FHWA-NJ-2017-011Design and Evaluation of Scour for BridgesUsing HEC-18(Volume 1 of 3)FINAL REPORTJuly 2017Submitted byJohn R. Schuring, PE, PhDProfessor of Civil &Environmental EngineeringNew Jersey Institute ofTechnologyRobert Dresnack, PE, PhDProfessor of Civil &Environmental EngineeringNew Jersey Institute ofTechnologyNJDOT Research Project ManagerMs. Pragna ShahIn cooperation withNew JerseyDepartment of TransportationBureau of ResearchAndU.S. Department of TransportationFederal Highway AdministrationEugene Golub, PE, PhDProfessor of Civil &Environmental EngineeringNew Jersey Institute ofTechnology
DISCLAIMER STATEMENTThe contents of this report reflect the views of the authors who are responsible for thefacts and the accuracy of the data presented herein. The contents do not necessarilyreflect the official views or policies of the New Jersey Department of Transportation orthe Federal Highway Administration. This report does not constitute a standard,specification, or regulation.
TECHNICAL REPORT STANDARD TITLE PAGE1.Report No.2.Government Accession No.3.Recipient’s Catalog No.5.Report DateFHWA-NJ-2017-0114.Title and SubtitleJuly 2017Design and Evaluation of Scour for Bridges Using HEC-18, Vol. 1 of 37.Author(s)6.Performing Organization Code8.Performing Organization Report No.John R. Schuring, Robert Dresnack, & Eugene Golub9.Performing Organization Name and Address10. Work Unit No.Department of Civil and Environmental EngineeringNew Jersey Institute of TechnologyUniversity HeightsNewark, NJ 07102-198211. Contract or Grant No.NJDOT TO-8912. Sponsoring Agency Name and AddressN.J. Department of Transportation1035 Parkway AvenueP.O. Box 600Trenton, NJ 08625-060013. Type of Report and Period CoveredFederal Highway AdministrationU.S. Department ofTransportationWashington, D.C.Final Report1/1/09- 9/30/1314. Sponsoring Agency Code15. Supplementary Notes16. AbstractThe overall objective of this research is the development of a new approach for evaluating bridge scour forNew Jersey's bridges on non-tidal waterways. The study commenced with a web-based survey of scourpractice within the U.S. and a literature review of predictive scour models. The major project deliverable is anew Scour Evaluation Model (SEM), which is a tiered, parametric, risk-based decision tool. A variety of geotechnical, hydrologic, and hydraulic data are analyzed to generate risk ratings for a particular bridge. Theseratings are then inputted into a Risk Decision Matrix to generate a scour priority level and recommendedactions, which may range from expedited installation of countermeasures to removal from scour critical status.Bridge importance is also factored into the final priority level. In addition, the New Jersey SEM providesstandard protocols for: (1) erosion classification of sediments; (2) application of scour envelope curves; and (3)analysis of hydrologic data. The model was validated and calibrated by inspecting scour critical bridges andcomparing actual field observations with model results. While the current model reflects New Jersey’s geologyand hydrology, it can be recalibrated to other regions or states. The model is principally designed to evaluatescour risk of existing bridges, but many model components are useful for designing new bridges as well.Included are example SEM applications for 12 bridges and two detailed example problems.17. Key Words18. Distribution StatementBridge Scour, Scour Analysis, Scour Critical Bridges19. Security Classification (of this report)UnclassifiedNo Restrictions.20. Security Classification (of this page)Unclassified21. No of Pages68Form DOT F 1700.7 (8-69)i22. Price
ACKNOWLEDGEMENTSThis project was conducted with the support and cooperation of the New JerseyDepartment of Transportation and the Federal Highway Administration.The Research Team gratefully acknowledges the contributions of: Project Managers Pragna Shah, Paul Thomas, Daniel LiSanti, and NazhatAboobaker for their guidance and skillful administration; Research Customers Richard Dunne, Scott Thorn, Nat Kasbekar, and AyodeleOshilaja for their direction and strong technical insight; Scour Project Implementation Committee for their valued input, especiallymembers Scott Deeck, Eddie Germain, Xiaohua “Hanna” Cheng, and EricKraehenbuehl; FHWA engineers Eric Brown (Baltimore Resource Center), Chester Kolota (NJDivision), and Dave Henderson (Washington, D.C. Office of Bridges andStructures) for their expert review and constructive comments; USGS New Jersey Water Science Center for their expert hydrologic support,especially Richard Kropp, Kara Watson, Blaine White, Jason Shvanda, andRobert Schopp.We are also indebted to the student research assistants for their wonderful efforts onthe project and significant contributions to this report, including Josh Tooker, MelissaSalsano, Shu Tham, Piotr Wiszowaty, Matthew Young, William Pennock, Brian Shiels,Dillion Collins, Yosef Portnoy, Abolfazl Bayat, Andrew Semanchik, and Joseph Kardos.Most have since graduated and moved into the professional world. It was our pleasureto work with them.ii
