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The Subsidence Problem INTRODUCTION This report reviews the land subsidence problem in the United States and the measures that have been applied to mitigate it, and it assesses the effectiveness of research, engineering, and technical solutions in reducing losses incurred by subsidence. It addresses subsidence that occurs naturally, as well as that caused by land development and resource extraction. Resource and land development practices, mainly underground mining, groundwater and petroleum withdrawal, and the drainage of organic soils, are key contributors to the problem. Public and private responsibilities and capabilities for reducing subsidence-related losses on the federal, state, and local levels are examined and their effectiveness evaluated. The report concludes that if its findings are implemented, they would reduce and more equitably distribute subsidence losses in the United States. ~ . . . . . . Land subsidence, loss of surface elevation due to removal of subsurface support, at rates that are of practical significance to man-made structures, affects most of the United States. Subsidence is one of the most varied forms of ground failure affecting the country, ranging from broad regional lowering of the land surface to local collapse. Its practical impact depends on the specific for of the surface deformation. Regional lowering may either aggravate Me flood potential or permanently inundate an area, particularly in coastal or riverine settings. Local collapse may damage buildings, roads, and utilities and either impair or totally destroy them. fortunately, subsidence Is more hazardous to property than to life, because of the typically low rates of lowering. It has caused few casualties. Subsidence, however, increases the potential for loss of life in flood-prone areas by increasing the depth and size of areas susceptible to flooding. Subsidence is caused by a diverse set of human activities and natural processes, including: mining of coal, metallic ores, limestone, salt, and sulfur; withdrawal of groundwater, petroleum, anal geothermal fluids; dewatering of organic soils; pumping of groundwater from limestone; wetting of dry, low-density deposits, which is known as hydrocompaction; natural sediment compaction; melting of permafrost; liquefaction; and crustal deformation. This diversity and the broad range of impacts from subsidence in fact probably are the major causes of the lack of a national focus on subsidence. Instead, many industries, professions, and federal, state, 3

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4 TABLE 1 Types of Land Subsidence* Collapse into voids Mining Sinkhole Compaction Underground fluid withdrawal Natural compaction Hydrocompaction Liquefaction Drainage of organic soils Melting of permafrost Crustal deformation Volcanism Seismic . Aselsmlc Postglacial deformation * Subsidence discussed in this report is in italics. and local agencies are independently involved with aspects of subsidence. Most occurrences of subsidence in the United States, however, are induced by human activity. Resource de- velopment and land-use practices, particularly underground mining of coal, groun(lwater and petroleum withdrawal, and drainage of organic soils, are the primary causes. The common association of land subsidence win either the exploitation of natural resources or land development practices is an important aspect of subsidence. These activities have economic benefits. Problems often arise because Hose who benefit from the activity that causes subsidence may not bear the full cost. In fact, some parties who incur damage may not profit at Al from the activity causing the subsidence. In addition to the equity issue, specific subsidence problems may be aggravated by legal and institutional barriers that prevent legal recourse to injured parties. Legal recovery theories conflict in some states with other doctrines Hat establish rights to resource recovery. This is He second report prepared under He auspices of the National Academy of Sci- ences/National Research Council Committee on Ground Failure Hazards Mitigation Research. These reports summarize ground failure problems that affect the United States and outline methods to mitigate their impact. In defining its purview, the committee was charged win reviewing those ground failure topics that were not receiving adequate scrutiny by other Academy committees. Thus, three causes of subsidence, including the melting of permafrost, crustal deformation, and liquefaction, were not treated, since they fall under the mandates of existing committees (Table 1~. The pane! also did not consider rising sea level, despite its obvious relevance to coastal flooding, an important aspect of the nation's subsidence problem. Such a study has been published by the National Research Council Committee on Engineering Implications of Changes in Relative Mean Sea Level (1987~.

