Sacramento Weir and Bypass
The development of a system-wide flood management plan in the Sacramento Valley began around 1850. Up until the flood of 1909, flood management activities focused primarily on confining flows within the existing river channels. This was a period with frequent levee failures, including failures in the 1909 flood event. As a result, the State of California and federal government decided that a bypass system was needed to divert waters from the main rivers during high flows to relieve stress along the levee system. The state approved the Sacramento and Yolo Bypass systems as part of the Sacramento River Flood Control Project in 1911. Congress then authorized the U.S. Army Corps of Engineers (USACE) to construct the remainder of the project in 1917 and incorporated it into the federal system.
The Sacramento Weir, completed in 1916, is situated along the right bank of the Sacramento River between the Yolo Causeway and the Garden Highway, approximately 3 miles upstream from the confluence of the American River and Sacramento Rivers. Its primary purpose is to protect the City of Sacramento from excessive flood stages in the Sacramento River by diverting river flows west into the two-mile-long Sacramento Bypass that connects to the Yolo Bypass. The Yolo Bypass takes 80% of the flood flows during storm events from major valley rivers including the Sacramento, American and Feather Rivers.
Just downstream of the Sacramento Weir is the confluence of the American and Sacramento Rivers. The design flood capacity of the American River is higher than that of the Sacramento River. During a major flood event, flows from the American River channel 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.
USACE and the State of California are planning to widen the Sacramento Weir and Bypass to allow more water to enter into the Bypass system during flood events, thereby reducing the water surface elevation in the Sacramento River. This work includes widening the existing weir by 1,500 feet and constructing a new 2-mile-long setback levee along the Sacramento Bypass. The first phases of construction began in 2020.
Sacramento River East Levee
The Lower Sacramento River East Levee System is comprised of approximately 12 miles of levee between downtown Sacramento and the Town of Freeport. This levee protects essentially all areas of the City of Sacramento south of the American River and west of Highway 99.
Between 1990-1993, the U.S. Army Corps of Engineers (USACE) constructed 15 miles of levee cutoff walls to address the potential of through seepage during high river flows. An additional 2 miles of cutoff wall work was completed between 2003-2004. However, research and studies conducted following the events of Hurricane Katrina in 2005 found that levees were more susceptible to failure from conditions created by deep under seepage than was previously understood. In light of this finding, state and federal agencies adopted new policies and standards requiring substantial changes to levee design for urbanized areas. In 2019, the U.S. Army Corps of Engineers (USACE) began constructing levee improvements to address embankment and foundation stability issues caused by deep under seepage and bring the levee system up to modern day standards. USACE is also planning to further strengthen the levee system by constructing erosion protection measures along the river bank.
In all, it is anticipated that USACE will construct an additional 9 miles of levee cutoff walls and 10 miles of riverbank erosion protection work. Once completed, the improvements will further reduce flood risk and meet current urban levels of flood protection requirements.
The Natomas Basin is surrounded by 42 miles of levees that provide protection from the American River, Sacramento River, Natomas Cross Canal and Natomas East Main Drain Canal. Improvements to the levees were constructed in the early 1990’s, which consisted of raising levees along the streams and canal systems. However, as other risk factors, including susceptibility to under seepage, began to generate increased concern, particularly following the 1997 flood event in the Sacramento Valley, it became clear that additional levee improvements would be needed in the Natomas Basin.
In 2006, the U.S. Army Corps of Engineers (USACE) determined the Natomas Basin had less than a 100-year level of flood protection due to the threat of deep under seepage. In 2007, SAFCA and the State of California began constructing levee improvements while USACE sought Congressional approval and appropriations for the work. These improvements included the construction of deep cutoff walls, landside berms, and a new adjacent levee along a portion of the Sacramento River East Levee to prevent under seepage.
By 2013, SAFCA and the state completed 18.3 of the 42 miles of levee improvements required to meet current flood control standards. In 2019, USACE began construction on the additional 24 miles of levee improvements necessary to provide a minimum 200-year level of flood protection to the Natomas Basin.
Folsom Dam & Reservoir
Completed in the mid-1950’s, Folsom Dam is the cornerstone of Sacramento’s flood control system on the American River. Considering larger flood events that occurred after its completion, it was recognized in the late 1980s that improvements to Folsom Dam and Reservoir, combined with improvements to downstream levees, were necessary in order to provide what flood control officials deemed a minimum 200-year level of flood protection to the Sacramento area.
A new dam and auxiliary spillway, called the “Joint Federal Project” or “JFP” (because the U.S. Army Corps of Engineers (USACE) and U.S. Bureau of Reclamation each constructed a portion of the project), was completed in 2017. The new spillway was constructed adjacent to Folsom’s main concrete dam and includes a 1,100-foot-long approach channel beginning in Folsom Reservoir with submerged gates embedded in a new control structure designed to discharge water down a 3,100-foot-long spillway chute. Because the auxiliary spillway’s gates are lower than those at the main Folsom Dam, dam operators are able to quickly create more available flood storage space in the reservoir in anticipation of increased inflows during storm events.
To further increase flood control space in the reservoir, the USACE is raising the existing main dam and reservoir’s surrounding dikes by 3.5 feet, which began in 2019.
American River Levees
The Lower American River Levee System comprises 26 miles of levees between Folsom Dam and downtown Sacramento. The levees play a crucial role in protecting communities in the Arden-Arcade and North Sacramento areas as well as communities to the south in downtown Sacramento, Land Park, Pocket, Meadowview and South Sacramento. As a result of the flood events of 1986 and 1997, it was recognized that robust improvements were necessary to bring the levee system up to modern day standards. Between 1998 and 2016, the U.S. Army Corps of Engineers (USACE) installed seepage cutoff walls to depths of 80 feet, raised and stabilized levees, and corrected erosion problems along 25 miles of the system.
USACE is planning to construct 11 miles of erosion protection along portions of both the north and south banks of the American River, beginning as early as 2021. Once completed, the cumulative flood control improvements will allow the levee system to safely handle sustained flows of up to 160,000 cubic feet per second in the event of an extreme flood event in the American River watershed.