

or dynes or kilograms per square centimeter; 

also, by extension, the external pressure that 

creates the internal force. (Reference 3)

sturzstroms (German language term for “fall 

stream”) A huge mass of rapidly moving rock debris and dust, derived from the 

collapse of a cliff or mountainside, flowing 

down steep slopes and across low ground, 

often for several kilometers at speeds of 

more than 100 km/hr. Sturzstroms are the 

most catastrophic of all forms of mass 

movement. (Reference 3)

subaqueous (submarine) landslide Conditions and processes, or features and deposits, 

that exist or are situated in or under water. 

Generally used to specify a process that 

occurs either on land (the slide extending 

underwater) or that begins under water; for 

example, slumping, gravitational slides. 

(Reference 3)
62 The Landslide Handbook—A Guide to Understanding Landslides

subsidence Sinking or downward settling 

of the Earth’s surface, not restricted in rate, 

magnitude, or area involved. Subsidence may 

be caused by natural geologic processes, 

such as solution, compaction, or withdrawal 

of fluid lava from beneath a solid crust or 

by human activity such as subsurface mining or the pumping of oil or ground water. 

(Reference 3)

surficial geology Geology of surficial 

deposits, including soils; the term is sometimes applied to the study of bedrock at or 

near the Earth’s surface. (Reference 3)

swelling soils These are soils or soft 

bedrock that increases in volume as they get 

wet and shrink as they dry out. They are also 

commonly known as bentonite, expansive, or 

montmorillinitic soils. (Reference 1)

tensile stress A normal stress that tends to 

pull apart the material on the opposite sides 

of the plane on which it acts. (Reference 3)

weathering The destructive process by 

which earth and rock materials exposed to 

the atmosphere undergo physical disintegration and chemical decomposition resulting 

in changes in color, texture, composition, or 

form. Processes may be physical, chemical, 

or biological. (Reference 4)

References for Glossary:

1. Creath, W.B., 1996, Homebuyers’ guide 

to geologic hazards: An AIPG issues 

and answers publication: Department of 

Natural Resources, Colorado Geological 

Survey, Miscellaneous Publication (MI) 

no. 58, 30 p. 

2. Chatwin, S.C., Howes, D.E., Schwab, 

J.W., and Swanston, D.N., 1994, A 

guide for management of landslideprone terrain in the Pacific Northwest, 

2d edition: Research Branch, Ministry 

of Forests, Province of British Columbia, Victoria, British Columbia, Crown 

Publications.

3. Jackson, Julia A., ed., 1997, Glossary of 

geology, fourth edition: Prepared by the 

American Geological Institute, Alexandria, Virginia, USA, Doubleday.

4. Jochim, Candice L., Rogers, William P., 

Truby, John O., Wold, Robert L., Jr., 

Weber, George, and Brown, Sally P., 

1988, Colorado landslide hazard 

mitigation plan: Department of Natural 

Resources, Colorado Geological Survey, 

Bulletin 48.

5. Shelton, David C., and Prouty, Dick, 

1979, Nature’s building codes, geology 

and construction in Colorado: Department of Natural Resources, Colorado 

Geological Survey Special Publication 

No. 48, 72 p.

6. Turner, A. Keith, and Schuster, 

Robert L., 1996, Landslides— 

Investigation and mitigation: National 

Research Council, Transportation 

Research Board, Special Report 247, 

National Academy Press, Washington, 

D.C., 673 p. 

weathering, differential When weathering across a rock face or exposure occurs at 

different rates mainly due to variations in the 

composition and resistance of the rock. This 

results in an uneven surface with the more 

resistant material protruding. (Reference 4)

weathering, mechanical The physical 

processes by which rocks exposed to the 

weather change in character, decay, and 

crumble into soil. Processes include temperature change (expansion and shrinkage), 

freeze-thaw cycle, and the burrowing activity 

of animals. (Reference 4)

zonation A term used generally, even 

vaguely, for a region of latitudinal character more or less set off from surrounding 

regions by some distinctive characteristic, for 

instance, the Earth’s torrid zone, two temperate zones, and two frigid zones. For hazards, 

zones are geographic regions or designations 

that are differentiated through a variety of 

different criteria, for example, residential 

zones, zones of low hazard, zones of high 

hazard. (Reference 3)
Part 2. Parts of a Landslide—Description of Features/Glossary 63

accumulation The volume of the displaced 

material, which lies above the original 

ground surface. 

crown The practically undisplaced material 

still in place and adjacent to the highest parts 

of the main scarp.

depletion The volume bounded by the 

main scarp, the depleted mass and the original ground surface.

depleted mass The volume of the displaced 

material, which overlies the rupture surface 

but underlies the original ground surface.

displaced material Material displaced 

from its original position on the slope by 

movement in the landslide. It forms both the 

depleted mass and the accumulation.

flank The undisplaced material adjacent 

to the sides of the rupture surface. Compass 

directions are preferable in describing the 

flanks, but if left and right are used, they 

refer to the flanks as viewed from the crown.

foot The portion of the landslide that has 

moved beyond the toe of the surface of rupture and overlies the original ground surface.

head The upper parts of the landslide along 

the contact between the displaced material 

and the main scarp. 

Part 2. Parts of a Landslide—Description of Features/Glossary

top The highest point of contact between 

the displaced material and the main scarp.

toe of surface of rupture The intersection 

(usually buried) between the lower part of 

the surface of rupture of a landslide and the 

original ground surface.

zone of accumulation The area of the landslide within which the displaced material lies 

above the original ground surface. 

zone of depletion The area of the landslide 

within which the displaced material lies 

below the original ground surface.

Sources of information on 

nomenclature: 

1. Cruden, D.M., 1993, The multilingual 

landslide glossary: Richmond, British 

Columbia, Bitech Publishers, for the 

IUGS Working Party on World 

Landslide Inventory in 1993.

2. Varnes, D.J., 1978, Slope movement 

types and processes, in Schuster, R.L., 

and Krizek, R. J., eds., Landslides—

Analysis and control: Transportation 

Research Board Special Report 176, 

National Research Council, Washington, 

D.C., p. 11–23.

main body The part of the displaced material of the landslide that overlies the surface 

of rupture between the main scarp and the 

toe of the surface of rupture.

main scarp A steep surface on the undisturbed ground at the upper edge of the landslide, caused by movement of the displaced 

material away from the undisturbed ground. 

It is the visible part of the surface of rupture.

minor scarp A steep surface on the displaced material of the landslide produced by 

differential movements within the displaced 

material.

original ground surface The surface of the 

slope that existed before the landslide took 

place. 

surface of separation The part of the 

original ground surface overlain by the foot 

of the landslide. 

surface of rupture The surface that forms 

(or which has formed) the lower boundary 

of the displaced material below the original 

ground surface.

tip The point of the toe farthest from the 

top of the landslide.

toe The lower, usually curved margin of 

the displaced material of a landslide, it is the 

most distant from the main scarp.

Transverse cracks

Minor scarp

Head

Main scarp

Crown cracks

Crown

Surface of rupture

Main body

Toe of surface of rupture Foot

Surface of separation

Toe

Radial

cracks

Transverse ridges

Right flank

Original ground 

surface

Figure A1. Parts of a landslide. (Modified from Varnes, 1978, reference 43).
64 The Landslide Handbook—A Guide to Understanding Landslides

Natural Causes

Geological causes

•	 Weak materials, such as some 

volcanic slopes or unconsolidated 

marine sediments, for example

•	 Susceptible materials

•	 Weathered materials

•	 Sheared materials

•	 Jointed or fissured materials

•	 Adversely oriented mass discontinuity (bedding, schistosity, and so 

forth)

•	 Adversely oriented structural 

discontinuity (fault, unconformity, 

contact, and so forth)

•	 Contrast in permeability

•	 Contrast in stiffness (stiff, dense 

material over plastic materials)

For further reading: 

References 9, 3, and 45

Part 3. Landslide Causes and Triggering Mechanisms

Physical Causes—Triggers

•	 Intense rainfall

•	 Rapid snowmelt

•	 Prolonged intense precipitation

•	 Rapid drawdown (of floods and 

tides) or filling

•	 Earthquake

•	 Volcanic eruption

•	 Thawing

•	 Freeze-and-thaw weathering

•	 Shrink-and-swell weathering

•	 Flooding

Human Causes

•	 Excavation of slope or its toe

•	 Use of unstable earth fills, for construction

•	 Loading of slope or its crest, such as placing earth fill at the top of a slope

•	 Drawdown and filling (of reservoirs)

•	 Deforestation—cutting down trees/logging and (or) clearing land for crops; 

unstable logging roads

•	 Irrigation and (or) lawn watering

•	 Mining/mine waste containment

•	 Artificial vibration such as pile driving, explosions, or other strong ground 

vibrations

•	 Water leakage from utilities, such as water or sewer lines

•	 Diversion (planned or unplanned) of a river current or longshore current by 

construction of piers, dikes, weirs, and so forth

Morphological causes

•	 Tectonic or volcanic uplift

•	 Glacial rebound

•	 Glacial meltwater outburst

•	 Fluvial erosion of slope toe

•	 Wave erosion of slope toe

•	 Glacial erosion of slope toe

•	 Erosion of lateral margins

•	 Subterranean erosion (solution, 

piping)

•	 Deposition loading slope or its crest

•	 Vegetation removal (by forest fire, 

drought)
Appendix B.

Introduction to Landslide Evaluation Tools—

Mapping, Remote Sensing, and Monitoring 

of Landslides
66 The Landslide Handbook—A Guide to Understanding Landslides

Part 1. Mapping

Maps are a useful and convenient tool for presenting information on landslide 

hazards. They can present many kinds and combinations of information at different 

levels of detail. Hazard maps used in conjunction with land-use maps are a valuable planning tool. Commonly, there is a three-stage approach to landslide hazard 

mapping. The first stage is regional or reconnaissance mapping, which synthesizes 

available data and identifies general problem areas. This regional scale (sometimes 

called “small-scale”) mapping is usually performed by a Provincial, State, or Federal 

geological survey. The next stage is community-level mapping, a more detailed 

surface and subsurface mapping program in complex problem areas. Finally, detailed 

site-specific large-scale maps are prepared. If resources are limited, it may be more 

prudent to bypass regional mapping and concentrate on a few known areas of concern. We discuss three types of general mapping; (1) Regional, (2) Community level, 

and (3) Site specific.

Regional mapping

Regional or reconnaissance mapping supplies basic data for regional planning 

by providing baseline information for conducting more detailed studies at the community and site-specific levels and for setting priorities for future mapping.

Such maps are usually simple inventory or susceptibility maps and are directed 

primarily toward the identification and delineation of regional landslide problem 

areas and the conditions under which they occur. They concentrate on those geologic units or environments in which additional movements are most likely. The 

geographical extent of regional maps can vary from a map of a State or Province to 

a national map, which delineates an entire country. Such mapping relies heavily on 

photogeology (the geologic interpretation of aerial photography), reconnaissance 

field mapping, and the collection and synthesis of all available pertinent geologic 

data. Map scales at this level are typically at scales ranging from 1:10,000 down to 

1:4,000,000 or even smaller.

