Marine Construction on Loch Lomond Inlet/Outlet Replacement Project at Newell Creek Dam, Ben Lomond, CA

The City of Santa Cruz Water Department in California replaced the existing Newell Creek Dam inlet/outlet works. The existing inlet/outlet works included a deteriorated 36-inch diameter pipeline under the dam and other components that did not meet current California DSOD dam safety reservoir drawdown requirements. To maintain the reservoir, the City’s engineer designed a new inlet/outlet works that could be constructed under full reservoir conditions, in water up to 115 feet deep. The underwater work began with the dredging of an “amphitheater” on the native 2:1 rock slope. After dredging, three large, critically located benches were excavated into the rock slope with a heavy digging bucket and specialty designed underwater tools. A marine rail was constructed, on which three large concrete donuts were precast and lowered into the water before picking with a crane. These donuts were then installed and grouted in place on the excavated benches. Six foot diameter shafts were drilled through the hole at each inlet foundation donut to the tunnel intersect elevation, and steel casings were installed and grouted into each shaft. Divers installed the three intake structures at design elevations. These 30” intake structures were 30-inch standpipes with 30” diameter hydraulic valves and 48” diameter by 56” tall drum screens. The piping was connected to a 20” diameter air vent line which is designed to prevent vacuum conditions and plumbing to provide a minimum downstream flow. Once the shafts and intakes were in place, a new tunnel was advanced by the general contractor from the downstream right abutment to intercept the shafts. 30” CISS piles were installed to support the 20” vent line from the intake locations to the dam crest. These piles were drilled from barges. Divers then installed Vortex designed articulating pipe supports on the piles, and finally the vent pipe to shore.