Guidance for Bridge Scour Studies 

In Reply Refer To:			July 23, 2003 
Mail Stop 415 


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. 


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 


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. 


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. 


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 
·        Memorandum – Subject: Summary of the Bridge Scour Assessment Work Group, 
May 11, 1993 (/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. 


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. 


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. 


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 
/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. 


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. 

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 

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. 

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. 

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. 


        Stephen F. Blanchard 
        Chief, Office of Surface Water 

Distribution Codes:  A, B, DC, CD, OSW Staff All, District SW Specialists, 
and Bridge Scour Group 

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