Levee Improvements for Sacramento’s Vulnerable Natomas Basin

The Natomas Basin is located at the confluence of the American and Sacramento Rivers within the greater metropolitan area of Sacramento, California. The levee system was originally created to promote agricultural development in the 53,000-acre basin over 100 years ago. Today, the Natomas Basin is home to over 80,000 people and over $8 billion in infrastructure, including the Sacramento International Airport, two interstate highways, the region’s sports complex (Arco Arena), numerous businesses, and residential neighborhoods. The basin is now considered one of the most vulnerable regions around Sacramento and in the country in terms of risk of significant flooding.
The Natomas Basin’s flood protection system consists of 43 miles of levees originally constructed using hydraulic and clam-shell dredging processes, which resulted in a relatively weak system of levees. As a result of several levee seepage-related near-failures during the 1986 flood, the levees were improved and certified by the United States Army Corps of Engineers (USACE) in 1998. However, subsequent levee investigations showed that the levees remain vulnerable to underseepage, erosion, and overtopping. The Natomas Basin now is rated as having less than a 3 percent annual chance (33-year) level of flood protection. The potential depths of flooding within the basin exceed 20 feet, and floodwaters would likely consist of cold snowmelt runoff from the mountains to the east. As a result, USACE decertified the Natomas levee system in 2006 and the basin was remapped as a floodplain by the Federal Emergency Management Agency (FEMA) in 2008, halting development in the basin and causing economic hardship for the region.
To address this very significant flood risk and threat to a major portion of Sacramento, the Sacramento Area Flood Control Agency (SAFCA) took the bold step of partnering with the State of California and rapidly began improving the levees ahead of a new federal flood control project currently in the planning stage. SAFCA will have successfully completed about half of the levee improvements necessary to provide a 0.5 percent annual chance (200-year) level of protection by the end of 2011 – all of this in advance of federal funding. The second half of the project is expected to be completed by USACE with the overall levee improvements in the basin expected to cost over $700 million.
This paper summarizes the history of the Natomas flood problems and outlines some of the engineering challenges associated with making major improvements to an urban levee system. These challenges include meeting the recent USACE vegetation and encroachment requirements, addressing potential seismic stability problems, resolving cultural resource issues, meeting endangered species needs, working with adjacent urban development, and complying with different, contradictory, and changing design criteria of multiple state and federal agencies. These challenges have resulted in design and mitigation approaches that include constructing set-back adjacent levees rather than upgrading existing levees, and the use of multiple measures to address the issue of underseepage (i.e., cutoff walls, seepage berms and relief wells). The levee improvement designs that have now been successfully constructed have balanced significant environmental, cultural resource, social and fiscal impacts with the required level of flood protection for the Natomas basin. 51 pp. 10 references.