Impacts of Climate Change and Land Use on the Southwestern United States

Las Vegas Valley: Land Subsidence and Fissuring Due to Ground-Water Withdrawal

John W. Bell
Nevada Bureau of Mines and Geology

This presentation is abstracted from: Bell, J.W., Price, J.G., and Mifflin, M.D., 1992, Subsidence-induced fissuring along preexisting faults in Las Vegas Valley, Nevada: Proceedings, Association of Engineering Geologists, 35th Annual Meeting, Los Angeles, p. 66-75; and from Bell, J.W., 1981, Subsidence in Las Vegas Valley: Nevada Bureau of Mines and Geology Bulletin 95, 84 p. The reader is urged to refer to the full paper and (or) contact the authors for more information.

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Rapidly growing populations in the semiarid to arid regions of the southwestern United States require water. The water is used not only for basic needs (drinking, sewage systems, etc.), but also to maintain lifestyles transferred from wetter areas of the country (suburban lawns, golf, backyard swimming pools). These uses can be met either by purchases of surface water in an already over-allocated water-supply system (e.g., the Colorado River) or, most readily, by tapping ground water. However, ground-water withdrawal can adversely affect the sedimentary deposits that fill typical ground-water basins in the desert. These effects include differential compaction, reactivation of old faults, and surface fissuring, and can have considerable impact on human infrastructures (Figs. 1 and 2).

Figure 1. Photograph of Las Vegas Valley Water district Well No. 5 showing well-head protrusion caused by subsidence. Photo by John W. Bell, 1989.

Land in the Las Vegas Valley has been subsiding due to ground- water withdrawal (Figs. 1 and 2) since about 1935. This subsidence and its relations to specific wells and faults have been very well documented by leveling studies conducted along survey lines, some of which were initially established in 1935 during studies related to the construction of Hoover Dam (Lake Mead). This report briefly sketches the magnitude of the subsidence problem and summarizes the ongoing studies, chiefly conducted by the Nevada Bureau of Mines and Geology. Studies by Bell and others (1992) and Bell (1981) are the chief sources of information and are freely quoted and paraphrased here without direct attribution.

Geologic and Urban Setting of Las Vegas Valley

Figure 2. Photograph of house in Windsor Park subdivision in North Las Vegas showing differential settlement to a combination of fault scarp movement and fissuring. Note fissure crossing road toward house. Photo by John W. Bell.

Las Vegas Valley is a fault-bounded structural and hydrologic basin containing hundreds of meters of late Tertiary and Quaternary lacustrine, paludal, and alluvial deposits. These sediments consist of poorly compressible, coarse-grained alluvial-fan deposits around the valley margins and of highly compressible, fine-grained sediment in the middle of the valley. Nearly all of the ground-water supply comes from a zone of confined and semi-confined principal aquifers at depths of 200-300 m (Maxey and Jameson, 1948).

Las Vegas Valley is the most rapidly growing metropolitan area in the U.S. (1995 Census Bureau report) and now has well over one million people. About 20% of the present water supply (~375,000 acre-feet/year) is from ground water. Annual ground-water withdrawals began to exceed estimated annual recharge in 1946 (Maxey and Jameson, 1948). Since 1968, annual withdrawals have been gradually reduced (in 1991 the water district began re-injecting water into the subsurface) but have consistently exceeded natural recharge levels by factors of two to three. As a result of continued long-term overdraft, water levels have declined more than 90 m in some portions of the valley.

Land subsides due to fluid withdrawal as a result of a decrease in sediment volume (Poland and Davis, 1969). This consolidation is related to an increase in effective stress within the deposit and to the grain size. As the effective stress increases, fine-grained deposits (silt and clay) undergo plastic deformation accompanied by a permanent rearrangement of sediment particles. For a given stress increase, the amount of deformation will be highest in fine-grained deposits because they have higher porosity and are more compressible than coarse-grained deposits. The fine-grained Tertiary and Quaternary basin-fill deposits that fill Las Vegas Valley have geologic and hydrologic properties that are very conducive to consolidation upon fluid extraction.

Land subsidence in the Las Vegas area is primarily related to ground-water withdrawal. Although the rate of subsidence has remained relatively constant for the last decade, recent urban development has intensified the occurrence of fissuring and structural damage. Areas within the valley that have been heavily pumped and show large water-level declines have also been the sites of major elevation change, surface deformation, and damage.

