Guidance for Bridge Scour Studies
In Reply Refer To: July 23, 2003
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 2003.06
Subject: Guidance for Bridge Scour Studies
This memorandum provides a summary of current bridge-scour research needs
and presents general guidance for defining the scope of several types of
common bridge-scour projects. The U.S. Geological Survey (USGS) has
played a major role in research and data collection associated with scour
of the streambed from around bridge foundations (bridge scour) caused by
floods and by long-term geomorphic changes in the watershed. The USGS has
worked cooperatively with the Federal Highway Administration and State
highway departments for nearly 50 years. During this time, the USGS has
helped improve procedures for the hydraulic and scour design of highways
and bridges and has provided hydraulic, hydrologic, and geomorphic
assessments of streams crossed by the national highway system. The Water
Resources Discipline (WRD) has identified hydrologic hazards as an area
for increased emphasis over the next 10 years.
The hazards associated with bridge scour are significant. Bridge scour is
the leading cause of bridge failure in the United States (Richardson and
Davis, 2001). The problem is not only relevant to existing bridges, but it
is also important to the economical design of new bridges that are
resistant to the effects of hydrologic hazards. On average, from 1990 to
1997, over 2,800 new bridges were built each year (Jorge Pagan, FHWA,
written commun., 1998). Research and data collection by the USGS to better
understand hydraulic and sediment transport processes at highway crossings
are important to the development and maintenance of safe, economical
bridges over our nation's waterways.
RESEARCH NEEDS
The USGS has been involved in geomorphic and scour assessments,
quantitative scour predictions, sediment transport and
multiple-dimensional hydrodynamic modeling, real-time and post-flood field
data collection, and fixed and portable scour monitoring at selected
bridges. These studies have helped define the limitations of existing
methods and identify areas requiring additional research. The following
research topics were identified, in part, in "Scour at Bridge Foundations:
Research Needs" (Transportation Research Board, 1996) of which the USGS
was a contributing author.
· Hydraulic modeling of contracted openings
· Three-dimensional modeling of pier and abutment scour
· Field measurements of scour, especially abutment and contraction scour
· Correlation of easily observable variables and scour potential
· Time development of scour
· Scour in cohesive bed materials
· Scour in coarse bed materials
· Quantitative predictions of geomorphic change
· Scour at wide piers
· Effect of drift accumulations on scour
GUIDANCE FOR BRIDGE SCOUR RELATED PROJECTS
All bridge scour studies undertaken by the USGS must be able to
demonstrate that there is an appropriate role for WRD. One of the key
roles for WRD is to provide new understanding, approaches, technology, and
research for defining and solving water-resources problems. If the scope
of the project only meets an operational, informational, or permit
requirement for the customer, with little or no value beyond this
immediate need, the project should not be undertaken by the USGS. However,
if the operational needs of the customer can be satisfied within a project
scope that includes one or more broader goals, such as, (a) advancing the
knowledge of the regional hydrologic system, (b) advancing field or
analytical methodology, (c) furnishing hydrologic data or information that
contribute to protection of life and property, and (d) contributing data
to national data bases that will be used to advance the understanding of
regional and temporal variations in hydrologic conditions, WRD should
undertake the project. The issue of competition with the private sector on
bridge scour related projects is very important. We should not undertake
projects that are more appropriate for the private sector. WRD Memorandum
No. 95.44 and No. 84.21 provide more information on avoiding competition
with the private sector and on projects that are not appropriate for WRD.
Specific guidance for different types of bridge-scour projects is provided
in the following paragraphs. The Office of Surface Water is available to
assist Districts by providing technical and policy related reviews of
bridge scour proposals. Please send bridge scour proposals that you would
like reviewed to the Chief, Office of Surface Water.
FIELD MEASUREMENTS OF LOCAL AND CONTRACTION SCOUR
Since the Federal Highway Administration mandate for the evaluation of
scour potential at all existing bridges and the scour resistant design of
new bridges, design engineers have repeatedly questioned the validity of
design methods based on laboratory flume studies of scour. The lack of and
need for reliable complete field data on scour at bridges has been a
recurring conclusion of many researchers (Hjorth, 1975; Melville and
others, 1989; Lagasse and others, 1991; Laursen and Toch, 1956; Shen and
others, 1969; and others). There is a need for field investigations of all
components of bridge scour based on measurements made during floods.
Recent studies by the USGS compiled and reported in "Channel Scour at
Bridges in the United States" (Landers and Mueller, 1996) have greatly
expanded the availability of reliable field data on local scour at bridge
piers. However, there is still a significant need for field data on scour
at skewed piers, piers with complex shapes, wide piers, and piers with
debris accumulations. Currently the data available on contraction,
abutment, and pressure scour is very limited. Projects to increase our
data base of field data are highly encouraged. Although, real-time
collection of scour data and the associated hydraulic parameters is
considered the best method of collecting scour data, post-flood data
collection can also yield valuable information.
