FEDERAL EMERGENCY MANAGEMENT AGENCY                                                                                                                    FEMA 182/August 198&9

  Landslide Loss Reduction:
 A Guide for State and Local
 Government Planning

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                                       EARTHQUAKE HAZARDS REDUCTION SERIES 52

         Issued by FEMA in furtherance of the Decade for Natural Disaster Reduction.
                       DISCLAIMER
This document has been reviewed by the Federal Emergency
   Management Agency and approved for publication. The
contents do not necessarily reflect the views and policies of
       the Federal Emergency Management Agency.
FEDERAL EMERGENCY MANAGEMENT AGENCY

                  Landslide Loss Reduction:
                  A Guide for State and Local
                  Government Planning
                  by:,
                  Robert L. Wold, Jr.
                  ColoradoDivision of DisasterEmergency Services
                  and
                  CandaceL. Jochim
                  Colorado Geological Survey
     II

ii
                                                       Contents

                                                                                                                                             14
                                                                                              Lahar ..............................
                                                                       7i
FOREWORD                                   .n...............................
                                                                                          Subaqueous Landslide .......................... 15
                                                                      vii
                                                    ...............................
ACKNOWLEDGIMENTS
                                                                                      Interrelationship of Landsliding With
   Advisory Committee ................................ vii
                                                                                      Other Natural Hazards (The Multiple
CHAPTER 1-Introduction.......................... 1
                                                                                      Hazard Concept) ...............................        15
   Purpose of this Guidebook ........................... 3
                                                                                                                 lDam Safety ............... 15
                                                                                          Landsliding and
CHAPTER 2-Landslide Losses and
                                                                                          Landsliding and Flooding .................... 17
the Benefits of Mitigation ............................. 4
                                                                                          Landsliding and Seismic Activity ....... 18
   The Landslide Hazard ................................. 4
                                                                                          Landsliding and Volcanic Activity ....... 19
   Econoamic and Social Impacts of
                                                                                  CHAPTER 4-Hazard Identification,
                                                                         4
   Landsliding .................................
                                                                                  Assessment, and Mapping .......................... 20
       Costs of Landslidling ............................... 4
                                                                                                                                             20
                                                                                      Hazard Analysis ..............................
       Impacts and 'Consequences of
                                                                                                                                             20
                                                                                          Map Analysis ...............................
       Landsliding .................................                     4
                                                                                          Analysis of Aerial Photography
   Long-Term Benefits of Mitigation ..........- 5
                                                                                                                                             20
                                                                                          and Imagery ...............................
       The Cincinnati, Ohio Study ................... 6
                                                                                          Analysis of Acoustic Imagery
       The Benefits of Mitigation in Japan ..... 6
                                                                                                                                             20
                                                                                          and Profiles ...............................
   Pla nning as a Means of Loss Reduetion.....6
                                                                                          Field Reconnaissance ........................... 20
       Local Government Roles ........................ 7
                                                                                          Aerial Reconnaissance ......................... 20
CHAPTER 3-Causes and Types of
                                                                                          Drilling ..............................            20
                                                                         9
Landslides .................................
                                7 ................................. 9                     Geophysical Studies ............................. 21
   What Is a Landslide
   Why Do Landslides Occur 7 .......................... 9                                 Computerized Landslide Terrain
                                                                                          Analysis ..............................            21
       Human Activities ................................. 9
                                                                                          Instrumentation ..............................     21
       Natural Factors ................................ .9
                                                                                      Anticipating the Landslide Hazard .......... 21
           Climate ................................                .9
                                                                                      Tlranslation of Technical Information
                                                                       10
           Erosion ................................
                                                                                      to Users ..............................                21
           Weathering ................................                 10
                                                                                          Regional Mapping .............................. 22
           Earthquakes ................................ 11
                                                                                          Commrunity-Level Mapping ................. 22
           Rapid Sedimentation ...................... 11
                                                                                          Site-Specifie Mapping .......................... 22
           Wind-Generated Waves ................... 11
                                                                                                                                             22
                                                                                          Types of Maps ..............................
           Tidal or River Drawdown ............... 11
                                                                                              Landslide Inventories .................... 22
   Types of Landslides ................................ 11
                                                                                              Landslide Susceptibility Maps ...... 23
                                                                       11
       Falls ................................
                                                                                              Landslide Hazard Maps ................. 25
                                                                       11
       Toopple ................................
                                                                                   CHAPTER 5-Transferring and
                                                                       11
       Slides ................................
                                                                                  Encouraging the Use of Information ....... 26
           Rotational Slide .............................. 12
                                                                                      Information Transfer ............................... 26
           Translational Slide ......................... 12
                                                                                          Users of Landslide Hazard
                Block Slide ................................ 13
                                                                                          Information ...............................        27
       Lateral Spreads ................................ 13
                                                                                      Developing an Information Base:
                                                                       13
       Flows ................................
                                                                                      Sources of Landslide Hazard
                                                                       13
           Creep ................................
                                                                                                                                             28
                                                                                      Information ..............................
                                                                       13
           Debris Flow ................................
                                                                                   CHAPTER 6-Landslide Loss-Reduc-
           Debris Avalanche ........................... 13
                                                                                                                                             30
                                                                                   tionTechniques ..............................
                                                                       14
           Earthflow ................................
                                                                                      Preventing or Minimizing Exposure
                                                                       14
           Mudflow ................................

                                                                                                                                           fli
                                                  Contents Continued
  to Landslides ................................        30              Local Needs ...............................     39
      Land-Use Regulations ......................... 30                 Step 6 - Formulation of Goals and
  Reducing the Occurrence of Landslides                                 Objectives ...............................      40
  and Managing Landslide Events .............. 30                          Local Landslide Hazard
      Building and Grading Codes ............... 30                        Mitigation ...............................   40
      Emergency Management ..................... 31                        Development of Mitigation
  Controlling Landslide-Prone Slopes                                       Projects ...............................     40
  and Protecting Existing Structures .......... 31                      Step 7 - Establishment of a
      Precautions Concerning Reliance                                   Permanent State Hazard
      on Physical Methods ............................ 32               Mitigation Organization ...................... 41
      Design Considerations and Physical                                Step 8 - Review and Revision .............. 43
      Mitigation Methods .............................. 33      CHAPTER 8-Review and Revision of
CHAPTER 7-Plan Preparation................ 35                   the Plan and the Planning Process .......... 44
                                                                   Inventory of Landslide Costs ..................... 44
     Determining the Need for a State Plan ... . 35
        Federal Disaster Relief and                                Evaluation of Mitigation Projects and
        Emergency Assistance Act                                   Techniques ...............................           44
        (Section 409) ................................     35         Examples of Innovative Mitigation
     The Planning Team ................................ 36            Approaches ...............................        45
     The Planning Process ................................ 37      Analyses of Local Mitigation Programs ....45
        Step 1 - Hazard Analysis ..................... 37       CHAPTER 9-Approaches for Over-
                                                                coming Anticipated Problems ................... 46
         Step 2 - Identification of Impacted
         Sites ................................          37        Organizational Problems ........................... 46
                                                                   Management Problems .............................. 46
         Step 3 - Technical Information
         Transfer ................................       38        Financial Problems ...............................   46
                                                                   Coordination Problems .............................. 47
         Step 4 - Capability Assessment ........... 38
                                                                REFERENCES CITED ...............................        48
         Step 5 - Determination of Unmet

