San Vicente Dam is currently 220 feet tall and when completed, will be 117 feet taller, increasing the reservoir’s capacity from 90,000 acre-feet to 242,000 acre-feet.
originally published August 30, 2011 at GrokSurf's San Diego; reprinted with permission
Everything is going according to plan, though, and right on schedule.
All this time has been devoted to prepping the dam and foundation, setting up the quarry where the ingredients for the concrete will be mined, constructing a new access road to the future new marina, and building a small “saddle dam” (because the eventual higher water level will reach a saddle-shaped depression in the hills on the reservoir’s west side).
San Vicente Dam is currently 220 feet tall and when completed, will be 117 feet taller, increasing the reservoir’s capacity from 90,000 acre-feet to 242,000 acre-feet. Although the larger capacity is billed as being enough to supply 300,000 households, the expansion is not really to accommodate additional customers from growth and development as might be implied by that statement.
Because around 80% of San Diego’s water must be imported it could be disastrous if something happened to cut off that supply. Our water managers take that risk very seriously. The real point of expanding the reservoir, therefore, is to have more water stored locally for use if something (like an earthquake) breaks the imported water infrastructure and repairs take months to complete. A secondary reason is to provide carryover storage to be used during extra dry years.
While the City of San Diego owns and operates San Vicente Dam and reservoir, the dam raise project is actually being managed and paid for by the San Diego County Water Authority (SDCWA) which will also own the rights to the additional water. The dam raise is one of several projects SDCWA is doing as part of its countywide Emergency Storage Project.
Work on the saddle dam was recently halted halfway at about the 20 foot mark (again, as planned) because the conveyor system that will deliver concrete from the quarry and concrete mixing operation to the big dam needs to go through that space. The conveyor will cross right over the top of the half-finished saddle dam. After the main dam raise is completed, the conveyor will be dismantled and work on the saddle dam will resume to bring it up to its full height.
Since the precisely formulated concrete will be delivered by conveyor instead of trucks it will minimize contamination from soil and other material. By mining the aggregate for the concrete from the hills next to the reservoir and producing the concrete on site instead of another location, residents living in the vicinity will be spared the dust, noise, and traffic impact that thousands of truck trips would otherwise cause.
Concrete will not be made and poured into forms the traditional way; rather, a technique known as “roller compacted concrete” (RCC) will be used. SDCWA already used this method to build the Olivenhain Dam in northern San Diego County. As explained in the above-linked story about Olivenhain, “Roller-compacted concrete is similar to traditional concrete, but is less expensive, requires less water and is much thicker when placed…Roller-compacted concrete is placed in layers. The layers are compacted with rollers similar to those used in road building. Interruption of work must be minimised to facilitate bonding of the layers. Therefore, crews worked 24 hours a day, seven days a week during a 10-12 month period to construct the dam.”
Similarly, once concrete placement begins at San Vicente Dam (probably in mid-September), work will be non-stop 24/7 until the dam is finished. The dam will rise at the rate of about one foot per day. When I asked about viewing opportunities to see the dam as it goes up, SDCWA Senior Public Affairs Representative Gina Molise told me there’s a possibility that they’ll set up a camera overlooking the job to capture time-lapse images. However, that’s only a possibility at this time.
The project schedule and more information can be found at SDCWA’s project web page.
A few weeks ago SDCWA invited the news media to a photo opportunity of work underway on the saddle dam. I was invited too :-) so I get to share some photos. Click images for enlargements (it makes a big difference with some of them).