Bridge Scour-Europe Scour

TRANSPORTATION RESEARCH BOARDNATIONAL RESEARCH COUNCIL

National Cooperative Highway Research Program

RESEARCH RESULTS DIGESTJuly 1999—Number 241

Subject Area: IIC Bridges, Other Structures, Responsible Senior Program Officer: Edward T. HarriganHydraulics and Hydrology

1998 Scanning Review of European Practice for Bridge Scourand Stream Instability Countermeasures

This digest summarizes the findings of an international technology scanning review conducted with the support ofNCHRP Project 20-36, “Highway Research and Technology—International Information Sharing.” The scanning

review team consisted of representatives from U.S. federal and state agencies as well as the private sector.Peter F. Lagasse, Report Facilitator for the review, prepared the digest.

SUMMARY

INTRODUCTION

This digest describes the findings of the 1998International Scanning Review of European Prac-tice for Bridge Scour and Stream Instability Coun-termeasures organized under the auspices ofFHWA’s International Outreach Program and theAmerican Association of State Highway andTransportation Officials (AASHTO) through theNational Cooperative Highway Research Program(NCHRP). This review was undertaken to reviewand document innovative techniques used to miti-gate the effects of scour and stream instability atbridges, evaluate these techniques for potential ap-plication in the United States, and share informa-tion on U.S. practice with European counterparts.

This review was performed by a team of repre-sentatives from FHWA, the state DOTs of Califor-nia, Illinois, Maryland, Minnesota, Oregon, andSouth Carolina, universities, and the private sec-tor. The review included visits to highway researchinstitutes, hydraulic research laboratories, and fieldsites in Switzerland, Germany, the Netherlands,and the United Kingdom.

Although the review concentrated on bridgescour and stream instability countermeasures, thescanning review team members’ inquiries werewide-ranging and included basic scour technologyfor evaluation and design, laboratory and field re-

search programs, environmental issues, and bio-engineering techniques.

GENERAL OBSERVATIONS

Design Philosophy

The general design approach in Switzerland,Germany, and the Netherlands is to prevent scourfrom occurring or move scour away from the struc-ture by including scour and stream instability coun-termeasures in the initial design and construction oftheir bridges. In general, these countries believethat they do not have a significant bridge scour prob-lem, largely because of this design approach. Thebridge scour problem in the United Kingdom issimilar to that faced in the United States. Both coun-tries are applying the latest scour technology to thedesign of new bridges, but both face significantproblems with scour and stream instability at thou-sands of bridges in an aging infrastructure.

Risk Analysis

Some form of risk analysis is used to deter-mine the level of effort and investment in counter-measure design and installation in all countries vis-ited. For example, the Netherlands uses both aregional risk analysis (considering such factors asdistance from the coast and vulnerability to storm

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CONTENTS

SUMMARY, 1

CHAPTER 1 Introduction, 61.1 Background, 61.2 Purpose, 71.3 Scanning Review Team Members, 81.4 Itinerary and Approach, 81.5 Report Format, 9

CHAPTER 2 Observations and Discussion, 92.1 General Observations, 92.2 Observations on Specific Countermeasures, 18

CHAPTER 3 Applications to U.S. Practice, 253.1 Techniques to Reduce Bridge Scour and to Enhance Stream Stability, 263.2 Countermeasure Techniques for Bridge Foundations, 263.3 Techniques to Address Environmental Issues, 26

CHAPTER 4 Recommendations and Implementation Plan, 274.1 High-Priority Recommendations, 274.2 Implementation Plan, 28

APPENDIX A Organizations and Contacts, 29APPENDIX B Glossary of Acronyms and Abbreviations, 30APPENDIX C References, 31APPENDIX D Design Methodology, 33

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surge) and a detailed analysis of the risk of failure of indi-vidual components of a project. These factors influence thedecision on investment in scour countermeasures.

Environmental Policy

Environmental impacts are considered for scour and streaminstability countermeasure selection, design, and installation inall countries visited. In general, the approach is to emphasizeenvironmental enhancement and sustainability, without creat-ing an undue risk to lives and property in applying environmen-tal policy to structures in a riverine system.

River Geomorphology

All four countries in Europe recognize the value of ageomorphic analysis in bridge and countermeasure design.In the United Kingdom, in particular, research in applyinggeomorphic reconnaissance techniques to river engineeringproblems has produced useful and practical guidance for thehydraulic engineer. Such techniques support geomorphicclassification of a river system and permit a detailed investi-gation of form and process for critical reaches where insta-bility could affect bridge design or countermeasure selec-tion, design, and maintenance.

