Land subsidence due to groundwater withdrawal continues to be a geotechnical issue in Las Vegas Valley. In 1981 the Nevada Bureau of Mines and Geology published a compilation of land subsidence data for Las Vegas Valley (NBMG Bulletin 95). This bulletin included a synthesis of all existing data through about 1979 as well as the results of first-order releveling of the benchmarks in the valley by the National Geodetic Survey in 1980. During the subsequent decade, the geodetic monitoring of fault movement continued through the efforts of the Nevada Department of Transportation, but no comprehensive research was conducted on the continuing nature of subsidence throughout the valley.
In 1989 rapid urbanization of Las Vegas Valley coupled with the appearance of new earth fissures prompted the Nevada Bureau of Mines and Geology to propose a research project to update the subsidence database for the period 1980-91. To ensure a comprehensive, wide-ranging evaluation of the subsidence problem, this project was to be wider in scope than earlier studies.
This project was sponsored by five local agencies (Cities of Las Vegas, Henderson, and North Las Vegas, Clark County Sanitation District, and Las Vegas Valley Water District), four federal agencies (Department of Defense-Nellis Air Force Base, Department of Energy-Nevada Operations Office, Department of Housing and Urban Development, and Department of Interior-Bureau of Reclamation), and two state agencies (State Engineer's Office in the Department of Conservation and Natural Resources and the Nevada Bureau of Mines and Geology). Additional assistance was provided by the Nevada Department of Transportation. Work was performed by scientists at the Nevada Bureau of Mines and Geology; Desert Research Institute; U.S. Geological Survey; Department of Civil Engineering at the University of Nevada, Reno; Geoscience Department at the University of Nevada, Las Vegas; and Mifflin & Associates. A project report, available through the NBMG Publications and Information Offices, was finalized early in 1992.
Subsidence in the valley has been geodetically monitored since 1935 when the National Geodetic Survey established a regional first-order level network. Based on this original leveling network, it was found that by 1963 the center of the valley (near downtown Las Vegas) had subsided as much as 3.4 feet. From 1963 to 1986/87, the downtown area continued to sink another 2.8 feet, and other areas near the Strip and the North Las Vegas Airport subsided 2.9 feet and more than 5.0 feet, respectively.
Broad valleywide subsidence and localized subsidence bowls do not, in and of themselves, present the most significant existing or potential hazard. The greatest subsidence hazard is posed by the occurrence and continued growth of earth fissures. New fissures, developed since 1980, have been mapped and correlated to preexisting geologic faults. The valleywide and localized subsidence bowls are triggering vertical and horizontal differential movements on the numerous Quaternary faults occurring throughout the valley, resulting in the formation of fissures. Once initiated as small tension cracks, fissures will continue to grow as erosional features even if subsidence is arrested, and the impact on structures will increase with time if the fissures are not detected and remedial action taken.
Land subsidence will continue to occur in Las Vegas Valley as long as the net annual groundwater withdrawal continues to exceed the net annual recharge. For the 1980-91 period, net withdrawals have exceeded recharge by factors of 2 to 3. In order to moderate or eliminate the effects of subsidence, the withdrawal-to-recharge ratio must be reduced either by reducing pumping or by artificially increasing recharge. Importation of surface water is the most direct means of reducing or arresting subsidence. Subsidence may continue for years after equilibrium is achieved because of a lag in sediment response.
On the basis of the nature of subsidence and fissuring and mitigation measures implemented in other areas of the western U.S. having similar subsidence problems, the following alternatives are considered realistic options for mitigating the subsidence hazard in Las Vegas Valley:
---Jonathan G. Price, Director/State Geologist
John W. Bell, Engineering Geologist
Donald C. Helm, Research Hydrogeologist