Folsom Dam Auxiliary Spillway

Project Description:

The project consists of constructing an auxiliary spillway to allow water to be released earlier and more safely from Folsom Lake during a high water event. Key features of the project include a control structure containing six submerged flood gates, a 3,000-foot spillway chute and a stilling basin, and a 1,100-foot approach channel.

BSK Roles/Responsibilities

BSK assisted Granite construction with designing and permitting the storm water management program for the 110-acre, Risk Level 3 project. The program consisted of conducting a hydrologic assessment to evaluate storm water runoff quantities, designing containment berms and settling basins, and developing the Storm Water Pollution Prevention Plan (SWPPP).


There were several challenges to this project. First, was to design and implement the program so that no storm water entering the site discharged to the American River. All water was required to be captured, treated, and discharged into Folsom lake. Second, potential tension cracks were observed on the surface of an approximately 500-foot long access ramp to allow vehicles to enter the 50-foot-deep spillway construction area

BSK Solutions:

BSK recommended a 170-foot-long containment dike be constructed to contain the rainwater collected in the spillway. This water was then pumped to a settling basin, through a 3,000-GPM active treatment system, and ultimately into Folsom Lake.