The City of Novato retained Geosyntec to evaluate the sources of seepage along the levee at Hamilton Field, a former military base that has undergone redevelopment. The levee was constructed over compressible soils in 1997, and was raised in 2007 to account for settlement since construction. The levee was originally constructed for flood control. However, there is a current U.S. Army Corps of Engineers (USACE) project in conjunction with the California Coastal Conservancy to develop wetlands along the outboard side of the levee, the Hamilton Wetland Restoration Project (HWRP). Two seeps were identified on the inboard side of the levee that separates the Hamilton Field residential/commercial area from the ongoing HWRP. The seeps were identified shortly after commencement of filling of Cell 2 of the HWRP in 2008. Two additional seeps were identified in 2009 during later filling stages.
Geosyntec's Scope of Services
Geosyntec reviewed the original design and construction of the levee using a variety of source materials. Our geotechnical engineers also reviewed the geotechnical design documents and construction observations associated with the filling program for the wetlands development. The goals of our review were to identify likely sources of seepage and to provide an independent assessment of the stability of the levees as a result of the HWRP filling program. Geosyntec submitted findings in two reports for the City of Novato. Both reports were also submitted to the USACE for review and comment.
A field investigation was performed in 2009 as the first stage of a project to address the seepage issues. It was found that the majority of seepage was due to preexisting conduits at depth below the levee (e.g., a 36" diameter former storm drain approximately 10 feet below ground surface) that had not been sealed off during original levee construction.
Findings from the review and field investigation were presented at two community forum meetings attended by members of the adjacent residential and business community, the Novato City Council, and the USACE. Geosyntec worked with the City of Novato and their contractor to develop and implement approaches for sealing the conduits, and for managing the remaining seepage water at the identified seep locations.