Overview

The Monterey County Water Resources Agency (MCWRA) has been utilizing a collaborative effort with Salinas Valley interests to develop the Salinas Valley Water Project (SVWP) to address the water resources management issues within the Salinas Valley. The SVWP provides for the long-term management and protection of groundwater resources in the basin by meeting the following objectives: stopping seawater intrusion and providing adequate water supplies and flexibility to meet current and future (year 2030) needs. In addition, the project provides the surface water supply necessary to attain a hydrologically balanced groundwater basin in the Salinas Valley. Future project elements may, however, be necessary to achieve this objective. The SVWP the Nacimiento Dam Spillway Modification Component, which includes enlargement of the spillway and installation of a rubber spillway gate at the dam and a diversion facility , which is another rubber dam on the Salinas River near Marina, to allow diversion of river water for treatment and piping to nearby farms for irrigation. The SVWP, approved by an overwhelming 85 percent of property valuation in the Salinas River Basin in 2003, is intended to help stop seawater intrusion, improve flood control and Nacimiento Dam safety, recharge the aquifers and improve river flow for migration of the federally designated threatened Steelhead trout. Construction of the Nacimiento Spillway Modification was completed in 2009 and Salinas River Diversion Facility began its operation in April 2010.

Nacimiento Dam Spillway Modification

Purpose

The purpose of the Salinas Valley Water Project's Nacimiento Dam Spillway Modifications Component is to address safety issues associated with the flood flows that must be conveyed through the reservoir and the spillway. The two main regulatory agencies involved include the Federal Energy Regulatory Commission [FERC] (due to the existing hydroelectric plant located at the base of the dam) and the State Department of Water Resources, Division of Safety of Dams (DSOD). These agencies have participated throughout the design phase to ensure the design meets the high safety standards they have established. They will continue to participate during the construction phase by reviewing and approving proposed design changes and conducting inspections. Dam safety and the protection of lives and property is the mission of both agencies and the Monterey County Water Resources Agency (MCWRA). Without this project, the MCWRA would have to provide additional flood storage equal to 17 feet of elevation by lowering the reservoir level as of January 1 every year to 765 feet.

Description Of Facilities

Construction of Nacimiento Dam was completed in 1956. It is an earthfill dam with a maximum height of about 215 feet from streambed to dam crest. The total crest length is 1,630 feet, including the spillway bridge. The dam crest is at elevation 825 feet and has a width of 30 feet. The existing empty space for flood flow retention is approximately 190 feet. The dam impounds about 378,000 acre-feet at the current spillway crest elevation of 800 feet. Nacimiento Dam is under the jurisdiction of the California Department of Water Resources, Division of Safety of Dams (DSOD) and the Federal Energy Regulatory Commission (FERC). The FERC has jurisdiction over the project due to the existence of the hydroelectric plant located at the right abutment. Exhibit 1 shows an aerial view of the dam and spillway.

Spillway

The spillway consists of a trapezoidal, unlined approach channel, an un-gated concrete ogee crest weir with a crest elevation of 800 feet, and a 540-foot long rectangular, concrete chute that terminates in a flip bucket at about elevation 630 feet. The flip bucket is approximately 20 feet above the streambed. A double lane bridge spans the spillway. The spillway crest is a 150-foot wide, ogee-shaped weir that is curved in plan with a 309.23-foot radius. Up to 60-foot-high retaining walls flank the ogee and the approach channel. The spillway chute tapers down to a 100-foot width and varies in slope from 0.15 ft/ft near the crest to 0.5 ft/ft near the flip bucket. The chute has sidewalls that vary in height from 13 to 25 feet. The flip bucket has a 50-foot vertical radius of curvature and directs the flow into a plunge pool.

Outlet Works

High- and low-level outlets are gated and can be opened and closed to regulate the flows out of the reservoir during low-flow conditions. The low-level outlet, intake elevation of 670 feet, is located adjacent to the powerplant. The high-level outlet is installed within the spillway chute. An 80-foot wide spillway approach channel conveys water to the high-level outlet. The invert of the approach channel is at elevation 750 feet. This channel delivers water to two 8-foot by 8-foot slide gates (inverts are at 755 feet) that discharge into a cast-in-place concrete conduit under the dam's crest before intersecting the spillway and discharging flows down the spillway chute.

Hydroelectric Plant

A 4-megawatt powerplant is constructed at the right dam abutment. The plant contains both large and small turbines that operate in the range of 25 cubic feet per second to 400 cubic feet per second. To maximize power production, two sets of runners have been provided. One set is used at low reservoir elevations and the second at high reservoir elevations. 

Construction Elements Of The Spillway Modification

• Partial demolition of the existing spillway ogee weir to accommodate a new Obermeyer (rubber) Gate System (no change in spillway elevation which remains at maximum reservoir elevation: 800 feet);
• Installation of the Obermeyer Gate System - Gate height of 12.25 feet;
• Raising and strengthening existing chute walls, and anchoring the chute walls with "Sister" walls ;
• Strengthening and anchoring approach channel walls;
• Modifying the High-Level Gate Operations System with an updated mechanical system; and
• Strengthening the Bridge pier with steel reinforced concrete.

The Salinas River Diversion Facility (SRDF)

Project Description

Seawater intrusion, due to over pumping of the aquifers has been recognized as a problem for the Salinas Valley since the middle of last century. The first stage of the solution was the construction of Nacimiento and San Antonio Dams for flood control and recharge of the aquifers. The second stage of the solution was construction of the Monterey County Water Recycling Project consisting of the Salinas Valley Reclamation Project (recycled water) and the Castroville Seawater Intrusion Project (distribution and supplemental well system) serving 12,000 acres of prime farmland in the Castroville area. Even with the recycling project providing two-thirds of the growers’ needs, there continued a heavy reliance for pumping groundwater for irrigation.

The Salinas River Diversion Facility was constructed to provide treated (filtered and chlorinated) river water, significantly reducing the need to pump groundwater except in periods of extremely high demand. Without this project, seawater intrusion would continue to advance and there would be a continuing need for groundwater extraction to meet the irrigation needs of the growers in the CSIP area. At the request of growers, MCWRA has constructed a project that supplies a water source of a quality which equals, if not exceeds, the recycled water they have been receiving for more than 10 years.

Project Elements

• Pneumatically controlled diversion dam spanning the width of the Salinas River
• Intake structure
• Fish bypass facilities, including a fish screen and ladder
• Pump station
• Pipeline connection to the 80 acre-foot recycled water storage pond
• Filtration facilities
• Chlorination facility

The SRDF was constructed to further reduce groundwater pumping. When SRDF is operational, well pumping has been reduced by about 80% during peak agricultural demand periods.

It takes 5 to 7 days for water released from the dams in South County to reach the SRDF. Full Impoundment (9 Feet depth at the dam) holds approximately 120 acre-feet of water and extends approximately 3 miles upriver. The SRDF dam has two main sections, the Regulating Weir (which has one gate) and the Main Section (which has 8 gates).

The SRDF incorporates a fish ladder to allow fish passage up and down river when the dam is raised. The operating permit requires that certain volumes of water be released from the SRDF at different times of the year. The required volume is entered into the SRDF control system and the control system opens the fish ladder and lowers the regulating weir to bypass the required volume of water.

At the end of the season, after the impoundment has been slowly emptied over a 28 day period, the dam is lowered.

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