FORENSIC GEOPHYSICAL ANALYSIS OF A 1969 LEVEE REPAIR, SOUTH SHERMAN ISLAND, SACRAMENTO ESTUARY, CALIFORNIA

The estuary of the Sacramento and San Joaquin rivers comprises 700 miles of waterways and intervening islands bound by over 1,100 miles of levees critical for the region’s agriculture. Since 1900 there have been 160 levee failures for various reasons related to three fundamental problems: heterogeneity of the levee materials, subsidence, and oxidation of peat incorporated into the levees. Students from CSU Stanislaus, Merced College, and San Joaquin Delta College are jointly conducting geophysical resistivity and seismic surveys on the south side of Sherman Island, to document the condition of a repair made in 1969, when the levee broke and the San Joaquin River flooded the island. The research area focuses on a 2,000-foot stretch of the levee. Field methods applied included dipole-dipole capacitively-coupled dynamic resistivity, galvanic stationary resistivity, surface shear wave (Love wave) reflection, and P-wave refraction surveys.

In the resistivity tomography, the repair of the 1969 break shows a higher resistivity (100 to 200 ohm-m) than the older portions of the levee, which are generally lower than 30 ohm-m. This reflects the difference between a levee built over the century with layer after layer of local soils rich in clay and peat, and the single-event repair made with boulders and river sand. In the seismic velocity tomography, the levee repair appears to have a shear wave velocity of 120 to 160 m/s, likely because the repair consisted of large boulders pushed into the break, and later injection of stream channel sand. The large boulders supported the framework of the repair, but impeded proper compaction of the sand.