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Sacramento Weir is the last of six weirs (as observed heading downstream) in the Sacramento River Flood Control Project. The Sacramento Weir was completed in 1916. It is the only weir that is manually operated – all others overflow by gravity on their own. It is located along the right bank of the Sacramento River approximately 4 miles upstream of the Tower Bridge, and about 2 miles upstream from the mouth of the American River. Its primary purpose is to protect the City of Sacramento from excessive flood stages in the Sacramento River channel downstream of the American River. The weir limits flood stages (water surface elevations) in the Sacramento River to project design levels through the Sacramento/West Sacramento area. The project design capacity of the weir is 112,000 CFS.
The Sacramento weir is 1,920 feet long and consists of 48 gates that divert Sacramento and American River floodwaters to the west down the mile-long Sacramento Bypass to the Yolo Bypass. Each gate has 38 vertical wooden planks "needles" (4 inches thick by 1 foot wide by 6 feet long), hinged at the bottom and retained at the top by a hollow metal beam. The beam is manually released using a latch. Flood forecasters provide the necessary predictive information to weir operators who manage the number of opened gates in order to control the river's water surface elevation. Closing the hinged gates is a more laborious process than opening them. While opening a gate takes only a matter of minutes, closing it can take up to an hour. Long, hooked poles are used to raise each gate from its free open position to the vertical upright position. The hollow metal beam is then replaced, and the gate is released and allowed to rest against it. Each open gate has a peak design flow of 1,500 cubic feet per second.
The Department of Water Resources operates the weir according to regulations established by the U.S. Army Corps of Engineers. The opening and closing criteria have been optimized to balance two goals: (1) minimize sediment deposition due to decreased flow velocities downstream from the weir to the mouth of American River; and (2) limit the flooding of agricultural lands in the Yolo Bypass until after they have been inundated by floodwaters over Fremont Weir.
Though the weir crest elevation is 24.75 feet, the weir gates are not opened until the river reaches 27.5 feet at the I Street gauge with a forecast to continue rising. This gauge is about 1,000 feet upstream from the I Street Bridge and about 3,500 feet downstream from the mouth of the American River. The number of gates to be opened is determined by the NWS/DWR river forecasting team to meet either of two criteria: (1) to prevent the stage at the I Street gauge from exceeding 29 feet, or (2) to hold the stage at the downstream end of the weir to 27.5 feet. Once all 48 gates are open, Sacramento River stages from Verona to Freeport may continue to rise during a major flood event. Project design stages are 41.3 feet at Verona, 31.5 feet at the south end of the Sacramento Weir, and 31 feet at the I Street gauge.
During a major flood, opening the weir gates at river stages below 27.5 feet does not reduce ultimate peak flood stages in the Sacramento River from Verona to Freeport. Diversion of the majority of upstream floodwaters to the Yolo Bypass from Fremont Weir controls Sacramento River flood stages at Verona. Downstream of the Sacramento Weir, the design flood capacity of the American River is 5,000 CFS higher than that of the Sacramento River. Flows from the American River channel during a major flood event often exceed the capacity of the Sacramento River downstream of the confluence. When this occurs, floodwaters flow upstream from the mouth of the American River to the Sacramento Weir.
The weir gates are closed as rapidly as practicable once the stage at the weir drops below 25 feet. This provides "flushing" flows to re-suspend sediment deposited in the Sacramento River between the Sacramento Weir and the American River during the low flow periods when the weir is open during the peak of the flood event.
Between 4:00 AM and 5:00 AM on January 10, 2017, the Sacramento Weir gates were opened for the first time in over ten years. The previous time the weir gates were opened was in January of 2006. The gates were subsequently closed on January 27, 2017.
10 of the 48 gates were reopened of February 8, 2017 around 11:00 PM. This marks the first time since the 1985-1986 water year that the gates were opened twice in the same water year. An additional 15 gates were opened on February 9, 2017 around 8:00 am. At around 7:00 PM, another 21 gates were opened, bringing the total to 46 out of 48 gates.