united states bridge scour agriculture evaluation ... · evaluation: screening, analysis, &...

Bridge ScourEvaluation:

Screening, Analysis,& Countermeasures

United StatesDepartment ofAgriculture

Forest Service

Technology &DevelopmentProgram

7700—Transportation SystemsSeptember 19989877 1207—SDTDC

Information contained in this document has been developed for the guidance ofemployees of the Forest Service, USDA, its contractors, and cooperating Federaland State agencies. The Department of Agriculture assumes no responsibilityfor the interpretation or use of this information by other than its own employees.The use of trade, firm, or corporation names is for the information and convenienceof the reader. Such use does not constitute an official evaluation, conclusion,recommendation, endorsement, or approval of any product or service to theexclusion of others that may be suitable.

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To file a complaint, write the Secretary of Agriculture, U.S. Department ofAgriculture, Washington, DC 20250, or call 1-800-245-630 (voice) or 202-720-1127 (TDD). USDA is an equal opportunity employer.

Bridge ScourEvaluation:Screening, Analysis,& Countermeasures

John Kattell, P.E.Regional Bridge EngineerUSDA Forest Service Region 1

Merv Eriksson, P.E.Project LeaderSan Dimas Technology and Development Center/Bridge EngineerWood in Transportation Program

USDA Forest ServiceSan Dimas Technology and Development CenterSan Dimas, California

September 1998

ACKNOWLEDGMENTS

A project of this scope requires the efforts of many people. The authors gratefully acknowledge the WashingtonOffice Engineering Staff and the Technology and Development Program Engineering Steering Committee forproviding guidance and funding.

The following persons are thanked for project guidance, reviewing documentation, and providing comments onuse and applications:

Dick Jones, Regional Hydraulic/Geotechnical Engineer, USDA Forest Service Region 8

Nelson Hernandez, Structural Engineer, USDA Forest Service Washington Office

Bill Grabner, Regional Bridge Engineer, USDA Forest Service Region 6

Scott Mitchell, Regional Bridge Engineer, USDA Forest Service Region 2

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CONTENTS

ACKNOWLEDGMENTS..................................................................................................................... iiiINTRODUCTION................................................................................................................................ 1SCOUR EVALUATION REFERENCE STANDARDS ......................................................................... 2FOREST SERVICE SCOUR EVALUATION PROGRAM ................................................................... 2

Required FHWA Reporting ..................................................................................................... 4Documentation and Program Monitoring................................................................................ 4

STEP 1 - OFFICE SCREENING AND MANAGEMENT PRIORITY ANALYSIS ................................. 4STEP 1A - OFFICE SCREENING ...................................................................................................... 4

Objective ................................................................................................................................ 4Office Screening Flow Chart .................................................................................................. 5Suggested Responsible Person(s) ......................................................................................... 5Recommended Documentation .............................................................................................. 5

STEP 1B - MANAGEMENT PRIORITY ANALYSIS ........................................................................... 5Objective ................................................................................................................................ 5Management Priority Analysis Flow Chart .............................................................................. 8Suggested Responsible Person(s) ......................................................................................... 9Recommended Documentation .............................................................................................. 9

STEP 2 - FIELD REVIEW, SCOUR VULNERABILITY ANALYSIS, & PRIORITIZE ........................... 9STEP 2A - FIELD REVIEW ................................................................................................................ 9

Objective ................................................................................................................................ 9Suggested Responsible Person(s) ....................................................................................... 10Recommended Documentation ............................................................................................ 10

STEP 2B - SCOUR VULNERABILITY ANALYSIS ........................................................................... 10Objective .............................................................................................................................. 10Scour Vulnerability Appraisal ................................................................................................ 10Available Methodologies ...................................................................................................... 10CAESAR............................................................................................................................... 10Colorado Highway Department Scour Vulnerability Ranking Flow Charts ........................... 10Rapid-Estimation Method For Assessing Scour at Highway Bridges Based on Limited Site Data ...... 10Suggested Responsible Person(s) ....................................................................................... 11Recommended Documentation ............................................................................................ 11

STEP 2C - PRIORITIZE BRIDGES .................................................................................................. 11Objective .............................................................................................................................. 11Priority Guidelines and Criteria ............................................................................................. 11Suggested Responsible Persons ......................................................................................... 11

STEP 3 - DETAILED SCOUR EVALUATION ................................................................................... 11Objective .............................................................................................................................. 11Evaluation Criteria, and Tools ............................................................................................... 12Suggested Responsible Person(s) ....................................................................................... 12

STEP 4 - PLAN OF ACTION ............................................................................................................ 12Objective .............................................................................................................................. 12Suggested Responsible Person(s) ....................................................................................... 12

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Recommended Documentation ............................................................................................ 12TYPICAL FOREST SERVICE BRIDGE ........................................................................................... 12COMMON PROBLEMS AND CHARACTERISTICS ........................................................................ 14

Stream Channel Instability ................................................................................................... 14Bridge Geometry and Scour ................................................................................................. 14Aggradation .......................................................................................................................... 15Long-Term Degradation........................................................................................................ 15Contraction Scour ................................................................................................................. 15Abutment Scour .................................................................................................................... 15Debris ................................................................................................................................... 17Abutment Fill Failures ........................................................................................................... 17

COMMON COUNTERMEASURES ................................................................................................. 18Riprap ................................................................................................................................... 18Spur Dikes, Barbs, Groins, and Vanes ................................................................................. 18Foundation Strengthening .................................................................................................... 18

LITERATURE CITED ....................................................................................................................... 20APPENDIX A ...................................................................................................................................A-1APPENDIX B...................................................................................................................................B-1APPENDIX C ................................................................................................................................. C-1

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1

INTRODUCTION

Scour, defined as “the erosion or removal ofstreambed or bank material form bridge foundationsdue to flowing water” is the most common cause ofhighway bridge failures in the United States. TheForest Service, U.S. Department of Agriculture,administers 7,650 bridges on National Forest landsand virtually all of them are over water. Scour isalso the single most common cause for bridgedamage and failure on National Forest lands(Figure 1). Many bridges will experience floodswhich can cause damage each year. To minimizefuture bridge flood damage and ensure public safetyrequires developing and implementing improvedprocedures for designing bridges and inspectingthem for scour. “Every bridge over water, should beassessed as to its vulnerability to scour in order todetermine the prudent measures to be taken for thatbridge and the entire inventory” (Richardson andDavis␣ 1995).

Realizing this need, the Federal HighwayAdministration (FHWA) issued a Technical Advisoryin 1988 revising the National Bridge InspectionStandards (NBIS) to require evaluation of all bridgesfor susceptibility to damage resulting from scour. Inaccordance with the Memorandum of Understandingbetween the Forest Service and the FederalHighway Administration, the Forest Service isrequired to implement the Technical Advisory,establish a scour evaluation program, and submitreports to FHWA discussing the progress of theevaluation program.

Prior to 1998, the Forest Service had notimplemented a scour evaluation program. In 1998,an Engineering Technology Development Proposalwas funded to develop a scour evaluation program,specifically for the Forest Service, that all Regionsof the Forest Service could implement. The projectwas to outline a single process and establish criteria,methods, and guidelines that would ensureconsistency throughout the agency and eliminateduplication of effort.

The project was completed in cooperation with theRegional Bridge Engineers and was organized intothree phases.

Phase 1. Review the FHWA guidelines andexisting public road agency scourprograms.

Phase 2. Develop a scour evaluation programspecifically for the Forest Service basedon the information from Phase I.

Phase 3. Provide support for the program duringimplementation by the Regions.

This document is the culmination of Phases 1 and 2.

Implementing this process will provide valuableinformation and initiate pro-active management ofour bridge inventories. Managers will be able toprioritize needs and avoid many future bridgeproblems and failures. In addition, the process willprovide valuable training, experience, and tools thatwill enhance the skills of employees who implementthe program. This will benefit many future projectsin our role of “Caring For The Land, and ServingPeople.”

Figure 1—Little Salmon River Bridge, Nez Perce National Forest. A January 1997 flood eventscoured the abutment and one of the intermediate piers causing failure.