iv
                                                     Figures
                                                                   15. Debris fan formed by debris flows .......... 15
       Map showing relative potential of
la.
                                                                                                                            15
                                                                   16a. Earthiflow .................................
       different parts of the conterminous
                                                                   16b. Roan Creek Eartlilow, DeBeque,
       United States to landsliding .................... 1
                                                                                                                            15
                                                                        Colorado ................................
       Potential landslide hazard in
lb.
                                                                   17. Damage from Slide Mountain
                                                               2
       Maine .............................
                                                                        landslide, Nevada ................................ 16
       Major damage from debris flow,
2.
                                                                   18. Jackson Springs landslide,
                                                               5
       Farmington,Utah ............................ 6
                                                                        Franklin D.Roosevelt Lake,
                                                  Utah ....... 5
       "Bucket brigade," Farmington,
3.
                                                                        Washington state ................................. 17
       Landslide losses in Japan 1938-1981 ...... 6
4.
                                                                   1L9. Aerial view of the Thistle landslide,
       The relationship of people, landslides,
5.
                                                                                                                            18
                                                                        Utah ................................
                                                        .      7
       and disasters ............................
                                                                   20. Landslide inventory map, Durango,
       Aerial view of the Savage Island
6.
                                                                                                                            23
                                                                        Colorado .................................
       landslide, Washington state .................. 10
                                                                   21. Landslide inventory map, La Honda,
       Ruins of home destroyed in Kanawha
7.
                                                                                                                            24
                                                                        California .................................
                                                             10
       City, West Virginia ............................
                                                                                      susceptibility map, King
                                                                   22. Landslide
       Rockfall .11
8a.
                                                                        County, Washington ................................ 24
       Rockfiall on U.S. Highway 6,
8b.
                                                                   23. Earthquake landslide hazard map,
       Colorado .11
                                                                        San Mateo County, California ................ 25
       Topple .12
9a.
                                                                   24. Hazardous area warning sign ................ 31
                                                              12
                                                   .
       Topple, western 'Colorado
9b.
                                                                   25. Warning system schematic ..................... 32
       Rotational slide .12
lOa.
                                                                   26. Rudd Creek debris basin, Farmington,
       Rotational slide, Golden, Colorado .                   13
lb.
                                                                                                                            32
                                                                        Utah ................................
       Translational slide .13
11.
                                                                   27. Retaining wall, Interstate 70,
       Block slide .1
12.
                                                                                                                            33
                                                                         Colorado ................................
       Lateral spread .14
13a.
                                                                   28. Executive Order establishing
                                                              14
       Lateral spread, Cortez, Colorado.
13b.
                                                                        Colorado Natural Hazards Mitigation
       Creep .14
14a.
                                                                                                                            42
                                                                         Council ....
       Creep, Mt. Vernon Canyon, Colorado .... 14
14b.

                                                        Tables
                                                                                                                             26.
                                                                       information .......................................
1. Estimates of minimum landslide
                                                                   4. Potential users of landslide hazard
   damage in the United States,
                                                                                                                        27
                                                                      information ..............................
                                                              2
   1973-1983 .............................
                                                                   5. Examples of producers and providers
2. Techniques for reducing landslide
                                                                      of landslide hazard information ............... 29
                                                              8
   hazards ..............................
                                                                   6. Physical mitigation methods ..................... 33
3. Examples of resources available for
                                                                   7. Capability assessment checklist ............... 38
   -obtaining/transferring landslide

                                                                                                                              V
                                       Foreword

There is a need for a comprehensive program          demonstrates that successful programs can be
to reduce landslide losses in the United States      put into place with reasonable costs. Numerous
that marshals the capability of all levels of gov-   examples of responsible landslide hazard
ernment and the private sector. Without such a       planning and mitigation by private developers
program, the heavy and widespread losses to          exist but are usually overshadowed by impro-
the nation and to individuals from landslides        per development that ignores the hazard.
will increase greatly. Successful and cost-effec-         Transfer of proven governmental and pri-
tive landslide loss-reduction actions can and        vate sector landslide hazard mitigation tech-
should be taken in the many jurisdictions fac-       niques to other jurisdictions throughout the
ing landslide problems. The responsibility for       nation is one of the most effective ways of help-
dealing with landslides principally falls upon       ing to reduce future landslide losses. This
state and local governments and the private          guide, prepared by the State of Colorado for the
sector. The federal government can provide re-       Federal Emergency Management Agency,
search, technical guidance, and limited funding      builds upon the impressive efforts taken by
assistance, but to meet their responsibility for     Colorado state and local governments in plan-
maintaining the public's health, safety and          ning for and mitigating landslide losses. The
welfare, state and local governments must            Federal Emergency Management Agency hopes
prevent and reduce landslide losses through          that this guide and the accompanying plan for
hazard mapping, land-use management, and             landslide hazard mitigation will stimulate and
building and grading controls. In partnership        assist other state and local governments, priv-
with public interest groups and governments,         ate interests, and citizens throughout the na-
the private sector must also increase its efforts    tion to reduce the landslide threat.
to reduce landslide hazards.
     Dramatic landslide loss reduction can be                          Arthur J. Zeizel
achieved. The effective use of landslide build-                        Project Officer
ing codes and grading ordinances by a few state                        Federal Emergency
and local governments in the nation clearly                               Management Agency

vi
                                Acknowledgments

                                                       butors included: David B. Prior of the Coastal
This project was funded in part by the Federal
                                                       Studies Institute of Louisiana State University
Emergency Management Agency TFEMA), the
                                                       and William J. Kockelman of the U.S. Geo-
Colorado Division of Disaster Emergency Serv-
                                                       logical Survey. Project management was pro-
ices (DODES), the Colorado Geological Survey
                                                       vided by Arthur Zeizel (FEMA) and Irwin
(CGS), and the U.S. Geological Survey (Grant
                                                       Glassman (DODES).
No. 14-08-0001-A0420).
                                                            Other essential project personnel included:
     The document was written and prepared
                                                       Cheryl Brchan (drafting and layout), Nora
by Robert L. Wold, Jr. (DODES) and Can-
                                                       Rimando (word processing), and David
dace L. Jochim (CGS). Staffl contributors
                                                       Butler (editing).
included: William P. Rogers, Irwin M. Glass-
man, and John 0. frulby. Additional contri-

                                       Advisory Committee
                                                       George Mader, President, William Spangle and
John Beaulieu, Deputy State Geologist, Oregon
                                                           Associates, California
John P. Byrne, Director, DisasterEmergency Services,
                                                       Dr. Robert L. Schuster, U.S. GeologicalSurvey,
    Colorado
                                                            Colorado
William J. Kockelman, Planner, U.&. Geological
                                                       Dr. James E. Slosson, ChiefEngineering Geologist,
    Survey, California
                                                            Slosson.andAssociates, California
Peter Lessing, Environmental Geologist, West
                                                       Darrell WalIer, State Coordinator,Bureau of Disaster
    Virginia GeologicalSurvey
                                                            Services, Idaho

                                                                                                        vii
viii
                                                                      Introduction

According to available information, landsliding      control. The failure to lessen the problem is
                                                     primarily due to the ever-increasing pressure
in the United States causes an average of 25 to
                                                     of development in areas of geologically hazard-
50 deaths (Committee on Ground Failure Haz-
                                                     ous terrain and the failure of responsible gov-
ards, 1985) and $1 to $2 billion in economic
                                                     ernment entities and private developers to
losses annually (Schuster and Fleming, 1986).
                                                     recognize landslide hazards and to apply ap-
Although all 50 states are subject to landslide
activity, the Rocky Mountain, Appalachian, and       propriate measures for their mitigation, even
Pacific Coast regions generally suffer the great-    though there is overwhelming evidence that
est landslide losses (Figures la, b). The costs of   landslide hazard mitigation programs serve
landsliding can be direct or indirect and range      both public and privatPinterests by saving
                                                     many times the cost of implementation. The
from the expense of cleanup and repair or
                                                     high cost of landslide damage (Table 1) will
replacement of structures to lost tax revenues
and reduced productivity and property values.        continue to increase if community development
                                                     and capital investments continue without tak-
     Landslide losses are growing in the United
States despite the availability of successful        ing advantage of the opportunities that cur-
techniques for landslide management and              rently exist to mitigate the effects of landslides.