The USGS, in cooperation with the FHWA, has developed equipment and
procedures for making scour measurements during floods (Landers and
Mueller, 1996; Mueller and others, 1995; Mueller and Landers, 1994;
Mueller, 1996). Quantitative data collected as part of a bridge scour
study should be collected and interpreted using standard or recommended
techniques. If standard and recommended techniques are not used, the value
of these data to other researchers and for national syntheses of data may
be lost.
Collection of post-flood data can also be valuable in evaluating scour at
bridges. Surface-geophysical techniques have been successfully used to
determine the minimum historic streambed elevation around bridge
foundations in Connecticut (Gorin and Haeni, 1989; Placzek and others,
1995), Oregon (Crumrine, 1991), and Indiana (Mueller and Miller, 1993;
Mueller and others, 1994). Crumrine (1991) and Placzek and Haeni (1995)
provide a good discussion of the applications and limitations of
surface-geophysical techniques to bridge scour studies. Where the
hydraulic conditions are known or can be accurately estimated, data
collected by use of surface-geophysical techniques provide a means of
documenting scour that is not dependent on real-time measurements during
floods. Detailed post-flood measurement of scour, high-water marks, and
flow directions soon after a major flood can provide valuable insights to
scour processes, particularly when coupled with numerical modeling.
Post-flood data can also provide valuable information for simple envelope
curves that define the maximum observed scour based on physically measured
bridge, stream, and (or) watershed characteristics (Benedict, 2003). This
type of study can be very helpful to State highway departments and can
provide a large amount of data to other researchers. These types of
studies may appear to be of only local or regional interest since they do
not include hydraulic data, but the development of a national distribution
of such data will allow relations with observable bridge and site
characteristics to be analyzed and may provide understanding of how the
bridge design and site characteristics contribute to the maximum potential
depth of scour.
GEOMORPHIC AND SCOUR ASSESSMENTS
The USGS has been involved in many geomorphic and scour assessment
studies. Two unnumbered memoranda provide guidance on developing and
conducting these types of studies:
· Memorandum – Subject: Projects—Reconnaissance Type Bridge-Scour
Projects, January 14, 1991
(http://water.usgs.gov/osw/techniques/bs/bsmemo91.html)
· Memorandum – Subject: Summary of the Bridge Scour Assessment Work Group,
May 11, 1993 (http://water.usgs.gov/osw/techniques/bs/bs-assessmemo.html)
The broad objective of geomorphic and scour assessment studies is to
assess the channel stability and scour potential of a stream site at or
near a bridge from observable characteristics of the site. State highway
departments use the assessments to help screen sites that do not need
further evaluation and to identify bridges where scour-critical conditions
exist. These methods are qualitative and do not reflect scour potential,
which requires a quantitative analysis, such as, local scour caused by the
100-year flood. These assessments are not equivalent to a scour evaluation
described in Richardson and Davis (2001). To avoid confusion with FHWA
terminology, proposals and other documents for assessment studies should
not use the term "scour evaluation", because this term encompasses
analysis of bridge foundation stability as described by Richardson and
Davis (2001). The term "channel-stability assessment" is recommended when
referring to the field assessment.
WRD's role in channel-stability assessments has been questioned in the
past. If the project involves only application of existing techniques to
many bridges in the State for the purpose of providing the State highway
department information about each of the selected bridges, then in
accordance with WRD Memorandum 84.21, the project should not be conducted
by the USGS. However, if properly scoped these assessments can provide a
valuable assessment of river basin morphology and habitat evaluation that
will be of value to a variety of local, State, and Federal agencies. The
earth science merits lie in the collection of transferable data, in
expanding our understanding of channel stability through national and
regional synthesis, in the long-term monitoring of the behavior of
individual river basins, and in developing and validating methodology for
accurately assessing channel stability. One or more of these scientific
components must be included to justify WRD's role in the investigations.
It is important that data collection, coding, and interpretation be
consistent among studies to facilitate analysis of the assessment
databases across regional boundaries. A 1993 USGS work group conducted a
literature review and concluded that the form developed and used in
Tennessee (Simon and Outlaw, 1989) should be the starting point for
standardized scour assessment variables. The inspection form developed for
west Tennessee streams may not be applicable to streams in every State.
The form should be reviewed and parameters should be added (but not
modified or consistency will be lost) as necessary to meet individual
project requirements. Changes to parameter definitions may improve the
accuracy of the results for a given region, but will seriously inhibit
comparisons of results between States where different parameter
definitions were used. Therefore, any changes in definitions should result
in adding new parameters rather than modifying existing parameters.