Scour Prediction

Although investigating improved scour prediction tech-niques was not the primary purpose of the scanning review,methods to calculate scour, particularly in complex flowsituations, were of interest to the scanning review team mem-bers. In Europe, the influence of turbulence in relation tothe structure (e.g., bridges and storm surge barriers) and thetime rate of scour are considered key factors for estimatingscour potential and designing scour countermeasures.

The problems of estimating scour at wide piers, the timerate of scour (particularly in cohesive materials) and theinteraction of the various scour components are recognizedas being among the most pressing U.S. research needs inscour. Comprehensive scour manuals obtained by the scan-ning review team members include techniques to analyzeseveral of these problems.

Modeling

Both physical hydraulic modeling in a laboratory andnumerical computer modeling are among the standard tech-niques available to analyze the scour problem and designcountermeasures. In Europe it is much more likely thatphysical modeling, often in conjunction with computer mod-eling, will be used as an integral part of the hydraulic designprocess for bridge foundations and countermeasures than istypical in the United States.

A major effort is underway in Europe to develop 1-, 2-,and 3-dimensional computer models with hydrodynamic,

sediment transport, and, in some cases, morphologic capa-bilities. As an initial reaction, it appears that U.S. 1- and 2-dimensional hydrodynamic modeling capabilities to supportscour predictions and countermeasures design are compa-rable with what is currently available in Europe.

Inspection and Monitoring

Most of the countries visited have initiated efforts todevelop a bridge inspection or scour evaluation programcomparable to the National Bridge Inspection Standards(NBIS) in the United States. In Germany, the Federal High-way Research Institute (BAST) is investigating the use ofthe FHWA PONTIS bridge management system.

An effort comparable to NCHRP Project 21-3 has beenmade in the United Kingdom to develop fixed instrumenta-tion for measuring and monitoring scour at bridge piers andabutments. This resulted in the patented Wallingford “Tell-Tail” device, which was installed on several railroad bridgesfollowing a catastrophic railroad bridge failure.

In none of the four countries was technology availableto determine the characteristics of unknown bridge founda-tions (i.e., foundations for which design or as-built drawingsdo not exist). In the United Kingdom, there are numerousunknown foundation bridges and the problem is consideredas serious as it is in the United States.

OBSERVATIONS ON SPECIFICCOUNTERMEASURES

Riprap

The use of riprap (i.e., armor stone in combination witha geotextile or granular filter) is by far the most commonscour and stream instability countermeasure in all countriesvisited in Europe. Its availability, economy, ease of installa-tion, and flexibility are considered highly desirable charac-teristics in all four countries visited. As a result, consider-able effort has been devoted to techniques for determiningsize, gradation, layer thickness and horizontal extent, filters,and placement techniques and equipment for revetment andcoastal applications. European hydraulic engineers considerriprap an effective and permanent countermeasure againstchannel instability and scour, including local scour at bridgepiers.

Great care is taken in placing the riprap at critical loca-tions, and, in many cases, stones are placed individually inthe riprap matrix. Highly specialized equipment has beendeveloped by construction contractors in Europe for placingriprap, particularly for coastal installations. The use of bot-tom dump or side dump pontoons (barges) is common inboth Germany and the Netherlands. Some of the smallerpontoon systems, particularly the bottom dump pontoonsdeveloped in Germany, could be used to place riprap in wa-ter at larger bridges.

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At the BAW in Germany, the scanning review team mem-bers observed wave tank testing of prototype scale partiallygrouted riprap. In general, the objective is to increase the sta-bility of the riprap without sacrificing all of the flexibility. Con-tractors in Germany have developed techniques and equipmentto achieve the desired grout coverage and the right penetration.Current guidance in the United States tends to discourage theuse of grouted riprap. However, BAW engineers believe thatpartial grouting, if done correctly, will ensure that the riprapretains sufficient flexibility while enhancing stability.

Filters

As in the United States, a properly designed geotextile orgranular filter is considered essential to the success of riprapand most other countermeasures on sand or fine-grained ma-terial. In Germany and the Netherlands, a significant invest-ment has been made in the development and testing ofgeosynthetic materials, and innovative installation techniqueshave been developed that could find application for bridgepier and abutment countermeasures in the United States.