Figure la. Map showing relativepotential of different parts of the conterminous United States
to landsliding (U.S. Geological Survey, 198 la).

                                                                                                       1
Table 1.Estimates of minimum amounts of                                                          EXPLANATION
landslide damage in the United States,
                                                                                                 *    High
1973-1983, in millions of dollars. All figures
are estimates. Figures queried are very
                                                                                                   ' RMedium
rough estimates (adapted from Brabb, 1984).
                                                                                                         Low ?
                                                                                                   m
                    Damage 1973-1983
                     State    Priv.               Ann.
                                                                                                    D    Low
                     Roads    Prop.      Total    Avg.
State                 ($M)     ($M)      ($M)     ($M)

Alabama               10.0      0.5      10.5      1.05
Alaska                10.0               10.0
                                0.0                1.0
Arizona                2.0      0.0       2.0      0.2
Arkansas               2.0      0.0       2.0      0.2
                                                                                     wJ
                                                                                      Figure lb.
                                                                                 w
California           800.0?   200.0?   1000.0?   100.0 ?
                                                                                        Potential landslide
Colorado              20.0                         7.0
                               50.0      70.0
Connecticut            0.0                         0.0
                                0.0       0.0
                                                                                        hazard in Maine
Delaware               2.0      0.0       2.0      0.2
                                                                                        (Wiggins et al., 1978).
Dist. of Columbia                                  0.8
                       0.1      0.1       0.01
Florida                                            0.0
                       0.0      0.0       0.0
Georgia                                            0.1 ?
                       1.0?     0.0       1.0?
Hawaii                                             0.45
                       4.0      0.5       4.5
Idaho                                              1.1 ?
                      10.0?     1.0?     11.0?
                                                                The widespread occurrence of landsliding,
Illinois                                           0.2 ?
                                          2.0?
                       1.0      1.0?
                                                           together with the potential for catastrophic
Indiana                                            1.1
                      10.0      1.0      11.0
                                0.3
Iowa                                               0.13
                       1.0                1.3              statewide and regional impacts, emphasizes
Kansas                                             0.13
                       1.0      0.3?      1.3?
                                                           the need for cooperation among federal, state,
Kentucky                                          19.0?
                     180.0     10.0?    190.0?
                                                           and local governments and the private sector.
Louisiana                       0.3                0.23
                       2.0                2.3
                       0.3      0.3                0.06
Maine                                     0.6              Although annual landslide losses in the U.S.
                      20.0
Maryland                        0.0                2.0
                                         20.0
                                                           are extremely high, significant reductions in
                       0.3                         0.03
Massachusetts                   0.0       0.3
                                                           future losses can be achieved through a comb-
Michigan               0.1                         0.01
                                0.0       0.1
                                                           ination of landslide hazard mitigation and
Minnesota              7.0                         0.7
                                0.0       7.0
Mississippi            3.0      0.5                0.35
                                          3.5
                                                           emergency management.
Missouri               2.0?     1.0?               0.3 ?
                                          3.0?
                                                                Landslide hazard mitigation consists of
Montana               10.0?     1.0?               1.1 ?
                                         11.0?
                                                           those activities that reduce the likelihood of
Nebraska               0.4      0.4?               0.08?
                                          0.8?
                       2.0?     0.5                0.25?
Nevada                                    2.5?             occurrence of damaging landslides and mini-
New Hampshire         10.0      0.0                1.0
                                         10.0
                                                           mize the effects of the landslides that do occur.
New Jersey             3.0      3.0                0.6
                                          6.0
                                                           The goal of emergency management is to mini-
New Mexico             3.0      1.0                0.4
                                          4.0
                                                           mize loss of life and property damage through
New York              20.0     50.0?               7.0 ?
                                         70.0?
North Carolina                  0.5                4.55
                                         45.5
                      45.0
                                                           the timely and efficient commitment of avail-
North Dakota                                       0.4
                                0.0
                       4.0                4.0
                                                           able resources.
Ohio                                              10.0
                      60.0?    40.0     100.0?
                                                                Despite their common goals, emergency
Oklahoma                                           0.2?
                       2.0?     0.0       2.0?
Oregon                                             4.0
                                         40.0
                      30.0     10.0                        management and hazard mitigation activities
Pennsylvania                                       6.0
                               10.0?     60.0?
                      50.0
                                                           have historically been carried out independ-
Rhode Island                                       0.0
                       0.0      0.0       0.0
                                                           ently. The integration of these two efforts is
South Carolina                                     0.0
                       0.0      0.0       0.0
South Dakota                                       1.8     most often demonstrated in the recovery phase
                      16.0      2.0      18.0
Tennessee                                         11.0 ?
                     100.0     10.0?    110.0?
                                                           following a disaster, when decisions about re-
Texas                                              0.8
                       8.0      0.0       8.0
                                                           construction and future land uses in the com-
Utah                                              21.0 ?
                                        210.0?
                     200.0?    10.0?
                                                           munity are made.
Vermont                         0.5                0.35
                       3.0                3.5
                                1.0
Virginia                                           1.2
                      11.0               12.0
Washington                     30.0?              10.0?              Emergency management, if well executed,
                                        100.0?
                      70.0?
West Virginia                                     27.5         can do much to minimize the loss and suffering
                     270.0      5.0     275.0
Wisconsin                       0.5                0.07
                       0.2                0.7                  associated with a particular disaster. However,
Wyoming                         0.0                0.4
                       4.0                4.0                  unless it is guided by the goals of preventing or
                                                               reducing long-term hazard losses, it is unlikely
Total (U.S.)        2010.3    442.2    2452.5    245.25
                                                               to reduce the adverse impact of future disasters

2
                                                          The purpose of this guidebook is to provide
    significantly. This is where mitigation becomes
    important (Advisory Board on the Built Envir-     a practical, politically feasible guide for state
    onment, 1983, p. 9).
                                                      and local officials involved in landslide hazard
                                                      mitigation. The guidebook presents concepts
                                                      and a framework for the preparation of state
     Purpose of this Guidebook                        and local landslide hazard mitigation plans. It
As mentioned above, the development and im-
                                                      outlines a basic methodology, provides informa-
plementation of landslide loss-reduction strate-
                                                      tion on available resources, and offers suggest-
gies requires the cooperation of many public          ions on the formation of an interdisciplinary
and private institutions, all levels of govern-
                                                      mitigation planning team and a permanent
ment, and private citizens. Coordinated and
                                                      state natural hazards mitigation organization.
comprehensive systems for landslide hazard            Individual states and local jurisdictions can
mitigation do not currently exist in most states
                                                      adapt the suggestions in this book to meet
and communities faced with the problem. In
                                                      their own unique needs.
most states, local governments often take the
                                                          Because of its involvement in identifying
lead by identiyring goals and objectives, con-
                                                      and mitigating landslide hazards, the state of
trolling land use, providing hazard information
                                                      Colorado was selected by the Federal Emer-
and technical assistance to property owners           gency Management Agency (FEMA) to produce
and developers, and implementing mitigation
                                                      a prototype state landslide hazard mitigation
projects as resources allow. State and federal        plan. The technical information contained in
agencies play supporting roles-primarily
                                                      the plan was designed to be transferable to
financial,, technical, and administrative. In
                                                      other states and local jurisdictions and suit-
some cases, however, legislation originating at
                                                      able for incorporation into other plans. The
the state or federal level is the sole impetus for
                                                      planning process can also serve as an example
stimulating effective local mitigation activity.
                                                      to other states and localities dealing with land-
      In many states there remains a need to de-
                                                      slide problems. The materials contained in the
velop long-term organizational systems at state
                                                      Colorado Landslide Hazard Mitigation Plan
and local levels to deal with landslide hazard
                                                      (Colorado Geological Survey et al., 1988) were
mitigation in a coordinated and systematic            intended to complement the information pre-
manner. The development of a landslide hazard
                                                      sented in this guidebook. In an effort to pro-
mitigation plan can be the initial step in the        mote landslide hazard mitigation nationally,.
 establishment of state and local programs that
                                                      FEMA has provided for the distribution of
promote long-term landslide loss reduction.
                                                                                                        L
                                                      these two documents to all states.

