The largest construction project of its kind in the U.S. is under way in San Diego County.Construction crews are almost halfway done with a "saddle dam" at the San Vicente Reservoir. The dam will eventually be 40-feet tall, according to officials.That project is nothing compared to what engineers and construction crews call the "Main Event." That's where the San Diego County Water Authority will raise the current San Vicente Dam an additional 117 feet.It's the largest project of its kind in the U.S., and once it's completed the San Vicente Reservoir will be able to hold more than twice its current capacity.The reservoir will become a valuable emergency water storage, especially in the event of a worst-case scenario like an earthquake or terrorist attack. San Diego County could be cut off from its water supplies in Northern California and the Colorado River.Right now, almost all of the county's water is imported. Several projects from saltwater desalination to recycling wastewater are being investigated to decrease San Diego's dependency on outside water sources."It's important because if we couldn't bring imported water in, we could tap into our local storage here, right in San Vicente," said project manager Kelly Rodgers.By 2015, the reservoir is expected to hold a year's worth of emergency water for 300,000 households.The $450 million project may not have been possible if it wasn't for the hills near the reservoir. That's where construction crews are mining the rocks that are the main ingredient in the roller-compacted concrete being used in the two dams. About 90 percent of the concrete comes from the hills which are less than half-a-mile away. The concrete is added at the top of the dam, one foot at a time."We're also not having to truck supplies onto site and that's real great for the Lakeside community, eliminating that truck traffic," said Rodgers.Otherwise, dump trucks would have to take 100,000 trips through the nearby community of Lakeside during the next six months.