AEG January Meeting: Lower American River Geomorphology 

Please join us on Tuesday, January 21, 2020 at Aviator's Restaurant! We will be hosting Mitchell Swanson from Northwest Hydraulic Consultants, who will be giving a presentation titled Lower American and Sacramento Rivers Geomorphology Applications for Bank Erosion and Levee Stability and Ecosystem Resotration.

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Lower American and Sacramento Rivers Geomorphology Applications for Bank Erosion and Levee Stability and Ecosystem Resotration

Mitchell Swanson

 Northwest Hydraulic Consultants

mswanson@nhcweb.com

Speaker Abstract

The City of Sacramento and billions of dollars in property value is protected by 26 miles of levee along the south bank Lower American River and the east bank of the Sacramento River. An ongoing effort to strengthen levees and protect them against erosion is being conducted by the US Army Corps of Engineers and the Sacramento Area Flood Control Agency (SAFCA). A recently completed auxiliary spillway on Folsom Dam and updated operations based upon weather forecasting now provides over 200 year level of flood protection. With the new operation manual, an ultimate level of 325-year flood protection will be provided by raising objective flood releases from 115,000 cfs to 160,000 cfs. In 2016, Congress provided funds to upgrade levees against bank erosion or direct erosion failure with the 160,000 cfs release.

The American and Sacramento Rivers have been the subject of detailed geologic mapping and subsurface investigations conducted after 2007 in order to assess levee foundation conditions and underseepage erosion potential. These geotechnical studies, along with detailed lidar surveys and new hydraulic modeling also helped with the analysis of bank erosion potential. They also contribute to past literature and data on the geologic history and evolution of the rivers and their geomorphic behavior over time. Analysis of historical data and accounts provide details of the significant alterations due to post 1850s Euro American land use history greatly affected channel form and processes. The historical period includes: severe sedimentation and channel aggradation due to hydraulic mining from the 1860s to 1950s, construction of levees and fill that afforded floodplain development and urbanization, and gravel mining and dredging. Over time many miles of bank armoring has been installed to protect levees and now many acres of densely urbanized floodplain. These developments have removed significant floodplain areas (up to over 1 mile wide) from flood conveyance and severely concentrated levee channel floodways (as narrow as 900 feet) hydraulic and erosional force.  A series of 5 Erosion Assessments were produced by NHC to identify locations where new bank protection will be needed in order to withstand future floods events and the 160,000 cfs LAR flood release. Through engagement of stakeholders including resource agency biologists, engineers, planners and geomorphologists, plans are being prepared to install needed protection before 2025. The ecological effects of new rip rap bank protection will be offset by incorporating shoreline native vegetation plantings and placement of instream woody materials. Past successes along both rivers demonstrate the viability of habitat creation.

Speaker Bio

Mitchell Swanson is a fluvial geomorphologist with over 35 years of experience in river, stream and wetland restoration and resource management in many locations in California and Nevada. Mitchell graduated with a Bachelors (1981) and Masters (1983) degrees in Earth Sciences from UC Santa Cruz. Since 1983, he has been in the consulting industry and involved in a wide range of flood control planning, resource management, environmental impact studies and restoration projects in Lake Tahoe, Central Valley and Coastal Watersheds. These include pioneering work to incorporate geomorphology and ecosystem processes into flood control facilities and O&M and gravel mining projects and completion of numerous stream restoration projects in the Lake Tahoe Basin. Presently, Mitchell is working for Northwest Hydraulic Consultants in Sacramento providing support for levee erosion protection and ecosystem restoration in the Lower American and Sacramento Rivers, meadow restoration along the Upper Truckee River in South Lake Tahoe and fish passage / dam removal projects in Southern California.