FEDERAL EMERGENCY MANAGEMENT AGENCY FEMA 182/August 198&9 Landslide Loss Reduction: A Guide for State and Local Government Planning ~ A % A AA A AA A A A A AA A 'A ~~~~~~~, I I I I I I I I I I .: A A. A A . AAIA AAAAAAAA SAAA A A A A A A A A A A A A A A A A A AAA AAAAAz 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