TABLE OF CONTENTSVOLUME 1PageEXECUTIVE SUMMARY . 1INTRODUCTION . 4Background . 4Project Objectives . 5Scope and Techniques of the Research Study . 6Phase 1 - Literature Search . 6Phase 2 – Investigative Research Study . 7SUMMARY OF NEW JERSEY’S SCOUR PROGRAM . 10Chronology of NJ’s Scour Program . 10Program Highlights and Current Status . 13SURVEY OF SCOUR PRACTICE . 16Survey Methodology. 16Survey Results . 16Selected Best Practices of Other States . 19REVIEW - TRADITIONAL & ALTERNATE METHODS OF SCOUR ANALYSIS . 22Overview of HEC-18 Scour Equations and Sources . 22HEC-18 Hydrologic and Hydraulic Requirements for Scour Evaluation . 24Comparative Studies of Observed vs. Predicted Scour in the U.S. 25GEOTECHNICAL EVALUATION OF BRIDGE SCOUR. 29Background . 29Summary of New Jersey Geology with Comments on Scour Potential . 30Description of Erosion Classes . 32Geological Materials with High Erosion Resistance . 33Geological Materials with Moderate Erosion Resistance . 38Geological Materials with Low Erosion Resistance . 42Compound and Stratified Erosion Classes . 44Long-term Channel Stability. 45Geotechnical Evaluation Procedure Steps . 46Step 1- Geotechnical Reconnaissance Study . 46Step 2- Field Scour Investigation . 47Step 3 - Detailed Investigation (Optional). 48Step 4 - Determination of Erosion Class and Scour Risk . 49GUIDELINES - HYDROLOGIC/HYDRAULIC EVALUATION OF SCOUR RISK . 50Background . 50Envelope Curves – Their Development and Applications to New Jersey . 51Selection of Envelope Curves Appropriate to New Jersey . 53Procedures for Reconnaissance Hydrologic/Hydraulic Analysis . 58iii
VOLUME 2PageNEW JERSEY SCOUR EVALUATION MODEL (SEM). 61Model Purpose and Overview . 61Assigning Geotechnical Risk Level – Module 1 . 63Assigning Hydrologic/Hydraulic Risk Level – Module 2 . 66Risk Decision Matrix – Module 3 . 69Bridge Importance Analysis – Module 4 . 70Recommended Actions – Module 5. 72Reporting Requirements for Existing Bridges . 74Scour Evaluation for New Bridges . 76EXAMPLE APPLICATIONS OF THE SCOUR EVALUATION MODEL (SEM) . 79Field Visits for Validation and Calibration of the Model . 79Example Model Applications to Selected Scour Critical Bridges . 79REFERENCES . 102VOLUME 3PageAPPENDICES . 110Appendix A: Selected Scour Analysis Methods from HEC-18 . 110Appendix B: Supplementary Materials . 127Appendix B1: USGS Envelope Curves Investigated . 128Appendix B2: Summaries of Envelope Curve Analyses andSupporting Data for the Coastal Plain & Piedmont Provinces . 142Appendix B3: Field Inspection Form for Bridge Scour Investigation . 148Appendix B4: Procedures for Completing the “Field InspectionForm for Bridge Scour Investigation” . 155Appendix B5: Web Survey Email Transmittal and Web Survey Form . 165Appendix C: Example Investigative ReportsSEM analyses require that three kinds of reports be generated for each bridgestudied: (1) Geotechnical Reconnaissance Study; (2) Field Scour Investigation; and(3) Reconnaissance Hydrologic Analysis. These reports are not included in thisdocument due to length restrictions. However, examples of each are available uponrequest from the Department of Civil and Environmental Engineering at the NewJersey Institute of Technology. Contact: Dr. John Schuring at [email protected]
LIST OF TABLESVolume 1Table 1 – Examples of Modified or Alternative Scour Evaluation MethodsTable 2 – Hydraulic Design, Scour Design, and Scour Design CountermeasureDesign Flood Frequencies (Table 2.3 from Arneson et al, 2012)Table 3 – Summary of Predicted vs. Observed Abutment Scour for Maine Study(modified from Lombard and Hodgkins 2008)Table 4 – Definition of SEM Erosion ClassesTable 5 – Summary of USGS Envelope Curve Studies ReviewedTable 6 – Hydraulic Analysis of BridgesPage202526345557Volume 2Table 7 – Priority Levels and Corresponding Recommended ActionsTable 8 – Common Protective MeasuresTable 9 – Coding Guide for Bridges - Item 113Table 10 – Summary of Model Input and Results for Example BridgesPage73737581LIST