EM 1110-2-2907
1 October 2003
then merged with remote sensing methods to proceed with the levee assessment. Levee to-
pology was assessed with the use of LIDAR, digital video, aerial photographs, soil maps,
and geological maps. Topographic deviations of 6 in (15.2 cm). or more along the centerline
of the levee were then targeted for detailed seismic field studies. In addition to targeting
segments with an undulating topography, several stretches of the levee (on the US side)
were seismically surveyed.
c. Field Work.
(1) Ground surveys were conducted at five sites ranging in length from 3000 to 5000
ft (0.91 to 1.5 km). Electrical resistivity, EM, and magnetic surveys were collected in con-
junction with the airborne EM and magnetic survey. The ground-based geophysical sites
were geo-referenced with the use of a global position unit.
(2) Much of the data acquired for the GIS database were collected from previous
sources. Information was taken from state and Federal survey maps, and from new and old
aerial photographs. Digital photography and aerial photographs were used to map Holocene
and Pleistocene deposits and geomorphologic structures. In areas with recent urban devel-
opment, older images dating to the 1930s were used to evaluate the underlying geology.
d. Sensor Data Acquisition. LIDAR was utilized to survey levee elevation to determine
deviations from the original design. Deviations in height indicate segments with potential
damage attributable to seepage or sediment voids. Floodwater overtopping, slope failure,
and seepage all potentially compromise levee stability. Seepage can create void spaces in
the sediment and soil, resulting in subsequent levee collapse.
e. Study Results.
(1) The levee was then mapped and tagged with a conditional assessment of good,
marginal, acceptable, or high-risk zones. The assessment was based on a numerical measure
of 10 features deemed important in determining levee stability. These 10 features were cho-
sen by agreement among Corps experts specializing in levee construction and repair. Table
6-2 lists the 10 features ranked in order of importance.
(2) Low scores in any one of the 10 features could result in a poor rating for a given
levee segment. The levee was divided into segments based on conductivity measurements
(shown to be controlled by levee material make-up); each segment was then given a numeri-
cal value based on a weighted measure of the 10 features. Segment ratings were color coded
and presented as a layer within the GIS database. The color-coded maps provided an easy to
interpret assessment of levee condition.
6-18
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