                                                                                                        3
Landslide Losses and
the Benefits of Mitigation
         The Landslide Hazar, d;                         CALIFORNIA-In 1982 in the San Fran-
Landsliding is a natural process whic h occurs      cisco Bay Region, 616 mm (24.3 in.) of rain fell
and recurs in certain geologic settingEhunder       in 34 hours causing thousands of landslides
certain conditions. The rising costs of landslide \ which killed 25 people and caused more than
damages are a direct consequence of t the in-       $66 million in damage (Keefer et al., 1987).
creasing vulnerability of people and s-tructures         TEXAS-In Dallas in the 1960s, a toppl-
                                                     :
to the hazard. In most regions, the ov erall rate   ing failure occurred in a vertical exposure of a
of occurrence and severity of naturallyy caused ; geological formation known as the Austin
landslides has not increased. What hi as increas-   Chalk. This closed two lanes of a major down-
ed is the extent of human occupation, of these      town thoroughfare for eight months. Costs of
lands and the impact of human activi ties on        construction of remedial measures and con-
the environment. Many landslide dan nages that      struction delays amounted to about $2.8 mil-
have occurred might have been prevelnited or        lion (Allen and Flanigan, 1986).
avoided if accurate landslide hazard i informa-      V UTAH-In 1983, a massive landslide dam-
                                                    med Spanish Fork Canyon, creating a lake.
tion had been available and used.
                                                    The landslide buried sections of the Denver and
                                                    Rio Grande Western Railroad and U.S. High-
    Economic and Social Iml aacts                    ways 6, 50, and 89 and inundated the town of
          of Landsliding                             Thistle. The estimated total losses and recon-
                                                     struction costs due to this one landslide range
            Costs of Landsliding                     from $200 million (University of Utah, Bureau
The most commonly cited figures on I andslide        of Economic and Business Research, 1984) to
losses are $1 to $2 billion in economic losses       $600 million (Kaliser and Slosson, 1988).
and 25 to 50 deaths annually. HowevEor, these             WEST VIRGINIA-In 1975, landslide
figures are probably conservative bec, iuse they     movements in colluvial soil damaged 56 houses
were generated in the late 1970s. Sini ce that       in McMechen, West Virginia, located on a hill-
time, the use of marginally suitable lEnd has
                                         i           side above the Ohio River. This landslide was
increased, as has inflation. Furthermi ore, there    attributed to above normal precipitation. Mit-
are no exhaustive compilations of lani islide loss   igation was accomplished by grading and
data for the United States, so these figgures are    surface and subsurface drainage (Gray and
basically extrapolations of the availal )le data.    Gardner, 1977).
     The high losses from landsliding; are illus-
                                                            Impacts and Consequences
trated in Table 1. Surveys indicate thuatdamage
                                                                    of Landsliding
to private property accounts for 30 to 50 per-
cent of the total costs (U.S. Geological l Survey,   Economic losses due to landsliding include both
1982). Examples of costs associated w ith indivi-    direct and indirect costs. Schuster and Fleming
dual landslide events from represents Ltive          (1986) define direct costs as the costs of re-
areas across the country include:                    placement, repair, or maintenance due to dam-
     ALASKA-It has been estimated I (Youd,           age to property or facilities within the actual
1978) that 60 percent of the $300 mill ion dam-      boundaries of a landslide (Figure 2). Such
age from the 1964 Alaska earthquake was the          facilities include highways, railroads, irrigation
direct result of landslides.                         canals, underwater communication cables,

4
offshore oil platforms, pipelines, and dams. The
cost of cleanup must also be included (Figure
3). All other landslide costs are considered to
be indirect. Examples of indirect costs given by
Schuster and Fleming (1986) include:
     (1) reduced real estate values,
     (2) loss of productivity of agricultural or
         forest lands,
     (3) loss of tax revenues from properties
         devalued as a result of landslides,
         costs of measures to prevent or mitigate
     (4)
         future landslide damage,
     (5) adverse effects on water quality in
         streams,                                   Figure 3. Local volunteers form "bucket
     (6) secondary physical effects, such as        brigade" to help clean mud and debris from
         landslide-caused flooding, for which       homes in Farmington, Utah in 1983
         the costs are both direct and indirect,    (photograph by Robert Kistner, Kistner and
     (7) loss of human productivity due to          Associates).
         injury or death.
Other examples are:
                                                    Long-Term Benefits of Mitigation
     (8) fish kills,
     (9) costs of litigation.                       Studies have been conducted to estimate the
     In addition to economic losses, there are      potential savings when measures to minimize
intangible costs of landsliding such as personal    the effects of landsliding are applied. One early
stress, reduced quality of life, and the destruc-   study by Alfors et al. (1973) attempted to fore-
tion of personal possessions having great sen-      cast the potential costs of landslide hazards in
timental value. Because costs of indirect and       California for the period 1970-2000 and the
intangible losses are difficult or impossible to    effects of applying mitigative measures. Under
calculate, they are often undervalued or ignor-     the conditions of applying all feasible measures
ed. If they are taken into account, they often      at state-of-the-art levels (for the 1970s), there
produce highly variable estimates of damage         was a 90 percent reduction in losses for a bene-
for a particular incident.                          fit/cost ratio of 8.7:1, or $8.7 saved for every $1
                                                    spent. Nilsen and Turner (1975) estimated that
                                                    approximately 80 percent of the landslides in
                                                    Contra Costa County, California are related to
                                                    human activity. In Allegheny County, Penn-
                                                    sylvania, 90 percent are related to such activity
                                                    according to Briggs et al. (1975).
                                                         Because most landslides triggered by man
                                                    are directly related to construction activities,
                                                    appropriate grading codes can significantly
                                                    decrease landslide losses in urban areas. Slos-
                                                    son (1969) compared landslide losses in Los
                                                    Angeles for those sites constructed prior to
                                                    1952, when no grading codes existed and soils
                                                    engineering and engineering geology were not
Figure 2. Major damage to homes in                  required, with losses sustained at sites after
Farmington, Utah as a result of 1983 Rudd           such codes were enacted. He found that the
Creek mudslide (photograph by Robert                monetary losses were reduced by approximat-
Kistner, Kistner and Associates).                   ely 97 percent.