QUANTITATIVE SCOUR EVALUATIONS AND MODELING
The USGS's involvement in quantitative scour evaluations and modeling has
also been questioned. It may appear that computing the depth of scour at
specific bridges or providing site-specific models is driven solely by the
operational need of the customer to meet some information requirement for
a permit, design, or regulation. However, a more thorough investigation of
the USGS's activities in this area reveals a proper balance that has
benefited the scientific and engineering community. The continued vitality
and relevance or our research depends on our close involvement and
responsiveness to our customers. As the USGS applies scour prediction
methodology and modeling techniques to projects, USGS personnel become
familiar with the models and procedures and the validity of the models and
procedures are evaluated. Valuable feedback from the USGS to the FHWA on
limitations and deficiencies of various techniques has lead to
corrections, enhancements, and further research to improve the design and
evaluation guidance and methodology. As models are developed and improved,
these models need to be applied to a wide variety of situations, so that
the limitations can be identified and errors corrected. Through
site-specific scour evaluation and modeling activities the USGS can
evaluate and test models and scour prediction techniques. USGS involvement
in the practical application of scour prediction and modeling techniques
provides the USGS with experience that is necessary to identify and direct
future research to improve the applicability and scientific basis of these
techniques. Without such involvement, the ability of the USGS to provide
timely and relevant solutions to practical problems related to scour at
bridges would be hampered. Therefore, the inclusion of models and scour
computations in field data-collection projects is encouraged because this
allows valuable comparisons between computed and measured scour and
hydraulic parameters. All projects that include scour computations or
modeling should include sufficient field data collection to allow
validation of the techniques being applied.
SCOUR MONITORING
The term "scour monitoring" has been used in the literature associated
with bridge scour to describe several types of data collection. However,
scour monitoring as defined by the Federal Highway Administration
(Richardson and Davis, 2001) is a countermeasure or temporary
countermeasure for a bridge determined to be scour critical. Although the
primary objective of scour monitoring is to provide for the safety of the
public without closing bridges during high flows and without installing
expensive countermeasures, it provides an excellent opportunity for the
USGS to meet an operational need of a customer while collecting much need
data on scour.
Real-time monitoring can be accomplished using one of two basic types of
equipment: fixed instrumentation installed on a bridge (such as falling
rods, sliding collars, sonar, conductance probes etc.); or portable
instrumentation used by field personnel to make measurements at any
bridge. The highway department needs only the elevation of the streambed
to evaluate the stability of the bridge foundations. However, when the
streambed elevation measurements are combined with hydraulic measurements,
the data becomes valuable for bridge-scour research. Sites with fixed
scour monitoring equipment, if supplemented with a continuous-record
streamgaging station, can provide valuable data on the initiation and rate
of scour, as well as, under what conditions scour holes refill (if the
installed technology allows measuring the refilling process). Likewise,
mobile field teams making measurements at selected bridges can supplement
the streambed elevation measurement with a discharge measurement and other
hydraulic observations to complete a limited-detailed data set. Therefore,
properly scoped scour monitoring projects represent a significant
opportunity for the USGS to collect field data that can be used for
scientific research, while meeting a fundamental need of many highway
departments. The extension of scour monitoring to include hydraulic
measurements for research purposes is an ideal application for
Federal-State cooperative funds. There is also potential for projects that
develop and test equipment that can be used for scour monitoring.
Instruments that work effectively in steep mountain streams and in streams
with ice are needed.
DISSEMINATION OF DATA
Office of Surface Water Technical Memorandum 92.03 provides guidelines for
the transmittal of bridge-scour information to cooperators, prior to
completion of an interpretive report. The data collected during a study is
often of significant value to other researchers. The value of data from
individual studies is often increased by combining it with data from other
similar studies. To the maximum extent possible, these data should be
published on the Internet for easy access by other researchers. Field data
on pier, abutment, and contraction scour processes can be stored in the
Bridge Scour Data Management System (BSDMS) (Landers and others, 1996).
The BSDMS has been updated and real-time access to the data via the
Internet is available at
http://water.usgs.gov/osw/techniques/bs/BSDMS/index.htm. Enhancements to the
BSDMS allow storage and retrieval of more detailed data and scanned
images. Level I, geomorphic assessment, data may be combined with the data
from completed studies by storing it in the database created for a
national synthesis of these data (Parker, 1998). When publication of the
actual data is not feasible, a summary of the data should be presented,
including information on how to obtain the data. The national coordinator
for bridge scour studies in the USGS is available to assist in making data
from USGS bridge scour projects available through the USGS web page on
bridge scour.