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s CAUSES AND EFFECTS The types of land subsidence considered in this report (Table 1) affect parts of at least 45 states (Figure I). More than 44,000 km2 of land, an area equal to half the state of Maine, has been lowered. In this section both the causes of and effects from subsidence are reviewed. (For detailed technical discussion of mechanisms the reader is referred to the references in the Appendix.) Collapse into Voids- Mines and Underground Cavities Collapse of surficial materials into underground voids is the most dramatic kind of subsi- dence. Buildings and other engineered structures may be damaged or destroyed, and land may be removed from productive use by such ground failure. Mines Underground excavations have been constructed in the United States since the early 1700s. Most of the voids with which subsidence has been associated in the United States were created by coal mining, and this report focuses on that activity. Abandoned tunnels and underground mining of metallic ores, limestone, and salt contribute to a much smaller extent, although problems may be severe in some regions. For example, hundreds of subsidence occurrences in the Midwest have been associated with failures of abandoned underground lead-zinc mines. A singular, but spectacular and complex, example was the catastrophic drainage of Lake Peigneur and disappearance of about 27.6 ha of lanct at Jefferson Island, Louisiana, on November 20, 1980 (Figure 2~. The collapse was triggered by the drilling of an oil well inadvertently into an underground salt mine in the Jefferson Island salt dome; damages totaled hundreds of millions of doDars (Autin, 1984~. In general, coal-mine subsidence is caused by collapse of Me mined-out or tunneler! void. It occurs as both steep-sided pits (Figure 3) and broad, gentle depressions. Subsidence clepends on Me number, type, and extent of the voids. For example, it is a planned consequence of the longwall mining method for coal, in which most of the coal seam is removed along a single face, the longwall. By this method, the roof above the mined-out seam is allowed to collapse as the longwall advances laterally as mining progresses. Subsidence above longwall mines is rapid, generally ending within a few months after the removal of subsurface support. Subsi(lence above mines with partial extraction is usually unplanned. By this memos, only parts of the coal, the rooms, are removed. The unmilled portions, the pillars, are left to provide support. Collapse into Me rooms occurs when the pillars, floors, or ceilings deteriorate. Subsidence resulting from collapse into rooms may take years to decades to manifest itself. Examples of collapse occurring 100 years after mines were abandoned have been documented. Coal is fount} in 37 states and mined underground in 22 states (HRB-Singer, 1980~. Approximately 32,000 km2 of land is undermined, and it is anticipated that the area will ultimately grow to about 162~000 km2. Approximately 8,000 km2 of the undermined area, most of which is in the eastern United States, already has experienced subsidence. The U.S. Bureau of Mines estimates that 1,600 km2 of land in urban areas is threatened (Johnson and Miller, 1979~. Seventy-one percent of this area is in Pennsylvania, Illinois, and West Virginia.

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6 W~.:-~::q A. Mining C. Underground fluid withdrawal Am, ~ ( 3 ~ , :: :: . :1 ~ it. r N: :: :::::,::-::::; ~ 22 ~ ~1 Ask ~ , fit ~ ~ . ~ 1 ~ ~ W~ Ha 0, B. Sinkholes D. Natural compaction ~E. Hydrocornpaction F. Drainage of organic soils LEGEND Costs ~ <$1 million .$~-10 million $10-100 million .~$100 million FIGURE 1 National distribution of subsidence problems by state. Costs were compiled from published and unpublished sources for the purpose of providing an order-of-magnitude, state-by-state comparison. Only relative importance is suggested by maps, because the time periods on which estimates are based vary by state, and costs were not converted to constant dollars. In general, costs are conservative estimates.