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SCOUR EVALUATION REFERENCESTANDARDS

The Technical Advisory issued by FHWA in 1988provided recommendations for developing andimplementing a scour evaluation program. Sincethat time, FHWA developed two additionaldocuments that have become the referencestandards for all scour evaluation programs. Thesedocuments are:

1. Hydraulic Engineering Circular No. 18 (HEC-18)- Evaluating Scour at Bridges. (Richardson andDavis 1995)

2. Hydraulic Engineering Circular No. 20 (HEC-20)- Stream Stability at Highway Structures.(Lagasse et al. 1995)

HEC-18 is the technical standard for knowledge andpractice in the design, evaluation, and inspection ofbridges for scour. HEC-20 provides guidelines foridentifying stream instability problems at streamcrossings that may cause scour damage to bridgesor culverts (Figure 2).

The Scour Evaluation Program outlined within thisdocument also uses the two HEC documents as theprimary reference standards. Successfulimplementation of the program will requireknowledge, understanding, and use of thesereferences.

FOREST SERVICE SCOUR EVALUATIONPROGRAM

The Forest Service Scour Evaluation Program hasbeen developed into a four-step process similar tothe five-step process recommended in HEC-18,Chapter 5. The objective of the process is to providea consistent, efficient method to review and evaluateall bridges over water, determine the scour potentialof each bridge, assist in establishing priorities andidentifying appropriate countermeasures, anddocumenting the results. Figure 3 presents a flowchart of the process. The four steps are:

Step 1 - Office Screening and Management PriorityAnalysis

Step 2 - Field Review, Scour Vulnerability Analysis,and Prioritizing.

Step 3 - Detailed Scour Evaluation.Step 4 - Plan of Action.

Each step will be discussed in more detail in thefollowing sections. Within each step of the process,bridges or major culverts are categorized withrespect to the determined scour potential.(Categories are discussed under section Step␣ 1)These categories also correspond to the ScourCritical Bridge field on the Structure Inventory andAppraisal form, Item␣ 113. As a bridge proceedsthrough the evaluation process, a structure may beplaced in a different category from the previous step

Figure 2—Left - HEC 18, Evaluating Scour at Bridges; Center - An AssessmentMethodology for Determining Historical Changes in Mountain Streams; Right - HEC 20,

Stream Stability at Highway Structures.

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R9800107

Bridges needing further evaluationCatagories 2 - Scour Susceptible

3 - Scour Critical4 - Unknown Foundation



Category 1Low

Scour Risk

Category 5Tidal

STOP STOP

Category 1Low Scour Risk

Category 5Tidal

STOPSTOP

Step 1aOffice Screening

Step 2bScour Vulnerability

Analysis

Step 2aField Review

Step 4Plan of Action

for Bridge

Step 2cPrioritize

Step 3Detailed Scour Evaluation

Step 1bMgmt Priority

Analysis

LowPriority

MonitorBridge

Figure 3—Forest Service Scour Evaluation Program Four Step Process.

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and the Scour Critical Bridges field code may alsochange. The process will be complete for a bridgewhen the coding of the Scour Critical Bridges fieldon the Structure Inventory and Appraisal form,Item 113, is any value other than code 6, which is“Scour calculation/evaluation has not been made.”

Steps 1 and 2 provide guidelines and criteria toefficiently and economically screen as many bridgesand major culverts as possible into the appropriatecategories, identify proper Scour Critical Bridgecodes, and prioritize bridges. Step 3, Detailed ScourEvaluations, requires an experienced inter-disciplinary team. In most instances, this team willinclude technical expertise outside the Forest Serviceand is anticipated to be expensive. Steps 1 and 2can be completed by Forest Service personnel orconsultants.

All bridges should be first evaluated using Steps 1and 2 since appropriate countermeasures can oftenbe easily identified before proceeding to Step 3 onmany of the single-span, smaller bridges that arerated scour critical.

Required FHWA Reporting

In addition to coding the Scour Critical Bridges fieldon the Structure Inventory and Appraisal form, Item113, for each bridge, FHWA requires the ForestService to submit progress reports outlining theprogress of the evaluation program. In Appendix Ais an outline of the format of the report andexplanations for the different reporting categories.The progress reports are to reflect the number ofbridges and the appropriate coding for each bridge.

Documentation and Program Monitoring

Documentation is recommended for each bridge ateach step of the process. Recommendeddocumentation methods are described for each stepin this report. In addition, INFRAstructure-Bridgeand Major Culvert (INFRA-BMC) will remain thedatabase and inventory for all Forest Service bridgesand major culverts. Monitoring of each bridgethrough the scour evaluation process should bepossible using INFRA-BMC with a few proposedchanges. The proposed changes are as follows:

Revise Scour Critical Bridge, Item 113:

• Add new code “U” for Unknown Foundation.• Add new code “T” for Tidal.• Add new code “LP” for Low Priority. (This code will

reflect bridges that are identified as low priority inStep 1b - Management Priority Analysis, in theproposed Forest Service scour evaluation process).

Add new field “Scour Vulnerability” to the Appraisalfields. This field will assist in prioritizing the bridgesduring Step 2c of the scour evaluation process.Proper codes are:

• H High Scour Vulnerability• M/H Moderate to High Scour Vulnerability• M Moderate Scour Vulnerability• M/L Moderate to Low Scour Vulnerability• L Low Scour Vulnerability• ND Not Determined

With the revisions to INFRA-BMC proposed, variousreports can be generated from the data to assist inmonitoring the program process. A supplement tothis report will be a standard report to query INFRA-BMC for the data needed to generate FHWAprogress reports. Maintaining the proper coding ofINFRA-BMC for each bridge during the scourevaluation process should result in adequatemonitoring capability.

STEP 1 - OFFICE SCREENING ANDMANAGEMENT PRIORITY ANALYSIS

STEP 1A - OFFICE SCREENING

Objective

The objective of Step 1a is to quickly review thecurrent available documents within the bridge ormajor culvert file and screen them into fivecategories. As mentioned above, each category hascorresponding codes with respect to the ScourCritical Bridge field on the Structure Inventory andAppraisal form, Item 113. It should be noted, thatany bridge which has not proceeded to Step 1 in theprocess, should have a code 6 for the Scour CriticalBridge field (scour calculation/evaluation has notbeen made) . The five categories and correspondingScour Critical Bridge (Item 113) codes are as follows:

1. Low Scour RiskItem 113 corresponding codes 4, 5, 7, 8, 9

2. Scour-susceptibleItem 113 corresponding codes 6, LP

3. Scour-criticalItem 113 corresponding codes 0, 1, 2, 3

4. Unknown FoundationsItem 113 corresponding codes U

5. TidalItem 113 corresponding codes T

These five categories are the same as recommendedin HEC-18, Chapter 5.

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Office Screening Flow Chart

Figure 4 provides a flow chart of Step 1 indicatingthe recommended steps in an office screening andthe criteria for placing bridges and major culvertsinto the five categories. Documents needed arebridge plans and past inspection reports. Below issome discussion on several of the various decisionnodes within the flow chart.

Pier/Abut/Ftg in Channel or Floodplain: Plansthat indicate all foundations are outside the channeland flood plain and well above flood water elevationscan be categorized as Low Scour Risk (Figure 5).

Pier/Abut/Ftg on Scourable Material: Plans thatindicate all foundations are on non-scourablematerial can be categorized as Low Scour Risk.Non-scourable material is considered to be durablerock that is not susceptible to significant deteriorationdue to weathering and that scours at such a slowrate that changes occur over a long period of time(measured in centuries).

Stream Velocity: “Slow” is generally associated withlakes, tidal zones, or ditches and canals whichexperience very slow moving, predominately static-flow conditions. All other streams, creeks, and riversshould be considered rapid (Figure 6).

Foundations: The foundation type is a primaryinfluencing factor in determining the vulnerability toscour damage. Deep foundations such as long pilesor drilled shafts are considered to have lowvulnerability to scour damage and may be placed inthe low risk category upon review of the inspectionreports, thus eliminating the bridges further from thescour evaluation process. Shallow foundations suchas spread footings, short piles, mud sills, or cribsare considered to have high vulnerability to scourand are not recommended to be considered low riskwithout proceeding to the Field Review, Step 2a.Lengths defining a long pile vs. a short pile havenot been provided. Guidance should be providedon a Regional basis, however, in the predominatelyglacier till soils of the Rocky Mountain States,15 to 20 feet is being used to distinguish betweendeep and shallow pile foundations.

Bridges and major culverts are separated prior toreviewing the foundations. Major culverts usuallyhave no foundations or are on shallow spreadfootings and are not recommended to be consideredlow risk without proceeding to the Field Review,Step 2a.