Geotextile containers (large sand bags) made of me-chanically bonded non-woven fabrics up to 1.25 cubic m involume have been used to provide a filter layer for riprapinstallation at several large projects in Germany. The con-tainers are placed in layers using a side-dump pontoon.Riprap is then placed over the layer of geotextile containers.A geotextile bag filter and riprap protection were used incombination as a countermeasure against pier scour at a newbridge on the Peena River in Germany.

Three countries (i.e., Germany, the Netherlands, andthe United Kingdom) use fascine mats, a very old, tradi-tional approach for scour protection, to place a geotextilefilter in deep water. The fascines consist of a matrix ofwillow or other natural material woven in long bundles (15to 20 cm in diameter) to form a matrix assembled over alayer of woven geotextile. The fascine mattress, sometimescalled a “sinker mat,” is floated into position and sunk intoplace by dropping riprap-size stone on it from a barge.

River Training

River training and stabilization techniques against lat-eral channel migration in the major navigable waterways ofEurope are similar to those employed by the U.S. ArmyCorps of Engineers on navigable waterways in the UnitedStates (e.g., the Mississippi, Ohio, and Missouri river sys-tems). Groins and jetties projecting roughly perpendicularto the river bank, dikes placed parallel to the river bank, orrevetment placed on the river bank are the most commonriver training works in Europe.

Given that river training has been ongoing on Europe’snavigable rivers (or on canals in the Netherlands) for hun-dreds of years, there are few unprotected reaches of river.Thus, lateral instability because of river meander is rare andis not considered a threat to bridges.

Riverbed Degradation

Sills, grade control structures, low check dams, or weirsconstructed of various materials are commonly used in Eu-rope to protect against vertical channel instability (degrada-tion) as they are in the United States. In Germany, theapproach to the problem of degradation on the Rhine Riverhas involved sediment management on a large scale. Theproblems are generally related to a deficiency in the supplyof sediment to a river reach or river system. As a result, asystemwide sediment management program has evolvedthat involves as one component, an attempt to replenish thesediment supply by “feeding the Rhine.”

The Swiss also recognize the effects of sediment defi-ciency on river system stability. Prior to 1970, gravel min-ing (or harvesting) from rivers was allowed in Switzerland,but when scour problems were noted in adjacent reaches,the practice was restricted. Currently, the allowed quantityof gravel extracted is fixed on the basis of the sedimentregime of the river.

Alternative Countermeasures

Among the areas of particular interest to the scanningreview team members during the scanning review were al-ternative countermeasures such as flow-altering devices oralternatives to riprap (particularly for the pier scour prob-lem). The following paragraphs summarize some of thescanning review team members’ observations.

In 1987, the Swiss experienced a near catastrophic fail-ure of a major highway bridge when the Reuss River mi-grated laterally and undermined the foundation of a bridgepier. The countermeasure system developed by the VAW/ETH laboratory included very large precast concrete prisms,triangular in cross section, placed individually as revetment.In lieu of smaller interlocking armor units that would becostly to fabricate, the decision was made to cast much largerprisms with a simple shape and use the mass of the prisms toprotect against river bank scour. The economics of thetradeoffs between smaller, high-cost interlocking shapes forartificial riprap and simpler shapes with more mass are worthfurther consideration.

Recent laboratory testing by NCHRP, FHWA, and oth-ers in the United States indicates that when articulating matproducts are used as a pier scour countermeasure, the jointbetween the mattress and the pier must be protected to pre-vent scour under the mat. The scanning review team mem-bers encountered two approaches to solving this problemthat justify further evaluation.

Recent laboratory research by NCHRP, FHWA, and oth-ers in the United States has shown that flow-altering devices(e.g., scour collars, sacrificial piles, and guide vanes) are onlymarginally effective as countermeasures against pier scour.The scanning review team members did not encounter anysuccessful applications of flow-altering devices as a pier scourcountermeasure in any of the countries visited.

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Protecting bridges from the accumulation of debris andpredicting the increase in scour at a bridge caused by debris isa problem worldwide. The scanning review team membersdid not encounter any applications of “debris deflectors” orother devices at a bridge during the scanning review.

Bioengineering

Although bioengineering techniques are integrated withtraditional engineering countermeasures for river system man-agement in Europe, hydraulic engineers in all the countriesvisited would not recommend reliance on bioengineeringcountermeasures as the only countermeasure technique ifthere is risk of damage to property or a structure or if there isthe potential for loss of life. A primary concern expressedwas a lack of knowledge about the properties of the materialsbeing used in relation to force and stress generated by flowingwater and the difficulties in obtaining consistent performancefrom countermeasures relying on living materials.