OF FIGURESVolume 1Figure 1. Summary of Scour Practice Survey ResultsFigure 2. Observed vs. Predicted Scour for Original FroehlichFigure 3. Physiographic Provinces of New Jersey (NJGS 2011)Figure 4. New Jersey SEM Erosion Classes for Soil and RockFigure 5. Process for Development and Application of New Jersey EnvelopeCurvesFigure 6. Location of Sample Bridge, Nearest Gage, and Distance DeterminationUsing Google EarthFigure 7. Statewide Location of Bridges and Gages Using Google EarthPage17273133546060Volume 2Figure 8. Overview Flow Chart of SEM ModulesFigure 9. SEM Erosion Classes for Soil and RockFigure 10. Flow Chart for Evaluation of Geotechnical Risk – Module 1Figure 11. Flow Chart for Evaluation of Hydrologic/Hydraulic Risk – Module 2Figure 12. Risk Decision Matrix – Module 3Figure 13. Bridge Importance Analysis - Module 4Figure 14. Risk Decision Matrix with Example Bridge Applications PlottedvPage62646568697183
LIST OF ACRONYMSAASHTO – American Association of State Highway and TransportationACBs – Articulated Concrete BlocksADT – Average Daily TrafficARF – Average Risk FailureASTM – American Society for Testing and MaterialsBIM – Bridge Importance MatrixCOF – Consequence of FailureCSU – Colorado State UniversityDOT – Department of TransportationDR – Detour RiskEFA – Erosion Function ApparatusFDOT – Florida Department of TransportationFEMA – Federal Emergency Management AgencyFHWA – Federal Highway AdministrationHEC-18 – Hydraulic Engineering Circular No. 18ICSE-5 – 5th International Conference on Scour and ErosionILDOT – Illinois Department of TransportationNBIS – National Bridge Inspection StandardsNBSD – National Bridge Scour DatabaseNCHRP – National Cooperative Highway Research ProgramNJDEP – New Jersey Department of Environmental ProtectionNJDOT – New Jersey Department of TransportationNJIT – New Jersey Institute of TechnologyNWS – National Weather ServicePennDOT – Pennsylvania Department of TransportationPOA – Plan of ActionRQD – Rock Quality DesignationSCDOT – South Carolina Department of TransportationSDI – Slake Durability IndexSEM – Scour Evaluation ModelSHA – State Highway AdministrationSI&A – Structure Inventory and AppraisalSRICOS – Scour Rate in Cohesive SoilsSRICOS-EFA – Scour Rate in Cohesive Soil – Erosion Function ApparatusTXDOT – Texas Department of TransportationUS – United StatesUSDA – United States Department of AgricultureUSDOT – United States Department of TransportationUSGS – United States Geologic SurveyUSSCS – United States Soil Conservation ServiceWMA – Water Management Areasvi
EXECUTIVE SUMMARYDesign and Evaluation of Bridges for Scour Using Hydraulic Engineering CircularNo. 18 (HEC-18)In an effort to improve scour design and evaluation methods within the State of NewJersey, the New Jersey Department of Transportation (NJDOT) engaged the NewJersey Institute of Technology (NJIT) to perform a bridge scour research study underTask Order No. 89. The NJIT research team was comprised of faculty, a consultant,and students within the Department of Civil and Environmental Engineering with diversespecialties including hydraulic engineering, hydrology, geotechnical engineering andbridge engineering, reflecting the multidisciplinary nature of the scour phenomenon. Allresearch was done in consultation with the NJDOT Research Project Manager and theNJDOT Research Customer. In addition, a Scour Project Implementation Committeewas formed consisting of members from several NJDOT divisions, as well as the officesof the Federal Highway Administration (FHWA) and the United States GeologicalSurvey (USGS). The Implementation Committee convened periodically to review theresearch results and provide feedback.The overall objective of this study was to develop a rational and defensible process forestimating scour depths for New Jersey's bridges on non-tidal waterways. The studycommenced with a comprehensive literature review of theory and predictive models forbridge scour. This included a web-based survey of scour practice for DOTs within theU.S. in order to assess the varied scour design and evaluation methods used bytransportation agencies. HEC-18 methods and other available models and bestpractices were critically reviewed and compared to develop the most appropriate scourevaluation procedure for New Jersey. The study also investigated the geotechnical,hydrologic, and hydraulic factors affecting scour behavior. In addition, a detailed reviewof the Stage II studies for the bridges on New Jersey’s Scour Critical List wasundertaken to identify