DEVELOPING A GEOMORPHIC APPROACH TO ASSESSING LEVEE UNDERSEEPAGE

The migration of water through levee foundation materials by underseepage can lead to piping and levee instability during critical high-water stages. As part of regional levee evaluation studies, a geomorphic assessment approach was developed to identify areas of potential vulnerability to underseepage along levees within the Sacramento and San Joaquin Valleys. The approach integrated existing and new Quaternary geologic mapping with soil hydrologic classes and geomorphic data to develop a criteria matrix of relative underseepage susceptibility classes (very high, high, moderate, and low). These classes were assigned to levee segments according to the criteria matrix, with late Holocene and historical channel deposits judged to have very high susceptibility, late Holocene natural levee, overbank, and crevasse splay deposits having high susceptibility, and older alluvial fan and fine-grained, basin deposits having moderate to low susceptibility. Documented past levee performance issues were also evaluated and spatially analyzed using GIS and compared to the underseepage susceptibility mapping to help calibrate the susceptibility rankings based on historical events. The GIS analysis of past performance was designed to include discrete (point) data as well as continuous (line) data. Epistemic uncertainties include the completeness and locational accuracy of the levee performance data and precision in the surficial mapping. Preliminary results suggest a strong correlation between paleochannel, historical natural levee, and peat deposits and documented levee performance issues.