Flood Protection - Sacramento Levee Armoring
California officials have discussed expanding the Sacramento River bypass and levee system for nearly 65 years, dating back to 1955. The original system was built after two floods in 1907 and 1909, was completed in 1955. Three more major floods, including the largest on record in 1986, proved the need for an expanded system with larger capacity.
California Department of Water Resources (DWR) lead the design, construction and permitting of the project, named the Lower Elkhorn Basin Levee Setback (LEBLS). The project widened the Sacramento Bypass and the east side of the Yolo Bypass.
DWR worked closely with the following partners:
• US Army Corps of Engineers (USACE)
• Sacramento Area Flood Control Agency (SAFCA)
• Central Valley Flood Protection Board
• West Sacramento Area Flood Control Agency
• California Department of Fish and Wildlife
• United States Fish and Wildlife Service
• Yocha Dehe Wintun Native American Tribe
• Reclamation District 537
• Yolo County
This project was the first phase of implementing recommendations from the 2012 Central Valley Flood Protection Plan (CVFPP). This project contributed to the CVFPP goals by providing improved safety for approximately 780,000 people and protecting $53 billion in assets. Additionally, these improvements also provided an increased habitat for fish rearing, birds, and other wildlife through improved connection to the Sacramento Bypass Wildlife Area.
DWR’s design set back five miles of the east levee of the Yolo Bypass and two miles of the north levee of the Sacramento Bypass. The two levees formed an “L” shape, visible in the video below, that were set back about 1,500 ft from the old levee system, totaling over seven miles of new levee. The project doubled the capacity of the Sacramento Bypass, added about 60,000 cubic feet per second of capacity to the Yolo Bypass and added hundreds of acres to the floodplain in the bypasses.
A challenge the LEBLS design team encountered was the available onsite soil to construct the levee. The area consists of fat clay that would not ordinarily meet the Army Corps of Engineers’ standards for levees construction. Trucking in millions of cubic yards of more suitable soil would have driven up costs and exceeded the project’s emissions requirements.
Engineers designed the new levees to accommodate this clay. The 100-year-old old levee had been built with 2:1 and 3:1 side slopes, and had seen multiple failures from slope slides. The new levee was designed to be wider and flatter: 28 ft wide at the crown with 4:1 side slopes.
The levee designers also needed to account for permanent erosion control for the water side of the levee slopes. Southwesterly winds cause waves against the levee. Installing rock riprap to protect the levee sides was not acceptable because rock would not support the desired native vegetation. Additionally, acquiring and trucking the necessary rock would exceed the project’s emission requirements. Instead, the design team used tied concrete block mats (Flexamat) to armor the levee. The tied concrete block mats allow native perennial growth to thrive within the matting while protecting from wave erosion.
Several years before the installation of the tied concrete block mats, the team started growing test plots in 12’ x 40’ Flexamat sections to determine the best method to grow the native grass cover. The vegetated armor solution would create an area for migrating birds, such as the Swaison’s hawk, and a safe habitat for animals such as the giant garter snake.
In the summer of 2020, the construction team began the expansion of the capacity of the levee system. They dug the new levee material from the area between the current and future levees. The construction team had to move more than 5 million cubic yards of earth in the construction of the new levees.
By the summer of 2022, the construction team was ready for the installation of the tied concrete block mats, Flexamat. To meet project emission requirements and to reduce transportation costs, Flexamat was manufactured on the project site. This saved over 625 trucks from being needed.
The Flexamat team provided a mat layout for the entire seven-mile project. Flexamat rolls were cut to specific lengths to match the levee slope lengths. The rolls were all tagged with a placard identifying the roll number and size that correlated with the provided mat layout. The rolls included geogrid extensions, extending from a long edge of the mat. During installation, the mats overlap the adjacent mat’s extension, interlocking the entire system together.
The construction team quickly developed an efficient system for installing the tied concrete block mats. The rolls were staged next to where they were to be installed. Utilizing four, four-man crews, consisting of an excavator and three laborers, each crew was able to install 75,000 square feet per 8-hour workday. The installation time of the tied concrete block mats went much faster than the construction team had planned. The final Flexamat mats were installed in December 2023.
Vegetation growth immediately started to grow within the tied concrete block mats. Now that the new levee is finished, the construction team will remove the old levee and place that material in the borrow pits.
In February 2024, a huge milestone for flood protection in the Sacramento Valley was reached. The newly expanded space in the Yolo Bypass setback made possible by DWR’s Lower Elkhorn Basin Levee Setback Project began filling with water to alleviate flooding from the Sacramento River. The seven miles of levee expansion was immediately utilized to protect the 780,000 people in Sacramento, West Sacramento, and Woodland and protecting $53 billion in assets. Beyond creating more space for floodwater to spread out, the project created more habit for native fish, birds and other wildlife.
