Posted by: Deirdre Des Jardins | April 2, 2017

DWR dams upstream of Oroville also had extensive spillway repairs, began spilling last week

The California Department of Water Resources owns three smaller dams upstream of Oroville, including two on tributaries of the Middle Fork of the Feather River, and one on a tributary of the North Fork of the Feather River.  Records show that all three upstream dams had extensive spillway repairs in 2009, at the same time that the Oroville main spillway had extensive repairs.  The repairs for all three spillways included sawcutting concrete, void repairs, and repairs of major cracks. Frenchman Dam also had repairs of the spillway lip. Records show the three dams were last inspected in October of 2014.

Two of the dams, Grizzly Valley and Frenchman, began spilling last week, according to news reports. There are a number of small communities on tributaries and the Middle Fork of the Feather River that could be inundated if the dams had major spillway erosion. The U.S. Army Corps’ National Inventory of Dams Database shows no Emergency Action Plan for any of the dams, although all appear to meet the definition of High Hazard dams, because failure could result in loss of life. The State of California does not require Emergency Action Plans for High Hazard dams, and the dams are not supervised by the Federal Energy Regulatory Commission.

Frenchman Dam, at the headwaters of the Middle Fork of the Feather River, is on Little Last Chance Creek. Little Last Chance Creek flows down past the small community of Chilcoot-Vinton (population 457) into the Sierra Valley, where it joins with other tributaries to form the beginning of the Middle Fork of the Feather River. Frenchman Dam is 139 feet tall and impounds Frenchman Lake, up to 55,477 acre-feet of water.

1 Frenchman Lake and Lake Davis                                 Source: Google Earth

Further downstream, just north of the City of Portola (population 2,100), Big Grizzly Creek joins the Middle Fork of the Feather River. Grizzly Valley Dam, on Big Grizzly Creek, is 115 feet tall and impounds Lake Davis, up to 83,000 acre-feet of Water. Numerous other communities are on the Middle Fork of the Feather River downstream of the City of Portola including Deleker, population 700, Mabie, population 161, Iron Horse, population 297, Graeagle, population 737, and Cromberg, population 261.

2 City of Portola, California

DWR’s third dam is Antelope Dam, on Indian Creek, a tributary of the North Fork of the Feather River, in Plumas National Forest. Antelope is 113 feet tall and holds 22,566 acre-feet of water. The small community of Taylorsville (population 150) is on Indian Creek below Antelope Dam.

3 Antelope Lake                              Source: Google Earth

Grizzly Valley Dam, which impounds Lake Davis, began spilling last week, as did Frenchman.

On September 14, 2009, the Department of Water Resources published a notice soliciting bids  for repairs of the concrete chute on the main Oroville spillway , as well as for repairs of the spillway chutes of Grizzly Valley, Frenchman, and Antelope Dams.  DWR also published a summary of received bids, which showed that the Antelope Dam spillway had 1,790 linear feet of sawcutting and 47 cubic feet of void repair, as well as 100 linear feet of major crack repair and 200 linear feet of subdrain repair. The Frenchman Dam spillway had 713 linear feet of sawcutting and 100 linear feet of major crack repair. Grizzly Valley Dam had 40 linear feet of sawcutting and 40 linear feet of major crack repair.



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