Community-level mapping

This type of mapping identifies both the three-dimensional potential of land

sliding and considers its causes. Guidance concerning land use, zoning, and building, and recommendations for future site-specific investigations also are made at this 

stage. Investigations should include subsurface exploratory work in order to produce 

a map with cross sections. Map scales at this level typically vary from 1:1,000 to 

1:10,000.

Site-specific mapping 

Site-specific mapping is concerned with the identification, analysis, and solution of actual site-specific problems, often presented in the size of a residential lot. It 

is usually undertaken by private consultants for landowners who propose site development and typically involves a detailed drilling program with downhole logging, 

sampling, and laboratory analysis in order to procure the necessary information 

for design and construction. Map scales vary but usually are about 1:600 or 25 mm 

(1 inch) equal to 16 m (50 feet).
Part 1. Mapping 67

Three Important Criteria for Landslide Maps 

The three types of landslide maps most useful to planners and the general public 

are (1) landslide inventories, (2) landslide susceptibility maps, and (3) landslide 

hazard maps. 

Landslide inventory maps

Inventories denote areas that are identified as having failed by landslide processes 

(fig. B1). The level of detail of these maps ranges from simple reconnaissance inventories that only delineate broad areas where landsliding appears to have occurred to 

complex inventories that depict and classify each landslide and show scarps, zones of 

depletion and accumulation, active and inactive slides, geological age, rate of movement, and (or) other pertinent data on depth and kind of materials involved in sliding.

 Simple inventories give an overview of the areal extent of landslide occurrence and highlight areas where more detailed studies should be conducted. Detailed 

inventories provide a better understanding of the different landslide processes operating in an area and can be used to regulate or prevent development in landslide-prone 

areas and to aid the design of remedial measures. They also provide a good basis for 

the preparation of derivative maps, such as those indicating slope stability, for rating 

landslide hazard, and to identify land use. One way is to use aerial photography with 

selective field checking to detect landslide areas and then to present the information 

in map form using a coded format. The maps show any or all of the following: state 

of activity, certainty of identification, dominant types of slope movement, estimated 

thickness of landslide material, type of material, and dates or periods of activity.

In the United States, regional maps are most often prepared at a scale of 

1:24,000 (1:50,000 in Canada) because high-quality U.S. Geological Survey 

topographic base maps at this scale are widely available and aerial photographs are 

commonly of a comparable scale. Other scales commonly used in the United States, 

for example, include 1:50,000 (county series), 1:100,000 (30 × 60 minute series), 

and 1:250,000 (1 × 2 degree series).

Study area

EXPLANATION

Drainages

Landslides

149−251

252−354

355−456

457−559

560−661

662−764

765−866

867−969

970−1,071

1,072−1,174

Elevation, in meters

Figure B1. Example of a landslide inventory map showing the locations of past 

landslides and including topographical information consisting of elevation (measured in 

meters) and drainage courses (map from U.S. Geological Survey).
68 The Landslide Handbook—A Guide to Understanding Landslides

Figure B2. An example of a landslide susceptibility map. This map shows an area in 

Canada, the Mackenzie River Valley, Northwest Territories. Graphic by Réjean Couture, 

Geological Survey of Canada. 

Landslide susceptibility maps

A landslide susceptibility map goes beyond an inventory map and depicts areas 

that have the potential for landsliding (fig. B2). These areas are determined by correlating some of the principal factors that contribute to landsliding (such as steep 

slopes, weak geologic units that lose strength when saturated or disturbed, and poorly 

drained rock or soil) with the past distribution of landslides. These maps indicate only 

the relative stability of slopes; they do not make absolute predictions.

Landslide susceptibility maps can be considered derivatives of landslide inventory maps because an inventory is essential for preparing a susceptibility map. For 

example, overlaying a geologic map with an inventory map that shows existing landslides can identify specific landslide-prone geologic units. This information can then 

be extrapolated to predict other areas of potential landsliding. More complex maps 

may include additional information such as slope angle and drainage.
Part 1. Mapping 69

For mapping references and further 

reading: 4, 12, 18, 19, 21, 25, 29, 33, 

34, 35, 41 and 46. 

Relative hazard

High (more than 75 shallow landslides/km2)

Medium (20−75 shallow landslides/km2

)

Low (fewer than 20 shallow landslides/km2

)

Failure locations from landslide database

Puget Sound

Figure B3. Portion of shallow landslide hazard map showing part of the Magnolia area 

of the city of Seattle, Washington, USA. (km2 is notation for square kilometers.)

Landslide hazard maps 

Hazard maps show the areal extent of threatening processes (fig. B3): where 

landslide processes have occurred in the past, recent occurrences, and most important, 

the likelihood in various areas that a landslide will occur in the future. For a given 

area, hazard maps contain detailed information on the types of landslides, extent of 

slope subject to failure, and probable maximum extent of ground movement. These 

maps can be used to predict the relative degree of hazard in a landslide area. Areas 

may be ranked in a hierarchy such as low, moderate, and high hazard areas.
70 The Landslide Handbook—A Guide to Understanding Landslides

Part 2. Remote Sensing and Other Tools that Show 

Features of Landslide Activity

Maps and other forms of information are sometimes overlaid on each other 

using a GIS (Geographical Information System) so that the different types of information can be viewed all at once. In the absence of a GIS computerized system, 

transparencies can be made of each map and then can be overlaid together. It is 

important that the maps and data be at the same scale. The following list describes 

many types of information that might be useful in constructing layers for GIS analysis of landslide potential.

•	 Topographic Map Indicates slope gradient, terrain configuration, drainage 

pattern.

•	 Terrain Map Identifies material, depth, geological processes, terrain configuration, surface and subsurface drainage, slope gradient (also called surficial 

geology or Quaternary geology maps).

•	 Bedrock Map Identifies bedrock type, surface and subsurface structure, 

surficial cover (overburden), and age of rock over a topographic map base.

•	 Engineering Soil Map Identifies surficial material type, drainage, limited 

engineering characteristics, soils characteristics, vegetation cover.

•	 Forest Cover Map Identifies surface vegetation, topographic features, surface drainage pattern, and in some cases, soil drainage character.

•	 Research Studies May provide information on all of the above, plus quantitative data on controlling factors and possibly local stability risk assessment.

•	 Aerial Photography Remote Sensing (Examples shown in figs. B4 through 

B7.) Identification can be made of vegetation cover, topography, drainage 

pattern, soil drainage character, bedrock geology, surficial geology, landslide 

type, and relationship to other factors. Careful study of a given area of terrain 

with the aid of oblique aerial photographs and vertical stereo pairs can yield 

significant information on landslide type and frequency and the effects of 

management practices. A review of recent and past aerial photographs of the 

area should be undertaken whenever possible, as older slides may not be evident on more recent photographs. Features discernible on aerial photographs 

can help users identify landslide type and develop a reasonable assessment of 

overburden characteristics. These, in turn, provide a means for estimating the 

landslide hazard at a site.

•	InSAR Imaging InSAR is an acronym for Interferometric Synthetic Aperture Radar. Both InSAR and LIDAR (description follows) use active sensors 

emitting a pulse of energy (from a satellite) and recording its return, from 

the ground, at the sensor. Most InSAR equipment is able to penetrate fog and 

rain and can be used in areas difficult to access by foot. By bouncing signals 

from a radar satellite off the ground, digital elevation model (DEM) maps 

can be produced that will show the ground terrain. Two images of the same 

place are taken at different times then merged, forming a map called an interferogram. The merging of the two images shows the ground displacement (if 

any) that would indicate any movement that has occurred between the time 

the two images were taken. In this way, one can determine if a hillside, for 

example, has moved.

 Ordinary radar on a typical Earth-orbiting satellite has a very poor 

ground resolution of about 3 to 4 miles because of the restricted size of the 

antenna on the satellite. Synthetic Aperture Radar (SAR) takes advantage of 

the motion of the spacecraft along its orbital track to mathematically reconstruct (synthesize) an operationally larger antenna and yield high-spatialresolution imaging capability on the order of hundreds of feet.
Part 2. Remote Sensing and Other Tools that Show Features of Landslide Activity 71

Figure B4. An example of an aerial photograph of the La Conchita landslide in California, 

USA, taken in 2005. Blue line delineates an older landslide, yellow a more recent 

landslide. (Photograph courtesy of AirPhoto USA and County of Ventura, California, and 

Randy Jibson, U.S. Geological Survey.)

Figure B5. Schematic showing satellite passes over an area of the Earth’s surface 

(graphic modified from Reference 41.) 
72 The Landslide Handbook—A Guide to Understanding Landslides

Figure B6. Interferogram from InSAR imaging process showing the area of uplift 

(1997–2001) at the Three Sisters volcanoes (red triangles) in the Cascade Range in 

central Oregon, USA (circles show locations of earthquakes). (Photograph modified from 

Reference 41.)
Part 2. Remote Sensing and Other Tools that Show Features of Landslide Activity 73

•	 LiDAR Imaging LiDAR is an acronym for Light Detection and Ranging, 

also known as ALSM or Airborne Laser Swath Mapping. Using a narrow 

laser beam to probe through dense ground cover, such as trees, LiDAR can 

produce accurate terrain maps even where forest cover gets in the way of 

traditional photography. The technique produces a very accurate Digital 

Elevation Model map (DEM) (fig. B7). Accurate bare-earth DEMs can be 

produced when the imagery is acquired during the leaf-off season in areas 

covered by deciduous forests.

 Essential elements of a LiDAR mapping system are a scanning laser 

rangefinder mounted in an aircraft, differential Global Positioning System 

(GPS) to locate the aircraft, and an internal measurement unit (IMU) to 

measure aircraft orientation. LiDAR is a useful topographic mapping tool for 

three reasons. First is accuracy, the second is productivity; measurements are 

made at rates of 10,000 to 80,000 laser pulses per second. Third, LiDAR is 

monoscopic and provides its own illumination. These characteristics overcome the major liabilities of photogrammetry in forested terrain. The maps 

produced by LiDAR are very clear and detailed and in many cases reveal 

evidence of past landslides that are virtually invisible by other means due to 

heavy vegetation cover. LiDAR is expensive and highly technical and is used 

mainly by government agencies, universities, and some private entities. One 

drawback is that many LiDAR systems use a near-infrared laser that does not 

penetrate fog or rain.

