Subsidence in Las Vegas Valley

Fall 1992

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:

1. Reduce the net annual groundwater withdrawal to the level of net annual recharge. This can be accomplished either through a reduction of dependence upon groundwater (increase dependence upon surface water) or through an increase in the artificial recharge.
2. Continue to define potential hazard zones based upon the spatial distribution of faults and fissures and require detailed geologic, geotechnical, and structural engineering studies within these zones. Definitions may include "high hazard" or "no-build" designations in areas presently exhibiting significant levels of differential movement or fissuring. Developable subsidence zones may be defined on the basis of lower levels of potential hazard but may require that the area is buildable only with specialized construction.
3. In already-built areas lying within high hazard zones, restrictions on the use of applied water may be necessary to prevent the enlargement of fissures. This may require the implementation of strict water conservation policies, such as no watering or desert landscaping ordinances in areas prone to fissuring.
4. Establish a Las Vegas Valley Subsidence District similar to the one in the Houston-Galveston area, which was established by State legislation. This local interagency group would ideally be responsible for setting water policy and priorities and for developing continued subsidence mitigation strategies.
5. Establish a long-term monitoring program to track the occurrence and distribution of subsidence and fissuring. Even if groundwater withdrawals were reduced to the level of estimated annual recharge in the near future, primary and residual subsidence would continue for 5 to 10 years, and fissures would continue to grow through erosion. Geodetic networks are presently established throughout the valley, and they can be resurveyed on an annual basis. Surveillance of known fissure areas is necessary to detect changes in runoff and erosion patterns that may necessitate remedial action.
6. Encourage continued research into the processes that cause horizontal movement, tension, cracks, and fissuring. Assess mitigation strategies using coherent step- by-step experimental and assessment procedures. In order to avoid costly duplication of effort, this research might be done in concert with research in other parts of the country that are similarly affected by damage due to fissures.

---Jonathan G. Price, Director/State Geologist
John W. Bell, Engineering Geologist
Donald C. Helm, Research Hydrogeologist