REFERENCES
Benedict, S.T., 2003, Clear-water abutment and contraction scour in the
Coastal Plain and Piedmont Provinces of South Carolina, 1996-99: U.S.
Geological Survey, Water-Resources Investigations Report 03-4064, 150 p.
Crumrine, M.D., 1991, Results of a reconnaissance bridge-scour study at
selected sites in Oregon using surface-geophysical methods, 1989: U.S.
Geological Survey Water-Resources Investigations Report 90-4199.
Gorin, S.R., and Haeni, F.P., 1989, Use of surface-geophysical methods to
assess riverbed scour at bridge piers: U.S. Geological Survey Water
Resources Investigations Report 88-4212.
Hjorth, P., 1975, Studies on the nature of local scour: Department of
Water Resources Engineering, Lund Institute of Technology, University of
Lund, Sweden Bulletin Series A No. 46.
Lagasse, P.F., Schall, J.D., Johnson, F., Richardson, E.V., Richardson,
J.R., and Chang, F., 1991, Stream stability at highway structures: Federal
Highway Administration FHWA-IP-90-014.
Landers, M.N., and Mueller, D.S., 1996, Channel scour at bridges in the
United States: Federal Highway Administration FHWA-RD-95-184.
Landers, M.N., Mueller, D.S., and Martin, G.R., 1996, Bridge scour data
management system user's manual: U.S. Geological Survey Open-File Report
95-754, 66 p.
Laursen, E.M., and Toch, A., 1956, Scour around bridge piers and
abutments: Iowa Highway Research Board No. 4.
Melville, B.W., Ettema, R., and Jain, S.C., 1989, Measurement of bridge
scour, in Proceedings of the Bridge Scour Symposium, McLean, Va., Federal
Highway Administration, Research Report FHWA-RD-90-035, p. 183-194.
Mueller, D.S., 1996, Scour at bridges - detailed data collection during
floods, in Proceedings of the Sixth Federal Interagency Sedimentation
Conference, Las Vegas, Nev., p. IV-41 - IV-48.
Mueller, D.S., and Landers, M.N., 1994, Real-time data collection of scour
at bridges, in Fundamentals and Advancements in Hydraulic Measurements and
Experimentation, Buffalo, NY, ASCE, p. 104-113.
Mueller, D.S., Landers, M.N., and Fischer, E.E., 1995, Scour measurements
at bridge sites during the 1993 upper Mississippi River basin flood:
Transportation Research Record, no. 1483, p. 47-55.
Mueller, D.S., and Miller, R.L., 1993, Evaluation of historical scour at
selected stream crossings in Indiana, in Shen, H.W., Su, T.H., and Weng,
F., eds., Hydraulic Engineering '93: San Francisco, Calif., ASCE, p.
2231-2236.
Mueller, D.S., Miller, R.L., and Wilson, J.T., 1994, Historical and
potential scour around piers and abutments of selected stream crossings in
Indiana: U.S. Geological Survey Water-Resources Investigations Report
93-4066.
Parker, G.W., 1998, Comparison of erosion and channel characteristics, in
Abt, S.R., Young-Pezeshk, J., and Watson, C.C., eds., Water Resources
Engineering '98: Memphis, TN, American Society of Civil Engineers, p.
315-319.
Placzek, G., and Haeni, F.P., 1995, Surface-geophysical techniques used to
detect existing and infilled scour holes near bridge piers: U.S.
Geological Survey Water-Resources Investigations Report 95-4009.
Richardson, E.V., and Davis, S.R., 2001, Evaluating scour at bridges --
fourth edition: Federal Highway Administration Hydraulic Engineering
Circular No. 18, FHWA-IP-90-017.
Shen, H.W., Schneider, V.R., and Karaki, S., 1969, Local scour around
bridge piers: Journal of the Hydraulics Division, v. 95, no. HY6, p.
1919-1940.
Simon, A., and Outlaw, G.S., 1989, Evaluation, modeling, and mapping of
potential bridge-scour, west Tennessee, Proceedings of the Bridge Scour
Symposium: McLean, Va., Federal Highway Administration, Research Report
FHWA-RD-90-035, p. 112-129.
Transportation Research Board, 1996, Scour at bridge foundations -
research needs: National Cooperative Highway Research Program,
Transportation Research Board NCHRP 24-8.
/signed/
Stephen F. Blanchard
Chief, Office of Surface Water
Distribution Codes: A, B, DC, CD, OSW Staff All, District SW Specialists,
and Bridge Scour Group
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Steve Blanchard Chief, Office of Surface Water
U.S. Geological Survey Phone: 703-648-5629
415 National Center Fax: 703-648-5722
Reston, VA 20192 Cell: 571-216-1423
sfblanch@usgs.gov
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