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8 FIGURE 3 Oblique aerial view of subsidence pits above abandoned coal mines in the Sheridan, Wyoming, area. Diameter of pits ranges from 5 to 50 m. (Photograph courtesy of C. Richard Dunrud.) Sinkholes The sudden formation of sinkholescatastrophic subsidence is usually caused by move- ment of overburden into underlying cavities in soluble bedrock (Figure 4~. Failure of the bedrock is rarely believed to be a major factor in catastrophic subsidence. Most catastrophic subsidence in the United States is associated with carbonates such as limestone, but occasion- ally it is associated with evaporites such as gypsum and halite (Figure 5~. Although most historical collapses are man-induced, Me cavities in the bedrock usually antedate human activ- ities. This is particularly true of carbonates, because rates of solution are so low. Cavities in halite can be an exception because of the high solubility. For example, several dozen sinkholes have formed in the last 30 years in Kansas as a result of solution of salt beds by leaks through casings of brine-disposal wells. A recent example is a 60-m-wide and 33-m-deep sinkhole that formed in the summer of 1988 near Macksville, Kansas (Geotimes, 1988~. Catastrophic subsidence is most commonly induced by water-table lowering, rapid water-table fluctuation, diversion of surface water, construction, use of explosives, or impoundment of water. Davies and others (1976) in(licate that more than 1.4 million km2 of land in 39 states is underlain by cavernous limestone and marble. More than 30,000 km2 of this area lies beneath Standard Metropolitan Statistical Areas inhabited by 33 million people (HRB-Singer, 1977~. Of course, only a small portion is actually underlain by voids and at risk. Newton (1986) estimates that more than 6,000 collapses have occurred in the eastern United States since about 1950 (Figure 6~. The states with the largest number of active sinkholes include Alabama, Florida, Georgia, Indiana, Missouri, Pennsylvania, and Tennessee.

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FIGURE 4 Catastrophic subsidence caused by collapse of overburden into voids in limestone near Barstow, Flonda, that destroyed two houses. (Photograph courtesy of Florida Sinkhole Research Institute.) Sediment Compaction Sediment compaction typically causes broad regional subsidence. Exceptions include ground rupture and hydrocompaction. Rates of subsistence usually are low, ranging from a few mildimeters to centimeters per year, but total subsidence may reach several meters as it accumulates over decades. Underground Fluid Withdrawal The weight of the overburden above underground fluid reservoirs is supported by both fluid pressures and stresses transmitted through the solid skeleton of Me reservoir soil or rock. When fluids are withdrawn, fluid pressures decline, and support of the overburden is transferred to the solid skeleton. If the reservoir soil or rock is compressible, large and permanent loss of pore volume or compaction will occur as it adjusts to the new stresses. In geothermal reservoirs, significant thermal contraction also may occur as the reservoir cools during exploitation. Most of this type of subsidence in the United States is caused by pumping of groundwater and petroleum. More than 31 areas in 7 states have subsided. The two largest areas are in the San loaquin Valley, Califomia, and Houston, Texas, areas, where 13,500 anti 12,000 kind, respectively, have subsided because of groundwater withdrawal. Maximum elevation loss from this type of subsidence has been 9 m in the San Joaquin Valley (Figure 7~. Two coastal areas in California and Texas where subsidence caused or threatened inun- dation and increased flooding potential have suffered the most from this type of subsidence.

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10 FIGURE 5 August 11, 1983, catastrophic subsidence over the Boling salt dome, Texas. Diameter of depression is about 75 m. (Photograph courtesy of Boyd V. Dreyer.) Petroleum withdrawal in Long Beach, California, caused parts of the city's harbor facility to subside almost 9 m from 1937 to 1966. Groundwater withdrawal in Houston, Texas, has caused some coastal areas to subside by more than 2 m. About 80 km2 of land has been inundated, and several hundred square kilometers, including the 500-unit Brownwood subdivision in Baytown, which was abandoned in 1983, have been allied to the area susceptible to flooding by storm surges (Figure 8~. Inland areas are not immune to damage from this subsidence. Changes of surface gradients can affect either the design or operation of canals. For example, canals in the State Water and the Central Valley Projects in California and the Central Arizona Project have been affected. Other causes of damage in inland areas include subsurface deformation and ground rupture. Subsurface deformation shears or crushes well casings and diminishes the productivity of water and oil wells. Ground rupture, including faults and earth fissures, is particularly devastating to man-made structures. Faulting has damaged hundreds of houses in Houston, Texas (Figure 9a), and caused the catastrophic failure in California in 1963 of the Baldwin HiUs reservoir that claimed five lives. Earth fissures (Figure 9b) are an increasingly common occurrence as groundwater withdrawals increase in alluvial basins in the desert parts of the Sun Belt. Natural Compaction Se(liments also compact naturally as they are buried by younger sediment (Figure 10~. Probably nowhere in North America is natural subsidence occurring more rapidly than in the

