Mather Dam Improvement Project

Updates

The Sacramento County Board of Supervisors (Board) approved advertising the project for construction bids on March 21, 2017. The project received nine bids on April 27, 2017. The County awarded the construction contract to the low bidder - Nordic Industries, Inc.

The Board approved Project Environmental Documents on December 6, 2016. The Documents can be viewed at the following website:

https://planningdocuments.saccounty.net/ViewProjectDetails.aspx?ControlNum=PLER2014-00067

Background

During the 1950s, the United States Air Force created Mather Lake near the eastern boundary of Mather Air Force Base (Base) by damming the flow of Morrison Creek with earthen embankments. Mather Dam consists of the original earthen embankments combined with a spillway structure in the northwest corner of the lake. Mather Lake is located east of Zinfandel Drive and south of Douglas Road in east Sacramento County. The lake is adjacent to Mather Regional Park and Mather Golf Course. Over the years, Mather Lake has become the centerpiece of Mather Regional Park and serves as a recreational amenity for the community (Attachment 1).

Since the Base decommissioning in September 1993, the Air Force worked with the County of Sacramento on property transfer and redevelopment of the Base including Mather Dam. Upon transfer of the property from Federal ownership to the County, the dam would fall under California State Division of Safety of Dams (DSOD) jurisdiction. During the property transfer negotiations, it was determined that Mather Dam does not meet DSOD standards. The County was concerned with accepting the dam “as-is” without the ability to identify and construct improvements that would be needed to bring the dam up to DSOD standards. In response to County concerns, the Air Force agreed to provide up to $5,350,000 through a Cooperative Agreement to allow the County to fund the effort to properly plan, engineer, and execute the work required to improve the dam. Under the Cooperative Agreement, if the County determined dam improvements were not feasible, the County could have used the funding to reduce the lake’s water level in order to reduce the required improvements. On December 20, 2012, the Air Force executed the Cooperative Agreement with the County and on May 8, 2013, Mather Dam transferred from Federal ownership to the County of Sacramento as part of an Economic Development Conveyance.

Since April 2012, DWR has retained AECOM Technical Services, Inc. (AECOM) as a design consultant for Mather Dam improvements. AECOM’s involvement with Mather Dam dates back to 2005 when they performed assessment work on behalf of the Air Force. Concurrent to the property negotiations, the Sacramento County Department of Water Resources (DWR) and AECOM were interacting with DSOD to determine the extent of improvements required to meet DSOD standards. Early exchanges with DSOD resulted in requirements for extensive dam improvements that would have cost more than double the funding provided by the Cooperative Agreement. DWR started to consider the alternative to modify the dam to lower the lake’s water level to reduce the required improvements into an acceptable cost range. This was not a desirable alternative, as it would have lowered the lake level a minimum of five (5) feet, which would have significantly decreased the size of the lake causing negative impacts to lake users and the habitats it supports. Fortunately, after extensive additional investigation, engineering, and coordination with DSOD, an alternative was developed that lowered the lake level one-and-half (1.5) feet and fell within the Cooperative Agreement funding range. Lowering the lake level one-and-half (1.5) feet will have negligible impacts to the lake as the lake naturally drops to a similar level in the summer. The proposed lake level modification was reviewed and approved by Department of Regional Parks (Parks).

Discussion

DWR has developed the proposed Project to modify the dam to meet DSOD standards and maintain most of the current functionality of the lake. The Project includes the removal and replacement of the existing spillway structure, installation of a permanent pedestrian/vehicular bridge over the new spillway structure, repair of the spillway outfall area, re-contouring improvements upstream and downstream of the new spillway structure, and reinforcement of some of the existing earthen embankments (Attachment 2).

The spillway improvements will upgrade the spillway configuration and lower the spillway one-and-half (1.5) feet in elevation. The existing spillway is made up of eight (8) pipes and the spillway outfall area is made up of an old concrete apron and concrete rubble. The new spillway will be a U-shaped concrete broad crested weir with a permanent pedestrian/vehicular bridge spanning the weir. The new spillway will be lowered one-and-half (1.5) feet to meet DSOD standards for lake water level requirements. The new spillway outfall area will be a concrete energy dissipation basin with one row of energy dissipation blocks. Directly downstream of the energy dissipation basin, improvements will transition into an erosion resistant, riprap rock lined, trapezoidal channel for approximately 130 feet and then into a natural earthen bottom channel for another 100 feet where it will tie into the existing natural channel of Morrison Creek. Since the spillway will be lowered one-and-half (1.5) feet, the approach channel upstream of the spillway will also require lowering. A new 20 foot wide, one (1) foot deep trapezoidal approach channel will be excavated for approximately 100 feet upstream of the new spillway structure.

The embankments will be reinforced by placing an earthen buttress along the existing main dam and wing dam embankments. The earthen buttress is engineered material that is installed directly onto the existing embankment to add size and strength to the embankment. The main dam will receive a buttress along the downstream face of the embankment. The wing dam will receive a buttress over the entire upstream, crest (top), and downstream faces of the embankment. This work will involve removal of existing vegetation on the embankment slopes and adding, moving, and compacting engineered material consisting of soil and rock. Reinforcement of the main dam embankment is expected to require excavation of some existing material and backfilling with more suitable material before grading and compacting occurs.

The Project impact area includes the areas of Mather Dam receiving improvements, proposed staging and borrows areas, and temporary equipment access roads. By using area adjacent to and south of the lake for staging and borrow areas, the Project will minimize impacts to the surrounding area (Attachment 2). Most construction traffic will be onsite moving between the construction and borrow areas. There will be intermittent construction traffic on the public roadways, but there are no planned lane or road closures. Advanced coordination with Mather Golf Course will minimize impacts to their operations during construction. Areas disturbed during construction will be restored to pre-construction conditions following project completion. Construction is expected to last four months and is scheduled for July – October 2017.

The Department of Community Development, Planning and Environmental Review Division (PER) performed an environmental review of the Project. PER issued a Negative Declaration and MMRP for the Project on August 24, 2016. Four sets of comments were received during the public review period. The comments were standard notifications from Central Valley Flood Protection Board, Central Valley Regional Water Quality Control Board, Sacramento Municipal Utility District, and State of California Governor’s Office of Planning and Research State Clearinghouse (Attachment 3).
DWR performed continuous outreach throughout the development of the Project. DWR coordinated with Parks, Mather Golf Course, and the Independence at Mather Homeowners Association (HOA). DWR gave a presentation on the Project at the November 14, 2016 HOA meeting. Public outreach will continue throughout the Project.