Gross Reservoir Expansion Project

The expansion of Gross Reservoir will supply an additional 77,000 acre feet (approximately 25 billion gallons) of water storage. This is accomplished by raising the existing dam by 131 feet.

The dam will be raised with what is called a downstream buttress raise, where the new concrete will be placed on the downstream face of the dam from the bottom up. First our team will establish an on-site office and staging area to store equipment, followed by development of an on-site quarry that will be used to obtain the majority of aggregate materials to build the dam.

Work will then begin preparing the existing dam surface for new concrete to be added, this is accomplished with high-power water jets. Crews will excavate rock at the base and abutments of the existing dam to prepare the foundation to support the expanded dam surface. Concrete will be produced in an onsite concrete batch plant from the locally-sourced raw materials and conveyed to the dam site where it will be added to the structure using roller compaction.

The concrete mix we plan to use – Roller Compacted Concrete or RCC – has much lower water content than what is commonly used on sidewalks and driveways in urban areas. Its properties act much more like soil, which speeds curing at a lower temperature and thus is less susceptible to cracking.

In addition to the main dam expansion, a new auxiliary spillway will be added to process extreme flood events. Additional modifications to accommodate potential flood events will be required at existing overflow outlets as well.

Once started, we anticipate being able to complete the bulk of dam raising activities within three years (spring, summer and fall) by working around the clock as weather and conditions allow.

Construction Process

Gross Dam is a 340 foot tall concrete gravity dam. The expansion project will raise the dam 131 feet to a final height of 471 feet.
This raise will be constructed with a construction method known as a downstream buttress, which will use the existing dam as formwork. Concrete will be made at an on-site concrete batch plant and then conveyed to the dam.

Roller-compacted concrete (RCC) will be used. RCC uses less cement, more fly ash, and a lower watercontent than conventional concrete. This mix has superior physical properties that includes lower curing temperatures and a product that is less susceptible to cracking.

Latest News

Water projects take patience, stewardship

Water projects take patience, stewardship

Journal Advocate – June 29, 2018 – "As a result of going above and beyond, the Gross Reservoir expansion includes an "environmental pool" of 5,000 acre feet of water to benefit 17 miles of aquatic habitat in South Boulder Creek..." Read more...

Governor Endorses Gross Reservoir Expansion Project

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