Reconnaissance Geophysical Investigations for the Assessment of Levee Conditions at the Canal Ranch Levee, Sacramento River Delta

The Sacramento River Delta is the second largest river delta in the United States. The earthen levees were built in the 1800's and were not built with today’s engineering standards. Local earthen materials ranged from a wide variety of materials and structure. Therefore, the levees are prone to failure and flooding or other catastrophic events. The State of California (Department of Water Resources) published a set of specifications in an attempt to find some of the subtle physical problems before they occur. The specifications focus on surface geophysical techniques to assess subtle levee conditions in reconnaissance mode with follow up higher resolution geophysical techniques. They concentrated on two zones. The shallow zone targets were beaver burrows and dens, disturbed material, loose sandy material, and other shallow features that may cause levee failure during winter conditions. A second and deeper zone of interest was levee materials and conditions (ergo saturated areas, voids, loose materials, etc.) throughout the thickness of the levee itself. Shaw used duo frequency GPR surveys to assess the shallow zone conditions and conducted multiple OhmMapper resistivity surveys along 15 miles of levee to assess the effectiveness of the geophysical technologies. The survey was followed by ground-truthing which correlated positively with interpretations of the geophysical data.. Why assess and monitor levees? Levees have an important role in flood control and are designed to protect crop, buildings and structures as well as humans from floodwaters. These structures may retain millions of gallons of water behind them and in many cases are old with deferred maintenance. A well developed levee monitoring system can detect leaks and voids as well as normal degradation; identifying problems before a disaster such as a total breach occurs. Figure 1 below shows an example of a levee failure. . Figure 1: An unanticipated levee failure.