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11 Lo 1 ; A ~ ~ ~ N 5 A S '' 1 , T E N N. ~ S 5 [: ~ ~ MA R O L ~; C U L r ~ ~ ~ X ~ ~ \ 0 1~ 30 4tp EXPLANATION SITE ~ IMP ~ SITES - 0, COLLA PSE AREA OCCUPIED BY ANCIENT . _~' FIGURE 6 Sites of active sinkhole development in the eastern United States (from Newton, 1986~. Mississippi River Delta area of southern Louisiana, where about 3,900 km2 of land is subsiding, at least in part through natural compaction. Estimated average rates of subsidence range from ~ to 11 mm per century (Peniand and others, 1988~. Maximum rates measured by geodetic surveys are about 12 mm per year (Figure 11~. The abandoned town of BaTize, approximately 130 km southeast of New Orleans on the tip of the Delta, is an example of the long-term implication of this subsidence. Balize, which was abandoned during a yellow fever epidemic in 188S, was more than I.2 m below marsh level in 1934 (Russell and others, 1936~. Today the abandoned town sits more than 3 m below sea level. Increased flooding potential is Me principal impact of this type of subsidence, because affected areas commonly are low lying and naturally subject to flooding. Thus, subsidence exacerbates a preexisting problem. The flood problem is particularly acute in coastal areas

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FIGURE 7 Approximate location of maximum subsidence in the United States caused by groundwater with- drawal. Point is west of Mendota, California, in the San Joaquin Valley. Subsidence of 9.0 m occurred from 1925 to 1977. Signs indicate the former elevations of the land surface in 1926 and 1955, respectively. (Photograph courtesy of Richard L. Ireland.)

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13 FIGURE 8 Flooding and damage in Houston, Texas, subsidence area caused by Hurricane Alicia, August 1983. Area had subsided about 2 m because withdrawal of groundwater increased its vulnerability to storm surges. (Photograph courtesy of H. Crane Miller.) where long recurrence intervals between large storm surges and tidal floods diminish public perception of the problem. Documenting subsidence problems in coastal areas may be difficult if other types of subsidence are occurring and sea level is changing. For example, subsidence caused by drainage of organic soil and withdrawal of underground fluids is common in many deItaic areas. A well-studied example of this complexity is Venice, Italy, where increases in the incidence of tidal flooding (Figures 12a and by prompted an investigation, which discovered that the mean elevation above sea level had decreased from 130 to Il0 cm since the turn of the century. About 14, 41, and 45 percent of the 20-cm average elevation loss was attributed to natural compaction, sea-level rise, and withdrawal of ground water, respectively (Gatto an Carbognin, 1981~. Another very important impact from this subsidence is the destruction of productive estuarine marsh and coastal wetlands by either inundation or erosion. Thousands of hectares of low-lying coastal land along the Gulf of Mexico are converted each year to open water by natural subsidence (Figure 13~. This process, referred to as coastal land loss, results in a significant loss of habitat for birds, fish, crustaceans, and reptiles and has a profound impact on the commercial fishing, shrimping, oystering, an(l fur trapping industries. In abolition, salt-water intrusion into these areas (lestroys agricultural usage. The most severe land-Ioss problem in the United States is in southern Louisiana. Chan- nelization of the Mississippi River has caused Overborne sediment, which normally offsets land loss by replenishing beaches an(l wetlands, to (discharge directly offshore into the Gulf of Mexico. The rate of land loss in southern Louisiana currency is about 130 km2 per year;

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FIGURE 9 Ground ruptures associated with land subsidence caused by groundwater withdrawal. (a) Damage .~ house from faulting in the Houston, Texas, area. (b) Tension crack in southcentral Arizona that has been enlarged by erosion into 1-m-wide gully. (Photograph courtesy of Thomas L. Holzer.)

