Southport Early Implementation Project Levee Improvements

Northern California flood control levees have historically been designed to United States Corps of Engineers (USACE) requirements. In some cases, adherence to these requirements can lead to significant costs associated with importing fill if soil from local borrow sources does not meet criteria.

The West Sacramento Area Flood Control Agency (WSAFCA) encountered this challenge with their Southport Early Implementation Project (EIP). The Southport EIP consisted of about 6 miles of improvements to a 100-year-old levee along the Sacramento River to mitigate seepage and stability deficiencies. This reach of the levee was considered the highest risk component to the flood control system surrounding West Sacramento, and the highest priority project to protect its 53,000 residents. The project plan included a new setback levee to create valuable habitat restoration land between the old and new levees and cost-sharing benefits with the California Department of Water Resources (DWR). However, under USACE design criteria, setback levee construction would require cost prohibitive import of nearly 1.4 million cubic yards of fill.

To solve the problem, Blackburn Consulting worked with the design team, WSAFCA, USACE, DWR, Central Valley Flood Protection Board, Reclamation District 900, and a board of senior consultants to engineer a zoned levee with seepage berms and cutoff walls by transforming local soil to meet USACE seepage and stability criteria, and could be constructed with available funds. The zoned levee consisted of an interior core of high plasticity clay from local borrow sources surrounded by a shell of lower plasticity clay and silty sand from excavations needed to create the habitat restoration area and from local borrow sources. Blackburn worked closely with the design team to balance the zoning geometry with the type and quantity of soil that was locally available to reduce costly imported soils. Blackburn performed multiple borrow investigations, a significant laboratory testing program, and many rounds of seepage and slope stability analyses to confirm the zoned levee would meet USACE slope stability and seepage criteria.

During construction, Blackburn helped assure compliance with design through geotechnical support and quality assurance materials testing for cutoff walls, earthwork, concrete, and borrow sites. Because of our in-depth knowledge of design, we were able to provide proactive solutions for materials acceptance to keep construction moving.

The new setback levee construction was completed in 2018. The success of this project has opened the possibility for creative use of locally available soils to significantly reduced project cost and meet USACE design criteria.

“Credit for the project earthwork savings on the SIP improvements, and the flexibility we have with the setback levee fill specifications goes to your [Blackburn’s] diligence and follow-through in these areas.” – Southport EIP Management