For remote sensing and other map 

references and further reading: 4, 12, 14, 

15, 18, 19, 21, 24, 35, 39, and 41

Figure B7. An oblique LiDAR image of La Conchita, California, USA, landslide, taken 

in 2005. Outline of 1995 (blue) and 2005 (yellow) landslides are shown; arrows show 

examples of other landslides in the area; red line outlines main scarp of an ancient 

landslide that involved the entire bluff. (Photograph by Airborne 1, El Segundo, California, 

USA, and Randy Jibson, U.S. Geological Survey.)
74 The Landslide Handbook—A Guide to Understanding Landslides

For more information: References 4, 

21, 25, 28, 35, 38, 39, and 46

Part 3. Real-Time Monitoring of Landslides and 

Landslide Instrumentation 

The immediate detection of landslide activity that is provided by real-time 

systems can be crucial in saving human lives and protecting property. Traditional 

field observations, even if taken regularly, cannot detect changes at the moment 

they occur. Moreover, active landslides can be hazardous to work on, and large 

movements often occur during storms when visibility is poor. The continuous data 

provided by remote real-time monitoring permits a better understanding of dynamic 

landslide behavior that, in turn, enables engineers to create more effective designs to 

prevent or halt landslides. Landslide monitoring can be expensive, and most monitoring systems require installation by experts. The advantage is that systems that 

detect landslide movement can be coordinated with warning systems.

Figure B8. Measuring landslide 

movement using an extensometer, an 

instrument that can detect movement of 

the ground surface between stable ground 

and sliding ground. (Photograph by Richard 

LaHusen, U.S. Geological Survey.)

Figure B10. Example of a network for measurement and transmission of real-time 

landslide data. (Schematic from U.S. Geological Survey.)

Monitoring Instruments

Rain

gage

Battery

Direction of movement

Slope

movement

sensor

Shallow 

pore pressure

Deep 

pore pressure

Geophone

Figure B9. Testing a solar-powered 

radiotelemetry system for remote 

transmission of real-time landslide data. 

(Photograph by Mark Reid, U.S. Geological 

Survey.)
Appendix C.

Introduction to Landslide Stabilization and Mitigation

Note: Much of the material that follows on slope stabilization methods has been reproduced directly from “A Guide for Management of 

Landslide-Prone Terrain in the Pacific Northwest,” published by the 

Research Branch of the Ministry of Forests, British Columbia, Canada. 

However, this volume contains a much more comprehensive overview 

of mitigation, and it is highly recommended by the authors for those 

desiring more detailed information on mitigation measures. Please see 

reference 11, Chatwin and others, for full publication citation.
76 The Landslide Handbook—A Guide to Understanding Landslides

Part 1. Earth Slope Stabilization/Mitigation

Some of the stabilization techniques that are currently available in North 

America are illustrated in this discussion. We highlight simple methods that can be 

used safely in the absence of detailed soil or bedrock analysis or in low-risk situations. Some stabilization methods are very expensive and require significant time to 

implement. This is an overview of stabilization methods; many other methods are 

in use around the world. Professional advice is essential before, during, and after 

implementation (where possible), as is further literature consultation.

The stability of any slope will be improved if certain actions are carried out. 

To be effective, first one must identify the most important controlling process that 

is affecting the stability of the slope; second, one must determine the appropriate 

technique to be sufficiently applied to reduce the influence of that process. The mitigative prescription must be designed to fit the condition of the specific slope under 

study. For example, installation of drainage pipes into a slope that has very little 

ground water is pointless. Slope stabilization efforts take place during construction 

or when stability problems develop unexpectedly following construction. Most slope 

engineering techniques require a detailed analysis of soil properties and a sound 

knowledge of the underlying soil and rock mechanics.

In any high-risk situation, where a landslide may endanger lives or 

adversely affect property, a professional landslide expert such as a 

geotechnical or civil engineer should always be consulted before any 

stabilizing work is undertaken.

The following sections provide a general introduction to techniques that can be 

used to increase slope stability.

Excavation 

 Figures C1, C2, and C3 provide a cross-sectional view, in schematic form, of 

general principles for slope excavation, showing the effects and consequences of 

where on a slope the excavation takes place. These graphics are general in nature, 

and a geotechnical engineer or other professional should always be a consulted if 

possible.

Removal of soil from the head of a slide

This method reduces the driving force and thereby improves stability. This 

method is suitable only for cuts into deep soil where rotational landslides (see “Basic 

Landslide Types” in Section I) may occur. It is ineffective on translational failures on 

long, uniform or planar slopes or on flow-type landslides. 

Reducing the height of the slope

Reducing the height of a cut bank reduces the driving force on the failure plane 

by reducing the weight of the soil mass and commonly involves the creation of an 

access road above the main road and the forming of a lower slope by excavation. 

Also, it is possible to excavate deeply and lower the main road surface if the rightof-way crosses the upper part of a landslide. This method is only moderately efficient in increasing stability, and a complete solution may involve additional modification of the land. According to Chatwin (Reference 11), it usually increases the 

Factor of Safety by only 10 or 15 percent. (“Factor of Safety” in its simple definition 

is the ratio of the maximum strength of a piece of material or a part to the probable 

maximum load to be applied to it.)
Part 1. Earth Slope Stabilization/Mitigation 77

Ground surface 

Slip surface 

Ground surface

Slip surface 

Removing material from head reduces

driving forces and increases stability

Removing material from toe reduces

resisting forces and reduces stability

Ground surface 

Slip surface 

Ground surface

Slip surface 

Load applied at head increases 

driving forces and decreases stability

Load applied at toe increases 

resisting forces and increases stability

Ground surface 

Slip surface 

Ground surface

Slip surface 

Falling water table/pore pressure increases 

resisting forces and increases stability

Rising water table/pore pressure reduces

resisting forces and reduces stability

Figure C3. Illustration of the importance of water in the stability of 

a slope. (Graphic by Rex Baum, U.S. Geological Survey.)

Figure C1. Illustration of the differences in stability resulting in 

excavation at the head and toe surfaces of a slope. (Graphic by Rex 

Baum, U.S. Geological Survey.)

Figure C2. Illustration of the difference in stability of loading either 

the head or the toe of a slope. (Graphic by Rex Baum, U.S. Geological 

Survey.)
78 The Landslide Handbook—A Guide to Understanding Landslides

Wood fiber

Till

Gravel

Capping

Underdrain

Backfilling with lightweight material

A technique related to height reduction is to excavate the upper soil and replace 

it with a lightweight backfill material such as woodchips or logging slash. Then, 

covered with a thin layer of coarse aggregate, the backfilled material can form a 

foundation for limited-use traffic (fig. C4).

Figure C4. Schematic and photograph of a lightweight backfill. There has been an 

increased growth in the use of recycled tire shreds in civil engineering applications. 

Highway applications include using shredded tires as lightweight fill over weak soils 

in bridge embankments and retaining wall reinforcements or, in very cold climates, as 

insulation of the road base to resist frost heaves and as a high-permeability medium 

for edge drains. (Graphic from reference 11, photograph from U.S. Department of 

Transportation, Federal Highway Administration.)
Part 1. Earth Slope Stabilization/Mitigation 79

Benches

Benches are a series of “steps” cut into a deep soil or rock face for the purpose 

of reducing the driving forces. They are mainly effective in reducing the incidence 

of shallow failures but generally are not very efficient in improving the overall slope 

stability for which other methods are recommended. Benches are useful in providing 

protection structures beneath rockfall-prone cliffs, for controlling surface drainage, 

or for providing a work area for installing drainpipe or other structures.

Please see figure C12 for a photo of benches cut into a slope.

Flattening or reducing slope angle, or other slope modification

This reduces the weight of material and reduces the possibility of stream/river 

undercutting or construction loading.

When not to excavate a slide mass

In some situations, removing the entire slide mass is an effective and economic 

solution. Generally, however, it is only practical on small slumps or small rotational 

failures. Large-scale excavation of larger landslide areas is usually not recommended 

for several reasons:

•	 Excavation is not always effective—for large planar failures, excavation may 

not cause movement to stop and may allow the landslide to expand.

•	 Excavation may trigger a larger landslide by removing the support provided 

by the toe of the landslide.

•	 Excavation may actually destabilize the ground farther upslope by undercutting, which weakens the slope.

•	 In deeper soils, especially soft clays, where there are two potential failure 

surfaces, one deep and one shallow, excavating down to the first failure surface might trigger a sudden slippage on the deeper failure surface. A stability 

analysis using soil strength data is advised and most always necessary for 

any major excavation project in deep clay soils.
80 The Landslide Handbook—A Guide to Understanding Landslides

Strengthening Slopes

Plastic mesh reinforcement

There are numerous synthetic soil reinforcement materials on the market, and 

one example is a reinforcement material of plastic polymer stretched to form a 

lightweight, high-tensile-strength grid. The grid acts similarly to reinforcing mesh in 

concrete, adding strength to the shear strength of the soil.

These types of materials have been used to reduce the amount of ballast needed 

over soft ground by increasing the bearing capacity of the subsoil. These types of 

grids also have a number of possible applications in slope stabilization, including soil strength reinforcement, soil drainage improvement, and retaining-wall 

construction.

Rock-fill buttresses 

A simple method to increase slope stability is to increase the weight of the 

material at the toe, which creates a counterforce that resists failure (fig. C5). A berm 

or buttress of earthfill can be easily dumped onto the toe of a slope. Broken rock 

or riprap instead of soil is preferable, however, because it has a greater frictional 

resistance to shear forces and is also free draining, which reduces the problem of 

impeding ground-water flow.
Part 1. Earth Slope Stabilization/Mitigation 81

Figure C5. Schematic and photograph of a rockfill buttress in Canada. (Graphic is from 

Reference 11.)
82 The Landslide Handbook—A Guide to Understanding Landslides

Stream channel linings

Channel linings are another way of stabilizing a stream or creek channel and 

the sides of the stream or creek. The lining is usually slush grouted with high-quality 

concrete, preferably reinforced by steel fiber mat to resist abrasion. Protruding boulders are set in the concrete to dissipate the energy of waterflow.

Channel linings can reduce the incidence and volume of debris flows (fig. C6). 

They are also effective in maintaining channel alignment upstream from a bridge 

and for protecting the abutments. Channel linings are most effective if applied over 

the entire reach of an unstable channel. Linings are usually much less costly than, 

for example, check dams, especially if a long reach is to be stabilized. Check dams 

are preferable, however, if the banks are very unstable because a dam can be keyed 

into the bank, providing toe support and thereby enhancing stability.

Figure C6. Example of creek channel lining using rock, Dickson Creek, Montana, USA. 

(Photograph and graphic, U.S. Department of Agriculture.)
Part 1. Earth Slope Stabilization/Mitigation 83

Check Dams

Check dams are small, sediment-storage dams built in the channels of steep 

gullies to stabilize the channel bed. They are commonly used in Europe and Japan 

to control channelized debris-flow frequency and volume. A less common use of 

check dams is to control raveling and shallow slides in the source area of debris 

slides. Check dams are expensive to construct and therefore are usually built only 

where important installations or wildlife habitat, such as a camp or unique spawning 

area, lie downslope. Channelized debris flows are associated with channel gradients 

over 25 degrees and obtain most of their volume by scouring the channel bed. Check 

dams serve three purposes when installed in the channels (following information 

quoted from Reference 11):

1. To mitigate the incidence of failure by reducing the channel gradient in 

the upper channel.