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15 approximately 3,200 km2 of land has been lost in the last 80 years. Nationally, more than 150,000 km2 of coastal marsh has been lost since 1954 (Gosselink, 1980~. Hydrocompaction Dry, low-density, fine-grained sediment may be susceptible when wetted to a loss of volume known as hydrocompaction (Figure 14~. These sediments, known as collapsible soils, generally are of two types: mudflow deposits in alluvial fans and wind-deposited, moisture- cleficient silt caned loess. Most collapsible soils have anomalously low densities because they remained moisture deficient throughout their postUepositional history. When water percolates through the root zone into this type of sediment, the soil structure collapses and the soil compacts. Very localized subsidence, typically 1 to 2 m, may result. Damaging hydrocompaction has been reported in 17 states. The three largest affected areas are the alluvial slopes of the western San Joaquin Valley and loess-covered areas in the Missouri River basin and Pacific Northwest. The major impact has been on design and operation of hydraulic structurescanals and dams. Locally significant impact has been incurred by buildings and highways. Irrigation for agriculture also has caused differential subsidence that required releveling of fields. Organic Soil Drainage of organic soil, particularly peat and muck, induces a series of processes, including biological oxidation, compaction, and desiccation, that reduce the volume of the soil. Biological oxidation usually dominates in warm climates (Figure 15~. The principal areas of organic soil subsidence in the United States are the greater New Orleans, Louisiana, area; the Sacramento-San Joaquin River Delta, California; and parts of the Florida Everglades. Maximum observed subsidence is 6.4 m in the Sacramento-San Joaquin River Delta. About 9,400 km2 of land underlain by organic soil has subsided in the United States because of drainage. An even larger area is susceptible to subsidence. About 101,000 km2 of He conterminous United Stairs is covered by peat and muck soils (Stephens and others, 1984~; more than 26,000 km2 of organic wetlands is in standard metropolitan statistical areas (HRB- Singer, 1977~. Increased flooding is the most serious problem caused by organic soil subsidence. The low relative elevation of organic soil areas makes them vulnerable to flooding even in their natural state. Levees and pumps to remove stone and groundwaters usually are required to protect areas developed on organic soils. For example, about 35 km2 of land in New Orieans is kept ciry by a protective levee system and 21 pumping stations. About 2,250 km of levees are required to protect 1,300 km2 of farmland underlain by peat in the Califomia Delta. Even construction of levees does not guarantee prevention of flooding. About 3,000 people lost Heir lives in 1926 ant} 1928 in hurricane flooding that swept away levees around the south shore of Lake Okeechobee, Florida, where subsidence of about I.5 m had occurred. The high compressibility of organic soils also makes them a poor foundation material for engineered structures. Consequen~dy, special construction practices that rely on piles driven to firm materials are commonly employed. These practices have led to subsiding of the land surface relative to He structures (Figure 16~. ~ ~ ~ ~ r

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16 1838 ~ O jre-openin6/ it/ jrJ1905~ ~ go\' al ~ 1~/ ~ ~~ ~~ \ - ?~.~ ~ Km 1922 FIGURE 10 Historic maps illustrating natural compaction at Cubits Gap in Mississippi River Delta. In 1862, an oyster fisherman dug a trench along a low section of river levee. Flooding in 1862 enlarged the trench and a small delta began to form. As the tributaries prograded seaward, compaction of deposits landward caused bays to form and by 1971, substantial submergence had occurred. By the late 1980s, about 75 percent of the original land was lost to subsidence (from Coleman, 1988~.