History of Subsidence Studies in Las Vegas Valley

Subsidence in Las Vegas Valley has been monitored since 1935 when the U.S. Coast and Geodetic Survey established a first-order vertical-control network across the valley as a regional monitoring program to document the effects of loading of water impounded in Lake Mead behind Hoover Dam. The entire network was releveled in 1940-41 and again in 1949-50. The network was extended and all or portions of it were releveled at first- or second-order accuracy in 1963, 1972, 1980, and 1986-87. From the 1935-1950 leveling data, Longwell (1960) found a broad, shallow sinking of the Boulder Canyon area centered about 19 km upstream of the dam. In the area of Las Vegas, northwest of Hoover Dam, this depression was expressed as a southeastward tilt of about 10-12 cm.

Subsidence due to ground-water withdrawal is superimposed on this broad regional depression. Maxey and Jameson (1948) first noted that the valley was subsiding in apparent relation to water withdrawals. By 1963, the center of the valley had subsided as much as 1 m and by 1980, about 1.5 m (Bell, 1981; Fig. 1). Subsidence still continues; a new compilation by Bell and Ramelli (1991) using 1986-87 data showed that the location and rates of subsidence have remained relatively constant at least since 1963 (fig. 3). A broad regional subsidence bowl occupies the central portion of Las Vegas Valley. Three localized subsidence bowls are superimposed on the broad pattern and are located in the central (downtown), southern (Las Vegas Strip), and northwestern parts of the valley. These localized bowls have had at least 75 cm of subsidence since 1963; the northwestern bowl has subsided more than 1.5 m since 1963.

Control of Subsidence by Pre-Existing Faults

Figure 3. Quaternary faults and fissure zones in the Las Vegas area. Contours show subsidence measured only from 1963 to 1986-87. Short lines are level lines across faults (see fig. 3 for leveling history of line 1).

A series of linear and curvilinear north- to northeast-trending late Quaternary faults cut the floor of Las Vegas Valley (fig. 3) creating a succession of prominent scarps as much as 50 m high. These faults were originally believed to be the result of climatically-induced compaction faulting within the valley fill (Maxey and Jameson, 1948). More recent work (e.g. Bell, 1981) suggests that these faults are in large part tectonic in origin. The Nevada Bureau of Mines and Geology established several short (1.5-4 km), second-order, vertical-control lines in 1978 across selected faults in order to test an hypothesis that the faults are sites of incipient vertical rupture. The lines were releveled annually for 14 years from 1978 to 1991 (Bell and Ramelli, 1991); releveling studies are ongoing.

Bell and others (1992) reported that the leveling history across the faults indicates they are preferred sites for localized, subsidence-induced, vertical differential movement. Data for four of the lines across three separate fault zones (Fig. 3) indicate that the faults moved at relatively constant rates for the entire 1978-1991 period. Line 1 (Fig. 4) crossed the large, northeast-trending, Eglington scarp. Relative elevation changes were measured along this line between 1978 and 1985, when the line was destroyed by suburban development. These changes indicate that the northwest (upthrown) side of the fault was subsiding at a rate of about 5 cm/yr. Along the four lines reported by Bell and others (1992), either the sharpest differential displacements closely coincide with the central portion of the fault scarp (Fig. 4), or the differential offsets are initiated at the scarp. In all four cases, the displacements are antithetic, opposite to the original geologic sense of displacement. The antithetic movement is attributed to the presence of adjacent localized subsidence bowls (Fig. 3).

Fissures

Figure 4. Level line 1 across the Eglington scarp in the northwestern part of Las Vegas Valley (fig. 3). Top shows benchmarks (small squares), topography, fault scarp, and zone of fissuring. Bottom shows relative change in elevation on benchmarks for the period 1978-1985.

Fissures have been observed in Las Vegas Valley since 1925 and documented in many reports (e.g. Mindling, 1971; Patt and Maxey, 1978; Mifflin et al., 1991). The fissures form as small (mm-scale) tension cracks in the sediment above the water table and are thought to enlarge due to mechanical piping. As piping continues and the fissure grows, it eventually breaks through the sedimentary cover bridging the concealed pipe. In Las Vegas Valley, enlarged fissures range up to 3 m in width and 4.5 m in visible depth. Exposures of near-surface pipes indicate that they are commonly 1-2 m in diameter. The sedimentary cover bridging the pipes may be punctuated by linear alignments of small potholes on the surface. Subsurface piping and fissuring has direct and obvious effects on man- made features, including buildings (Fig. 4), highways, and buried pipelines or electric lines.

