Salinas Valley Water Project (SVWP)
Nacimiento Dam Spillway Modifications Component
Construction began April 1, 2008

Construction on the Nacimiento Dam Spillway Modification Component of the Salinas Valley Water Project began on Tuesday, April 1, 2008, and will continue during the non-rainy seasons over the next two years.

Because of construction activities, there will be periods when traffic on Nacimiento Lake Road will be affected by construction truck traffic, and there will be variations in the levels of Lake Nacimiento. However, the construction schedules and the water levels will not have significant effects on traffic around the reservoir and on recreational activities, particularly on weekends and holidays.

The purpose of the Nacimiento Dam Spillway Modification Component, which includes enlargement of the spillway and installation of a rubber spillway gate at the dam, is to address safety issues associated with the flood flows that must be conveyed through the reservoir and the spillway.

State and federal regulators require that the dam be modified so that if a large flood event were to occur, the dam's spillway could release enough water to ensure flood protection and safety of the dam. In addition, the spillway modification would provide additional water releases during the year for water conservation in the Salinas Valley.

Dam safety and the protection of lives and property is the mission of both the Federal Energy Regulatory Agency (FERC) and the State Department of Water Resources, Division of Dam Safety (DSOD) and also the Monterey County Water Resources Agency (MCWRA). Without this project, the MCWRA would have to provide an additional flood storage equal to 17 feet of elevation by lowering the reservoir level as of January 1 every year to 765 feet.

"The Agency's goal has been to stop seawater intrusion and to increase flood protection at the dam.This project allows us to meet that goal," according to Curtis Weeks, general manager of the Water Resources Agency.

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.

Modifications to the spillway at Nacimiento Dam are expected to be completed by Fall 2009. Until then, construction will occur between April and September 2008, and April and September 2009.

 

 

 

 

 

 

 

The second component of the Salinas Valley Water Project is construction of 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.
Project Information: Frequently Asked Questions:
Purpose 1. Will the lake be drained completely during construction?
Descriptions of Facilities 2. How will the Project affect residents and visitors to the lake?
Construction Elements of the Spillway Modification 3. Where can I get the current reservoir/lake elevations?
Photographs 4. Where will the construction work be located?
Salinas Valley Water Project EIR 5. What is the construction schedule?
Groundbreaking Program 6. Will any local roads be closed?
Monthly Construction Progress Report for Oct. 08 7. Who benefits from this project?
Location Map 8. Will I have to pay additional property taxes for these improvements?
  9. Who is requiring this project be constructed?
  10. Have all the Environmental Documents been completed?
  11. Is the construction on the north side of the Dam related to this project?
  12. Where can I get updated information on this Project?
Purpose (top)
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. (See Description of Facilities below) (Also see "Project Overview: SVWP Groundbreaking Ceremony Program")

Description of Facilities (top)
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.

1) 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.

2) 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.

3) 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 (top)
  •  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.

Frequently Asked Questions:

1. Will the lake be drained completely during construction? (top)
No. The MCWRA is required by the Federal Energy Regulatory Commission (FERC) and the State's Department of Water Resources, Division of Safety of Dams (DSOD) to maintain a maximum reservoir (lake) level of 770 feet during the first construction season (summer 2008) and 764 feet during the second season in 2009 (the maximum reservoir elevation at the Spillway is 800 feet). This is dependent upon the amount of rainfall and inflow received by the reservoir from the Nacimiento Watershed area. Maximizing reservoir elevations will benefit conservation efforts and recreation. Normal conservation releases will continue throughout construction.   (See Release Schedule)

2. How will the Project affect residents and visitors to the lake? (top)
There will be minimal effects to residents and visitors. Normal working hours will be adhered to during a normal workweek (Monday - Friday). No work will be allowed on weekends or on holidays. Traffic controls will be in place during working hours, allowing for through traffic at all times. The North Shore Boat Launch Area will remain open to all visitors during its normal operating hours. (See Monterey County Parks Dept. at Nacimiento)

3. I'm planning to visit Nacimiento or San Antonio dams, where can I get the current reservoir/lake elevations? (top)
(See Reservoir Elevations)

4. Where will the construction work be located? (top)
Work will be concentrated on the north side of the dam and below Nacimiento Lake Drive Bridge, which is part of the top of the dam, outside of any public roads or property. It will be entirely contained on property owned and maintained by the Monterey County Water Resources Agency. (See Location Map)

5. What is the construction schedule? (top)
In order to comply with FERC and DSOD worker safety requirements, the construction schedule will extend from April 1, to September 30, 2008, and April 1 to September 30, 2009. All work will be completed by September 30, 2009.