APPLICATION TO U.S. PRACTICE

The scanning review team members identified severalpotential European bridge scour techniques that could im-prove U.S. practice. These techniques should be consideredfurther by appropriate research funding agencies (e.g., TRB,NCHRP, FHWA, and state DOTs and other bridge owners)or agencies such as FHWA or AASHTO that establish trans-portation policy, code, guidelines, and specifications.

Techniques To Reduce Bridge Scourand To Enhance Stream Stability

• Conduct a thorough review of European literature onbridge scour and stream instability technology, particu-larly the comprehensive scour manuals obtained duringthe scanning review.

• Encourage increased use of risk analysis in the designand evaluation process including accepting a variabledegree of protection depending on the importance ofthe structure.

• Adapt stream reconnaissance techniques to the evalua-tion of stream stability in the vicinity of highway struc-tures, and continue to encourage a geomorphic approachfor stream system analysis, bridge design, and counter-measure selection.

• Improve techniques to analyze and predict scour, par-ticularly for complex flow situations (e.g., wide piers,pressure flow, debris, and the interaction of general andlocal scour components) by a more detailed evaluationof European practice.

• Investigate the characteristics of time rate of scour innon-cohesive and cohesive materials.

• Consider the applicability of sediment management as astrategy to counteract long-term riverbed degradationproblems.

Countermeasure Techniques for Bridge Foundations

• Evaluate the economics of including scour and streaminstability countermeasures in the initial construction ofa bridge.

• Re-evaluate design and installation techniques forriprap, and reconsider its viability as a permanent coun-termeasure against pier scour.

• Evaluate and test European techniques for the design andinstallation of partially grouted riprap, and re-evaluate itsapplicability to United States practice.

• Evaluate and test the use of innovative techniques for plac-ing filters under riprap and other countermeasures, includ-ing geotextile containers, geotextile mattresses, the use offascine mats, and hydrodynamically sand tight filters.

• Investigate the economics of tradeoffs betweensmaller, high-cost interlocking shapes for artificialriprap and simpler shapes with more mass to resisthydraulic stress.

• Evaluate and test European techniques to prevent scourat the “joint” between articulating mattresses and abridge pier when these products are used as a pier scourcountermeasure.

Techniques To Address Environmental Issues

• Consider risk to the structure, lives, or property in ap-plying environmental policy to bridge scour protectionand countermeasures.

• Evaluate and test bioengineering and biotechnical engi-neering techniques as bridge scour countermeasures forsituations where public safety considerations would notpreclude their use.

RECOMMENDATIONS

The scanning review team members recommend thatseveral elements of European practice be considered on ahigh-priority basis to improve U.S. capabilities to deal withstream instability and bridge scour problems. An imple-mentation plan is also suggested to ensure that the technol-ogy acquisition activities initiated by the scanning reviewwill continue and will be disseminated to bridge owners andtheir engineering staff.

High-Priority Recommendations

Riprap and Filters

Currently, policy in the United States considers riprapplaced at bridge piers to be only a temporary countermea-

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sure against pier scour, and guidance dictates that riprapplaced at bridge piers must be monitored by periodic inspec-tion or with fixed instruments. During the scanning review,it was apparent that the European counterparts considerriprap as a permanent pier scour countermeasure. The dif-ference between U.S. and European practice is not necessar-ily derived from the availability of better techniques for siz-ing riprap, but rather from the higher standard of care andquality control in placing the stone and providing an appro-priate filter on sandbed channels. In addition, Europeanpractice includes inspection and monitoring to verify thatriprap is performing properly. European hydraulic engi-neers have developed innovative techniques for placing aneffective filter beneath the riprap in flowing or deep water;these techniques include the use of such products as largegeotextile sand containers, geotextile mattresses filled withgranular filter material, and fascine sinker mats.

As state DOTs in the United States develop Plans ofAction for their scour-critical bridges, improved techniquesto use riprap effectively as a pier scour countermeasure couldresult in significant savings, particularly where the only al-ternative may be rehabilitation or replacement of the af-fected bridge. A high-priority evaluation of European prac-tice for the design and installation of riprap with anappropriate filter as a permanent pier scour countermeasureis warranted.