                                                                                                     5
        The Cincinnati, Ohio Study                   in the Kobe area. However, since the Japanese
                                                     program went into effect, losses have decreased
In 1985, the U.S. Geological Survey, in cooper-
                                                     dramatically. In 1976-one of Japan's worst
ation with the Federal Emergency Managament
                                                     years for landsliding-only 2000 homes were
Agency, conducted a geologic/economic develop-
                                                     destroyed with fewer than 125 lives lost
ment study in the Cincinnati, Ohio area. This
                                                     (Schuster and Fleming, 1986).
study developed a systematic approach to
quantitative forecasting of probable landslide
activity. Landslide probabilities derived from a                            NUMBER OF DEAD OR MISSING [TOP BAR)
reproducible procedure were combined with
property value data to forecast the potential                                        0      C0         00
                                                                                                     000o
                                                                                            C                                  0       -               -
                                                                                                                     >
                                                                                                                     -CD                       -

                                                                                 U UU U
                                                           JULY 1938
economic losses in scenarios for proposed                                                                           - II-130,000
                                                                                                                        '--
                                                                                   _                         -                             -
                                                           JULY 1945
development and to quantitatively identify the             SEPT. 1947
potential benefits of mitigation activities.                                         r_      _       _       _---
                                                           JULY 1951                                                                           _ _ ,15,141
                                                                                                                                I-
      The study area was divided into 14,255               JUNE 1953

grid cells of 100-square meters each. Informa-        E JULY 1953
                                                      o    AUG.1953
tion calculated for each cell included: probabil-
                                                                                                                           ..
                                                                                 _ _ _ U em                           U                            _       r19.754
                                                                        *
                                                      0 SEPT. 1958
ity of landslide occurrence, economic loss in the                       W
                                                      Z    AUG. 1959
event of a landslide, cost of mitigation, and         Ln JUNE 1961
                                                                        r
economic benefit of mitigation. This informa-         C0 SEPT. 1966
                                                      1    JULY 1967
tion was used to develop a mitigation strategy.
                                                      Ad JULY 1967
In areas where both slope and shear strength               JULY1972
                                                      -)
                                                                        I-
information were available, the optimum strat-        z AUG.1972
egy required mitigation in those cells with           -I   JULY 1974

                                                                        r
                                                      = AUG.1975
slopes steeper than 14 degrees or where mater-        0
                                                      e    AUG.1975
ials had effective residual stress friction angles         SEPT.1976
of less than 26 degrees. This strategy yielded              MAY1978
                                                                        I
$1.7 million in estimated annualized net bene-              OCT.1978

fits for the community. In areas where only                AUG.1979
                                                                                         1111
slope information was used, the best strategy               AUG.1981

required mitigation in those cells where slopes                                                  0           0   0N        0       0       0           0
                                                                             -
                                                                                                                    rcc        0       o
                                                                                                         0          N                  R
                                                                                 ;       CM CO           e

were greater than 8 degrees. This yielded an                            NUMBER OF HOUSES DESTROYED OR BADLY
estimated annualized net benefit of $1.4 mil-                                  DAMAGED [BOTTOM BAR]
lion. Therefore, using regional geologic inform-
ation in addition to slope information resulted
                                                     Figure 4. Losses due to major landslide
in an additional $300,000 net benefit. The
Cincinnati study cost only $20,000 to prepare        disasters (mainly debris flows) in Japan from
                                                     1938-1981. All of these landslides were
(Bernknopf et al., 1985).
                                                     caused by heavy rainfall, most commonly
    The Benefits of Mitigation in Japan              related to typhoons, and many were assoc-
Japan has what is considered by many to be           iated with catastrophic flooding (data from
the world's most comprehensive landslide loss        Ministry of Construction, Japan, 1983).
reduction program. In 1958, the Japanese gov-
ernment enacted strong legislation that provid-
                                                            Planning as a Means of Loss
ed for land-use planning and the construction
of check dams, drainage systems, and other
                                                                     Reduction
physical controls to prevent landslides. The
                                                     The extent and severity of the landslide hazard
success of the program is indicated by the
                                                     in a particular area will determine the need for
dramatic reduction in losses over time (Figure
                                                     a landslide hazard mitigation plan.
4). In 1938, 130,000 homes were destroyed and
more than 500 lives were lost due to landslides

6
                                                      for actions, or failures to act, that are deter-
          Communities that have landslide prob-
    lems are encouraged to assess the costs of        mined to contribute to personal injuries and
    damage to public and private property and
                                                      property damages caused by natural hazards.
    weighl those costs against the costs of a land-
                                                      Consequently, a model community landslide
    slide reduction program. The prevention of a
                                                      hazard management planning process should
    single major landslide in a community may
    more than compensate for the effort and cost of   encourage citizen participation and review in
    implementing a control program (Fleming and       order to identity and address the perspectives
    Taylor, 1980, p. 20).
                                                      and concerns of the various community groups
      Avoiding the costs of litigation is an addi-    affected by landslide hazards.
tional incentive to undertaking a local program            Because most landslide damages are relat-
                                                      ed to human activity-mainly the construction
of landslide hazard mitigation.
      When landslide disasters do occur, the ex-      of roads, utilities, homes, and businesses-the
istence of a program for loss reduction should        best opportunities for reducing landslide
help ensure that redevelopment planning takes         hazards are found in land-use planning and
                                                      the .administration and enforcement of codes
existing geologic hazards into account.
      In the U.S., only a few communities have         and ordinances.
                                                           The vulnerability of people to natural haz-
established successful landslide loss reduction
programs. The most notable is Los Angeles,             ards is determined by the relationship between
where, as mentioned above, loss reductions of         the occurrences of extreme events, the proximi-
97 percent have been achieved for new con-            ty of people to these occurrences, and the
struction since the implementation of modern           degree to which the people are prepared to cope
grading regulations (Slosson and Krohn, 1982).        with these extremes of nature. The concept of a
                                                      hazard as the intersection of the human sys-
      In communities that have achieved loss
reductions, decisions about building codes,           tem and the physical system, is illustrated in
                                                      Figure 5. Only when these two systems are in
zoning, and land use take into account identi-
                                                       conflict, does a landslide represent a hazard to
fied landslide hazards. The U.S. Geological
                                                       public health and safety.
Survey (1982) has found that these conuni-
ties have in conmnon four preconditions leading
to successful mitigation programs: (1) an
adequate base of technical information about
the local landslide problem, (2) an "able and
concerned" local government, (3) a technical
community able to apply and add to the tech-
nical planning base, and (4) an informed pop-
ulation that supports mitigation program ob-
jectives. While the technical expertise to reduce
landslide losses is currently available in most
 states, in many cases it is not being utilized.
 Still, the success of loss reduction measures
 clearly depends upon the will of leaders to
                                                      Figure5 The relationshipof people, land-
 promote and support mitigation initiatives.
                                                      slides, and hazards(modified from Colorado
                                                      Water Conservation Board et a., 1985).
          Local Government Roles
At the local government level, hazard mitiga-
tion is often a controversial issue. Staff and             The effectiveness of local landslide mitiga-
elected officials of local governments are
                                                      tion programs is generally tied to the ability
usually subjected to diverse and sometimes
                                                      and determination of local officials to apply the
conflicting pressures regarding land use and
                                                      mitigation techniques available to them to
development. Local officials, as well as build-
                                                      limit and guide growth in hazardous areas. A
ers, realtors, and other parties in the develop-
                                                      list of 27 techniques that planners and mana-
ment process, are increasingly being held liable
                                                                                                      7
gers may use to reduce landslide hazards in          Regulating new development in hazardous
their communities is presented in Table 2. The       areas by:
key to achieving loss reduction is the identifica-       Enacting grading ordinances
tion and implementation of specific mitigation           Adopting hillside-development regulations
initiatives, as agreed upon and set forth in a           Amending land-use zoning districts and
local or state landslide hazard mitigation plan.            regulations
                                                         Enacting sanitary ordinances
Table 2. Techniques for reducing landslide               Creating special hazard-reduction zones
hazards (Kockelman, 1986).                                  and regulations
                                                         Enacting subdivision ordinances
Discouraging new developments in hazardous               Placing moratoriums on rebuilding
areas by:
    Disclosing the hazard to real-estate buyers      Protecting existing development by:
    Posting warnings of potential hazards                Controlling landslides and slumps
    Adopting utility and public-facility                 Controlling mudflows and debris-flows
       service-area policies                             Controlling rockfalls
    Informing and educating the public                   Creating improvement districts that
    Making a public record of hazards                       assess costs to beneficiaries
                                                         Operating monitoring, warning, and
Removing or converting existing development                 evacuating systems
through:
    Acquiring or exchanging hazardous
       properties                                         Although certain opportunities for
    Discontinuing nonconforming uses                 reducing landslide losses exist at the state
    Reconstructing damaged areas after               government level (selection of sites for schools,
       landslides                                    hospitals, prisons, and other public facilities;
    Removing unsafe structures                       public works projects that protect highways
    Clearing and redeveloping blighted areas         and state property), the greatest potential for
      before landslides                              mitigation is in the routine operations of local
                                                     government: the adoption and enforcement of
Providing financial incentives or disincentives      grading and construction codes and ordinances,
by:                                                  the development of land-use and open-space
    Conditioning federal and state financial         plans, elimination of nonconforming uses,
       assistance                                    limitation of the extension of public utilities,
    Clarifying the legal liability of property       etc. For this reason, state mitigation plans
       owners                                        should emphasize mitigation activities that
    Adopting lending policies that reflect risk      will essentially encourage and support local
       of loss                                       efforts. Local mitigation plans should provide
    Requiring insurance related to level of          guidelines and schedules for accomplishing
       hazard                                        local mitigation projects, as well as identify
    Providing tax credits or lower assessments       projects beyond local capability that should be
       to property owners                            considered in the state plan.                    U