Unknown foundations should be placed in the samecategory as shallow foundations, requiring furtherreview.

Inspection Reports: Prior to placing any structurein Category 1, Low Scour Risk, the inspection reportsshould be reviewed for indications of past or currentscour problems. Scour damage should include notonly damage to the structure itself, but also toapproach fills. An abutment on a deep foundationmay have a low vulnerability to scour damagestructurally, but the approach fills may scour awayleaving a serious safety hazard even though thebridge itself is not damaged. Specific items in theinspection reports to be reviewed are theSubstructure, Channel & Channel Protection, andWaterway Adequacy fields. A numeric code of 5 orless in any of these fields (substructure must be dueto observed scour) indicates potential scourproblems and those bridges should proceed to thenext step of the process. If a bridge has deepfoundations, and the inspection reports do notindicate any potential scour problems, the bridgesare placed in Category 1, Low Scour Risk and nofurther review is necessary.

Suggested Responsible Person(s)

The office screening can be completed by ForestService Bridge Inspection Team Leaders or ProgramManagers or consultants with assistance from theRegional Bridge Engineer.

Recommended Documentation

A simple method of documenting the office screeningstep is to highlight the decisions, path, and endingCategory that the bridge was placed in on the OfficeScreening Flow Chart, Figure 4. A flow chart shouldbe prepared for each individual bridge. Notes couldalso be written on the flow chart. If electronicdocumentation is preferred, highlighting, shading,and typed notes can be added to the flow chartindicating the decisions, path, and ending Categorythat the bridge was placed in.

STEP 1B - MANAGEMENT PRIORITYANALYSIS

Objective

Realizing that funding and resources for detailed fieldreviews, scour evaluation studies, and implementingon-site scour countermeasures will be limited, theobjective of Step 1b is to recognize that some ForestService bridges and major culverts will have a muchlower priority, regardless of the bridge’s scourvulnerability rating. Many Forest Service bridges are

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Region_________________________Forest__________________________Route ID & MP___________________Name__________________________Feature Crossed__________________

NO

NO

NO

STOPBridge overwater

StreamVelocity

YES

YES

YES

Pier/Abut/Ftg.in channel

or Floodplain

Pier/Abut/Ftg.on Scourable

material

Slow (lake, canal, tidal)

Item 60Substructure

≤ 5 due toobserved scour

Item 61Channel &Channel

Protection≤ 5

Item 71WaterwayAdequacy

≤ 5



To Step 1b

Bridge Major Culvert

Dee

p F

ound

atio

ns(lo

ng p

iles,

dril

led

shaf

ts)

Unk

now

n or

sha

llow

foun

datio

ns (

spre

adfo

otin

gs, s

hort

pile

s,m

ud s

ills,

crib

s)

Rapid

STOPSTOP

Category 1Low Scour Risk

Category 5Tidal

Inspection Reports

No

No

No

No

Item 60Substructure

≤ 5 due toobserved scour

Item 61Channel &Channel

Protection≤ 5

Item 71WaterwayAdequacy

≤ 5

Inspection Reports

No

No

No

No

YES

YES

YES

YES

YES

YES

Figure 4—Forest Service Scour Evaluation Program -Office Screening Flow Chart. This figure breaksdown step 1a from the overall diagram in Figure 3.

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Figure 6—Lake Koocanusa Bridge, Kootenai National Forest. Illustration of bridge crossing a body ofwater with “slow” stream velocities.

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Figure 5—Libby Creek Bridge, Kootenai National Forest. Bridge footings are set into non-erodible bedrockas well as being outside the channel and well above flood waters. Bridge is categorized as Low Scour Risk.

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behind locked gates, have low traffic volumes, arenot vital access routes, are older, or are small bridgeswith low present-worth values, where scour damageor complete washout would not create significantresource damage. These structures are of lowpriority and do not economically justify furtherevaluation or installation of any scourcountermeasures. An acceptable mitigation plan forthese structures is monitoring after flood events andclosure if necessary. Figure 7 outlines a process forStep 1b in which bridges and major culverts that havebeen screened into Categories 2, 3 or 4 in Step 1a,are quickly evaluated with respect to priority.Structures meeting certain criteria can be consideredlow priority without further review or evaluation. Theaction plan for these structures is monitoring.

Management Priority Analysis Flow Chart

Figure 7 presents a flow chart of the recommendedprocess and criteria for the Management PriorityAnalysis. Below is some discussion on several ofthe various decision nodes within the flow chart.

NBIS vs. Non-NBIS Structure: To evaluate bridgesand major culverts with respect to traffic volumesand public use, distinguishing between a NBIS(National Bridge Inspection Standards) or Non-NBISbridge is recommended. (A NBIS bridge or majorculvert is one that is considered “open to publictravel” and subject to the National Bridge InspectionStandards). A NBIS structure should remain in thescour evaluation process and proceed to the FieldReview of Step 2.

Potential For Resource Damage: The bridge ormajor culvert should be reviewed for potential toresource damage if significant scour or completewashout occurs. Several possibilities to considerare:

• The amount of sediment that would be added tothe creek or river with scour damage. In general,sediment comes from the approach fills, whichmany times are small; however, major culvertsmay be buried in high fills that would contributemuch more sediment (Figure 8).

• The debris from a bridge or major culvert that maydamage other structures below.

Figure 7—Forest Service Scour Evaluation Program - Management Priority Analysis. Thisfigure breaks down Step 1b from the overall diagram shown in Figure 3.

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Region__________________________

Forest___________________________

Route ID & MP____________________

Name___________________________

Feature Crossed__________________

Bridges needing further evaluation

Categories 2, 3, or 4

(From Step 1a)

NBIS orNon-NBIS

Potential forResource Damage

OtherConsiderations

Vital Access

Monitor Bridge

(Low Priority)

Completed by_____________________

Date____________________________

Non-NBIS Acceptable No Low Acceptable

Proceed to Step 2aField Review

NBIS Unacceptable

Bridge Present Worth

or Value

YesUnacceptable High

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The potential resource damage should be evaluatedas Acceptable or Unacceptable. Structures withunacceptable potential for resource damage shouldremain in the scour evaluation process and proceedto the Field Review of Step 2.

Vital Access: A bridge or major culvert that may beclosed to public travel (Non-NBIS) but is on a vitaladministrative route and would severely impactaccess to critical management areas should remainin the scour evaluation process and proceed to theField Review of Step 2.

Bridge Present Worth / Value: Non-NBIS bridgesthat are large, in good condition, or can beeconomically rehabilitated and would have a highreplacement cost should remain in the scourevaluation process and proceed to the Field Reviewof Step 2. However, Non-NBIS bridges that aresmall, in poor condition, or can not be economicallyrehabilitated and would have a low replacement costwould be good bridges to rate as low priority andmonitor.


The Management Priority Analysis should be theresponsibility of the Engineering Forest Staff Officer,with assistance from the Forest Bridge InspectionTeam Leader or Program Manager, ForestTransportation Planner, and Resource Specialists.


A simple method of documenting the ManagementPriority Analysis is to highlight the decisions andpath for the bridge on the Management PriorityAnalysis Flow Chart, Figure 7. Notes could also bewritten on the flow chart. If electronic documentationis preferred, highlighting, shading, and typed notescan be added, indicating the decisions.

STEP 2 - FIELD REVIEW, SCOURVULNERABILITY ANALYSIS, & PRIORITIZE

The bridges and major culverts placed inCategories 2, 3, and 4 in Step 1a and identified toproceed to Step 2 in Step 1b, may be screenedfurther by completing Field Reviews and ScourVulnerability Analyses. In addition, bridges areprioritized for further evaluation and/orimplementation of scour countermeasures.

STEP 2A - FIELD REVIEW

Objective

The objective of the Field Review is to verify theinspection report information used during Step 1 andto gather additional field data necessary to completea Scour Vulnerability Analysis. The Field Review isa comprehensive study of current scour problemsas well as an analysis of the fluvial geomorphologyof the stream.

Figure 8—Moss Creek, Idaho Panhandle National Forest. Piping and subsequent scour of this culvert resulted infailure of the entire fill introducing a large amount of sediment into the stream. With Step 1b, Management Priority

Analysis, the potential for resource damage at similar sites could be considered as unacceptable.