Partially Grouted Riprap

Current practice in the United States discourages theuse of grouted riprap, primarily because total grouting con-verts a flexible revetment material into a rigid mass suscep-tible to undermining and failure. Ongoing tests in Germanyat BAW, experience on German inland waterways, and de-velopment of design guidance for partial bituminous andcement grouted riprap in the United Kingdom indicate thatdesign guidelines and installation experience are availableor are being developed in Europe. These European designguidelines, specifications, and installation techniques forpartially grouted riprap should be investigated on a high-priority basis

Risk Analysis

The increased use of risk analysis in countermeasureselection and design and the use of techniques such as faulttree analysis could result in more economical design ofbridge scour countermeasures. These concepts should beevaluated and disseminated, as appropriate, to bridge own-ers in the United States.

Scour Prediction

In the United States, the problems of estimating scour atwide piers, the time rate of scour in cohesive and non-cohesive materials, and the interaction of the various scour

components are among the most pressing U.S. researchneeds in scour. Several scour manuals that provide a com-prehensive treatment of the European approach to theseproblems were obtained by the scanning review team mem-bers. A detailed review of the Dutch Scour Manual (8) andother comprehensive treatments of the scour process is war-ranted.

Update of the FHWA HECs

In the United States, bridge scour technology is con-tained primarily in three FHWA HECs:

• HEC-18 Evaluating Scour at Bridges• HEC-20 Stream Stability at Highway Structures• HEC-23 Bridge Scour and Stream Instability Counter-

measures

FHWA is revising and updating these manuals, withdraft revisions scheduled for completion in October 1999.The scope of work for these revisions includes reviewingand evaluating the European literature on bridge scour andstream instability obtained by the scanning review teammembers during the scanning review. The FHWA Hydrau-lic Engineering Circulars and NHI training courses are themost efficient means of disseminating new technology tostate DOTs and other bridge owners. Information gainedfrom the countermeasures scanning review on Europeanpractice that does not require further research or laboratoryor field testing should be incorporated into the current revi-sions of the HECs and training course materials.

Other activities to continue the technical contacts withcounterparts in Europe and disseminate information gainedduring the scanning review are outlined in the Implementa-tion Plan.

Implementation Plan

During the final scanning review team meeting, initialsteps were taken to develop an implementation plan. Sincereturning from the scanning review, several implementationactivities have been completed and others are being planned.The final section of this report summarizes these activitiesand suggests other implementation actions that should beconsidered for the future.

CHAPTER 1

INTRODUCTION

1.1 BACKGROUND

Of the more than 575,000 bridges in the national bridgeinventory, approximately 84 percent are over water. Eachyear in the United States, highway bridge failures cost mil-

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lions of dollars as a result of both direct costs necessary toreplace or restore bridges and indirect costs related to dis-ruption of transportation facilities. Of even greater conse-quence is the loss of life from bridge failures. Hydraulicfactors (e.g., stream instability, degradation, contractionscour, and local scour) account for more U.S. bridge failures(approximately 60 percent) than all other factors combined.

Ongoing screening and evaluation of the vulnerabilityof the nations’ highway bridges to scour by state DOTs haveidentified more than 18,000 bridges considered scour-criti-cal (i.e., the bridge foundation is unstable for the calculatedor observed scour condition) and in need of repair or re-placement. Almost 100,000 bridges with unknown founda-tions have been identified. As state DOTs develop Plans ofAction to identify adequate countermeasures for scour andstream instability, innovative, effective, and economicalcountermeasures should be considered for the design of newbridges and for repairing existing bridges.

Although considerable research has been done on thedesign of countermeasures for stream instability and scourproblems in the United States, many have evolved through atrial-and-error process. With the publication of HydraulicEngineering Circular 23 (HEC-23) (1997) (1), FHWA tookthe initial step toward sharing countermeasure experience,selection, and design guidelines among Federal, state, andlocal highway agency personnel through the development ofa countermeasure matrix. Although the scour countermea-sure matrix will serve as FHWA interim guidelines on scourcountermeasures for bridge owners to use in protectingbridge foundations from scour, the matrix represents, prima-rily, practice in the United States. There is a need to reachout to other countries to identify and evaluate countermea-sures being used by bridge owners for potential implemen-tation in the United States.

With this in mind, FHWA, AASHTO, and TRB spon-sored a scanning review of European practice for bridge scourand stream instability countermeasures in October 1998. Thecountries selected—Switzerland, Germany, the Netherlands,and the United Kingdom—include a wide range of hydro-logic, hydraulic, and geomorphic settings. Thus, the scanningreview team members were able to observe and evaluatebridge scour and stream instability countermeasures on steep,coarse bed mountain streams (Switzerland); lower gradient,larger rivers (e.g., the Rhine in Germany); and tidal scourproblems in the coastal zone (e.g., bridges and storm surgebarriers in Germany and the Netherlands). In the United King-dom, the scanning review team members were able to interactwith researchers and practitioners, dealing with scour andstream instability problems on an aging rail and highway in-frastructure where efforts are underway to determine the scopeof the problem and evaluation, inspection, and design tech-nology are being developed.