8
                      Causes and Types of Landslides

                                                     ground-water regimes. Changes in slope result
         What is a Landslide?
                                                     from terracing for agriculture, cut-and-fill
The term "landslide" is used to describe a wide
                                                     construction for highways, the construction -of
variety of processes that result in the percept-
                                                     buildings and railroads, and mining operations.
ible downward and outward movement of soil,
                                                     If these activities and facilities are ill-conceiv-
rock, and vegetation under gravitational influ-
                                                     ed, or improperly designed or constructed, they
ence. The materials may move by: falling, top-
                                                     can increase slope angle, decrease toe or lateral
pling, sliding, spreading, or flowing.
                                                     support, or load the head of an existing or pot-
     Although landslides are primarily associ-
                                                     ential landslide. Changes in irrigation or sur-
ated with steep slopes, they also can occur in
                                                     face runoff can cause changes in surface drain-
areas of generally low relief. In these areas
                                                     age and can increase erosion or contribute to
landslides occur as cut-and-fill failures (high-
                                                     loading a slope or raising the ground-water
way and building excavations), river bluff fail-
                                                     table (Figure 6). The ground-water table can
ures, lateral spreading landslides, the collapse
                                                      also be raised by lawn watering, waste-water
of mine-waste piles (especially coal), and a wide
                                                      effluent from leach fields or cesspools, leaking
variety of slope failures associated with quar-
                                                     water pipes, swimming pools or ponds, and
ries and open-pit mines. Underwater landslides
                                                      application or conveyance of irrigation water. A
on the floors of lakes or reservoirs, or in
                                                     high ground-water level results in increased
offshore marine settings, also usually involve
                                                     pore-water pressure and decreased shear
areas of low relief and small slope gradients.
                                                      strength, thus facilitating slope failure. Con-
                                                     versely, the lowering of the ground-water table
     Why Do Landslides Occur?                         as a result of rapid drawdown by water supply
Landslides can be triggered by both natural          wells, or the lowering -ofa lake or reservoir, can
and man-induced changes in the environment.           also cause slope failure as the buoyancy pro-
The geologic history of an area, as well as          vided by the water decreases and seepage
activities associated with human occupation,          gradients steepen.
directly determines, or contributes to the con-
                                                                     Natural Factors
ditions that lead to slope failure. The basic
causes of slope instability are fairly well known.   There are a number of natural factors that can
They can be inherent, such as weaknesses in          cause slope failure. Some of these, such as
the composition or structure of the rock or soil;    long-term or cyclic climate changes, are not dis-
variable, such as heavy rain, snowmelt, and          cernible without instrumentation and/or
changes in ground-water level; transient, such       long-term record-keeping.
as seismic or volcanic activity; or due to new
                                                     Climate
environmental conditions, such as those
                                                     Long-term climate changes can have a signifi-
imposed by construction activity (Varnes and
                                                     cant impact on slope stability. An overall de-
the International Association of Engineering
                                                     crease in precipitation results in a lowering of
Geology, 1984).
                                                     the water table, as well as a decrease in the
                                                     weight of the soil mass, decreased solution of
              Human Activities
                                                     materials, and less intense freeze-thaw activity.
Human activities triggering landslides are
                                                     An increase in precipitation or ground satura-
mainly associated with construction and invol-
                                                     tion will raise the level of the ground-water
ve changes in slope and in surface-water and
                                                                                                       .9
                                                                            Figure 6.
                                                                            Aerial view of the
                                                                            Savage Island land-
                                                                            slide on the east
                                                                            shore of the
                                                                            Columbia River,
                                                                            Washington, 1981.
                                                                            This landslide was
                                                                            caused by irrigation
                                                                            water (photograph
                                                                            by Robert L.
                                                                            Schuster, U.S.
                                                                            Geological Survey).

                                                   ing), streams, rivers, waves or currents, wind,
table, reduce shear strength, increase the
                                                   and ice removes toe and lateral slope support of
weight of the soil mass, and may increase
                                                   potential landslides.
erosion and freeze -thaw activity. Periodic
high-intensity precipitation and rapid snow-
                                                   Weathering
melt can signifcantly increase slope instability
                                                   Weathering is the natural process of rock deter-
temporarily (Figure 7).
                                                   ioration which produces weak, landslide-prone
                                                   materials. It is caused by the chemical action of
Erosion
                                                   air, water, plants, and bacteria and the physical
Erosion by intermittent running water (gully

              Figure 7.
      The remains of a
   house where three
     children died in a
 mudflow in Kanawha
   City, West Virginia.
   The movement was
   triggered by heavy
 rainfall from a cloud-
 burst on July 9, 1973
 (Lessing et al., 1976).

10
                                                        I
action brought on by changes in temperature
                                                                                                    ~

(expansion and shrinkage), the freeze-thaw                                              FIRM BEDDED ROCK

cycle, and the burrowing activity of animals.
Earthquakes
Earthquakes not only trigger landslides, but,
over time, the tectonic activity causing them can
create steep and potentially unstable slopes.
Rapid sedimentation
Rivers supply very large amounts of sediment
to deltas in lakes and coastal areas. The rapid-
ly deposited sediments are frequently under-
consolidated, and have excess pore-water
pressures and low strengths. Such deltaic
sediments are often prone to underwater
delta-front landsliding, especially where the
sediments are rich in clay and/or contain gas
                                                        Figure 8a. Rockfall (Colorado Geological
from organic decomposition.
                                                        Survey et al., 1988).
Wind-generated waves
Storm waves in coastal areas are known to
trigger underwater landsliding in deltas by
cyclically loading weak bottom sediments.
Tidal or river drawdown
Rapid lowering of water level in coastal areas
or along river banks due to tides or river dis-
charge fluctuations can cause underwater land-
sliding. The process in which weak river bank
or deltaic sediments are left unsupported as
the water level drops is known as "drawdown."