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The Field Review can be completed by ForestService Bridge Inspection Team Leaders, orconsultants. With some training, Forest ServiceBridge Inspection Team Leaders should be able togather the information in conjunction with theregularly scheduled bridge inspections.


Documentation of the Field Review will be necessaryto provide pertinent information for the ScourVulnerability Analysis. More information is providedin the Step␣ 2b discussion.

STEP 2B - SCOUR VULNERABILITYANALYSIS

Objective

With the information provided by the Field Review,bridges and major culverts may be further screenedinto appropriate categories and proper Scour CriticalBridge field codes. In addition, for many single-span,smaller bridges, the Scour Vulnerability Analysis maybe adequate to determine the nature of a scourproblem and the appropriate countermeasure ormitigation.

Scour Vulnerability Appraisal

Scour vulnerability is defined as “the degree to whicha bridge is open to attack or damage from forcesand conditions causing scour.” Scour vulnerabilityis related to the scour critical codes but are not thesame. If appropriate countermeasures are installed,the degree of scour vulnerability will probablydecrease. Also, over time, site conditions maychange, generating new factors that effect the degreeof scour vulnerability. In addition, a bridge’s scourvulnerability will influence a manager’s prioritydecision. Therefore, a new field in the INFRA-BMCinventory has been proposed that will indicate thedegree of scour vulnerability. The codes are:

• H High Scour Vulnerability• M/H Moderate to High Scour Vulnerability• M Moderate Scour Vulnerability• M/L Moderate to Low Scour Vulnerability• L Low Scour Vulnerability• ND Not Determined

Available Methodologies

Various scour vulnerability analyses have been usedby many other agencies at similar points in their scourevaluation programs. A number of methodologiesare available to complete the analysis and toestimate scour potential with limited information and

without completing a full scour evaluation with aninterdisciplinary team. Of the methodologiesreviewed, a computer program developed by theUniversity of Washington, called CAESAR, isrecommended to the Forest Service at this step inthe program. A discussion of the CAESAR programand two alternative methodologies follows.

CAESAR

The University of Washington has developed acomputer program called Cataloging and ExpertEvaluation of Scour Risk and River Stability at BridgeSites (CAESAR). The program operates in aWindows environment and is structured in a questionand answer format. Basic bridge data and FieldReview data are required as input. The programoutputs weighted recommendations pertaining toscour vulnerability, stream stability, and waterwayadequacy. The program has two parts:

1. The user interface, through which site informationis collected, stored, and retrieved. Textual andvisual (graphs and photographs) help is provided.The Field Review data can be documented andstored within the program.

2. An evaluation module assesses the site conditionsand provides recommendations (with confidencevalues) and suggested actions.

The program has been “beta” tested on severalForest Service bridges and was found to be anefficient and effective tool for completing anddocumenting the Field Review and ScourVulnerability Analysis.

Colorado Highway Department ScourVulnerability Ranking Flow Charts

The Colorado Highway Department developed aseries of Scour Vulnerability Ranking Flow Chartsas part of their scour evaluation program. The flowcharts outline a method to determine a vulnerabilityscore for general site conditions, abutment scourvulnerability, and pier scour vulnerability. The flowcharts are not as comprehensive as the CAESARanalysis and do not document the site conditions ofthe Field Review. Appendix␣ B provides a copy ofthe flow charts and accompanying documentation.

Rapid-Estimation Method For AssessingScour at Highway Bridges Based onLimited Site Data

The Montana U.S. Geologic Survey (USGS), incooperation with the Montana Department ofTransportation, developed a methodology for

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estimating scour depths that would (1)␣ require onlylimited on-site data, (2)␣ provide estimates of scourdepth that would be reasonably comparable toestimates from more detailed methods and wouldtend to overestimate rather than underestimate scourdepths, and (3)␣ provide estimates for each site in afew hours or less (Holnbeck and Parrett␣ 1997). Themethod was developed using calculated scourdepths from 122␣ detailed scour evaluations of bridgesites in 10␣ states and formulating relationshipsbetween scour depth and hydraulic variables thatcan be rapidly measured in the field. “Although themethod was developed specifically for bridges inMontana, it is believed to be valid for a wide rangeof hydrologic and hydraulic conditions throughoutthe United States” (Holnbeck and Parrett␣ 1997). Themethod uses a Standardized Scour Analysis andReporting Form which includes a worksheet forcalculating the scour depths and a general summarysheet for general field investigation information.

This method will provide a good assessment of scourvulnerability for a bridge with good documentation.The method will require more time than either theCAESAR program or the Colorado flow charts, goodjudgement, and a high level of expertise.


• The CAESAR program can be used by fieldpersonnel with little formal training in rivermechanics and scour processes. Forest ServiceBridge Inspection Team Leaders, ProgramManagers, or consultants can complete the ScourVulnerability Analysis. Completion of a trainingsession in stream stability and bridge scour isrecommended.

• Colorado DOT scour vulnerability ranking flowcharts should be completed by Forest ServiceBridge Inspection Team Leaders, ProgramMangers, or consultants. Completion of a trainingsession in stream stability and bridge scour isrecommended.

• Rapid-Estimation Method For Assessing Scour atHighway Bridges Based on Limited Site Data,developed by USGS, requires a higher degree ofexpertise than the CAESAR program or theColorado flow charts. To complete thevulnerability analysis using this method, qualifiedForest Service hydraulic or bridge engineers orqualified consultants are recommended.


Documentation will be dependant on the methodused. An advantage of the CAESAR program is thatthe Field Review report and any subsequent Field

Review data can be stored electronically along withthe evaluation of the bridge. The proposed new fieldin INFRA-BMC (Scour Vulnerability) will provide ameans for documentation within the bridge inventory.

STEP 2C - PRIORITIZE BRIDGES

Objective

Throughout the scour evaluation process, bridgeprogram managers will need to evaluate availableresources, personnel, and the funding required tocomplete more detailed bridge evaluations andimplement countermeasures. Step␣ 1b provides aninitial priority screening. At this point in the process,after the Scour Vulnerability Analysis, bridgesremaining in Categories␣ 2,␣ 3,␣ and␣ 4, which requiremore detailed evaluations, need to be prioritized withrespect to safety. Safety will be evaluated withrespect to the determined scour vulnerability, thefunctional classification, and the road the bridgeresides on, along with National Forest roadmanagement policies. Due to the varied roadmanagement policies within the National Forests, astrict method for developing a prioritized list is notprovided or recommended. Some generalguidelines and criteria are suggested.

Priority Guidelines and Criteria

Each National Forest will need to determine how toprioritize and evaluate the safety risks of a bridge. Asimple listing of bridges separated between thoseon arterial, collector, or local roads and sorted byscour vulnerability from high to low will providemanagers with a basis to plan for needed resourcesand prioritize individual bridges for further evaluation.Additional information accompanying the listing, suchas the scour critical code, substructure conditioncode, and average daily traffic (ADT) values may behelpful. A supplement to this document will be astandard report to query INFRA-BMC as describedabove.

Suggested Responsible Persons

Prioritizing bridges should be the responsibility ofthe Forest Staff Officer for engineering activities withassistance from the Forest Bridge Inspection TeamLeader or Program Manager, Forest TransportationPlanner, and Resource Specialists.

STEP 3 - DETAILED SCOUR EVALUATION

Objective

Bridges that have gone through Steps␣ 1␣ and␣ 2 andremain in Categories␣ 2,␣ 3␣ and␣ 4 will require aDetailed Scour Evaluation. HEC-18 indicates this

12

evaluation is to be completed by an interdisciplinaryteam of hydraulic, geotechnical and structuralengineers. The evaluation typically includes adetailed site review, estimated scour calculations,structural evaluation of the foundations under theestimated scour conditions, and the design of anynecessary scour countermeasures. In addition, theresults of a Detailed Scour Evaluation willsubsequently establish proper Scour Critical Bridgefield codes for the INFRA-BMC database.

Evaluation Criteria, and Tools

Criteria and guidelines for a Detailed ScourEvaluation are outlined in the reference, HEC-18.FHWA recommends that a bridge be evaluated forthe design flood and superflood conditions and havesuggested the 500-year flood event.

HEC-18 presents the state-of-the-art in scourcalculation methods and equations. There are anumber of hydraulic computer programs availablefor assisting in the calculation of scour depths, suchas WSPRO, HEC-RAS, and BRI-STARS. Most ofthese programs use one-dimensional models anddo not have the capability to evaluate lateral flowsand channel instabilities or meanders of the stream.Engineers must account for these effects separately,as well as evaluate the structure foundations forinstability at the calculated scour depths.