• HEC-18 Evaluating Scour at Bridges (1995) (2)• HEC-20 Stream Stability at Highway Structures (1995)

(3)• HEC-23 Bridge Scour and Stream Instability Counter-

measures (1997) (1)

In addition, results of NCHRP Project 21-3 to develop,test, and evaluate fixed instrumentation that would be bothtechnically and economically feasible for use in measuringor monitoring maximum scour depth at bridge piers andabutments are presented in three NCHRP reports:

• NCHRP Report 396, “Instrumentation for MeasuringScour at Bridge Piers and Abutments” (1997) (4)

• NCHRP Report 397A, “Sonar Scour Monitor” (1997)(5)

• NCHRP Report 397B, “Magnetic Sliding Collar ScourMonitor” (1997) (6)

Copies of the HEC manuals and NCHRP reports wereprovided to the host agency in each country visited as abasis for technology exchange and as an indication of cur-rent practice in the United States.

1.2 PURPOSE

The scanning review included visits to highway researchinstitutes, hydraulic research laboratories, and field sites inthe four countries visited. Scanning review objectives wereas follows:

• Review and document innovative techniques used tomitigate the effects of scour and stream instability atbridges,

• Evaluate these techniques for potential application inthe United States, and

• Share information on U.S. practice with counterparts inthe countries visited.

Although the review concentrated on bridge scour andstream instability countermeasures, the scanning reviewteam members’ inquiries were wide ranging and includedbasic scour technology for evaluation and design, laboratoryand field research programs, environmental issues, andbioengineering techniques.

In preparation for the scanning review, the scanningreview team members developed a list of amplifying ques-tions which were provided to the countries visited, to high-light the scanning review team members’ areas of interest.This list (Appendix A of the Final Report, not includedherein) included questions on the following topics:

• Bridge scour and stream stability technology (includingbasic technology, design, and inspection/training).

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• Countermeasures for bridge scour and stream instability(including bridge scour countermeasures, stream insta-bility countermeasures, flow-altering devices, rock riprapand alternatives, and instrumentation/monitoring),

• Laboratory and field research (including research pro-grams, laboratory research, and field (site) research)and,

• Environmental issues and bioengineering.

1.3 SCANNING REVIEW TEAM MEMBERS

The scanning review involved representatives fromFHWA, state DOTs (i.e., California, Illinois, Maryland,Minnesota, Oregon, and South Carolina), universities, andthe private sector. The Scanning review team included aState Bridge Engineer and three members of the AASHTOTask Force on Hydraulics and Hydrology. The list of scan-ning review team members follows:

Don Flemming State Bridge Engineer Minnesota(Co-Chairman) Department of Transportation

Jorge E. Pagan-Ortiz Hydraulics Engineer Office of(Co-Chairman) Engineering, Bridge Division,

FHWA

Catherine Avila Senior Bridge EngineerCALTRANS

Jean-Louis Briaud Professor, Department of CivilEngineering Texas A and MUniversity

David W. Bryson Hydraulics Engineer OregonDepartment of Transportation

Daniel Ghere Hydraulics Engineer IllinoisDepartment of Transportation

William H. Hulbert Hydraulics Engineer SouthCarolina Department ofTransportation

J. Sterling Jones Research Hydraulics EngineerOffice of Engineering Researchand Development, FHWA

Andrzej J. Kosicki Bridge Hydraulics EngineerMaryland State HighwayAgency

Peter F. Lagasse Senior Vice President(Report Facilitator) Ayres Associates

Curtis Monk Bridge EngineerFHWA, Iowa Division Office

Arthur Parola, Jr . Associate Professor, Departmentof Civil and EnvironmentalEngineering, University ofLouisville

Brief biographical sketches of scanning review teammembers are provided in Appendix B of the Final Reportbut are not included herein.