          Types of Landslides
The most common types of landslides are des-            Figure 86. Rockfall on U S . Highway 6,
cribed below. These definitions are based               Colorado (photograph by Colorado
mainly on the work of Varnes (1978).                    Geological Survey).
                       Falls
Falls are abrupt movements of masses of                 separates from the main mass, falling to the
geologic materials that become detached from            slope below, and subsequently bouncing or
steep slopes or cliffs (Figures 8a, b). Movement        rolling down the slope (Figures 9a, b).
occurs by free-fall, bouncing, and rolling. De-
                                                        Slides
pending on the type of earth materials invol-
ved, the result is a rockfall, soilfall, debris fall,   Although many types of mass movement are
earth fall, boulder fall, and so on. All types of       included in the general term "landslide," the
falls are promoted by undercutting, differential        more restrictive use of the term refers to move-
weathering, excavation, or stream erosion.              ments of soil or rock along a distinct surface of
                                                        rupture which separates the slide material
                      Topple                            from more stable underlying material. The two
A topple is a block of rock that tilts or rotates       major types of landslides are rotational slides
forward on a pivot or hinge point and then              and translational slides.
                                                                                                        11
                                         Figure 9b. Topple, western Colorado (photo-
                                         graph by Colorado Geological Survey).
                                         Rotational slide
                                         A rotational slide is one in which the surface of
                                         rupture is curved concavely upward (spoon
                                         shaped) and the slide movement is more or less
                                         rotational about an axis that is parallel to the
                                         contour of the slope (FigureslOa, b). A "slump"
                                         is an example of a small rotational slide.
                                         Translational slide
                                         In a translational slide, the mass moves out, or
                                         down and outward along a relatively planar
                                         surface and has little rotational movement or
                                         backward tilting (Figure 11).The mass com-
                                         monly slides out on top of the original ground
                                         surface. Such a slide may progress over great

Figure 9a. Topple (Colorado Geological   Figure 1Oa. Rotational landslide (modified
Survey et al., 1988).                    from Varnes, 1978).

12
                                                    materials and are distinctive because they
                                                    usually occur on very gentle slopes. The fail-
                                                    ure is caused by liquefaction, the process
                                                    whereby saturated, loose, cohesionless sedi-
                                                    ments (usually sands and silts) are trans-
                                                    formed from a solid into a liquefied state; or
                                                    plastic flow of subjacent material. Failure is
                                                    usually triggered by rapid ground motion such
                                                    as that experienced during an earthquake, or
                                                    by slow chemcal changes in the pore water and
                                                    mineral constituents.
I$
          A                                                                              SLIP SURFACE
     "m

Figure 1Ob. Rotational landslide, Golden,
Colorado (photograph by Colorado
Geological Survey).

distances if conditions are right. Slide material
may range from loose unconsolidated soils to
extensive slabs of rock.

           GROUND SURFACE

                                                    Figure 12. Block slide (Colorado Geological
                                                    Survey et al., 1988).

                                                                        Flows
                                                    Creep
                                                    Creep is the imperceptibly slow, steady
                                                    downward movement of slope-forming soil or
                                                    rock. Creep is indicated by curved tree trunks,
                                                    bent fences or retaining walls, tilted poles or
                                                    fences, and small soil ripples or terracettes
                                                    (Figures 14a, b).
                                                    Debris flow
Figure 11. Translational slide (Colorado            A debris flow is a form of rapid mass movement
Geological Survey et a/., 1988).                    in which loose soils, rocks, and organic matter
                                                    combine with entrained air and water to form a
                                                    slurry that then flows downslope. Debris-flow
Block Slide. A block slide is a translational
                                                    areas are usually associated with steep gullies.
slide in which the moving mass consists of a
                                                    Individual debris-flow areas can usually be
single unit, or a few closely related units that
                                                    identified by the presence of debris fans a t the
move downslope as a single unit (Figure 12).
                                                    termini of the drainage basins (Figure 15).
               Lateral Spreads                      Debris avalanche
Lateral spreads (Figures 13a, b) are a result of    A debris avalanche is a variety of very rapid to
the nearly horizontal movement of geologic          extremely rapid debris flow.

                                                                                                        13
Earthflow                                          of snow and ice due to heat from volcanic vents;
Earthflows have a characteristic "hourglass"       or by the breakout of water from glaciers, crat-
shape (Figures 16a, b). A bowl or depression       er lakes, or lakes dammed by volcanic eruptions.
forms a t the head where the unstable material
collects and flows out. The central area is                                              SPREADING

narrow and usually becomes wider as it reach-
es the valley floor. Flows generally occur in
fine-grained materials or clay-bearing rocks on
moderate slopes and with saturated conditions.
However, dry flows of granular material are
also possible.
Mudflow
A mudflow is an earthflow that consists of
material that is wet enough to flow rapidly and
that contains at least 50 percent sand-, silt-,                                     OUT OF ALIGNMENT
and clay-sized particles.
Lahar
                                                   Figure 14a. Creep (Colorado Geological
A lahar is a mudflow or debris flow that origin-
                                                   Survey et al., 1988).
ates on the slope of a volcano. Lahars are
usually triggered by such things as heavy rain-
fall eroding volcanic deposits; sudden melting

Figure 13a. Lateral spread (Colorado
Geological Survey et al., 1988).

                                                   Figure 146. Creep, vicinity o Mt. Vernon
                                                                                f
Figure 136. Lateral spread, Cortez, Colorado.
                                                   Canyon, Jefferson County, Colorado (photo-
(Photograph by Colorado Geological
                                                   graph by Colorado Geological Survey).
Survey).

14
                                                Subaqueous landslide *
                                                Landslides which take place principally or tot-
                                                ally underwater in lakes, along river banks, or
                                                in coastal and offshore marine areas are called
                                                subaqueous landslides. The failure of subaque-
                                                ous slopes may result from a variety of factors
                                                acting singly or together, including rapid lacus-
                                               trine or marine sedimentation, biogenic meth-
                                                ane gas in sediments, surface water storm
                                               waves, current scour, water level drawdown,
                                                depositional oversteeping, or earthquake
                                                stresses. Many different types of subaqueous
                                               landslides have been identified in different
Figure 15. Debris fan formed by debris flows
                                               locations, including rotational and translation-
(Colorado Geological Survey et al., 1988).
                                               al slides, debris flows and mudflows, sand and
                                                silt liquefaction flows. There is also evidence
                                               that, in some circumstances, subaqueous land-
                                                slides evolve into or initiate turbidity currents,
                                               which may flow underwater a t high speeds for
                                               long distances. Subaqueous landslides pose pro-
                                               blems for offshore and river engineering, parti-
                                               cularly for the construction and maintenance of
                                               jetties, piers, levees, offshore platforms and
                                               facilities, and for sea-bed installations such as
                                               pipelines and telecommunications cables.

                                                Interrelationship of Landsliding
                                                with Other Natural Hazards (The
                                                   Multiple Hazard Concept)
                                               Natural hazards often occur simultaneously or,
7gure 16a. Earthflow (modified from Varnes,
                                               in some cases, one hazard triggers another. For
1978).
                                               example, an earthquake may trigger a land-
                                               slide, which in turn may block a valley causing
                                               upstream flooding. Different hazards may also
                                               occur at the same time as the result of a com-
                                               mon cause. For example, heavy precipitation or
                                               rapid snowmelt can cause debris flows and
                                               flooding in the same area.
                                                    The simultaneous or sequential occurrence
                                               of interactive hazards may produce cumulative
                                               effects that differ significantly from those ex-
                                               pected from any one of the component hazards.