significant parameters and trends.The major project deliverable is a new Scour Evaluation Model (SEM) that reflects NewJersey’s unique geologic and hydrologic/hydraulic conditions. In general, the NewJersey SEM is a tiered, parametric, risk-based decision tool. In applying the model, avariety of geotechnical, hydrologic, and hydraulic data are inputted for a particularbridge. These data are analyzed to determine two risk ratings, one geotechnical andthe other hydrologic/hydraulic. The user then enters the risk ratings into a twodimensional Risk Decision Matrix to generate a priority rating that varies according torisk level. This, in turn, generates recommended actions, which may include priorityinstallation of countermeasures, real time scour monitoring, or removal from the ScourCritical List. Bridge importance (ADT and detour length) is also evaluated and factoredinto the final priority rating. A complete set of flowcharts are provided for application ofthe SEM. Although the model is principally designed to evaluate the scour risk ofexisting bridges, some of the model components are useful for designing new bridgesas well.1
The overall purpose of the New Jersey SEM is to improve bridge safety and allow theNJDOT to expend repair funds more strategically. The method, which is documented inthis report, will allow the Department to discern more precisely those bridges which arescour critical and require protective measures. The SEM procedure is also capable ofidentifying other bridges that can be returned to a normal or modified monitoringprogram. While the current model reflects New Jersey’s geology and hydrology, it canbe recalibrated to other regions or states.The SEM procedure also assures that scour evaluations for bridges are performed in auniform manner. Standard protocols are provided for conducting geotechnicalreconnaissance studies and field scour investigations to evaluate and document scourrisk. Included is a new classification system that rates the erosion resistance of thestreambed according to the kind of soil or rock present. Seven different erosion classesare defined ranging from sound rock to soft clay.The SEM also provides standard methods for conducting hydrologic and hydraulicevaluation of scour risk. One method is envelope curve analysis, which defines theupper range of observed scour depths in a specific geologic region. This studyrecommends that envelope curves be applied to certain bridges in State’s Coastal Plainand Non-glaciated Piedmont/Highlands provinces. The study also employshydrologic/hydraulic analyses, which determine whether a bridge has experienced a100-year storm. Several data sources were utilized including stream gages,StreamStats runs, and weighted USGS flows.The majority of this research study was conducted when HEC-18, 4th edition was theprevailing guidance document. This edition contained very limited information foranalyzing scour in some of the geologic conditions present in New Jersey, such asbedrock, boulder trains, and hard cohesive soils. With the publication of the 5th editionin 2012, new scour relationships became available and were reviewed and validated.SEM now incorporates those parts of HEC-18 appropriate for New Jersey geology andbridges.During the final phase, the New Jersey Scour Evaluation Model was validated andcalibrated by inspecting 34 bridges on the Scour Critical List. Bridges were selected inall four of the New Jersey’s physiographic provinces to examine a range of geologic andhydrologic conditions. Actual field observations were compared and correlated withmodel results. The study report also presents example SEM applications for 12selected scour critical bridges, determining preliminary risk and priority levels for each.In addition, two detailed example problems are provided to further instruct the user inthe application of the SEM.The New Jersey SEM will be applied to the remaining 142 bridges on the State's ScourCritical List for possible status change. Those bridges determined to have the highestscour priority will be placed on a list for expedited monitoring, repair, or replacement.Preliminary results also suggest that a significant number of bridges are candidates for2
