Big Creek Dam No. 4 is located on Big Creek, a tributary to the San Joaquin River in Fresno County. The nearest city is Bishop, located approximately 70 miles northeast of Big Creek No. 4 Dam. Big Creek No. 4 Dam is located at the tailrace of Big Creek Powerhouse No. 1 (Powerhouse No. 1) and forms the forebay for discharges from Big Creek Powerhouse No. 2 (Forebay). Operations of Big Creek No. 4 Dam are regulated by the Federal Energy Regulatory Commission (FERC) as part of the Big Creek Nos. 1 and 2 Hydroelectric Project, FERC Project No. 2175.
The Project involves two main activities: (1) repair Big Creek No. 4 Dam’s existing low-level outlet (LLO) system to comply with California Department of Water Resources Division of Safety of Dams regulations and future flow requirements associated with the new FERC license that is anticipated for the Big Creek Nos. 1 and 2 Hydroelectric Project; and (2) resurface the downstream face of the dam to repair aging concrete. The Project is located on lands administered by the United States Forest Service in the Sierra National Forest.
The majority of Project work will be supported by a 110-ton barge-mounted crane positioned adjacent to Big Creek No. 4 Dam in the Forebay and by a 500-ton crane positioned in the existing paved area along the north side of the Forebay and adjacent to Powerhouse No. 1. The barge-mounted crane will be used to set and retrieve materials downstream of Big Creek No. 4 Dam. Site mobilization and setup activities include: (1) temporarily reinforcing the bridge over Pitman Creek on Huntington Lake Road to allow heavy equipment to pass; (2) removing up to 16 trees adjacent to Powerhouse No. 1 to allow crane operations; and (3) establishing dry work conditions around and downstream of the LLO using coffer dams and turbidity curtains.
To resurface the dam, the existing deteriorated concrete surface will be removed to an approximate depth of six inches. Hydro demolition using ultra high-pressure water jets, also known as water blasting or hydro blasting, will be used to remove the concrete without damaging the underlying rebar or the surrounding metal infrastructure. Following hydro blasting demolition of the concrete dam surface, the dam will be resurfaced with new shotcrete. Access to the downstream dam face will include traditional scaffolding on both abutments for worker transport as well as modular swing stage scaffolding across the dam. Scaffolding will serve as both a staging area and work area. The barge-mounted crane in the Forebay will set and retrieve materials from all work and staging areas downstream of the dam. Concrete removed by hydro blasting demolition will be collected in watertight hauler trucks and disposed of offsite. Seepage water that enters the downstream work area and process water will be treated using Baker tanks and a portable water treatment facility before it is released into Big Creek.