6. Will any local roads be closed? (top)
No. However, there will be some delays. Nacimiento Lake Drive will not be closed at any time. Traffic personnel (flaggers) and signage will be present on Nacimiento Lake Drive to guide construction vehicles/equipment when necessary for safe traffic flow.
  • The construction contractor is required to provide traffic control devices that will safely direct traffic movements in/near the project site. In addition, construction activities will be coordinated between emergency response personnel, such as police and fire, for access through the project site at all times.
  •  Signage will be posted in advance notifying motorists of significant truck activity (more than six truck trips per day). This increased truck activity will occur Mondays through Thursdays.
  • One-way road closures may occur but not on Fridays or weekends during the peak recreation season (May 1st to September 30th) nor during peak commuting times (7 a.m. to 9 a.m. and 4 p.m. to 6 p.m.) Monday through Thursday.
  • To optimize safety, traffic flaggers will direct and signage will be provided in advance, informing motorists of the days and hours of potential closures and duration (less than 15 minutes).
  • Two-way closures of Nacimiento Lake Drive at the top of the dam may occur but will be limited to less than 15 minutes and a maximum of four times a day. These closures can only occur Monday through Thursday, between 9 a.m. and 4 p.m., and not on Fridays and weekends. These measures will minimize any traffic inconvenience.

7. Who benefits from this project? (top)
Residents living downstream of Nacimiento and Salinas rivers will benefit most from an increased level of flood protection and also from recharge of aquifers through water conservation. Monterey County residents will also benefit by slowing, if not, stopping seawater intrusion entirely, which contaminates their groundwater supply. Residents and visitors to Lake Nacimiento will continue to enjoy recreational benefits from the project and use the reservoir as a source of water. San Luis Obispo County residents who participate in the Nacimiento Water Project will benefit by drawing water from the reservoir for consumption. For more information on San Luis Obispo County's Nacimiento Water Project, please visit the following website: SLO Water Project.

8. Will I have to pay additional property taxes for these improvements? (top)
The Salinas Valley Water Project, and its components, will be paid entirely by Monterey County residents who voted to assess themselves, based on benefits derived from the project. Monterey County residents originally paid for the entire construction of Nacimiento Dam in 1957 and continue to pay for its operation and maintenance. San Antonio Dam was also paid for by Monterey County residents in 1967. The hydroelectric plant was constructed in 1987. No property taxes will be paid by San Luis Obispo County residents for the Spillway Modifications Component.

9. Who is requiring this project be constructed? (top)
The two main regulatory agencies involved include the Federal Energy Regulatory Commission (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. They have participated throughout the design phase to ensure that the design meets the highest 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 MCWRA. Without this project, the MCWRA would have to provide an additional flood storage equal to 17 feet of elevation by lowering the reservoir level as of January 1 every year to 765 feet.

10. Have all the Environmental Documents been completed? (top)
The Agency has a completed the California Environmental Quality Act (CEQA) document for the Salinas Valley Water Project (SVWP), the Nacimiento Dam Spillway Modifications Component being one element - certified by Monterey County's Board of Supervisors. All Federal, State, and local permits have been acquired, allowing construction to begin in 2008. Please visit MCWRA's website to view the environmental documents for the entire SVWP: SVWP on MCWRA Website

11. Construction is already occurring on the north side of the Dam; is this related to this project? (top)
That project is titled the Nacimiento Water Project (NWP) sponsored by the San Luis Obispo County Flood Control and Water Conservation District, in conjunction with various stakeholders and five local entities representing the communities of Paso Robles, Atascadero, Templeton, San Luis Obispo, and County Service Area No. 10, Benefit Zone A (Cayucos). The project is a regional raw water transmission facility that will deliver water from Lake Nacimiento to communities within San Luis Obispo County. The project involves the development of an intake and pump station at Nacimiento Reservoir, approximately 45 miles of transmission pipeline ranging in size from 36 to 12 inches in diameter, three storage tanks, two intermediate pump stations, turnouts, control center, new Supervisory Control and Data Acquisition (SCADA) and Project control system, and associated appurtenant facilities. The project is broken down into five contracts, each contract representing a major project component. San Luis Obispo County is entitled to 17,500 acre-feet of water per year per agreement with Monterey County. For more information, please visit the following website: SLO Water Project.

12. Where can I get updated information on this Project? (top)
We will be providing occasional project updates and other pertinent information on our website located at MCWRA Website.

Location Map (top)

Spillway Before and After Modification (Simulated) (top)
 
Looking Downstream at Spillway (top)
Before: After: 
 
Looking Upstream at Spillway (top)
Before:  After: