The Use Of Geophysics In Levee Assessment

Abstract

Levee inventories are often based on visual inspection and contain only basic data about the levee, usually location and height based on traditional elevation surveys. Current levee inventories generally do not include information about the underlying soils, or the engineering properties of the levee and its foundation. The U.S. Army Engineer Research and Development Center, (ERDC), and the U.S. Section of the International Boundary and Water Commission (USIBWC) have conducted research on airborne and ground-based geophysical methods to assess the condition of levees and their foundations. More than 800 km of levees in New Mexico and Texas were evaluated using airborne and surface geophysics, geologic studies, ground-truth sampling, and an Enterprise Geographic Information System (eGIS). Airborne geophysical survey methods include multi-frequency electromagnetic induction (EM), magnetics, digital video, and light detection and ranging (LIDAR) surveys acquired at speeds up to 130 km/hr. Airborne geophysical methods permit investigation of the subsurface to depths of about 30 m. Signatures from multi-frequency EM surveys were used to characterize soils in the levees and their foundations. Geologic studies included detailed mapping of floodplain features and the identification of paleochannels to determine locations where underseepage potential is greatest. Ground-truth information was obtained with an instrumented cone penetrometer and direct sampling. Geophysical, geographic, geologic, and other levee related data are managed efficiently through an eGIS, and is the basis for decisions about pre-emptive levee rehabilitation and risk analysis. A condition assessment model was applied to levee reaches based on the results of airborne geophysics, geologic data, and historic performance and construction. This model was used to determine levee reaches most at risk, to prioritize limited maintenance resources, and target areas for focused geotechnical studies. Subsequent geotechnical investigations have included electrical and seismic instrumentation of levee reaches, and simulation of a maximum flood event against a levee test section to observe and monitor the behavior of the levee soils under simulated flood conditions. This paper will present a summary overview of levee research using airborne and ground based geophysical methods and geologic studies by ERDC and the USIBWC.