For many streams and rivers, typically associatedwith mountainous bridge sites of the Forest Service,the evaluation of scour at a site is considered moreof an art than a science. At these sites, streammorphology is a significant factor. Also, hydrologicestimates of these mountainous stream flood-eventflows can have significant error. Therefore, inaddition to the criteria and guidelines outlined inHEC-18 for a Detailed Scour Evaluation, thefollowing is recommended:

• The interdisciplinary team should include a personwith expertise in stream morphology such as aWildland Hydrologist or a Fluvial Geomorphologistto assist in evaluating the potential of scour fromlateral stream instabilities, long-term aggregation,or degradation, etc.

• The scour evaluation should envelop estimatedscour depths by calculating depths for the 50-year,100-year, and 500-year flood events and applyengineering judgement to achieve a reasonableand prudent evaluation of the bridge.


In most instances, the Detailed Scour Evaluation isrecommended to be completed by consultants withthe interdisciplinary expertise required. Dependingon the Forest or Region the responsible person maybe the National Forest Bridge Program Manager,Forest Engineer, or Regional Bridge Engineer.

STEP 4 - PLAN OF ACTION

Objective

The final step in the Forest Service Scour EvaluationProcess is to develop a Plan of Action for a bridge tocorrect scour problems. The Plan of Action is thetool that “closes the loop” from evaluating andstudying a bridge, to acknowledging and recognizinga problem, and, finally, to implement field correctivemeasures. The Plan of Action may include interimscour countermeasures until permanent measuresare installed, monitoring plans and/or inspectionsafter flood events, and procedures for closing bridgesif necessary.


Depending on the Forest or Region, the responsibleperson to prepare the Plan of Action for a bridgemay be the Forest Bridge Program Manager, ForestEngineer, or Regional Bridge Engineer.


A simple Plan of Action form is included inAppendix␣ C. The form is to be completed for eachbridge and retained in the bridge file. The formincludes:

• Basic bridge identification information.• The BMC/INFRA Scour Critical field coding.• The proposed Scour Vulnerability coding with a

brief description of the critical elements vulnerableto scour.

• Recommended scour countermeasures andimplementation plan. Any design or drawingsshould be referenced. If scour countermeasureshave been completed, some basic informationshould be included for future reference.

• Bridge Closure Plan. A bridge closure plan shouldidentify the acceptable method of closing thebridge, such as gates or barricades, and anyneeded detour or safety signing.

TYPICAL FOREST SERVICE BRIDGE

The Forest Service road system is similar in someways to other public road systems because itincludes roads “open to the public” with standardsand traffic volumes similar to local county systems.

13

But the Forest Service road system is also uniquebecause it includes many roads not open to thepublic, which have reduced standards and very lowtraffic volumes. Forest Service bridges also reflectthis diversity with some major, important, arterialbridges and many small, local road bridges. Oneobjective in developing the Forest Service ScourEvaluation Program was to maintain flexibility in theprocess so that the wide variety of bridges could beevaluated efficiently with practical results.The most significant difference in this program, whencompared to the programs developed by each state,is Step 1b - Management Priority Analysis. Thisstep recognizes the fact that many Forest Servicebridges are of a low priority, regardless of the scourvulnerability of the bridge. The step providesflexibility early in the process to assess prioritiesand resources needed to complete the program.

Even though there are a wide variety of bridges inthe Forest Service, a review of the Forest Serviceinventory indicates a few basic characteristics of atypical Forest Service bridge (Figure␣ 9).

• Typically a single-span bridge.• Virtually all cross a stream or river.• Average span is 50␣ feet with 80% of the inventory

under 80␣ feet.• Most common abutment substructure type is vertical

walls supported on spread footings or mud sills.• Bridges are on typically low volume roads with

ADT’s less than␣ 100.

The majority of Forest Service stream crossings areon mountain streams with channel gradientsbetween 0.01 and 0.10. “Mountain streams aresubject to highly variable discharges and aresusceptible to large sediment loads from slopefailures and debris flows” (Smelser and Schmidt1998). The streams typically flow between steepforest slopes with incised channels and nearlyvertical, non-cohesive banks composed of gravel andcobbles before entering valleys where the streamsflow into larger streams and rivers. As the channelgradient drops, stream transport drops, causingaggradation and channel braiding. Meandering, slowmoving streams also occur in the large flatter valleybottoms as well as occasionally in higher elevationopen parks and swamps. Each of these streamchannel types (steep incised, braiding, andmeandering) have their own scour problems. Steepincised streams experience long-term degradation,braided streams (occurring at gradient changes)experience aggradation and stream instability, andmeandering streams experience lateral channelmigration and have very wide flood plains.

Scour associated with typical Forest Service bridgesin similar channel types will many times be of similarnature and cause. Therefore, it is also reasonableto assume that there may be common solutions orcountermeasures. The following is a discussion ofsome of the most common problems andcharacteristics that may be evident on a typicalForest Service bridge.

Figure 9—LaMarche Creek Bridge, Beaverhead National Forest. Typical single-span, treated timber bridge supportedon vertical wall abutments, with wingwalls, on mudsills crossing a mountainous stream.

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COMMON PROBLEMS ANDCHARACTERISTICS

Stream Channel Instability

As described above, stream channel instability is aproblem most commonly associated with braidingstreams. Many Forest Service roads are locatedadjacent to larger rivers, and thus, many ForestService bridges cross the tributaries. As describedabove, many of these tributaries have grade changesas they approach the flood plain of the larger rivers,have braiding characteristics, and experience shiftingand lateral migration. Bank erosion and changingangles of attach of the stream to the bridge causelocal scour problems.

Bridge Geometry and Scour

Scour depth equations in HEC-18 use a coefficientfor abutment shape. The coefficients are(Richardson and Davis 1995):

• Vertical wall abutment 1.00• Vertical wall abutment with wingwalls 0.82• Spill-through abutment 0.55

The coefficients indicate that spill-through abutments(trapezoidal-shaped channel through a bridge)decrease local scour depths significantly, comparedto vertical wall abutments. Spill through abutmentsprovide a smoother transition through a bridgeopening, eliminating abrupt corners that causeturbulent areas. Recent stream mechanics theorysuggests that bridge abutments should span outsidethe “bankfull” stage of the stream, which“corresponds to the discharge at which channelmaintenance is the most effective, that is, thedischarge at which moving sediment, forming orremoving bars, forming or changing bends andmeanders, and generally doing work that results inthe average morphologic characteristics of thechannels” (Rosgen 1996). Flows above the“bankfull” stage are accommodated with adequatefreeboard through the bridge or overflow channels.Again, spill-through abutments are more efficienthydraulically at higher flood stages, allowing morearea and capacity than a vertical wall abutment(Figure 10).

Figure 10—Independence Creek Bridge, Idaho Panhandle National Forest. Example of a spill through(trapezoidal) abutment configuration. See Figure 9 for an example of a vertical wall abutment with wingwalls.

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Aggradation

Mountainous streams generally will have variablegrades. Many will have steep grades, yet flattenout substantially within a short distance of theirconfluence with a larger river. Aggradation withinthis flatter stream section can be a problem as thestream transports bedload off the steeper grades anddeposits it as velocities slow along the flatter grades.High flows in a larger river where a smaller tributaryjoins can cause back-water in the smaller tributary,which can also cause aggradation. Over time, theaggradation may be balanced by isolated stormevents which will flush (downgrade) out the tributarystream section. However, in the short term, thisaggradation can be a problem to bridges. Continuedaggradation can minimize clearance for debrispassage, cause overtopping or scour damage to thebridge, and approach roadways (Figure 11).

Long-Term Degradation

Another characteristic of mountainous streams islong-term degradation. Steep, incised channels willexperience long-term degradation. When evaluatingbridges for scour vulnerability, long-term degradationshould be a factor. Many typical Forest Servicebridges were built on spread footings with anembedment depth of only a few feet. Today, manyof these bridges are of the age in which these footingswill be exposed or undermined, mainly due to long-term channel degradation (Figure 12).

Contraction Scour

Contraction scour occurs when a channel narrowsand stream velocities increase. Many Forest Servicebridge spans are undersized by today’s standardsand contraction scour is present (Figure 13).