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17 100 o -100 - LL z -200 LL c, oh ~ -300 An -400 -500 N ~ Terrain _ I\ ~ ~ / - __ Subsidence ~ Ott' ~ 19~-1959 I I 1 1 1 1 0 20 40 60 80 DISTANCE (km) 100 o 100 120 An cc cr: LLI FIGURE 11 Subsidence profile from Oskya, Mississippi (north), to New Orleans, Louisiana (south), illustrating regional subsidence Tom 1934 to 1959 of Mississippi River Delta. Subsidence is believed to be caused by crustal deformation and compaction of sediments beneath the Delta. ECONOMIC AND LEGAL ASPECTS The average annual damage cost from all types of subsidence is conservatively estimated to be at least $125 minion (Table 2~. The costs are dominated by subsidence from underground mining of coal, drainage of organic soils, and withdrawal of underground water and petroleum. These costs consist primarily of direct structural and property losses and depreciation of land values, but they also include business and personal losses that result from periods of repair. Although total annual damage cost of subsidence to the nation is small relative to the nation's economy, subsidence imposes substantial costs on both individual cities and neighborhoods. Examples of cities where cumulative damage costs from subsidence are more than $100 million include Long Beach, California; Houston, Texas; and New Orieans, Louisiana. In other areas, the hazard is more localized. Seemingly random collapses threaten both the safety and viability of selected neighborhoods and businesses. Examples include Scranton, Pennsylvania, and Seattle, Washington, where collapse of abandoned coal mines has damaged surface structures. Most mine-subsidence damage in the United States is associated with abandoned coal mines over which urban growth has occurred. Damage in urban areas has been estimated to cost more than $30 minion annually (HRB-Singer, 1977~. Even in rural areas, however, subsidence affects field drainage, reduces crop yields, and lowers property values. A study in Illinois indicates that property values in rural areas affected by subsidence were discounted an average of 16 percent (Illinois Department of Energy and Natural Resources, 19851. Costs of preventive measures and damages from sinkhole activity or catastrophic subsi- dence, which was incurred primarily in the eastern United States since 1970, are more than $170 million (Newton, 1986~. The costs are dominated by a $130 million expenditure at five dam sites to minimize or eliminate sinkhole activity. A factor that helps to keep damage costs low is that sinkhole potential is usually evaluated during siting of major engineered structures in limestone terrane. This cost is considered a type of mitigation cost. Damage costs from compaction caused by withdrawal of underground fluids are dominated

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18 200 t90. ~ ~0 '7O, 160 . - > o E t30 120 _ 110 100_ I! I ! 1. 1908 ~ ~ W' 2 2 c m _~L 1 ~ 9 1. 1980 ~~Wk . in, , 1910 t920 1930 1940 19S0 191S0 1970 1980 Y E ~ R FIGURE 12 Subsidence and flooding in Venice, Italy. (a) Flood heights and frequency, 1908-1980. Frequency of floods, locally known as acqua alla, increased with time as subsidence lowered mean elevation of Venice (Gatto and Carbognin, 1981). (b) Acqua alla in Piazza San Marco. (Photograph courtesy of Laura Carbognin.)

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19 Pit ~ ~F(s~ any.. ~x ~:~ A' ~ an' ' ~ G,~P 43-,_ .3 :~ ~ 8 40~5~ Judd_ , ~_ ._ - id :. ... I -- - T. ~ ~ FIGURE 13 Regional land subsidence arid diversion of sediment earned by Mississippi River directly to the Gulf are contributing to the disappearance of more than 130 km2 of wetlands annually in southern Louisiana. 1 by large losses in a few areas. For example, although subsidence caused by petroleum withdrawal at Long Beach presently is under control, mitigation costs were about $150 million from 1937 to 1966 (Mayuga, 1970~. Jones (1977) estimates that damages in the Houston area averaged $31.7 million per year from 1969 to 1974, the period covered by this study. Costs were approximately equally divided between decreased property value -and damage; most derived from flooding and inundation. Costs in inland subsidence areas are caused primarily by changes of gradients of canals, repair of subsurface damage to wells, and ground rupture. These costs probably have been smaller than flooding-related costs, although few firm estimates are available. Prokopovich and Marriott (1983) estimate that postconstruction rehabilitation of Califomia's Central Valley Project canals, one of the largest canal systems affected by subsidence, cost $34 million. The only available estimate of damage to well casings is that by Roll (1967) for the Santa Clara Valley, California, where more than $4 minion was spent to repair or replace well casings. Estimates of damage from ground rupture also are incomplete. CIanton and Amsbury (1975) estimate losses in He Houston area at several million dollars. Failure of the Baldwin HiDs reservoir because of ground rupture caused losses of $15 million. Losses from natural compaction, particularly in the Mississippi River Delta, are difficult to estimate because of the uncertain value of coastal wetlands. Annual revenue losses are possibly on the order of millions of dollars. Nationwide hydrocompaction damages and prevention expenditures are very poorly doc- umented. The most costly individual impact that has been documented was on the California