2. To reduce the volume of channel-stored material by preventing down 

cutting of the channel with subsequent gully sidewall destabilization and 

by providing toe support to the gully slopes.

3. To store debris-flow sediment, when installed in the lower part of the 

channel. 

When installed on debris slides, the dams store raveled material, which eventually creates small terraces on the slide, reducing the surface slope. Check dams can 

be constructed of reinforced concrete or log cribs (figs. C7 and C8). Concrete mortared rock dams do not usually exceed 8 m in height, whereas log crib dams must 

not exceed 2 m (6 feet). The spacing of dams depends on channel gradient and dam 

height. For example, a 2-m-(6 foot) high dam in a 20-degree channel with 10-degree 

sloping channel infill will be spaced every 12 m (36 feet). Lateral stream erosion and 

scour by spillway water are the main drawbacks.

To prevent check dam failure

During construction, the concrete wingwalls and log crib ends must be tied 

securely into the canyon wall and streambed to withstand backfill pressures and 

lateral scour. Wingwalls should slope at about 70 percent and extend a minimum 

of 1–2 meters (3 to 6 feet) into the banks. The foundation of the dam should have 

a minimum width of one-third the total height of the dam and be deeper than any 

scour holes likely to develop.

Backfilling the dam, rather than allowing it to fill naturally, reduces the 

dynamic loading on the structure and results in a more stable design. The slope 

of the backfill should be less than one-half the channel gradient. Dams that have 

been back-filled usually will survive a debris flow. The backfill material will not be 

scoured during or after a torrent.
84 The Landslide Handbook—A Guide to Understanding Landslides

Figure C7. Schematic and photograph of a crib wall check dam, which is one kind of 

check dam. (Graphic from Reference 11, photograph taken in Trafoi, Italy, courtesy of 

“Erosion Control,” Forester Communications, Santa Barbara, California, USA.)

Original gradient

Spillway

Rip-rap apron

H

L
Part 1. Earth Slope Stabilization/Mitigation 85

Figure C8. Upstream view of concrete crib-type check dam with low-flow center 

section in southern California, USA. (Photograph by Los Angeles County Flood Control 

District.)
86 The Landslide Handbook—A Guide to Understanding Landslides

Drainage Techniques

Ground water probably is the most important single contributor to landslide 

initiation. Not surprisingly, therefore, adequate drainage of water is the most important element of a slope stabilization scheme, for both existing and potential landslides. Drainage is effective because it increases the stability of the soil and reduces 

the weight of the sliding mass. Drainage can be either surface or subsurface. Surface 

drainage measures require minimal design and costs and have substantial stability 

benefits. They are recommended on any potential or existing slide.

The two objectives of surface drainage are to prevent erosion of the face, reducing the potential for surface slumping, and to prevent infiltration of water into the 

soil, thereby reducing ground-water pressures. Subsurface drainage also is effective 

but can be relatively expensive. It is therefore essential that ground water be identified 

as a cause of the slide before subsurface methods are used. The various methods of 

drainage include the following:

Site leveling

Smoothing the topography of the slide surface can prevent surface water from 

ponding or connecting with the ground water. Any depressions on the slope that 

might retain standing water must be removed. Infilling and sealing large cracks in the 

soil surface by grading the soil mass are beneficial and prevent surface water from 

reaching the failure plane.

Ditches and drains

Surface drainage can be through either surface ditches or shallow subsurface 

drains (fig. C9). Surface drainage is especially important at the head of the slide, 

where a system of cutoff ditches that cross the headwall of the slide, and lateral drains 

to lead runoff around the edge of the slide are effective. Ditch gradient should be at 

least 2 percent, to ensure rapid flow away from the unstable area.

The simplest type of subsurface drain is the lateral trench constructed above an 

unstable slope. Drainage trenches are economical only for shallow soils overlying 

bedrock or hard impermeable till. The trenches should be excavated to the base of 

the shallow soil to intercept any ground-water flow along the failure plane. They are 

backfilled with coarse gravel to prevent sloughing of the ditch sidewalls. An improvement is to use drainpipe and then backfill the area with coarse gravel.
Part 1. Earth Slope Stabilization/Mitigation 87

Figure C9. Schematic and photograph of a drain trench. (Graphic from Reference 11, 

photograph by Department of Transport, Energy and Infrastructure, South Australia.)

Unstable slope

Gravel

backfilled

trench

Open ditch

1.5 m

Hard till
88 The Landslide Handbook—A Guide to Understanding Landslides

Drainpipes

Horizontal drainpipe is a widely used device for landslide prevention in highway 

construction (fig. C10). It is most effective when installed during initial excavation. 

Because of the long lag times to lower ground-water tables, the drains are effective 

only if the pipe is carefully installed, the failure surface is intersected, and the pipe 

actually drains the soil. As most slopes have varying soil and hydraulic and geometric 

conditions, drainage systems must be individually designed. After drilling has been 

carried out to the desired depth and the casing installed, the latter is cleared of soil, 

and sections of slotted PVC drainpipe are covered with filter cloth, then pushed into 

the casing and coupled together. The casing is then withdrawn and screen is installed 

over the end of the drain. Drain holes must be thoroughly cleaned of drill cuttings and 

mud. Uncleaned holes may be only 25 percent effective.

In clay soils, the full change in ground-water tables can take up to 5 years, with 

50 percent of the improvement taking place in the first year. Once water tables are 

lowered in clay soils, the change is fairly permanent; however, seasonal fluctuations can occur: rainfall will not alter the ground-water level in the slope provided 

the drains do not clog. In sandy soils, the ground-water table will lower within a few 

months but will also fluctuate with rainfall. 

Figure C10. Schematic of drainpipes. (Schematic from Reference 11.) Photograph of 

drainpipes in a landslide in California, USA, by Andrew Alden.

Tension crack

Horizontal

drain pipe

Failure

surface
Part 1. Earth Slope Stabilization/Mitigation 89

Straw wattles and straw bales

Straw wattles, also known as straw worms, bio-logs, straw noodles, or straw 

tubes, are manufactured cylinders of compressed, weed-free straw (wheat or rice), 

20 to 30 centimeters (8 to 12 inches) in diameter and 7 to 9 meters (20 to 25 feet) 

long (fig. C11). They are encased in jute, nylon, or other photodegradable materials and have an average weight of 16 kilograms (35 pounds). They are installed in a 

shallow trench forming a continuous barrier along the contour (across the slope) to 

intercept water running down a slope. Straw wattles should be effective for a period 

of 1 to 2 years if they can be installed on slopes up to 70 percent; however, their 

effect diminishes greatly on slopes steeper than 50 percent. Soils can be shallow 

but not less than about 8 inches. Straw wattles increase infiltration, add roughness, 

reduce erosion, and add short-term protection on slopes where permanent vegetation will be established to provide long-term erosion control. Straw bales are easily 

obtainable in most areas of the world, are very portable, and have a modular-type 

application for slope erosion and drainage control (fig. C12).

Figure C12. Straw bales have a similar application and are widely available. Individual 

bale size can be seen in the pile of bales, right center of photograph. (Photograph 

of slope in New Mexico, USA, New Mexico Department of Mining, Minerals, and 

Natural Resources.)

Figure C11. Straw wattles on the side of a road capture sediment and hold it onsite, 

enabling seeds to settle and germinate, aiding the revegetation process. (Photograph by 

Lynn Highland, U.S. Geological Survey.) 
90 The Landslide Handbook—A Guide to Understanding Landslides

Retaining Walls

For all types of retaining walls, adequate drainage through the structure is 

essential because very high ground-water pressure can build up behind any retaining 

wall, leading to its failure. Drainage can be provided simply with a coarse backfill 

and foundation material.

Timber crib

Timber crib walls are box structures built of interlocking logs and backfilled 

with coarse aggregate (fig. C13). They work by intersecting the critical sliding 

surface, thus forcing the potential failure surface to a deeper, less critical depth. The 

structure must be able to withstand: (1) shearing, (2) overturning, and (3) sliding 

at the base. It must, therefore, be strongly built by burying to sufficient depth and 

extending beyond the critical failure plane. Crib walls are only effective where the 

volume of soil to be stabilized is relatively small. They are most efficient where a thin 

layer of unstable soil overlies a deeper, more stable layer of soil. Crib wall structures 

should have a volume equal to 10 to 15 percent of the volume of the soil to be stabilized. This relatively small volume provides little counterweight support at the toe; 

therefore, virtually the entire resistance to failure comes from the strength of the crib.

Figure C13. Schematic and photograph of a timber crib. (Schematic from Reference 11, 

photograph is courtesy of PHI Group, United Kingdom.)

Note: The advice of a civil engineer is 

needed for any walls higher than 

3 meters (9 feet), or for those in 

complex foundation soils.
Part 1. Earth Slope Stabilization/Mitigation 91

Steel bin wall

A steel bin wall is formed from corrugated galvanized steel components bolted 

together to form a box and then filled with earth (fig. C14). The stability of a gravity wall is due to the weight of the wall itself, perhaps aided by the weight of soil in 

front of the wall. The bulk of the weight is from the contained soil, not the steel, and 

this should be kept in mind when the foundation is prepared. Large walls must be 

individually engineered, with load and foundation requirements calculated. Structural 

and civil engineering design charts provide stringer (horizontal member) specifications and height-to-width ratios for typical loading conditions. The widths of walls 

vary from 2 to 5 meters (6 to 15 feet) and are one-half to three-fifths the height of the 

wall. To provide additional sliding resistance, the foot of the wall is usually 0.5 to 

1.0 meter (1.5 foot to 3 feet) below grade, although the design should not rely on 

the additional toe support, as it can erode or be removed inadvertently. The factor of 

safety is improved if the wall is at a 1:6 slope. Fill material must be well drained and 

compacted, preferably in 20-centimeter (7.8 inches) lifts. Material behind the wall 

also should be well drained and moderately compacted.

Figure C14. Schematic and photograph of a steel bin wall. (Schematic from 

Reference 11.)
92 The Landslide Handbook—A Guide to Understanding Landslides

Reinforced earth wall 

Reinforced Earth is a patented system for constructing fills at very steep to vertical angles without the use of supporting structures at the face of the fill (fig. C15). 

The system uses horizontal layers of flexible metal strips within the fill to form a 

composite earth-metal system with high strength.

Figure C15. Schematic and photograph of a reinforced earth wall. (Schematic from 

Reference 11.)

