Probable Maximum Flood and Spillway Adequacy Determination Study of Nacimiento and San Antonio Dams

GEI is working on a Probable Maximum Flood and Spillway Width Determination Study of Nacimiento and San Antonio Reservoirs for the Monterey County Water Resources Agency (Agency). The primary purpose of this study is to re-evaluate the Probable Maximum Flood (PMF) estimates for these two reservoirs, based on the Probable Maximum Precipitation (PMP) data published in the draft Hydrometeorological Report (HMR) 58 released by the National Weather Service.

Nacimiento Dam is 210 feet high and San Antonio Dam is 201 feet high, and by virtue of their large size (350,000 and 335,000 acre-feet, respectively) and location, they are key components of the Agency's water supply facilities. Both dams are earthfill structures. Currently, the water supply storage in both Nacimiento and San Antonio Reservoirs is limited by the rule curve operations that require the Agency to maintain large portions of these reservoirs empty during the winter months to accommodate flood storage, due to emergency spillway deficiencies. As a key component of this project, GEI is closely interacting with the regulatory agencies that have jurisdiction over these reservoirs, including the California Department of Water Resources, Division of Safety of Dams (DSOD), and the Federal Energy Regulatory Commission (FERC).

The tributary basins to the dams have been heavily instrumented in recent years with self-recording rainfall and streamflow gages. In March of 1995, one of the wettest periods of record was recorded with good precipitation and runoff measurements in the basins. This storm event has been estimated at about a 300-year interval, resulting in an excellent database to develop a unit hydrograph and runoff model using HEC-l. The spatial distribution of this storm was well distributed and showed the expected orographic effects associated with this region. It is interesting to note that the 1997-1998 wet season that has been characterized as being heavily influenced by the El Nino phenomena has not produced the intensity of rainfalls experienced in 1995. Additionally, the draft HMR58 PMF data indicates that the 72-hour rainfall volume is about 14 percent less than the existing HMR36.

In 1991, the Agency initiated the preparation of a Basin Management Plan (BMP) for the Salinas River Basin. The Salinas Valley is one of the nation's major producers of vegetables. One of the goals of the BMP was to provide a sufficient water supply for the Salinas Valley for another 20 to 30 years. In order to meet this goal, the Agency has identified potential sources for increased water supply, including alternatives for developing increased yield from these two existing reservoirs. Restoring the water supply storage in the reservoirs is an integral part of the BMP. The potential of linking the two reservoirs, which are less than 1 mile apart, with an inter-lake tunnel is also being explored. 12 pp., 11 references.