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20 FIGURE 14 Hydrocompaction of test plot in San Joaquin Valley, California. Water has infiltrated from pond and caused collapsible soils to compact as they became wet. Note extensive ground cracking in background around margin of subsidence depression. Poles were used to measure compaction at different depth intervals. (Photograph courtesy of California Department of Water Resources.) Aqueduct along the western margin of the San Joaquin Valley, where an aggregate length of 155 km of canal was built on collapsible soils (lames, 1974~. Investigations and prewetting to compact foundations resulted in a mitigation cost of $20 million (Curtin, 1973~. Design modi- fications to We nearby Central Valley Project led to an $8 million mitigation cost (Prokopovich and Marriott, 1983~. The most costly reported urban incident is a $3 million decrease of property value in Cellar City, Utah; property damages were about $1 million (Kaliser, 1982~. Even areas with humid climates have incurred significant costs. For example, collapsible soils added more man $2.5 million in mitigation costs to interstate highway construction in Louisiana (arm en and Thomton, 1972~. The incremental increase in the cost and prevention of flood damage caused by organic soil subsidence is difficult to separate from total flood-related costs in areas developed on organic soils. Flood prevention measures in these areas, such as levee construction, clearly are expensive. In the Everglades region of south Florida, a special flood-contrl! district with an annual budget of $69 million is required to manage an extensive water-control system, including 1,284 km of levees, with a replacement value of about $1.5 billion. Just one part of New Orieans's levee system, the Lakefront Levee, cost $27 million ($240 million in 1976 dollars) to buil(l in 1926 (Wagner and Durabb, 1976~. Levee repair, dewatering of flooded areas, and flood damage in the California Delta cost about $177 million from 1969 to 1983 (Prokopovich, 1985~. Structural damage costs from differential subsidence caused by drainage of organic soils

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21 FIGURE 15 Concrete monument set on rock beneath organic soil in Belle Glade, Florida, an active subsidence area. Elevation painted on monument is feet above mean sea level. Dates show former elevation of land surface. Photograph was taken October 1987. (Courtesy of George H. Snyder.) appear to be high. HRB-Singer (1977) estimates that approximately $30 million (in 1977 dollars) is spent annually in New Orieans to repair damages and maintain property. Earle's (1975) study in New Orieans indicates that costs are disproportionately distributed. Forty-five percent of his homeowner sample incurred problems; 5 percent encountered serious problems. The dolBar value of economic losses from subsidence reveals ondy part of the nation's subsidence problem. Inequitable aspects of both economic incentives and the American legal structure are major factors in the nation's subsidence problem, particularly where subsidence is caused by resource extraction or land use. Often existing legal or market incentives do not encourage the developer of a resource to consider the costs imposed on others who do not receive the benefit of development. These are termed extemal costs. External costs cause misallocation of society's resources. Social costs, the costs to society as a whole, are greater than private costs when there are external costs. Thus, for example, a mine operator who has caused subsidence on land that he does not own receives all of the benefits of ore production but pays only the private cost, a portion of the social cost. The surface property owner affected by subsidence, however, receives none of the benefits, yet bears the extemal costs from the actions of the mine operator. This misallocation of resources is corrected when the mine operator pays all of He social costs, private and external, of the mine operation.

