General Facts of this Project prepared
by the U.S. Geological Survey in
Cooperation with Vermont Agency of Transportation and Federal Highway Administration
By Erick M. Boehmler
What is Scour?
Scour is a general colloquial term used to identify the process of streambed
erosion under and near bridges. Scour occurs when moving water lifts,
and rolls or carries sand and rocks (streambed material) in the streamflow
direction. During certain flow conditions, material is carried away by
the moving water and the streambed erodes. This process of streambed erosion
or degradation is called general scour.
Under most natural conditions,
the general scour process occurs gradually. However, scour is enhanced,
in some cases, at bridges over waterways because of the constant interaction
of water with the structure. In addition to general scour, there are two
other types of scour that occur at bridge sites, contraction scour and
local scour.
Contraction scour is a process
of streambed degradation which results from the acceleration of water
approaching and flowing under a bridge. This process occurs when the width
of the bridge opening is narrower than the width of the waterway approaching
the bridge. An example of the flow acceleration may be observed by putting
a finger over the end of a running garden hose. Similarly, water approching
a narrower bridge accelerates. Water that flows more quickly can lift,
and carry or roll more sand and rocks out from under a bridge.
Local scour also is a process
of streambed degradation. However, it results from turbulence in the water,
which occurs mostly at the walls supporting a bridge. An example of the
turbulence is similar to that of a tornado. Just as tornadoes are formed
in air by the interaction of wind blowing in different directions at different
altitudes, eddies or whirlpools are formed in water by the interaction
of water flowing in different directions at different depths. In the same
way that objects may be lifted from the ground and thrown a mile away
by a tornado, eddies lift sand and rocks from the streambed near the walls
that support a bridge and carry them out from underneath the bridge.
Our Mission, This
Project, and You
The mission of the U.S. Geological Survey is to provide sound scientific
information to Federal, State, and local officials, and the public regarding
scour processes that pose a potential threat to public safety. The results
of this project increase the awareness and understanding of the interaction
between human activities and the environment of rivers and streams. Ultimately,
such information helps State and local authorities minimize and reduce
the adverse effects of scour and stream instability on bridges and improves
the safety of Vermont residents.
One of the more extreme examples
of scour and its effects was the collapse of the Interstate Route 90 bridge
over Schoharie Creek in New York State during a flood in 1987. Ten lives
were lost and traffic was routed around the site during a lengthy reconstruction
period, costing millions of dollars. The Vermont scour project is being
done in response to such costly tragedies so that hazardous scour situations
might be avoided at bridge sites throughout the state.
Our Assessment Process
Bridge scour potential is being evaluated at bridge sites in Vermont by
use of a two-leveled approach. The first level is an assessment of the
stream channel. The assessment consists of an evaluation of stream channel
features that are indicators of the stability or instability of the channel.
In the stream reach near a bridge, changes in the channel shape, changes
in channel depth, the degree of channel bends, the accumulation or absense
of excess streambed material at channel bends, the extent of densely rooted
vegetation on the banks, and changes in channel width and bank slope are
evaluated. These indicators provide evidence of channel movement or instability.
The channel under a bridge also is examined for remnant scour holes along
the walls supporting a bridge and exposure of the foundation of the bridge.
Information from the first-level
evaluation is considered in part with the second level of the assessment
process. In the second level of the assessment, the degree of streambed
erosion that might be expected under a bridge during a rare flood is determined.
This is done by applying a computer program designed to model the flow
of water under a bridge. The model results are used to evaluate the depth
of streambed material that may be removed from underneath the bridge.
In Vermont, approximately
400 of the 2,400 bridges over waterways were selected by the Vermont Agency
of Transportation for evaluation of scour potential. The bridge sites
are examined by a team consisting of personnel from the fields of engineering,
hydrology and geology from our New Hampshire / Vermont District office
in Pembroke, N.H. Reports of the stream stability and scour assessments
are on file with the Vermont Agency of Transportation in Montpelier. All
reports have been completed.
Conclusion of Project
Collection of historical data, Level I and Level II field data for all
400 scour susceptible bridges in Vermont is complete. Flood frequency
estimates and analyses of grain size distributions at each of the 400
sites is also complete. Quality assurance checks and processing of Level
I data into report format has also been completed for the 400 sites. Hydraulic
models and scour analysis have also been done for 400 sites. Fourhundred
reports (one report per bridge) have been approved and released as Open-File
Data Reports.
Vermont Bridge Scour Open-File Reports listing is available in .pdf format (162 KB), published 1996-1998.
More Information
For questions regarding this project, contact
Kenneth Toppin: Supervisory
Hydrologist
New Hampshire / Vermont District
U.S. Geological Survey, Water Resources Division
361 Commerce Way
Pembroke, NH 03275-3718
Phone: (603) 226-7800
Related USGS links:
Geology
-- Water -- Geography
5/30/00