1.4 ITINERARY AND APPROACH

The scanning review team members departed the UnitedStates on October 16, 1998, and convened in Zurich, Swit-zerland, on October 17, 1998. The itinerary and primaryhost agency at each location were as follows:

Zurich, SwitzerlandOctober 16-20, 1998Laboratory of Hydraulics, Hydrology and Glaciology(VAW); Swiss Federal Institute of Technology (ETH)

Karlsruhe, GermanyOctober 21, 1998Federal Waterways Engineering and Research Institute(BAW)

Rhine River—Koblenz to Mainz, GermanyOctober 22, 1998Federal Waterways Engineering and Research Institute(BAW) and Federal Highway Research Institute (BAST)

Bergisch-Gladbach (Cologne), GermanyOctober 23, 1998Federal Highway Research Institute (BAST)

The Hague, the NetherlandsOctober 24-25, 1998Mid-Tour Scanning Review Team Meeting

Eastern Scheldt Barrier, the NetherlandsOctober 26, 1998Directorate General of Public Works and Water Management

Delft Hydraulics, Delft and New Waterway Storm SurgeBarrier, Rotterdam, the NetherlandsOctober 27, 1998Delft Hydraulics

Wallingford, United KingdomOctober 28-29, 1998H.R. Wallingford

Nottingham, United KingdomOctober 30, 1998University of Nottingham

London, United KingdomOctober 31 - November 1, 1998Final Scanning Review Team Meeting

A list of key contacts at each organization visited isprovided as Appendix A of this digest; acronyms are identi-fied in Appendix B of this digest.

Visits were made to national research institutes or di-rectorates; federal, university, and private hydraulic research

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laboratories; and field sites. A typical visit began with apresentation by the U.S. scanning review team (Co-chairand facilitator) on the magnitude of the scour problem in theUnited States, current technologies, and primary areas ofinterest. This was generally followed by a series of presen-tations by host agency professionals or invited speakers ad-dressing issues raised in the amplifying questions. Ampletime was available for questions from scanning review teammembers as well as group debate and dialog.

At hydraulic laboratories, the scanning review teammembers went on guided tours of facilities, concentrating onbridge scour modeling and related testing apparatus. Thevisits with several agencies also included discussion and dem-onstrations of hydraulic computer modeling capabilities.Field site visits included the Koblenz-to-Mainz reach of theRhine River in Germany (to observe and discuss extensiveriver training works, bank protection, and bridge scour coun-termeasures) and both the Eastern Scheldt and Rotterdam NewWaterway storm surge barriers in the Netherlands.

The mid-tour scanning review team meeting providedthe opportunity to summarize the results of discussions inSwitzerland and Germany, evaluate the response to the am-plifying questions, and identify areas of particular interestfor the Netherlands and the United Kingdom. The finalscanning review team meeting was used to consolidate andprioritize findings, discuss applications of technology to U.S.practice, and develop a preliminary implementation plan.

1.5 REPORT FORMAT

Chapter 2 presents general observations related to scourand stream instability technology and observations on spe-cific countermeasures in the four countries visited. Chapter3 suggests applications to U.S. practice in the categoriesidentified in the amplifying questions (Appendix A of thefinal report). Finally, Chapter 4 contains recommendationson high-priority technology that could significantly affectU.S. practice, as well as an Implementation Plan for bothimmediate and long-range activities that would continue thedialog initiated during the scanning review and contribute tophasing new technology into U.S. practice.

Appendix C of this digest cites references used in thisdigest, and Appendix D of this digest summarizes designmethodology for selected topics.

CHAPTER 2

OBSERVATIONS AND DISCUSSION

2.1 GENERAL OBSERVATIONS

2.1.1 European Scour Manuals and Literature

During the scanning review, several recently published(or draft) documents were obtained that summarize bridge

scour and countermeasure technology in the countries vis-ited. These documents may enhance basic U.S. scour tech-nology and do suggest several potentially useful counter-measure design and installation techniques. Other relevantarticles and publications are cited in the sections that follow.Comprehensive manuals or scour-related documents includethe following:

• Hints on promoting the stability of structures in waterwith recommendations for the preservation and mainte-nance of existing structures and tips for the constructionof new structures recently issued (1998) by the SwissFederal Offices of Highways, of Transport, and of Wa-ter Management and Swiss Federal Railways (7).

