Modifications of Storrie Lake Dam

Storrie Lake Dam is located near Las Vegas, New Mexico. The reservoir is a multi-purpose facility providing 23,500 acre-feet of storage for agricultural use, wildlife conservation at the Las Vegas National Wildlife Refuge, municipal water supply for the City of Las Vegas, and water-based recreation at the Storrie Lake State Park. State Highway 518 is routed across the dam. The Ownership of the dam reflects the multi-purpose nature of the reservoir. The facility is owned by the Storrie Project Water Users Association (SPWUA), which includes the U.S. Fish and Wildlife Service (USFWS) and private shareholders. The USFWS has 57 percent of the shares in the project; the private shareholders, all irrigators, have 43 percent of the shares in the project.

The dam is a 90-foot-high embankment, originally built using hydraulic-fill methods in the 1920s. It was modified in 1965 to accommodate the State Highway and raised and stabilized in 1975 in conjunction with widening of Highway 518.

In 1990 the State Engineer restricted the storage level in the reservoir by 8 feet (Elevation 6600 to Elevation 6592) because of inadequate riprap on the upstream slope. This action reduced usable storage by about 9,500 acre-feet. During an inspection conducted for the USFWS by GEI Consultants, Inc. (GEI), a longitudinal crack, up to 1.5 inches wide, was observed in the crown along most of the concrete horseshoe outlet conduit. Cracks were initially observed in 1972, but the main crack was wider during the 1991 inspection. Although this crack was not believed to be an imminent threat to safety of the dam, the SPWUA decided to install a bracing system in the conduit, based on recommendations from another consultant. In 1991 the New Mexico State Engineer imposed a release restriction of 25 cfs.

The USFWS retained GEI to study the feasibility and cost of modifying the dam to improve safety and lift restrictions. GEI examined a broad range of alternatives from which the USFWS and SPWUA selected the following modifications:
1. Remove and replace the upstream slope protection.
2. Remove and reconstruct the upper 22 feet of the gate tower.
3. Line the existing concrete conduit with a steel conduit.
4. Remove the corrugated metal pipe (CMP) comprising the lower portion of the outlet conduit and replace it with a concrete-encased steel conduit.
5. Construct a new outlet works stilling basin.
6. Replace the old gates in the gate tower.
7. Install a new intake structure in the reservoir.

GEI designs included many innovations: a steel intake riser installed underwater to avoid expensive cofferdam construction; a 40-foot-deep supported excavation that enabled removal of the existing CMP while maintaining embankment stability; and detailed pipe liner fabrication geometry. The construction contractor, Bradbury and Stamm Construction of Albuquerque, NM, proposed several modifications that reduced cost and speeded construction time, including use of cellular concrete to grout the annular space between the steel pipe liner and the concrete horseshoe conduit and use of flowable fill to backfill around the concrete-encased conduit. 11 pp.