Abutment Scour

Abutment Scour is commonly termed local scour.Local scour involves removal of material fromisolated areas caused by an acceleration of flow pastan obstruction and the subsequent turbulent water(vortices). Local scour is accentuated by debrisbuildup or stream instabilities that shift the streamtowards one abutment or change the angle of attack.The most common locations for local scour on atypical Forest Service single-span bridge with verticalwall abutments is adjacent to the upstream anddownstream corners intersecting the wingwalls.

HEC-18 provides equations for predicting abutmentscour, however, HEC-18 also states that thelaboratory research to date has failed to replicatefield conditions and these equations generally giveexcessively conservative estimates of scour depths.Therefore, engineering judgement is required in theuse of these equations when evaluating or designingabutment foundations for scour.

Figure 11—Eagle Creek Bridge, Idaho Panhandle National Forest. Backwater effects from the main river causedaggradation at the mouth of this tributary and inadequate clearance for the bridge. Note, the following year the tributary

flushed and the stream re-established the normal channel depth.

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Figure 12—Shepherd Creek Bridge, Flathead National Forest. Long-term degradation has exposed the mudsill of thistreated timber bridge with vertical wall abutments.

Figure 13—Vigilante Bridge, Beaverhead-Deerlodge National Forest. This 40 foot bridge constricts the channel andhas caused upstream aggradation and contraction scour through and below the bridge.

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17

Debris

Debris can have a significant impact on bridge scourin a number of ways (Figure 14):

• A buildup of debris can reduce a bridge’s waterwayopening causing contraction scour of the channel.

• A buildup of debris can increase the obstructionarea of a pier or abutment and increase localscour.

• Debris can deflect the flow of the water, changingthe angle of attack, and increase local scour orshift the entire channel around the bridgealtogether.

• Action of water against debris can place asubstantial lateral force on the bridge.

In general, debris is associated with many scourproblems on a typical Forest Service bridge and mustbe considered carefully. However, debris problemsand the associated scour are difficult to anticipateand remedy on existing bridges. New bridge designscan account for potential debris problems byoversizing spans, providing additional freeboard, andminimizing or eliminating piers in the channel. Typicalcountermeasures for an existing bridge with a debrisproblem include:

• Monitoring debris buildup for prompt removal.• Clearing upstream debris.• Installing debris catchers/deflectors.

A debris catcher/deflector on mountainous streamsrequires maintenance and its use must be carefullyconsidered with respect to stream mechanics, sincea catcher/deflector could cause the stream channelto shift, resulting in other scour problems.

Abutment Fill Failures

During many flood events, the structure andfoundations of the bridge will not be damaged, butthe fill behind an abutment will scour (Figure␣ 15).This commonly occurs on a typical Forest Servicesingle-span bridge with vertical wall abutments.Local scour occurs around the wingwalls orundermines the abutment footing and subsequentlyscours the approach fill away. To a user on the road,an abutment fill failure can be just as hazardous asa bridge failure. For this reason, abutment fill failuresdue to scour should be included in determining thescour vulnerability of a bridge.

Figure 14—Irene Bridge over Cascade River, Mt. Baker - Snoqualmie NationalForest, Region 6. Drift build-up on upper side of pier.

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COMMON COUNTERMEASURES

The Federal Highway Administration saw the needto identify common scour countermeasures andprovide guidelines for their use. They recentlypublished HEC-23, Bridge Scour and StreamInstability Countermeasures (Lagasse et al. 1997).HEC-23 provides guidance for scour counter-measure applicability, design, installation, andmaintenance, highlighted by a countermeasurematrix. Many of the countermeasures that will applyto typical Forest Service bridges are included.

Riprap

Within HEC-18, riprap is included under Local ScourArmoring. Riprap is, and probably will remain, oneof the primary scour countermeasures to resist localscour forces at abutments of typical Forest Servicebridges. Riprap is generally abundant, inexpensive,and requires no special equipment. However, properdesign and placement is essential. HEC-18 andHEC-23 provide guidelines for proper sizing andplacement (Figure 16).

When designing riprap countermeasures,maintaining an adequate hydraulic opening throughthe bridge must be considered. Many times,

improperly placed riprap will reduce the hydraulicopening significantly and create contraction scourproblems.

The use of riprap to protect intermediate piers is nowconsidered only a temporary solution. Again, ifplaced improperly, riprap can increase local scourforces.

Spur Dikes, Barbs, Groins, Vanes

Spur dikes, barbs, groins and vanes are consideredriver training structures that alter stream hydraulicsto mitigate undesirable erosional and/or depositionalconditions. They are commonly used on unstablestream channels to redirect stream flows to a moredesirable location through the bridge (Figure 17).

Foundation Strengthening

On a typical Forest Service bridge, foundationstrengthening requires extending the footing deeperto offset long-term degradation, providing additionaltiebacks on a vertical wall abutment if scour hascaused loss of support and the abutments havebegun to “kick in,” or installing a new abutment withdeeper footings or piles.

Figure 15—Monture Bridge, Lolo National Forest. An example of abutment fill failure withlittle damage to the bridge.

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Figure 16—Monture Bridge, Lolo National Forest. Repair of abutment fill failure and use of riprapas a scour countermeasure.

Figure 17—Fisher River Bank Stabilization, Kootenai National Forest. Use of rock vanes toprotect the stream bank from scour.

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LITERATURE CITED

CAESAR, an Expert system for the Cataloging and Expert Evaluation of Scour Risk and River Stability at BridgeSites. University of Washington, Department of Civil Engineering, National Cooperative Highway ResearchProgram, Project 24-6. Transportation Research Board, National Research Council.http://maximus.ce.washington.edu/~scour/.

Colorado Highway Department. 1990. “Colorado Bridge Safety Assurance Procedure.” Ref: 1514. April 1990.

Holnbeck, S.R., and C. Parrett. 1997. “Method For Rapid Estimation of Scour At Highway Bridges Based OnLimited Site Data.” U.S. Geological Survey, Water-Resources Investigations Report 96-4310, pp. 1-3.

Lagasse, P.F., J.D. Schall, F. Johnson, E.V. Richardson, and F. Chang. 1995. “Stream Stability at HighwayBridges, Second Edition.” US Department of Transportation, Publication No. FHWA-IP-90-014. HydraulicEngineering Circular No. 20.

Lagasse, P.F., M.S. Byars, L.W. Zevenbergen, and P.E. Clopper. 1997. “Bridge Scour and Stream InstabilityCountermeasures.” US Department of Transportation, Publication No. FHWA-HI-97-030. Hydraulic EngineeringCircular No. 23.

Richardson, E.V., and S.R. Davis. 1995. “Evaluating Scour at Bridges, Third Edition.” US Department ofTransportation, Publication No. FHWA-IP-90-017. Hydraulic Engineering Circular No. 18, pp. 2, 48.

Rosgen, D. 1996. “Applied River Morphology.” Lakewood, CO: Western Hydrology.

Smelser, M.G., and J.C. Schmidt. 1998. “An Assessment Methodology for Determining Historical Change inMountain Streams.” USDA, Forest Service, Rocky Mountain Research Station, General Technical ReportRMRS-GTR-6, p. 1.

APPENDICES

APPENDIX A

FHWA Scour Report

A-1

REQUIRED FHWA REPORTING ENCLOSURE A

BRIDGE NEEDING UNDERWATER INSPECTION

REGION

DATE

REPORTING CATEGORIES

In Master ListInitial Inspection

CompletedObserved to Have

Structural ProblemsObserved to Have

Scour ProblemsCorrective Actions

Completed

NBICODE

92B

93B

---

---

---

NBIS NON-NBIS*TOTALNUMBER

NUMBER OF BRIDGES

* Includes bridges over waterways which are less than 20’ in length.

Note 1: The total number of bridges needing underwater inspection is for those bridges requiring specialmanpower, techniques or equipment for determining the condition of underwater elements with certainty. Thistotal would not include bridges than can be examined from above by wading, probing, or adequate visualinspection.