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FIGURE 16 Organic soil subsidence in Florida Everglades. Piles supporting structures rest on firm layer beneath organic soil. As organic soil decreased in thickness, ground surface subsided, leaving structures elevated. (Photograph courtesy of John C. Stephens.) TABLE 2 Estimated Annual Losses (in millions) from Land Subsidence Mines Sinkholes Underground fluid withdrawal Natural compaction Hydrocompaction Organic soils TOTAL $ 30 10 35 10 Not available 40 $125 SOURCES: Jones (1977), Newton (1986), Propokovich and Marriott, (1983), HRB-Singer (1977). In evaluating external costs, several dimensions are worth noting. Extemal costs from subsidence can stem from past activities, as in the case of abandoned mines, and can be viewed as a one-time imposition of some level of external costs on the future. Alternatively, past and current activities can cause a continuing subsidence problem. These types of external costs are direct. Extemal costs from subsidence also may be indirect. These include situations where subsidence increases the potential for economic damage from other natural events. Increased susceptibility to flooding from storm surges of low-lying coastal areas is an example.

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23 Another category of extemal costs potentially presents a special dilemma. This involves situations where Me external cost is home in perpetuity by future generations. For example, consider where drainage of organic soil causes land to subside below sea level or alters the freshwater-saltwater balance. In such cases, land may simply disappear because of inundation, or the ecology may change, and Bus a permanent loss is passed on to future generations. At common law the principal means to prevent externalities by transferring external costs to the actor who created them is the tort system. That system generally has a two-fold purpose: (~) to compensate worthy victims for damages they have suffered by the negligence of others (thus imposing the costs of the damage on the actor who created them), and (2) to act as a deterrent to negligent activity, Mat is, to encourage the actor to decide not to engage in an activity if it creates a cost that can be transferred back to Mat actor in a tort or lawsuit. There are at least Me reasons the legal system does not provide an adequate mechanism for transferring subsidence costs onto the actors that create them (Amandes, 1984~. First, in many states, such as those applying the English common law doctrine of absolute ownership of groundwater, the legal system does not allow a party injured by subsidence to recover from the person causing the subsidence. In those states, ownership of Be groundwater is said to be absolute, and any effects created by groundwater withdrawal must be home where they fan. Similarly, the legal system may not provide a recovery mechanism for a surface owner injured by the subsidence caused by underground mining. Second, even in those jurisdictions that allow recovery of subsidence damage, lack of public understanding about the causes and effects of subsidence may prevent members of the public from recognizing their damages and the identity of those who caused them. Even if some members of the public can successfully recover their losses, any members of the public who do not or cannot recover their losses win leave external costs that are not internalized. Third, because many of the damages caused by subsidence are indirect, the fun cost of subsidence may not be recognized. For example, a flood may inundate 1,000 ha when it would have inundated only 500 ha in Be absence of subsidence. The owners of the property whose inundation was caused by the subsidence may not recognize the role played by subsidence in expanding the flooded area. From an economic point of view, these indirect costs are as significant as direct costs and, if they are not recognized and recovered, they will not be internalized. When traditional legal mechanisms fail to provide adequate control over external economic costs, governments may create statutory or regulatory controls to compensate for the deficien- cies in the common law. These are not always limited to recovery for negligent performance of an activity. Legislation can remove the element of negligence by establishing strict liability for surface damages. Altematively, administrative regulation may be used to address local subsidence problems by either imposing taxes that internalize Be external cost or regulating resource removal or usage (Amandes, 1984~. Given that subsidence commonly is a phenomenon charactenzed as imposed external cost due to legal conflicts or market failure, what are the barriers to mitigating external costs? At least Tree barriers inhibit the design of appropriate public policy measures. First, a significant barrier is simply the lack of availability and public understanding of the scientific literature; it is difficult to mitigate the unknown. Second, subsidence is not typically viewed as a catastrophic event or even necessarily the primary cause of large economic damages. As such, on the agenda of preparations for natural hazards, subsidence is low. Third, the potential for long delays in the observance of phenomena causes significant problems for public policy response. 1