removal from the Scour Critical List over the next few years, with the potential to savethe Department tens of millions of dollars.Planning is currently underway at NJDOT to launch an “Implementation Phase” totransfer the results of this research into state-wide practice as expeditiously as possible.The project will be divided into three principal tasks. The first will be to evaluate selectedscour critical bridges using SEM to fully demonstrate the method. The key finding foreach bridge will be the Priority Level generated by the model (1 thru 4) along with theRecommended Actions. The second task of the project will be to develop envelopecurves for selected bridges in the Coastal Plain and Non-glaciated Piedmont/Highlandsprovinces of the New Jersey. These state-specific data will be added to the nationaldatabase of envelope curves, adding yet another degree of confidence to the method.The third and final task of the Implementation Phase will be to present an instructionalseminar in the use of New Jersey SEM to NJDOT personnel and design consultants.3
INTRODUCTIONBackgroundPrevention of bridge scour has now been a national priority for 2 full decades.Beginning in 1990 with the Federal Highway Administration’s (FHWA’s) issuance ofTechnical Advisory T5140.20, transportation agencies across the U.S. have beendeliberately engaged in evaluating the scour susceptibility of bridges within theirinventories (USDOT, 1988). Those bridges found to be scour critical are now in variousstages of remediation, ranging from monitoring to outright replacement. While progressis being made, many state and county DOTs are still in the process of implementingtheir action plans. The reason for the delay is the sheer number of bridges that detailedscreening has determined to be scour susceptible, which number into the hundreds insome states.Prudent action is warranted, since scour remains a leading cause of bridge failure in theU.S. Fortunately, the large majority of the failures are not sudden or catastrophic. Morecommonly, the responsible agency observes progressive erosion and scour, and thendecides to repair the bridge or replace it preemptively.For riverine flow the principal scour tool for U.S. bridge designers is HydraulicEngineering Circular No. 18 (HEC-18) published by the FHWA (Arneson et al., 2012).Increasingly, practitioners recognize that some of the standard equations in HEC-18over-predict scour depth for certain hydraulic and geologic conditions. One reason foroverly conservative or erroneous calculated scour depths is poor estimation of scourvariables. Misuse of methods can also be a culprit, such as applying a HEC-18equation to a bed sediment or hydraulic condition that does not actually fall within theusable range of the relationship.Another explanation for over-prediction of scour depth is that most of the HEC-18relationships are based on laboratory flume studies conducted with sand-sizedsediments increased with factors of safety. It is fair to ask whether scale modeling caneffectively represent a phenomenon as complex as scour, especially in view of the widediversity of hydrologic, hydraulic, and geotechnical conditions that exists across thenation. Indeed, the scour behavior of a bridge spanning a mile-wide river with siltysediments in the Midwest is quite different from a bridge crossing a boulder-filled streamin the Mountain States, which differs yet again from another bridge spanning a modestsize river choked with coarse glacial outwash in the Northeast. Recognizing suchregional differences, and driven by the funding limitations, it is prudent to re-examinepredictive scour models.The impact of over-predicting scour depth for new bridges can be significant, sincedesigners have only two general options: (1) extend and/or stiffen the substructure; or(2) provide countermeasures. Either option increases construction costs substantially.When retrofitting existing bridges for scour, additional complications may beencountered. One is the acquisition of right-of-way easements, since installed4
countermeasures typically extend beyond the bridge limits. A second is theenvironmental impact of the countermeasure on the flora and fauna present within thestream channel. Lengthy permit approval times can occur for bridges located alongenvironmentally sensitive watercourses.A principal motivation for this current research project was to develop a more discerningscour evaluation procedure to ensure that bridges on the scour critical list are actuallycritical. For example, the Research Team found that the majority of Stage II studies forthe bridges on the scour critical list did not satisfactorily characterize the grain size ofthe stream bed materials on account of inadequate sampling methods. This caused abias towards finer grain sizes in at least half of the bridges studied, which, in