REQUIRED FHWA REPORTING ENCLOSURE B

BRIDGES SCOUR EVALUATIONS

REGION

DATE


1. Over waterways

2. Evaluation Total

A. Low Risk Total

B. Scour Susceptible

C. Unknown Foundations

D. Scour Critical

E. Tidal

3. Analyzed for Scour

4. Countermeasures Installed

5. Monitoring Planned

NOTE: LOW RISK TOTALIS INCLUDED ABOVE


2A. Low Risk Total

(1) Calculated or

Assessed

(2) Screened

(3) Culverts

NBIITEM 113

CODE

---

---

4,5,7,8,9

6

U

0 - 3

T

---

7

---

NBIITEM 113

CODE

---

4, 5, &

7-9

6

8

NBIS NON-NBIS*TOTALNUMBER

NUMBER OF BRIDGES

* Includes bridges over waterways which are less than 20’ in length.

A-2

NBIS NON-NBISTOTALNUMBER

NUMBER OF BRIDGES

Notes for Enclosures A and B

The following notes are keyed to the National Bridge Inventory (NBI) as documented in the Recording andCoding Guide for the Structure Inventory and Appraisal of the Nations Bridges, December 1995 (metric version).Item numbers and codes are described in the coding guide. Category number refer to the reporting form categories.

CATEGORY EXPLANATION1 Equals sum of bridges with NBI Item 42B, coded 5-9.2 Equals the sum of Categories 2A, 2B, 2C, 2D, and 2E. This sum should also equal Category 1, Over

Waterways, when screening is complete. A bridge should be included in only one of the categoriesunder 2.

2A Equals the sum of Categories 2A (1), 2A(2) and 2A(3).2A(1) Equals the sum of (a), (b), and (c) described below:(a) bridges assessed during scour screening with code 8 (spread footing on competent rock) code 9

(foundation well above flood elevations)(b) bridges analyzed as stable : codes 4, 5, and 8 and(c) bridges protected by countermeasures, code 72A(2) Bridges assessed as low risk during scour screening activity, code 6. (These bridges are candidates

for scour analysis, but lower priority than category 2B)2A(3) Culverts assessed during scour screening as code 8.2B Bridges assessed during scour screening as requiring scour analysis, code 62C Bridges assessed with unknown foundations, code 62D Bridges analyzed as scour critical, codes 0-33 Total number of bridges that have been analyzed for scour4 Scour critical bridges that have been protected with a structural countermeasure (riprap, paving, etc.)5 Scour critical bridges to be monitored. (structural countermeasures have not been constructed.)4&5 The sum of categories 4 and 5 should equal category 2D, scour critical bridges.

Definitions

Assessed The structure has been screened for obvious conditions and evaluated using engineeringjudgement.

Analyzed The structure has received a full engineering evaluation which includes calculation of hydrology,hydraulics, scour and foundation stability.

A-3

APPENDIX B

Scour Vulnerability Ranking Flow Charts taken from:

Colorado Bridge Safety AssuranceProcedure for Colorado Highway Department

April 1990

Ref. 1514

Pages 15-24

2. Step Two

Ranking the bridges in each category as to scour vulnerability.

a. Scour Vulnerability Ranking Flow Charts.

The ranking of the scour vulnerability of those bridges determined to be scour susceptible, is obtained by flowcharts that evaluate the vulnerability on the basis of the bridges geologic, hydraulic and river conditions as wellas the conditions of the bridges foundation (abutments and piers).

The purpose of the Vulnerability Ranking Flow Charts is to provide a procedure to prioritize the list of scoursusceptible bridges by determining the relative scour vulnerability of all bridges in each scour susceptibilitycategory. The numerical values included in the flow chart were selected to give the relative effect of eachparameter on the potential to produce scour. For example, the river slope/velocity parameter for steep, mediumand mild conditions is valued at “2,” “1” and “0” respectively because a steep slope will produce deeper scourthan a mild slope. The values in each parameter are such that the most scour vulnerable bridge will have thelargest value. More than one bridge can have the same value of vulnerability.

The value of the vulnerability ranking is that it orders a bridge relative to other scour vulnerable bridges, andother things being equal (traffic counts for example) determines what bridge should be repaired or replaced first.

The Scour Vulnerability Ranking has three flow charts. They are: 1) General Considerations, 2) Abutments and3) Piers, which proceed sequentially. It is expected that field evaluation of the bridge will be required to completethe ranking.

b. General Conditions Flow Chart.

The General Conditions Flow Chart addresses parameters that have a general impact on the potential scourdepth. The need for intermediate scour countermeasures is included in the flow chart to remind the evaluatorto identify this need. No vulnerability ranking value is assigned to this parameter because it is expected that thecountermeasures will be implemented before the detailed scour evaluation and installation of remedial measureis complete. The intermediate scour countermeasures are intended to protect the bridge from catastrophicfailure until the design and construction of remedial measures is completed.

The remaining parameters are included for the following reasons:

(1) River Slope/Velocity - A steeper/faster flowing stream is expected to experience more severe scour thanone with a medium or mild slope. The stream slope is defined as follows:

i) Steep S>0.0015 ft/ftii) Medium 0.0015 < S > 0.0004 ft/ftiii) Mild S < 0.0004 ft/ft

(2) Channel Bottom - An aggrading condition is given a value of 0 because the slight deposition representedreflects a decrease in scour potential. Severe deposition that restricts capacity is addressed later in theflow chart. A stable channel condition is, therefore, given a value of 1 because it represents a morescour prone condition than aggradation. Similarly, a degrading channel is given a value of 2.

(3) The channel bed material are ranked because rock would take more time to erode to maximum scourthan sand. The other material also would take more time. Thus, in ranking bridges to scour vulnerabilitythe bridge that takes longer for scour to reach its maximum value would be less vulnerable.

(4) Channel Configuration - A meandering or braided channel is given a value of 2 because they have themost potential to have scour problems. A straight channel, defined as exhibiting a sinuosity of less than1.5, is given a value of 0 because it is the least likely to affect scour. However, if a straight channel hasbar formations that shift the thalweg, it should be given a values of 1.0.

(5) Debris/Ice Problem - Watershed, river conditions or pier and abutment configurations that promote debris

B-1

B-2

and ice accumulation, primarily as indicated by historic records or field observations, warrant a value of 1because the accumulation increases potential scour depth by either reducing the conveyance area or byincreasing the effective pier width.

(6) Near River Confluence - The potential for increased flow and river velocity near a river confluence and theresultant scour potential, warrants use of the value of 1 for this condition.

(7) Affected by Backwater - Locations affected by backwater for all flow conditions, primarily resulting fromproximity to a dam, warrants use of a value of 0. For this condition backwater from a downstreamwaterway should not be considered because it may not occur concurrently with peak flow and velocity onthe tributary and at the location being studied.

(8) Historic Scour Depth - Historic scour indicates a clear potential for continued and increased scouractivity. Historic scour depths in excess of 3’ are a concern because spread footings are seldom deeperthan this.

(9) Historic Maximum Flood Depth - Flow depth is a parameter in the scour prediction equations. Deeperflow is expected to produce greater scour.

(10) Adequate Opening - An inadequate opening is expected to produce greater scour than a restricted one,therefore, a value of 2 is assigned to this condition. This parameter also addresses the deposition ofmaterial in the channel at the structure to the point that the capacity of the bridge opening is restricted.Bridges that experience overtopping and thus have pressure flow should also be given a 2.

(11) Overflow/Relief Available - The ability of the design flow to proceed downstream by a means other thanthrough the structure, usually by way of a relief structure or by overtopping the roadway embankment,reduces the scour potential at the structure being evaluated because the resultant discharge and velocityare less than would otherwise be the case.

(12) Simple Spans - This parameter recognizes that the ramifications of scour at simple span structures ismore severe than would occur for structures with alternate load paths that probably would not experiencecatastrophic failure due to the loss of some foundation material.

The sum of the vulnerability ranking scores is tabulated at the bottom of the form before proceeding to theabutment vulnerability ranking flow chart.

Figure B-1—General Conditions Scour Vulnerability Ranking Flow Chart

Bridge # Feature Carried Stream

Community County

Bridge Type Spans

General ConditionsScour Vulnerability Ranking Flow Chart

River Slope/Velocity

Medium1

Mild0

Steep2

Channel Bottom

Aggrading0

Degrading2

Stable1

Channel Configuration

Meandering2

Braided3

Straight0

Channel Bed Material

Cobbles2

Glacial Till3

Sand4

Boulders1

Rock0

Historic Scour Depth

Medium1'-3'

2

Large>3'3

Small =<1'

1

None

0

Historic Maximum Flood Depth

Adequate Opening

10'-19'2

20'-40'3

> 40'4

5'-9'1

=< 5'0

Intermediate Scour Counter Measures RequiredNo

Debris/Ice Problem

Near River Confluence

Yes1

No0

Yes1

No0

Yes1

No0

Yes1

No0

Yes1

No0

Yes1

No0

Effected by Backwater

Overflow/Relief Available

Simple Spans

Implement ScourCounter Measures

Yes

General ConditionVulnerablility Score____________(Proceed to Abutment Scour Vulnerability

Ranking Chart)

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B-3

c. Abutment Vulnerability Flow Chart.

