Abstract
During the 2011 Mississippi River Basin flood, the
U.S. Geological Survey evaluated aspects of critical river
infrastructure at the request of and in support of local, State,
and Federal Agencies. Geotechnical and hydrographic data
collected by the U.S. Geological Survey at numerous locations
were able to provide needed information about 2011 flood
effects to those managing the critical infrastructure. These data
were collected and processed in a short time frame to provide
managers the ability to make a timely evaluation of the safety
of the infrastructure and, when needed, to take action to secure
and protect critical infrastructure. Critical infrastructure surveyed
by the U.S. Geological Survey included levees, bridges,
pipeline crossings, power plant intakes and outlets, and an
electrical transmission tower.
Capacitively coupled resistivity data collected along the
flood-protection levees surrounding the Omaha Public Power
District Nebraska City power plant (Missouri River Levee
Unit R573), mapped the near-subsurface electrical properties
of the levee and the materials immediately below it. The
near-subsurface maps provided a better understanding of the
levee construction and the nature of the lithology beneath
the levee. Comparison of the capacitively coupled resistivity
surveys and soil borings indicated that low-resistivity value
material composing the levee generally is associated with lean
clay and silt to about 2 to
4 meters below the surface, overlying
a more resistive layer associated with sand deposits. In
general, the resistivity structure becomes more resistive to the
south and the southern survey sections correlate well with the
borehole data that indicate thinner clay and silt at the surface
and thicker sand sequences at depth in these sections. With
the resistivity data Omaha Public Power District could focus
monitoring efforts on areas with higher resistivity values
(coarser-grained deposits or more loosely compacted section),
which typically are more prone to erosion or scour.
Data collected from multibeam echosounder hydrographic
surveys at selected bridges aided State agencies in
evaluating the structural integrity of the bridges during the
flood, by assessing the amount of scour present around piers
and abutments. Hydrographic surveys of the riverbed detected
scour depths ranging from zero (no scour) to approximately
5.8 meters in some areas adjacent to North Dakota bridge
piers, zero to approximately 6 meters near bridge piers in
Nebraska, and zero to approximately 10.4 meters near bridge
piers in Missouri. Substructural support elements of some
bridge piers in North Dakota, Nebraska, and Missouri that
usually are buried were exposed to moving water and sediment.
At five Missouri bridge piers the depth of scour left
less than 1.8 meters of bed material between the bottom of
the scour hole and bedrock. State agencies used this information
along with bridge design and construction information to
determine if reported scour depths would have a substantial
effect on the stability of the structure.
Multibeam echosounder hydrographic surveys of the
riverbed near pipeline crossings did not detect exposed pipelines.
However, analysis of the USGS survey data by pipeline
companies aided in their evaluation of pipeline safety and
led one company to further investigate the safety of their line
and assisted another company in getting one offline pipeline
back into operation. Multibeam echosounder hydrographic
surveys of the banks, riverbed, and underwater infrastructure
at Omaha Public Power District power plants documented the
bed and scour conditions. These datasets were used by Omaha
Public Power District to evaluate the effects that the flood had
on operation, specifically to evaluate if scour during the peak
of the flood or sediment deposition during the flood recession
would affect the water intake structures. Hydrographic
surveys at an Omaha Public Power District electrical transmission
tower documented scour so that they could evaluate the
structural integrity of the tower as well as have the information
needed to make proper repairs after flood waters receded.
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First posted July 2, 2014
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