Faults and fissures are spatially associated within Las Vegas Valley (Fig. 3). The eight principal zones of fissuring and many of the minor zones are closely coincident with known or inferred geologic faults, confirming that the faults may be preferred sites for the tensile strains required to induce fissuring. A small percentage of the known fissures occur as concentric features encircling high-yield water wells.

In 1989, the U.S. Department of Housing and Urban Development began requiring special subsidence hazard assessments for property located in close proximity to known subsidence features. This requirement was primarily a consequence of the structural damage caused by fissuring in the Windsor Park subdivision of North Las Vegas (Fig. 2); estimated total costs for repair or replacement of more than 240 damaged or threatened homes in this area were $12-13 million. This subdivision lies within fissure zone E (Fig. 3) at the junction of two major faults. Other areas exhibiting incipient signs of structural distress in homes in 1991 include fissure zones B, along the Eglington scarp, and H, in the southern part of the valley (Bell and others, 1992). The guidelines specified detailed subsidence studies and specialized construction design for all new developments within 150 m of a mapped fault. A statistical analysis of fissures throughout the valley, however, indicates that the specified zone would include only about 45% of all mapped fissures, and that a zone 610 m wide around mapped faults is required to include about 90% of the fissures.

Summary

Subsidence induced by ground-water withdrawal has continued in Las Vegas Valley since about 1935 and presently is as much as 2 m in the center of the valley. Based on annual surveys of short level lines, much of the subsidence is preferentially focused on pre-existing faults. These faults are also the preferred sites for fissuring, which has affected several existing residential developments and poses a potential threat to numerous other potential development sites in the valley. Similar effects may be expected in other structural basins filled with fine-grained sediment in the desert Southwest where ground-water supplies are extensively tapped to support growing populations located (for example, Phoenix and basins in the Mojave Desert northeast of Los Angeles).

References

Bell, J.W., 1981, Subsidence in Las Vegas Valley: Nevada Bureau of Mines and Geology Bulletin 95, 84 p.
Bell, J.W., Price, J.G., and Mifflin, M.D., 1992, Subsidence-induced fissuring along preexisting faults in Las Vegas Valley, Nevada: Proceedings, Association of Engineering Geologists, 35th Annual Meeting, Los Angeles, p. 66-75.
Bell, J.W., and Ramelli, A.R., 1991, in Bell, J.W., and Price, J.G., eds., Subsidence in Las Vegas Valley, 1990-1991, final project report: unpublished Nevada Bureau of Mines and Geology Report, p. E1-E31.
Longwell, C.R., 1960, Interpretation of the leveling data, in Smith, W.O., Vetter, C.P., Cummings, G.B., and others, Comprehensive survey of sedimentation in Lake Mead, 1948-49: U.S. Geological Survey Professional Paper 295, p. 33-38.
Maxey, G.B., and Jameson, C.H., 1948, Geology and water resources of the Las Vegas, Pahrump, and Indian Springs Valley, Clark and Nye Counties, Nevada: Nevada Department of Conservation and Natural Resources, Water Resources Bulletin 5, 128 p.
Mifflin, M.D., Adenle, O.A., and Johnson, R.J., 1991, Earth fissures in Las Vegas Valley, 1990-91 inventory, in Bell, J.W., and Price, J.G., eds., Subsidence in Las Vegas Valley, 1990-91, final project report: unpublished Nevada Bureau of Mines and Geology Report, p. C1-C31.
Mindling, A.L., 1971, A summary of data relating to land subsidence in Las Vegas Valley: University of Nevada Desert Research Institute Publication, 55 p.
Patt, R.O., and Maxey, G.B., 1978, Mapping of earth fissures in Las Vegas Valley, Nevada: University of Nevada Desert Research Institute Project Report 51, 19 p.
Poland, J.F., and Davis, G.H., 1969, Land subsidence due to withdrawal of fluids, in Varnes, D.J., Reviews in engineering geology, v. II: Boulder Colorado, Geological Society of America, p. 187-268.

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