Steel strips

Repose

angle

without

strips

Bedrock
Part 1. Earth Slope Stabilization/Mitigation 93

Gabion walls

Gabions are wire mesh, boxlike containers filled with cobble-sized rock that are 

10 to 20 centimeters (4 to 8 inches) size (fig. C16). A gabion retaining wall can also 

be constructed from stacked gabions. Gabion walls usually are inexpensive and are 

simple and quick to construct. Due to their flexibility, they can withstand foundation 

movement, and they do not require elaborate foundation preparation. Because of 

their coarse fill, they are very permeable and thus provide excellent drainage.

Gabion walls work because the friction between the individual gabion rows is 

very high, as is the friction between the basal row and the soil underneath. When 

failure occurs, it is almost always in the foundation soil itself. Three-tiered walls up 

to 2.5 meters (8 feet) high can usually be constructed without consulting any detailed 

engineering analysis. Higher walls are very heavy by nature of their added bulk and 

need larger base foundations and possibly counterforts for bracing of the wall. (A 

counterfort is a buttress bonded to the rear of walls, designed to improve stability.) 

Gabion walls built on clay soils require counterforts, which can be constructed as 

gabion headers extending from the front of the wall to beyond the slip circle. The 

counterforts serve as both structural components and as drains.

Design charts are available for various combinations of hillslope angle and 

retaining-wall height. 

Figure C16. Schematic and photograph of 

a gabion wall along a highway. (Schematic 

from Reference 11.) (Photograph of 

gabions located in the Pocono Mountains, 

Pennsylvania, USA, by Lynn Highland, 

U.S. Geological Survey.)
94 The Landslide Handbook—A Guide to Understanding Landslides

Piles

Large-diameter piles can be placed into the toe of a slope to form a closely 

spaced vertical pile wall (fig. C17). Pile walls are normally used as a preexcavation restraint system—the cut slope excavation takes place in front. Whereas 

large-diameter concrete pile and culvert pile walls have been used successfully on 

highways, wood or steel piles that are small in diameter have not. For most earth or 

rock movement, wood piles are not adequate to provide enough shearing resistance. 

They are suitable only where the volume of soil to be stabilized is small. On average, one wood pile is necessary for every 50 cubic meters (65.3 cubic yards) of soil, 

which is not sufficient for large stabilization projects. Too few piles can result in 

toppling and (or) breakage by the moving mass of soil, as well as by soil movement 

between the piles.

A major limitation when log piles are used is depth, as many failure surfaces lie 

below the height of the piles. Wood piles are the best for shallow soil failures over 

deeper stable soils. The piles should extend well below the potential failure surface 

and be firmly driven into firm subsoil. If the depth of placement is not sufficient to 

allow the piles to act as a cantilever system, then the piles must be tied back with an 

additional anchor system.

Figure C17. A concrete-filled pile wall. Reinforcing mesh has been hung over face 

of piles, ready for shotcrete spraying. Location is Brighton, Melbourne, Australia. 

(Photograph courtesy of Basement Construction Services, Victoria, Australia.)
Part 1. Earth Slope Stabilization/Mitigation 95

Slope Stabilization Using Vegetation

Seeding with grasses and legumes reduces surface erosion, which can under 

certain conditions lead to landslides. Planting with shrubs adds vegetative cover and 

stronger root systems, which in turn will enhance slope stability. If not controlled, 

surface erosion and small, shallow slope failures can lead to larger problems that 

cannot be controlled. Large-scale erosion requires applied engineering technology 

to correct and control. The terms “bioengineering” and “biotechnical slope protection” refer to the use of vegetation as slope protection to arrest and prevent slope 

failure and surface erosion. Bioengineering is discussed in detail in Section III of the 

handbook.

Planning is required for the successful implementation of a revegetation 

program. Before undertaking seeding, a person with local experience should be 

consulted for advice. Local knowledge based on successes and failures of projects is 

invaluable. Seed application should begin immediately following a disturbance, at a 

minimum of approximately 6 weeks before periods of drought or damaging frost.

A slope made as stable as possible before seeding will be of benefit in making 

the slope resistant to future erosion and failure. Controlling surface-water drainage, 

removing cut-bank overhangs, reducing slope angles, and benching all should be 

done before seeding begins.

There are two basic types of seeding: dry seeding and hydraulic, also known as 

hydroseeding:

Dry Seeding Dry seeding is done with rotary disk and air-blown 

seeders. These methods are less costly than hydraulic seeding but are 

limited to rough soil surfaces and gentler slopes. Rotary disk seeders 

spread seed and fertilizer by centrifugal force. The simplest seeder is 

the cyclone-type, hand-held seeder. Air-blown seeders use air to blow 

or shoot seed and fertilizer a distance of 5 to 8 meters (15 to 24 feet). 

Equipment can be adapted for motorized vehicles.

Hydraulic Seeding, or Hydroseeding This type of seeding is the 

application of seed in a water slurry that contains fertilizer, soil binder, 

and (or) mulch. The system requires a mixing tank with mechanical 

hydraulic agitation and volume pumping capacity. Hydraulic seeding is 

effective for seeding slopes 1:1 and steeper, where tacking of the seed to 

the slope is necessary. 

Types of seeds

A combination of two to five species is the normal grass-legume mix used for 

erosion control. Suitability of seeds depends on soil type, climatic conditions, species compatibility, and species replacement. Local conditions will vary, and no universal type of grasses or legumes can be recommended. The types of vegetation can 

vary from locality to locality, and it is best to get advice from locals who are familiar 

with local growing conditions. 

Mulching

Mulch is a nonliving material spread over the soil surface to provide protection 

from surface erosion by rain and retention of soil moisture. Various types of mulches 

will work—straw, grass fibers, wood fibers, seaweed, and paper products.
96 The Landslide Handbook—A Guide to Understanding Landslides

Biotechnical Slope Protection

This type of slope protection is used to reduce the environmental consequences 

of landslide-mitigation measures. When used for landslide remediation or mitigation, conventional earth-retaining structures made of steel or concrete usually are 

not visually pleasing or environmentally friendly. These traditional “hard” remedial 

measures are increasingly being supplanted by vegetated composite soil/structure 

bodies that are environmentally more friendly. This process has come to be known 

as biotechnical slope protection. Common biotechnical systems include geonets 

anchored by soil nails that hold in place soil seeded with grass and geocells with 

seeded soils in the interstices.

Research has been done on using plants to stabilize soil to prevent excessive 

erosion and also to mitigate the effect of landslides. One of the most promising types 

of plants is Vetiver, a type of grass that works very well in many different environments, to stabilize slopes against erosion. See Appendix C for more information on 

this plant, its uses, and its geographical suitability. 

Biotechnical slope protection consists of two elements: biotechnical 

stabilization and soil bioengineering stabilization, both of which entail the use of 

live materials—specifically, biotechnical vegetation stabilization uses mechanical 

elements (structures) in combination with biological elements (plants) to prevent and 

arrest slope failures and erosion. Mechanical and biological elements must function 

together in a complementary manner. Soil bioengineering stabilization, on the other 

hand, can be regarded as a specialized subset of biotechnical stabilization in which 

live plant parts, that is, roots, stems, and branches, serve as the main structural/

mechanical elements in the slope protection system. Biotechnical slope-protection 

systems blend into the landscape. They emphasize the use of natural, locally available materials, such as soil, rock, timber, and vegetation, in contrast to manufactured 

materials such as steel and concrete. The structural or mechanical components do 

not visually intrude upon the environment as much as conventional earth-retaining 

structures. Examples of biotechnical vegetation structures, which commonly 

incorporate vegetation into the structure itself, include log and timber cribs, gabion 

and rock-breast walls, welded wire walls, and reinforced earth. Internal, tensile 

reinforcements using the principles of bioengineering permit construction of oversteepened fill slopes to as much as 70o. A general guide to different bioengineering 

stabilization methods and more detailed information can be found in reference 30. 

As noted previously, soil bioengineering relies mainly on the use of native 

materials such as plant stems or branches, rocks, wood, or soil. Appropriate vegetation for bioengineering can be obtained from local sources of willow, alder, and 

other native, easily propagated varieties. In addition, soil bioengineering systems 

commonly are environmentally compatible during the construction process because 

they generally require minimal access for equipment and workers and cause relatively minor disturbance. With time, the bioengineering systems become visually 

nonintrusive and blend into the natural surroundings. This is a favorable attribute in 

environmentally sensitive areas such as parks, riparian areas, and scenic corridors 

where esthetic quality, wildlife habitat, and ecological restoration are important. As 

bioengineered structures that utilize tree species become older, they have the added 

benefit that they become more stable and eventually assist in the natural succession 

and long-term colonization of forest species. In most cases, native grasses, shrubs, 

and trees are used as the vegetation in bioengineering stabilization. Willow has been 

very successful in many parts of the world. In tropical and subtropical areas, Vetiver 

grass hedgerows (VGHR) for stabilization have become very popular because of 

the fast growth and deep root penetration of this grass. However, if exotic species of 

plants or trees are introduced, there is a real danger that they will conflict with native 

plant life. 

It is suggested that potential users 

consult the Vetiver grass Web site at: 

http://www.vetiver.org
Part 1. Earth Slope Stabilization/Mitigation 97

Figure C18. A Vetiver grass system is being used in the Democratic Republic of the 

Congo for gully control in urban areas and for highway stabilization. These gullies are a 

major problem in this area and other West African countries (top); the same slope now 

has improved drainage, and the slope has been planted in Vetiver grass (middle); this 

planting of Vetiver grass is about 3 months old (bottom).
98 The Landslide Handbook—A Guide to Understanding Landslides

While detailed slope-stability assessments normally have been carried out by 

geotechnical engineers and engineering geologists, the organic interactions between 

vegetation, soil, and structures that must be evaluated in applying the technique of 

bioengineering stabilization are perhaps better understood by soil scientists, agriculturists, foresters, and hydrologists. Thus, the bioengineering approach to slope stabilization requires cooperation of geotechnical and plant-science disciplines working 

in parallel and in unison.

Publications are available that tell how effective it is in different regions of the 

world. A good beginner overview on Vetiver grass is the book, “Vetiver Grass: A Thin 

Green Line Against Erosion.” The full reference for the book is in Reference 22. (See 

also reference 47.) 

Figure C19. Worldwide distribution of active Vetiver grass programs. Graphic is from the 

Vetiver grass Web site (http://www.vetiver.org).

