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Wawona Utility River Crossing Replacement
Yosemite National Park » Wawona Utility River Crossing Replacement » Document List
During the flood of 1997 three #2 electrical feeder lines in a 4-inch PVC conduit, under the South Fork of the Merced River, were stretched and broken by the flood waters. These under-river electrical lines provided power to the Number 2 lift station at Wawona. Temporary overhead power-lines were installed to the lift station until a new under-river crossing could be installed. The reason for a replacing the overhead lines with an under-river crossing are due to the fact that the South Fork of the Merced River is designated as a wild and scenic river-way and overhead power lines distract from the scenic views.
Alternative #1: Install a utility crossing, consisting of three 4-inch Schedule 40 PVC conduits encased in concrete, under the South Fork of the Merced River. This crossing would be completed via open-cut trenching to an approximate depth of 9-feet, diverting the river flow through a 24-inch steel pipe by temporary constructing a diversion dam (using an aqua bladder dam or sand bag system) on both the upstream and downstream sides of the trenching. Dewatering of the open trench will be completed by driving sand point casings parallel to the trench and removing water via pumping water through these sand point casings. The spoils from the trenching will be contained within the diversion dams, thus eliminating any silt from the excavation from flowing into the river. (see attached Wawona River Crossing Cross-Section and Wawona Electrical River Crossing Concept Drawing sketches for initial concept).
Alternative #2: Install a utility crossing, consisting of two 10-inch ductile iron (DI) mains and three 4-inch Schedule 40 PVC conduits encased in concrete, under the South Fork of the Merced River. This crossing is similar to Alternative #1 except the trench would need to be wider to accommodate the additional 10-inch DI mains (see attached Wawona River Crossing Concept Drawing with Elect/Water Options for initial concept).
The work for this under-river crossing will be conducted during the seasonal low water periods. The historical average low flows for the South Fork of the Merced River, at Wawona, are from August through October. During the months of August and September average flows are less than 1 cubic foot per second (cfs) with October averaging 1.2 cfs. Peak flows during this period will range from 2 to 3 cfs. The window to complete this crossing will be from August 1st through October 15th.
Alternative #1: Install a utility crossing, consisting of three 4-inch Schedule 40 PVC conduits encased in concrete, under the South Fork of the Merced River. This crossing would be completed via open-cut trenching to an approximate depth of 9-feet, diverting the river flow through a 24-inch steel pipe by temporary constructing a diversion dam (using an aqua bladder dam or sand bag system) on both the upstream and downstream sides of the trenching. Dewatering of the open trench will be completed by driving sand point casings parallel to the trench and removing water via pumping water through these sand point casings. The spoils from the trenching will be contained within the diversion dams, thus eliminating any silt from the excavation from flowing into the river. (see attached Wawona River Crossing Cross-Section and Wawona Electrical River Crossing Concept Drawing sketches for initial concept).
Alternative #2: Install a utility crossing, consisting of two 10-inch ductile iron (DI) mains and three 4-inch Schedule 40 PVC conduits encased in concrete, under the South Fork of the Merced River. This crossing is similar to Alternative #1 except the trench would need to be wider to accommodate the additional 10-inch DI mains (see attached Wawona River Crossing Concept Drawing with Elect/Water Options for initial concept).
The work for this under-river crossing will be conducted during the seasonal low water periods. The historical average low flows for the South Fork of the Merced River, at Wawona, are from August through October. During the months of August and September average flows are less than 1 cubic foot per second (cfs) with October averaging 1.2 cfs. Peak flows during this period will range from 2 to 3 cfs. The window to complete this crossing will be from August 1st through October 15th.