                                                      Landsliding and Dam Safety
                                               The safety of a dam can be severely compromis-
                                               ed by landsliding upstream from the dam or on
Figure 166. Roan Creek earthflow near          slopes bordering the dam's reservoir or abut-
DeBeque, Colorado, 1985 (photograph by         ments. Possible impacts include (1)the forma-
Colorado Geological Survey).
                                               *Discussionby D.B. Prior

                                                                                               15
tion of wave surges that can overtop the dam,      flow. After traveling about four kilometers and
(2) increased sedimentation with resulting loss    dropping 600 meters in elevation, the debris
of storage, and (3) dam failure.                   flow emerged from the canyon onto the alluvial
     Flood surges can be generated either by the   fan of Ophir Creek (total time-15 minutes).
sudden detachment of large masses of earth         One person was killed, four injured, and num-
into the reservoir, or by the formation and        erous houses and vehicles were destroyed
subsequent failure of a landslide dam across an    (Figure 17) (Watters, 1988).
upstream tributary stream channel. Waves                Rapid changes in the water level of res-
formed by such failures can overtop the dam        ervoirs can also trigger landslides. When the
and cause serious downstream flooding without      water level in the reservoir is lowered (rapid
actually causing structural failure of the dam.    drawdown), the subsequent loss of support
                                                   provided by the water and increased seepage
     Landsliding into upstream areas or reser-
                                                   pressure can initiate sliding (Figure 18). Al-
voirs can greatly increase the amount of sedi-
                                                   ternatively, the increase in saturation caused
ment that is deposited in the reservoir, ulti-
                                                   by rising water can trigger landslides on slopes
mately reducing storage capacity. This increas-
es the likelihood that the dam will be over-       bordering the reservoir.
                                                        Eisbacher and Clague (1984) describe an
topped during periods of excessive runoff, caus-
                                                   excellent example of the potential impacts of
ing downstream flooding. Excessive sedimenta-
                                                   landsliding on dam safety: the 1963 Vaiont
tion can also damage pumps and intake valves
                                                   dam disaster in Italy. The Vaiont Dam, a
associated with water systems and hydroelec-
                                                   hydroelectric dam, was completed in 1960 to
tric plants.
                                                   impound the Vaiont Torrent, a major tributary
     Actual dam failure could be caused by
landsliding at or near the abutments or in the     of the Piave River in the southern Alps of Italy.
                                                   The dam is 261 m high and spans a steep
embankments of earthen dams.
                                                   narrow gorge. The southern wall of the valley
     In 1983 a large mass of rock detached from
                                                   behind the dam is a steep dip slope. Within two
Slide Mountain in Nevada. The mass slid into
                                                   months after the reservoir was filled, a 0.7 x
Upper Price Lake, an irrigation reservoir, dis-
                                                   106 m3 mass of rock slumped away along the
placing most of the water which overtopped
                                                   submerged toe of the southern embankment.
and breached the dam, flowing into Lower Price
                                                   Over time, deep-seated movement of the slope
Lake. This lake's dam was also breached. The
                                                   occurred in response to changing levels of the
water flowed into Ophir Creek where it collect-
                                                   reservoir. As a result of these movements,
ed large amounts of debris and became a debris
                                                                            Figure 17.
                                                                            House destroyed by
                                                                            1983 Slide Mountain,
                                                                            Nevada landslide
                                                                            (photograph by
                                                                            Robert J. Watters,
                                                                            University of
                                                                            Nevada, Reno).

16
Figure 18. Jackson Springs landslide on the Spokane arm of Franklin D. Roosevelt Lake,
Washington, 1969. This landslide was triggered by extreme drawdown of the lake (photograph
by the U S . Bureau of Reclamation).

                                                           Landsliding and Flooding
monitoring instruments were set up on the
slope.In August and September of 1963, preci-      Landsliding and flooding are closely allied
pitation in the Piave Valley was three times       because both are related to precipitation, run-
higher than normal and infiltration of the         off, and ground saturation. In addition, debris
precipitation into the slope probably contribut-   flows usually occur in small, steep stream
ed to its eventual failure. The day before the     channels and often are mistaken for floods. In
catastrophic slope failure creep rates of          fact, these events frequently occur simultane-
40cdday were registered.                           ously in the same area, and there is no distinct
     On October 9-10, 1963, in the night, a        line differentiating the two phenomena.
large slab of the unstable slope failed and             Landslides and debris flows can cause
slipped into the reservoir. The volume of mater-   flooding by forming landslide dams that block
ial was estimated to be 250 x 106 m3 (a slab       valleys and stream channels, allowing large
250 m thick). A wall of water 250 m high           amounts of water to back-up (Figure 19). This
surged up the opposite side of the valley, then    causes backwater flooding and, if the dam
turned and overtopped the dam. The concrete        breaks, subsequent downstream flooding. Also,
dam held, and the wall of water (30 x lo6 m3)      soil and debris from landslides can "bulk" or
dropped into the narrow gorge below, scouring      add volume to otherwise normal stream flow or
                                                   cause channel blockages and diversions crest-
loose debris as it went and destroying several
communities below the dam. At least 1,900          ing flood conditions or localized erosion. Fin-
                                                   ally, large landslides can negate the protective
people were killed.
     The site of the dam has been left as it       functions of a dam by reducing reservoir capa-
remained after the disaster, as a monument.        city or creating surge waves that can overtop a
                                                                                                17
dam, resulting in downstream flooding (as           Landslide materials can be dilated by seismic
                                                    activity and thus be subject to rapid infiltration
described above).
                                                    during rainfall and snowmelt. Some areas of
    In turn, flooding can cause landsliding.
Erosion, due to rapidly moving flood waters,        high seismic potential such as the New Madrid
often undercuts slopes or cliffs. Once support is   Seismic Zone of the lower Mississippi River
removed from the base of saturated slopes,          valley may be subject to liquefaction and relat-
landsliding often ensues.                           ed ground failure.The Great Alaska Earth-
                                                    quake of March 27, 1964 caused an estimated
  Landsliding and Seismic Activity                  $300 million in damages. As mentioned eariler,
Most of the mountainous areas that are vul-         60 percent of this was due to ground failure.
nerable to landslides have also experienced at      Five landslides caused about $50 million dam-
least moderate seismicity in historic times.        age in the city of Anchorage. Lateral spread
                                                    failures damaged highways, railroads, and
The occurrence of earthquakes in steep
landslide-prone areas greatly increases the         bridges, costing another $50 million. Flow fail-
                                                    ures in three Alaskan ports carried away
likelihood that landslides will occur and in-
                                                    docks, warehouses, and adjacent transporta-
creases the risk of serious damage far beyond
that posed individually by the two processes.       tion facilities accounting for another $15

Figure 19. Aerial view of the Thistle landslide, Utah, 1983. This landslide dammed the Spanish
Fork River creating a lake which inundated the town of Thistle and severed three major
transportation arteries (photograph by Robert L. Schuster, U.S. Geological Survey).

18
million. Much of the landsliding was a direct      waters turned into steam and magmatic gases
result of the effect of the severe ground shak-    also expanded, resulting in a giant explosion
ing on the Bootlegger Cove Formation. The          (U.S. Geological Survey, 1981b).
                                                        Because human activity had been restrict-
shaking caused loss of strength in clays and
                                                   ed in the Mount St. Helens area due to pre-
liquefaction in sand and silt lenses (U.S.
                                                   dictions of an eruption, loss of life was mini-
Geological Survey, 1981a).
                                                   mized. However, the eruption devastated land
   Landsliding and Volcanic Activity               as far as 29 km from the volcano. The resulting
                                                   lateral blast, landslides, debris avalanches,
The May 18, 1980 eruption of Mount St. Helens
                                                   debris flows, and flooding took 57 lives and
in Washington state triggered a massive land-
                                                   caused an estimated $860 million in damage
slide on the north flank of the mountain. The
                                                   (Advisory Committee on the International
volume of material moved was estimated to be
2.73 km 3 . The landslide effectively depressur-   Decade for Natural Hazard Reduction, 1987). 0
ized the interior of the volcano; superheated

                                                                                               19