Additional Notes on Vetiver grass: For developing nations, soil erosion that 

encompasses, in its extreme form, landslides is one of the most damaging natural 

processes that must be dealt with. Little has been accomplished in dealing with 

erosion that can be widely applied, in inexpensive, long-lived, and appealing applications. Vetiver, a tropical grass, offers a practical and inexpensive way to prevent 

erosion. Planted in lines along the contours of sloping lands, Vetiver quickly forms 

narrow but very dense hedges. Its stiff foliage then blocks the passage soil and 

debris. This deeply rooted, persistent grass has restrained erodible soils for decades 

in Fiji, India, and some Caribbean nations. Figure C18 shows photographs of a 

Vetiver grass project in the Democratic Republic of the Congo and how it is used 

for stabilization for gullies and roads. The project is sponsored by various government agencies. These and additional photographs are on the Vetiver grass Web site, 

cited at the end of this section.
Part 2. Rock Slope Stabilization/Mitigation Techniques 99

Part 2. Rock Slope Stabilization/Mitigation 

Techniques

Rockfall can range from a few fist-sized rocks to large cliff sections and 

boulders which, depending on size and shape, can roll, bounce, and careen down 

slopes, landing in areas at great distances from the fall lines. Recreation areas such 

as beaches near cliffs, parks, and open spaces are affected by rockfall, and people 

are frequently exposed to these hazards. People venturing too near the edges of cliffs 

and rocky slopes can add pressure to already weak overhangs and cause rockfalls 

to land on people below or sustain injuries themselves on these collapsing edges. 

Whether hiking, camping, walking, or working around cliffs or rock faces, people 

encounter the hazard many times without warning. A variety of engineering techniques can be implemented to help mitigate the effects of rockfalls, and some of 

these are discussed here. In some cases, more than one type of engineered solution 

is the best, and a combination of these remediation measures applied to one area of 

rockfall hazard is shown in figure C20.

Figure C20. This photograph shows rockfall countermeasures that include mass 

concrete retaining walls, gabion walls (both wall types are at top of photograph), check 

fences, boulder treatment, and buttressing. (Photograph is the Pen-y-Clip tunnel on a 

highway in North Wales, United Kingdom. Photograph by Dave Giles, Engineering Geology 

Consultancy Group, University of Portsmouth, United Kingdom.)
100 The Landslide Handbook—A Guide to Understanding Landslides

Safe Catching Techniques

Catch Ditches

Wide catch ditches are effective in containing rockfall, but the ditches must be 

designed with the cliff geometry taken into account, and it is best to consult a professional about specifications. The bottom of the catchment ditch should be covered with 

loose earth to prevent falling rock from bouncing or shattering into pieces or shards. If 

there is not enough space to construct as wide a ditch as is specified, then a combination of smaller ditches with a gabion or rock wall along their downhill edges can be used.

Cable, Mesh, Fencing, and Rock Curtains

Cable lashing and wire nets are simple, low-cost methods for protecting a road or 

path from rockfall. For large, unstable blocks, strands of metal cable are wrapped around 

the blocks and anchored to the slope. Where the rock is too fractured to be restrained by 

individual cables, cable nets are used. Wire mesh (closely spaced interwoven wires) can 

be used to prevent smaller rocks, less than 0.75 meter (2.4 feet) in size, from falling. (See 

figure C21 for a photograph of an example of wire mesh). The standard mesh is doubletwisted gabion wire mesh or a heavy gage metal chain link. The mesh is either loosely 

draped over a uniform rock face or bolted or otherwise firmly secured where the cliff face 

is irregular and the mesh cannot make close contact with the rock. Bolting the mesh to 

the rock face can prevent rock from becoming dislodged and provides overall stability of 

the slope or rock face. Wire mesh also is useful on steep soil cuts, especially beneath talus 

slopes. 

Catch nets made of cable and wire mesh can be constructed to catch falling rock 

at the bottom of gullies and slopes. When suspended from an anchored cable, the mesh 

forms a flexible barrier to dissipate the energy of the falling rock and will usually stop 

boulders up to 1 m in diameter, if properly secured. Additionally, catch nets can be used in 

conjunction with roadside catch ditches.

Rock fences such as the ones shown in figure C22 are fairly easy to install and can 

restrain small rocks from falling onto roads but do not stop rocks that bounce out over 

the top of the fence barrier. Rock curtains, such as the one shown in figure C23, are more 

effective in directing rocks down to a catch ditch or other catching structure, preventing 

them from bouncing outward onto the highway or structures below.

Figure C21. Example of wire mesh placed over a rocky slope to contain the rocks that 

may come down.
Part 2. Rock Slope Stabilization/Mitigation Techniques 101

Figure C23. Example of a “rock curtain” that controls rockfalls in problem areas. 

(Photograph by Doug Hansen, High Angle Technologies, Inc.)

Figure C22. Protective rock barrier fencing along (A) a dirt trail in Pennsylvania, USA, 

and (B) a roadway in coastal California, USA. (Trail photograph, Lynn Highland, U.S. 

Geological Survey; roadway photograph from Federal Highway Administration, USA.)

A

B
102 The Landslide Handbook—A Guide to Understanding Landslides

Figure C24. Pitquah rock sheds, British Columbia, Canada. These sheds enclose 

sections of railroad, protecting it from rockfall and rock avalanches. (Photograph by John 

Carter, www.trainet.org.)

Retaining Walls

Retaining walls can work much like those described for soil slope-stabilization 

techniques in keeping rockfall debris out of an area. They are similar to rockfall 

fences but are in most cases more substantial and stronger. Retaining walls can be 

made out of steel, concrete, timbers, or other materials and must be anchored properly so as not to tip over during rockfalls.

Rock Sheds/Shelters

These are built over roads, railways, and sometimes structures to shield the area 

from rockfall and rock avalanches. Shed structures are either open ended or completely envelope the rockfall area in a concrete or steel (or other material) structure 

that will deflect rockfall away from the road, railway, or structure. Figures C24 

through C27 show examples of rock sheds/shelters.

Rock Ledge Reinforcement

These are not commonly used because they work only for unique situations and 

must be carefully engineered and structurally strong. 

Figure C27 is an example. 
Part 2. Rock Slope Stabilization/Mitigation Techniques 103

Figure C25. Example of an open rock shed in New Zealand (Photograph courtesy of 

Richard Wright, rock climber).

Figure C26. A rock avalanche shelter in the Montenyard area of France. The length 

of the shelter may be insufficient as a debris flow has damaged the road, shown to the 

right of the shelter. (Photograph by Dave Giles Engineering Geology Consultancy Group, 

University of Portsmouth, United Kingdom.)

Figure C27. Example of a reinforced rock ledge, Chapmans Peak Drive, Cape Town, 

South Africa. Photograph attribution: http://commons.wikimedia.org/wiki/

Image:Chapmans_peak_dr.jpg.
104 The Landslide Handbook—A Guide to Understanding Landslides

Excavation of Rock 

Benches

Horizontal benches excavated into a rock face are among the most effective 

kinds of protection from rockfall. In addition to intercepting rockfall, benches reduce 

tensional forces in the surface rock and reduce surface erosion rates. They also 

reduce the rate of occurrence of rockfall. However, they have little or no effect on 

potential deep-seated rock failure. 

Bench faces can be constructed steeper than the overall slope angle, as rocks 

that fall will likely remain on the benches (fig. C28). Vertical bench-face angles 

should be avoided, however, as tension cracks, dangerous overhangs, and excessive rockfall can result. The placing of bench faces should be stopped at the base 

of weaker rock layers, fractured rock zones, or water-bearing zones. A minimum 

width of 4 m is recommended for the benches, and all benches should have drainage 

ditches to divert water away from the slope.

Figure C28. Schematic and photograph of rock benches. (Schematic from Reference 11, 

photograph of a mine in Tucson, Arizona, USA, by Steve Dutch, University of Wisconsin, 

USA.)
Part 2. Rock Slope Stabilization/Mitigation Techniques 105

Scaling and Trimming

Loose, unstable, and (or) overhanging blocks of rock, which may pose a danger to passing traffic and (or) pedestrians, can be removed by scaling or trimming. 

Scaling is the removal of loose blocks by the use of hand-held pry bars and small 

explosive charges. Trimming involves some drilling and light blasting by explosives, 

followed by scaling, to remove larger areas of overhanging or potentially dangerous rock. The necessity of scaling and trimming can be reduced with the use of 

controlled blasting, but blasting is not always feasible. Overhanging rock is either 

removed or trimmed back to a stable part of the face. Scaling operations are usually 

carried out by workers suspended by ropes or other means, using pry bars, jacks, and 

explosives. These operations can be time-consuming and expensive (sometimes dangerous) and on active slopes may need to be repeated every few years, or as needed. 

Scaling is highly skilled work and can be dangerous; scaling crews should be trained 

and the work performed by professionals.

Photographs C29 and C30 show rock scaling and trimming operations.

Figure C29. Rocks dislodged by scaling and blasting. (Photograph by Washington 

Department of Transportation, USA.)
106 The Landslide Handbook—A Guide to Understanding Landslides

Figure C30. A hydraulic rock hammer in action, bringing down rock from the slope. 

This is an alternative to blasting rocks down. (Photograph by Washington Department of 

Transportation, USA.)
Part 2. Rock Slope Stabilization/Mitigation Techniques 107

Figure C31. Shotcrete operation to stabilize rockfall area of Wolf Creek Pass in the Rocky 

Mountains of Colorado, USA. (Photograph by Colorado Department of Transportation.)

Reinforcing Potential Rockfall Areas

Shotcrete and Gunite

Shotcrete and gunite are types of concrete that are applied by air jet directly onto 

the surface of an unstable rock face. Shotcrete is an all-inclusive term to describe the 

spraying of concrete or mortar either by a dry- or a wet-mix process. Gunite refers 

only to the dry-mix process in which the dry cementitious mixture is blown through 

a hose to the nozzle, where the water is injected immediately before application. This 

is a rapid and relatively uncomplicated method commonly used to provide surface 

reinforcement between blocks of rock and also to reduce weathering and surface 

scaling. Shotcrete contains aggregate up to 2 cm in size and is more commonly used 

than gunite, which has smaller aggregate. Both materials can be applied rapidly by 

air jet so that large areas can be covered in a short time. Figure C31 shows a shotcrete 

operation on the side of a highway.
108 The Landslide Handbook—A Guide to Understanding Landslides

Anchors, Bolts, and Dowels 

These are tools composed of steel rods or cables that reinforce and tie together 

a rock face to improve its stability. Anchors are post-tensioned members used to support large blocks of rock, whereas bolts are shorter and support surface rock. Dowels 

are similar to bolts but are not post-tensioned. Reinforcing a rock slope with steel 

requires a specialist’s knowledge of rock stability analysis, of grouting techniques, 

and of testing procedures. The determination of the orientation of the potential 

failure surfaces is crucial to a successful anchor system and requires a considerable 

amount of engineering experience. Figures C32 and C33 show rock bolts and rock 

bolt installation along a highway.

Figure C33. A rock bolting operation; notice broken-up (crumbling) nature of the rock. 

(Photograph by Washington Department of Transportation, USA.)

Figure C32. Closeup photograph of a rock anchor on a rock face, with mesh over the 

surface for more protection. The anchor is set in the rock about 5 meters (15 feet) deep. 
Part 3. Debris-Flow Mitigation 109

Figure C34. Wildfire-burned slopes and debris flow that occurred shortly after the fire at 

Lytle Creek, California, USA. (Photograph by Sue Cannon, U.S. Geological Survey.)

erosion The processes where the materials of the Earth’s crust are loosened, 

dissolved, or worn away and then are moved from one place to another. Process 

includes actions of wind, rain, freeze-thaw, weathering, and physical abrasion.