The abutment vulnerability assessment flow chart is intended to evaluate the relative vulnerability of a bridge toscour considering factors that affect abutment scour. A separate evaluation is provided for each abutmentbecause the scour producing parameters may vary at each one, although it is expected that the abutmentfoundation configuration will remain the same. The left and right directions are established looking downstream.The parameters evaluated in the abutment vulnerability ranking flow chart reflect their relative effect on scourvulnerability as discussed for the office review flow chart. The rationale for their use follow:

(1)Scour Countermeasures - Installation of a wall or spur dike (guide bank) represent a relatively permanentcountermeasure and are, therefore, provided the lowest value. Riprap and other countermeasures are con-sidered temporary and are, therefore, given a higher value. The absence of scour countermeasures warrantassignment of the highest value. Location that do not require scour countermeasures, as indicated in thegeneral conditions flow chart, should be given a value of 0 for this parameter.

(2)Abutment Foundation - The value assigned to each classification of abutment configuration and foundationtype reflects their relative susceptibility to scour as discussed for the office review flow chart.

(3)Abutment Location on River Bend - An abutment located on the outside of a bend is more susceptible to scourthan one on the inside of the bend or one on a straight channel and is, therefore, given a higher value than theother conditions.

(4)Angle of Inclination - The angle of inclination is determined in accordance with Figure 4.11 of the TechnicalAdvisory. Relative values are assigned to each range of angles.

(5)Embankment Encroachment - The magnitude of the scour encroachment is reflected in most of the abutmentscour equations, therefore, this parameter is included in the chart. A large encroachment would be consid-ered one that substantially reduces the overbank flow area available for the conveyance of peak discharges.A small encroachment would be considered one that impacts less than 10 percent of the total discharge forthe design discharge.

The abutment vulnerability score for each abutment is tabulated and summarized at the bottom of the form. Theintermediate vulnerability score from the general conditions flow chart is also tabulated and added to the totalabutment score to yield the subtotal, which is the final score, if the bridge does not have any piers. Thepresence of piers necessitates continuation of the evaluation by proceeding to the pier vulnerability rankingflow chart.

B-4

Figure B-2—Abutment Scour Vulnerability Ranking Flow Chart

B-5

Bridge # Feature Carried StreamCommunity CountyBridge Type Spans

Abutment Scour Vulnerability Ranking Flow Chart

Left Abutment

Scour Countermeasures

Abutment Foundation (Left)

Spur

0

Other

1

None

2

Wall

0

Riprap

1

VerticalWall Long

Piles>20' Wood

3

VerticalWall LongPiles >20' Not Wood

2

Spill thruSpread

UnknownWood or

Short Piles

1

Spill thruOther

0

VerticalWall

Short Piles<19'

4

VerticalWall

SpreadUnknown

5

VerticalWall Long

Piles>20' Wood

3

VerticalWall LongPiles >20' Not Wood

2

Spill thruSpread

UnknownWood or

Short Piles

1

Spill thruOther

0

VerticalWall

Short Piles<19'

4

VerticalWall

SpreadUnknown

5

20-44

2

45-90

3

>90

4

0-19

1

0

0

Large

2

Medium

1

Small

0

Abutment Location on River Bend

Angle of Inclination (Degrees)

Inside0

Outside1

Embankment Encroachment

Left Abutment Vulnerablility Score____________

Right Abutment


Abutment Foundation (Right)

Spur

0

Other

1

None

2

Wall

0

Riprap

1

20-44

2

45-90

3

>90

4

0-19

1

0

0

Large

2

Medium 1

Small0

Abutment Location on River Bend

Angle of Inclination (Degrees)

Inside0

Outside1

Embankment Encroachment

Right Abutment Vulnerablility Score____________

Abutment Scour Vulnerability Left Abutment____________ Right Abutment____________ Total____________General Conditions Vulnerability Score Total____________

Proceed to Pier Scour Vulnerability Ranking Flow Chart if Necessary

Subtotal____________(Final score if there are points)

Left and Right are established looking downstream

R9800146

B-7

Figure B-3—Pier Vulnerability Ranking Flow Chart

Bridge # Feature Carried Stream

Community County

Bridge Type Spans

Pier Vulnerability Ranking Flow Chart

R9800148


Cofferdam0

Other1

None2

Wall0

Riprap1

Pier Foundation

Piles0

Spread/Unkown1

15-201

>200

10-142

3-54

6-93

<35

Skew Angle (Degrees)

0-91

10-202

>203

00

Pier Width

>104

8-93

3-41

5-72

<30

Pier #1 Vulnerability

Score: __________

Pier #1

Pier/Pile BottomBelow Streambed


Cofferdam0

Other1

None2

Wall0

Riprap1

Pier Foundation

Piles0

Spread/Unkown1

15-201

>200

10-142

3-54

6-93

<35


0-91

10-202

>203

00

Pier Width

>104

8-93

3-41

5-72

<30


Score: __________

Pier #2



Cofferdam0

Other1

None2

Wall0

Riprap1

Pier Foundation

Piles0

Spread/Unkown1

15-201

>200

10-142

3-54

6-93

<35


0-91

10-202

>203

00

Pier Width

>104

8-93

3-41

5-72

<30


Score: __________

Pier #3



Cofferdam0

Other1

None2

Wall0

Riprap1

Pier Foundation

Piles0

Spread/Unkown1

15-201

>200

10-142

3-54

6-93

<35


0-91

10-202

>203

00

Pier Width

>104

8-93

3-41

5-72

<30


Score: __________

Pier #4


Pier Vulnerability Ranking Score SummaryPier #1_____ Pier #2_____ Pier #3_____ Pier #4_____

Pier with maximum score: Pier #______

Subtotal from abutment scour vulnerability:

Total Vulnerability Score:

B-6

d. Pier Vulnerability Flow Chart.

The pier vulnerability assessment flow chart is intended to evaluate the relative vulnerability of a bridge to scourconsidering factors that affect pier scour. A separate evaluation is provided for each pier because the scourproducing parameters may vary at each one. The piers are numbered sequentially from the left abutment, withthe left side established looking downstream.

The parameters evaluated in the pier vulnerability ranking flow chart reflect their relative effect on scour. Therationale for their use follows:

(1) Scour Countermeasures - The rationale is the same as presented for the abutment flow chart.

(2) Pier Foundation - A spread footing or unknown foundation condition warrants a higher value than a pilefoundation.

(3) Skew Angle - The skew angle ranges reflect the relative effect on scour potential as indicated in Table 4.3of the Technical Advisory (FHWA 1987).

(4) Pier/Pile Bottom Below Streambed - This parameter reflects the relative susceptibility to scour based onthe depth of the footing or pile bottom to the streambed elevation. The highest value is assigned to adepth of three feet or less because this is the normal depth of spread footings. Deeper footing or pilebottom elevations warrant lower ranking values. Depths greater than twenty feet are arbitrarily assignedthe lowest value.

(5) Pier Width - The pier width reflects the maximum expected scour in accordance with pier scour questionsas indicated in the Technical Advisory. The range of three to five feet in the pier width represents thenormal dimensions expected. No adjustment for debris or ice accumulation is used here because it isreflected in the general conditions flow chart.

The pier vulnerability score is tabulated for each pier evaluated. The values are summarized and the value ofthe most vulnerable pier added to the subtotal from the abutment vulnerability flow chart to determine the totalvulnerability score.

APPENDIX C

FOREST SERVICE SCOUR EVALUATION PROCESS

PLAN OF ACTIONRegion___________________________________

Forest ___________________________________

Route ID & MP __________________________

Name ___________________________________

Feature Crossed _________________________

NBIS / Non-NBIS ________________________

BMC / INFRA Codes

Scour Critical ____________

Scour Vulnerability ________

Description of Bridge Vulnerability____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Recommended Scour Countermeasures____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Implementation Plan_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Bridge Closure Plan______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

C-1