turn,inflated predicted scour depths. In addition, numerous inconsistencies were found inthe hydraulic and hydrologic analyses. As an example, stream discharges weredeveloped using many different methodologies (e.g. extreme value, regressionanalysis), as well as data from different agencies (e.g. FEMA, USSCS, USGS). Theseinadequacies in the Stage II studies had the effect of compounding, even further, thedegree of conservatism already built in the HEC-18 relationships. Note that someinstances of under-conservatism were also encountered in the Stage II studies, whichwere also of concern.Project ObjectivesIn an effort to improve scour design and evaluation methods within the State of NewJersey, the New Jersey Department of Transportation (NJDOT) engaged the NewJersey Institute of Technology (NJIT) to perform the current research study under TaskOrder No. 89. The NJIT research team was comprised of faculty, a consultant andstudents within the Department of Civil and Environmental Engineering with diversespecialties including hydraulic engineering, hydrology, geotechnical engineering andbridge engineering, reflecting the multi-disciplinary nature of the scour phenomenon. Allresearch was done in close consultation with the NJDOT Research Project Managerand the NJDOT Research Customer. In addition, a Scour Project ImplementationCommittee was formed consisting of members from several NJDOT divisions, as wellas the offices of FHWA and USGS. The Implementation Committee convenedperiodically to review the research results and provide feedback.The overall objective of this study was to develop a rational and defensible process forestimating scour depths for New Jersey's bridges on non-tidal waterways. The studycommenced with a comprehensive literature review of theory and predictive models forbridge scour. This included a web-based survey of scour practice for DOTs within theU.S. in order to assess the varied scour design and evaluation methods used bytransportation agencies. HEC-18 methods and other available models and bestpractices were critically reviewed and compared to develop the most appropriate scourevaluation procedure for New Jersey. The study also investigated the geotechnical,hydrologic, and hydraulic factors affecting scour behavior. In addition, a detailed review5
of the Stage II studies for the bridges on New Jersey’s Scour Critical List wasundertaken to identify significant parameters and trends.The majority of this research study was conducted when HEC-18, 4th edition(Richardson and Davis 2001) was the prevailing guidance document. This editioncontained very limited information for analyzing scour in some of the geologic conditionspresent in New Jersey, such as bedrock, boulder trains, and hard cohesive soils. TheResearch Team developed new methods for dealing with these special conditions. Withsubsequent publication of the 5th edition of HEC-18 in 2012, new guidance becameavailable for a wider range of geotechnical conditions, and this document nowincorporates selected HEC-18 methods appropriate for New Jersey geology andbridges.The major project deliverable is a new Scour Evaluation Model (SEM) that reflects NewJersey’s unique geological and hydrologic/hydraulic conditions. The method, which isdocumented in this report, will allow NJDOT to discern more precisely those bridgeswhich are scour critical and require protective measures. The SEM procedure is alsocapable of identifying other bridges that can be returned to a normal or modifiedmonitoring program. The model is risk-based and encompasses geotechnical,hydrologic, and hydraulic factors. It is planned to apply the New Jersey ScourEvaluation Model to the 140 bridges remaining on the State's Scour Critical List forpossible status change. The SEM is useful for the design of new bridges as well. Whilethe current model reflects New Jersey’s geology and hydrology, it can be recalibrated toother regions or states.Scope and Techniques of the Research StudyPhase 1 - Literature SearchThe Literature Search Phase was an important first step in the study, and it formed theessential foundation for later analyses and development of the new scour evaluationmodel. There is clearl
Eugene Golub, PE, PhD Professor of Civil & Environmental Engineering New Jersey Institute of Technology . The overall objective of this research is the development of a new approach for evaluating bridge scour for New Jersey's bridges on non-tidal waterways. The study commenced with a web-based survey of scour . Chronology of NJ's Scour .