Part 3. Debris-Flow Mitigation

This section describes some simple mitigation methods for debris-flow hazards 

for homeowners, businesses, and others. A short section on erosion and fire control 

is included since erosion, fire, and subsequent debris flows and flooding are interrelated hazards. 

Strengthening Slopes for Erosion/Debris Flows

Erosion may cause the steepening and lengthening of gullies and cause the 

loosening of soil, plant debris, rocks, and boulders, which can intensify the effects 

of debris flows. Keeping an area free of excess fuel for fires can also help in the 

mitigation of debris flows, as burned slopes become more vulnerable to the effects 

of debris-flow initiation and erosion (fig. C34). Loss of vegetation that holds soil in 

place and physical and chemical changes to the soil that result from intense heat and 

burning by fires make this soil more prone to debris flows.
110 The Landslide Handbook—A Guide to Understanding Landslides

Note: It should be remembered that floods, earthflows, and debris flows (some of 

these known commonly as “mudslides”) have many of the same characteristics and 

generally can be dealt with in similar manners. Floods, “mudslides,” and debris 

flows sometimes accompany each other, but not always. 

Strengthening the soil to resist erosion

Erosion is a process that must be taken into account when reinforcing an 

area, and some simple steps can be taken to lessen the effects of erosion. Erosion 

can sometimes lead to slope failures and drainage problems; trying to prevent it is 

something a homeowner can do, proactively, before bigger slope-failure problems 

are encountered. Straw or wood chips are effective in holding the soil in place. They 

have the further value of increasing the organic content of the soil. Place a covering 

of chips or straw about one-half centimeter (one-quarter inch) thick as slope and 

soil conditions indicate. Fertilizer may be added. Work the material into the top few 

centimeters (or inches) of the soil. 

Woven burlap (a loosely woven, fibrous material of generally low cost) can be 

laid on the slope and tied down with stakes to prevent lifting by wind or water. Regular planting procedures can be followed before laying the burlap because the fabric 

will not interfere with establishing a growth on the slope. The burlap will decompose 

eventually but will remain long enough for vegetation to become well established.

Proper planting of vegetation on slopes can prevent erosion

Keep plants watered, but do not overwater. Replant barren areas or areas that 

have been burned. Make inspections during rains. Watch for gullying. Correct problems as soon as possible. 

Keeping slopes free from fuel for wildfires

Burned slopes in debris-flow-prone areas can become hazardous as they 

increase the likelihood and intensity of debris flows when slopes become saturated 

by rainfall. Homeowners and businesses can act to keep properties free of excess 

fuel for wildfires, actions that may stop wildfires from spreading or burning large 

areas. Wildfires can denude slopes of vegetation and can change the soil chemistry, 

which may result in intensifying the hazard from debris flows. Piles of deadwood, 

dead vegetation, and other types of fuel that can accumulate on properties should be 

kept at a minimum to keep wildfires from starting or spreading. Many communities 

have local guidelines on controlling excess wildfire fuel and feature practical advice 

on the cleanup of property. Local municipalities can take community action to penalize unauthorized burning of trash, for example, through local ordinances. Lightningcaused wildfires are naturally occurring, but steps can be taken that preclude the 

spread of this type of wildfire through keeping available fuel at a minimum. For any 

necessary burning of farmland, for example, it is recommended that residents that 

live in areas of steep slopes be vigilant so a wildfire does not become uncontrolled 

and spread to other areas.
Part 3. Debris-Flow Mitigation 111

Structures for Mitigating Debris Flows

Debris-flow basins

These catchment basins are commonly built at the base of slopes where debris 

flows are frequent (fig. C35). They are used especially in areas where the debris must 

be contained so that soil and debris are stopped from flowing into sensitive ocean or 

river shorelines areas or where there are structures at the base of the slope that are 

vulnerable to debris-flow damage. These basins will eventually fill with the debrisflow deposits and must be emptied periodically or they will overflow. Commonly, 

large pieces of equipment such as dump trucks and power shovels are needed to 

empty the debris and carry it away. However, small basins can be emptied manually. 

They should be designed to be able to contain the maximum flow volumes of an area 

to prevent overtopping during a flow event.

Figure C35. Aerial photograph of a debris-flow basin, constructed at the bottom of a 

slope in San Bernardino, California, USA. (Photograph by Doug Morton, U.S. Geological 

Survey.)

Check dams

See Appendix C, “Part 1. Earth Soil Slope Stabilization/Mitigation,” for 

an explanation of how check dams can also be used to reduce the hazards from 

debris flows.
112 The Landslide Handbook —A Guide to Understanding Landslides

Debris-flow retaining walls

These are structures that can be built of various kinds of materials. They are 

designed to stop the progress of the debris fall, either by blocking the flow or diverting it around a vulnerable area. These structures should be carefully designed as any 

deflection of material may be unintentionally redirected into additional vulnerable 

areas (figs. C36 and C37). 

Figure C36. Caution is required in locating and constructing debris-flow retaining walls. 

This is a photograph of the partial failure of a retaining wall, caused by a landslide in 

Iztapalapa, a suburb of Mexico City, Mexico. The landslide slid onto a house at the toe 

of the slope, killing two people, and was triggered by heavy rains in the area. The house 

was below the 5-meter-high (15 yards) wall, but the wall could not withstand the mass of 

rocks and soil. (Photograph by Chinagate/Xinhua.)

Figure C37. A debris-flow retaining wall in Kamikochi Basin, Japan. (Photograph 

courtesy of Goncalo Vieira.)
Debris-Flow Mitigation for the Homeowner 113

Debris-Flow Mitigation for the Homeowner

This section provides some simple steps that a homeowner can take, or an emergency manager can recommend, 

to help individuals mitigate the effects of erosion, which in many cases may lead to debris flows and some landslides.

The following figures C38 through C52 show schematics of debris-flow mitigation techniques that may be helpful for protecting structures. The figures are modified from those found in Reference 20, a booklet published for Los 

Angeles, California, USA, homeowners by the Los Angeles County Department of Public Works, showing simple 

methodology for reducing hazards from floods, erosion, and debris flows. 

Debris Flows

Erosion

Destructive Flows

planted

 slopes allow

largest

path for

debris

engineered

block

wall

slope (bench)

drain

watershed

seeded and

watered where

possible

wooden

deflector

boarded up

door and

window

Figure C38. Schematic of an unprotected home,

in the path of a debris flow and (or) “mudslide.” 

Suggested methods to reduce the hazards from 

debris flows are shown in figure C39.

Figure C39. Schematic example of a house with 

protective structures in place. Shows construction 

of deflection walls and debris fences. Because 

of extreme force of impact associated with some 

debris flows, these and similar structures should 

be carefully engineered and constructed. 
114 The Landslide Handbook—A Guide to Understanding Landslides

sandbags sand

lumber plywood

Figure C40. Schematic of typical materials, usually available in many regions of the 

world, for helping to reduce damage from flood/debris-flow events.
Debris-Flow Mitigation for the Homeowner 115

Note: The ideal material for sandbags is sand, and bags should not be filled with 

wood chips, paper, trash, or other materials. Sand- and soil-filled burlap sandbags 

deteriorate when exposed for several months to continued wetting and drying. If 

bags are placed too early, they may not be effective when needed. 

water

Figure C41. Sandbags are basically for low-flow protection of up 

to 0 .6 meter (2 feet). Protection from higher flows requires a more 

permanent type of structure. It should be noted that sandbags will not 

seal out water. 
116 The Landslide Handbook—A Guide to Understanding Landslides

Figure C42. Schematic of a typical placement of sandbags for home protection 

(individual situations may vary in layout and orientation).

Caution: It is not advisable to use straw or bales of hay instead of sandbags. They 

do not perform as well as sandbags and may be washed away.

continue 

bags to

hill

debris

Figure C43. Sandbags help in directing debris away from buildings.
Debris-Flow Mitigation for the Homeowner 117

debris

Figure C44. Controlling debris or stormflows in streets with sandbags.

debris

clear

path

for

debris

Figure C45. Directing flows between buildings by using sandbags.
118 The Landslide Handbook—A Guide to Understanding Landslides

sliding

glass door plastic sheet

sandbags

or plywood

barrier

Figure C46. Sliding glass door sealing—Control of flows to prevent seeping into sliding 

glass door by using sandbags and plastic sheeting.
Debris-Flow Mitigation for the Homeowner 119

debris

Figure C48. Nailing up plywood or lumber for window and door protection.

removable door barrier

window protection

Figure C47. Typical window and door protection, using removable plywood.
120 The Landslide Handbook—A Guide to Understanding Landslides

debris

deflector

compacted earth for added strength

Figure C49. This is a timber deflector, which is more permanent than sandbags.

1/2 stake length

limit to

3 feet high

Figure C50. Closeup schematic of a timber deflector installation technique.
Debris-Flow Mitigation for the Homeowner 121

1/2 pole

underground

telephone

poles

or railroad ties

metal

post

concrete

cement

ground

removable driveway barrier wall

driveway

Figure C52. Removable driveway barrier. The metal posts can be removed and restored 

as needed, as they loosely fit into concrete casings in the ground

Figure C51. An alternative to timber deflectors—telephone pole sections or railroad tie 

barrier.
122 The Landslide Handbook—A Guide to Understanding Landslides

Basic Things to Remember Concerning Mitigation and Response to 

Debris-Flow and Other Landslide Hazards/Emergency Response

Sandbags, tools, and sheets of plastic may be useful during heavy rains (the plastic can be used to protect and 

waterproof some items). Keep them available, where possible. Help others in the community that may not be 

able to reinforce their dwellings, such as the disabled or elderly. Educate children in mitigation techniques. 

Mitigation techniques are the most effective when used by as many members of a community as possible.

•	 If unusual cracks, settling, or earth slippage starts, it is recommended that people consult a municipal agent (such as an emergency manager) and (or) a qualified civil engineer or geologist as soon as 

possible.

•	 It is suggested that slopes or drainage areas not be altered without expert advice. It is always best to 

consult a professional or someone with experience in this type of work.

•	 Normal property drainage usually flows to the street or an approved drainage device. When landscaping, homeowners or others should avoid disrupting flow patterns created when the property was originally graded. Obstructions such as patios, sidewalks, and decks should not be placed in side swales 

unless an alternate method of drainage is provided. 

•	 Post illustrations in the form of simple schematics in public places, to help people implement 

mitigation techniques.

•	 It is advisable to have an emergency response plan for evacuation and relocation of populations that 

are threatened by landslide hazards. It is generally best to make sure everyone is aware of these 

procedures. 
Part 3. Debris-Flow Mitigation 123

For more information and further reading: 

References 11, 12, 13, 20, 25, 26, 39, 42, and 46

Landslide Dam Mitigation

As previously mentioned, the primary hazard from landslide dams is flooding 

that can occur when a landslide dam fails, or flooding that occurs when the dam is 

overtopped by the ongoing flow of water backing up behind the dam. The following 

measures can be implemented when communities are faced with potential hazards 

from landslide dams:

Diversion of inflow water before it reaches the lake formed by the 

landslide dam

This can be done by diverting water from the stream into upstream reservoirs or 

irrigation systems. Although usually only a temporary measure, diversion may slow 

the filling of the lake enough to allow the application of a more long-term solution.

Temporary drainage from the impoundment by pumps or siphons 

 The rising water level can be controlled temporarily by means of pumps or 

siphons, causing the water to flow over the low point of the dam. This is usually a 

short-term (less than 1 to 2 years) measure that provides time for more extensive, 

long-term solutions.
124 The Landslide Handbook—A Guide to Understanding Landslides

Construction of an erosion-resistant spillway

The most common method of stabilizing a landslide dam is to construct an 

erosion-resistant open-channel spillway either across the dam or across an adjacent 

abutment. When the overtopping by water occurs, flow is controlled by the spillway in much the same way that emergency spillways are constructed on engineered 

dams to control water level. An additional advantage of this type of spillway is that 

it allows for the lowering of the water level behind the dam, which helps lessen the 

upstream flooding that landslide dams may cause.

Spillways are not always successful in preventing dam breaching and downstream flooding; they sometimes fail due to retrogressive erosion (erosion from the 

spillway outlet to its intake) caused by high-velocity outlet flow. To prevent erosion 

by minimizing flow velocity, the spillway should be wide and shallow. If possible, 

it should be lined with erosion-resistant materials (commonly riprap), especially at 

the outlet. Often, check dams are installed along steeper grades of the spillway to 

prevent erosion. Spillways that fail due to erosion may have been partially successful 

because they limit the total volume of the water behind the dam, thus reducing total 

discharge even if the dam breaches entirely.

Open-channel spillways across the landslide dam commonly are excavated by 

bulldozers; however, draglines, backhoes, explosives, and hand labor all have been 

used. Excavation can be dangerous in rough terrain, so an access road has to be 

constructed.

Drainage tunnel through an abutment

A long-term method of preventing overtopping and breaching of a landslide 

dam is construction of a diversion tunnel through an adjacent dam abutment. 

Because large landslide dams commonly occur in mountain canyons, they usually 

have bedrock abutments; thus rock-tunneling methods commonly are used. Figure 

C53 shows the Thistle, Utah, landslide in the United States, triggered by the El Niño 

conditions of 1983. Heavy rains the previous autumn and rapid snowmelt caused the 

massive failure. For further reading please see Reference 31.

The Thistle landslide also destroyed sections of both a major highway and a 

main line of a railroad track. After a tunnel was excavated through the mountain 

trains could continue to travel the route. The motorway (highway) was rerouted over 

a saddle, away from the landslide deposit (fig. 54). 

The landslide will be left as it is, as it is too massive to remove. The landslide 

is still being monitored with instrumentatino by the State of Utah, and it has recently 

reactivated. Reactivation is another peril from landslide dams such as Thistle 

(fig. C55).
Part 3. Debris-Flow Mitigation 125

Figure C53. The Thistle landslide in Utah, USA, 1983. This landslide dammed a river, which formed a lake (called “Thistle Lake”) behind 

the dam, flooding the town of Thistle. (Photograph by Robert L. Schuster, U.S. Geological Survey.)
126 The Landslide Handbook—A Guide to Understanding Landslides

Figure C54. Closeup view of mitigation measures taken to reduce the impact of the 

Thistle landslide dam, showing the tunnel for the river, and the diversion overflow tunnel. 

(Photograph courtesy of the Utah Geological Survey).

Figure C55. Photograph with annotations showing the reactivation and enlargement of 

the Thistle landslide dam. (Photograph courtesy of the Utah Geological Survey.)
Appendix D.

Sample Safety Information for Landslides/

Debris Flows —

Suitable for Posting and (or) Distributing

Adapt as needed to suit local conditions and (or) circumstances

Information in Appendix D is compiled by U.S. Geological Survey.
128 The Landslide Handbook—A Guide to Understanding Landslides

What Can You Do If You Live Near Steep Hills?

Prior to Intense Storms:

•	 Become familiar with the land around you. Learn whether debris flows have occurred in your area by contacting local 

officials, emergency management officials, State geological surveys or departments of natural resources, and university 

departments of geology. Slopes where debris flows have occurred in the past are likely to experience them in the future.

•	 Support your local government in efforts to develop and enforce land-use and building ordinances that regulate construction 

in areas susceptible to landslides and debris flows. Buildings should be located away from steep slopes, streams and rivers, 

intermittent-stream channels, and the mouths of mountain channels. 

•	 Watch the patterns of stormwater drainage on slopes near your home and note especially the places where runoff water 

converges, increasing flow over soil-covered slopes. Watch the hillsides around your home for any signs of land movement, 

such as small landslides or debris flows or progressively tilting trees.

•	 Contact your local authorities to learn about the emergency-response and evacuation plans for your area and develop your 

own emergency plans for your family and business.

During Intense Storms:

•	 Stay alert and stay awake! Many debris-flow fatalities occur when people are sleeping. Listen to a radio for warnings of intense 

rainfall. Be aware that intense short bursts of rain may be particularly dangerous, especially after longer periods of heavy 

rainfall and damp weather.

•	 If you are in an area susceptible to landslides and debris flows, consider leaving if it is safe to do so. Remember that travel 

during an intense storm is hazardous.

•	 Listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A 

trickle of flowing or falling mud or debris may precede larger flows. If you are near a stream or channel, be alert for any 

sudden increase or decrease in water flow and for a change from clear to muddy water. Such changes may indicate debris-flow 

activity upstream, so be prepared to move quickly. Don’t delay! Save yourself, not your belongings.

•	 Be especially alert when driving. Embankments along roadsides are particularly susceptible to landslides. Watch the road for 

collapsed pavement, mud, fallen rocks, and other indications of possible debris flows.

What to do if you suspect imminent landslide danger: 

•	 Evacuate, if possible.

•	 Contact your local fire, police, or public works department.

•	 Inform affected neighbors.

After landslides occur:

•	 It is best to stay away from the slide area, as there may be danger of additional slides; however, this is not always possible 

or desirable. 

•	 Check for injured and trapped persons near the slide area. Give first aid if trained, and call for help.

•	 Remember to help your neighbors who may require special assistance—infants, elderly people, and people with disabilities.

•	 Listen to a battery-operated radio or television for the latest emergency information.

•	 Remember that flooding may occur after a mudflow or a landslide.

•	 Check for damaged utility lines. Report any damage to the utility company.

•	 Check the building foundation, chimney, and surrounding land for damage.

•	 Replant damaged ground as soon as possible because erosion caused by loss of ground cover can lead to flash flooding.

•	 Seek the advice of geotechnical expert for evaluating landslide hazards or designing corrective techniques to reduce 

landslide risk.
Index 129

Index

A

acoustic imagery 47

alluvial fan 60

B

backcasting 129

backfill 78, 83, 86, 90

bedding surface/plane 60

bedrock 20, 22, 29, 46, 47, 60, 62, 70, 

86, 124

benches 53, 104

borehole 47, 60

C

check dams 60, 82, 83, 111, 124

colluvium 60

D

debris avalanche 20, 21, 37

debris basin 60

debris flow 5, 12, 16–18, 20, 26, 30, 50, 

60, 61, 83, 103, 109, 112, 113

delta-front landsliding 60

differential 6, 8, 60, 62, 63, 73, 129

digital elevation model (DEM) 60, 70

drains 78, 86, 88, 93

drawdown 22, 60, 64

drilling 47, 66, 88, 105

E

earthflow 5, 22–25, 39

electronic distance meter (EDM) 60

emergency 1, 35, 49, 50, 113, 122, 124, 

128

epicenter 60

excavation 6, 8, 22, 35, 52, 76, 77, 79, 

88, 94

expansive soils 60

F

factor of safety 60, 91

field reconnaissance 46

fracture 60

G

geodesic/geodetic measurements 60

Geographic Information System (GIS) 

60

geologic hazard 60

geologic map 60, 68

geomorphology 46, 60

geophysical studies 60

glossary 57, 60, 63

H

hydraulic 60, 61, 88, 95, 106

hydraulic seeding 95

hydrology 60

I

InSAR 58, 70, 72

Instrumentation 47, 74

interstitial water 61

L

lahar 18–20, 33, 61

landslide dam 32, 36, 53, 60, 123, 124, 

126

landslide hazard map 61, 69

landslide inventory maps 61, 68

landslide maps 52, 67

landslide risk map 61

landslide susceptibility map 61, 68

lateral spread 15

LiDAR 57, 73

liquefaction 14, 18, 32, 61

lithology 60, 61

loess 61

M

mapping 46, 49, 61, 66, 69, 73

mitigation 1, 6, 8, 20, 22, 24, 26, 34, 

36, 52–54, 56–58, 61, 62, 75, 96, 

109, 113, 122, 126

monitoring 10, 18, 46, 47, 52, 58, 74

mudflow 61, 128

mudslide 61, 113

P

perched ground water 61

pore water, 61

R

reconnaissance geology/mapping 61

relief 61

remote sensing 73

retaining walls 6, 10, 12, 24, 25, 44, 52, 

53, 90, 99, 112

risk 5, 26, 48, 58, 61, 70, 76, 128

rockfall 5, 7, 49, 50, 53, 79, 99, 100, 

102, 104, 107, 129

rock mechanics 61, 76

rotational landslides 76

S

safety 1, 36, 48–50, 60, 91, 127

sag pond 61

sandbags 115–118, 120

sea cliff retreat 61

seepage 61

shear 6, 10, 16, 22, 24, 26, 61, 80

slurry 16, 61, 95

soil mechanics 61

stress 24, 60–62

sturzstroms 62

subsidence 14, 60, 62

surficial geology 26, 46, 62, 70

swelling soils 44, 60, 62

T

tensile stress 62

timber cribs 96

topple 5, 8, 9

triggering mechanism 18

W

weathering 6, 8, 24, 62, 64, 107, 109

wildfire 28, 109

Z

zonation 58, 62
Publishing support provided by: 

Denver Publishing Service Center

Manuscript approved for publication May 5, 2008

Edited by Mary Kidd

Designed and prepared by Margo VanAlstine 

with assistance from Carol Quesenberry 

For more information concerning this publication, contact:

Team Chief Scientist, USGS Geologic Hazards 

Box 25046, Mail Stop 966

Denver, CO 80225

(303)273-8579

Or visit the Geologic Hazards Team Web site at:

http://geohazards.cr.usgs.gov/
 Highland, Lynn M., and Bobrowsky, Peter—The Landslide Handbook—A Guide to Understanding Landslides—Circular 1325

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