• Recently published (1997) Scour Manual by hydraulicengineers of Delft Hydraulics and the Ministry of Trans-port, Public Works and Water Management in theNetherlands (8)

• Handbook 47 for Bridge Scour Assessment prepared bythe British Railways Board (9)

• Railtrack Southern standards and procedures manual for“Managing the Danger to the Railway from Floodingand Tidal Action” (10)

• Scour Assessment Comparison prepared for BritishRailtrack by Jeremy Benn and Associates (11)

• Course notes on scour risk assessment and protectiondesign prepared for British Railtrack by Jeremy Bennand Associates (12)

• Draft Advice Notice on assessment of scour at highwaybridges being prepared for the Highways Agency in theUnited Kingdom (13)

• River and channel revetment design manual publishedrecently by the H.R. Wallingford Laboratory in theUnited Kingdom (14)

• Waterway bank protection manual prepared by the Brit-ish Environment Agency (15)

• A new publication on dikes and revetments by Pilarczykin the Netherlands (16)

• Engineers in all four countries visited referred to amanual on the use of rock in hydraulic engineering(CUR Report #169) as an extremely important refer-ence book (17)

On the subject of river geomorphology and channel sta-bility, several manuals and handbooks were obtained:

• Interim guidelines on design of straight and meanderingcompound channels prepared by H.R. Wallingford andothers for the British National Rivers Authority (18)

• Practical guide to river geomorphology prepared by theBritish National Centre for Risk Analysis and OptionsAppraisal (19)

• Stream Reconnaissance Handbook prepared by C.R.Thorne, Department of Geography, University ofNottingham, United Kingdom (20)

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• Recently published collection of papers on applied flu-vial geomorphology for river engineering and manage-ment, edited by Thorne, Hey, and Newson from theUnited Kingdom (21)

2.1.2 Design Philosophy

The general design approach in Switzerland, Germany,and the Netherlands is to prevent scour from occurring ormove scour away from the structure by including scour andstream instability countermeasures in the initial design andconstruction of their bridges. In general, these countriesbelieve that they do not have a significant bridge scour prob-lem, largely because of this design approach. The fact thattheir major navigable waterways and canals have been sta-bilized by extensive river training works contributes to thesuccess of this approach.

When designing hydraulic structures in the Netherlands,the following aspects are considered (8, 16, 22). (This ap-proach appears to be representative of European practice ingeneral):

• Function of the structure—erosion as such is not theproblem as long as the structure can fulfill its function.

• Physical environment—the structure should offer therequired degree of protection against hydraulic loading,with an acceptable risk and, when possible, meet therequirements resulting from landscape, recreational, andecological viewpoints.

• Construction method—construction costs should beminimized to an acceptable level and legal restrictionsmust be adhered to.

• Operation and maintenance—it must be possible tomanage and maintain the hydraulic structure.

The cost of construction and maintenance is generallyconsidered a controlling factor in determining the type ofstructure to be used. Therefore, the starting points for thedesign are carefully examined in cooperation with the fu-ture manager of the project. Most projects dealing withhydraulic structures are considered multidisciplinary incharacter (as characterized by all relevant interactions be-tween the soil, water, and structure) and may require com-bined hydraulic, geotechnical, and structural analyses.These interactions are often presented in a diagram similarto Figure 1.

Increased demand for reliable design of protective struc-tures in the Netherlands has stimulated preparation of animproved design policy (especially regarding safety aspects)and development of more reliable technical design methodsand design codes. The proper engineering strategy to befollowed is based on the total balance of the possible effectsof the countermeasures for the area considered, includingenvironmental issues and economic effects. It is part of theengineer’s philosophy to minimize the negative effects ofthe solution chosen (22).

The bridge scour problem in the United Kingdom issimilar to that faced in the United States. Both countries areapplying the latest scour technology to the design of newbridges, but both face significant problems with scour andstream instability at thousands of bridges in an aging infra-structure. As a result of a catastrophic railroad bridge fail-ure with fatalities at Glanrhyd in 1987, Railtrack has takenthe lead in developing scour assessment techniques andcountermeasures in the United Kingdom (9, 10, 11, 12). TheBritish Highways Agency has recently developed an AdviceNote for the assessment of scour at highway bridges (13).

In Europe, in general (16), and the United Kingdom,specifically (14), there are two principal alternative ap-proaches to the design of erosion and scour protection works:deterministic and probabilistic. In the deterministic ap-proach, the worst conditions of loading are determined andthe system is designed to withstand such loads with a certainmargin of safety. This is a simpler but usually more conser-vative approach than the approach based on probabilisticconsiderations. Probabilistic design requires a statisticalanalysis of the various loads (i.e., the estimation of the prob-ability of occurrence of loads and combinations of loads thatmay lead to the failure of the erosion or scour protectionsystem). This approach involves the consideration of sev-eral scenarios. Its higher complexity renders it more suit-able for major protection schemes, particularly those involv-ing extreme wave heights. In most river situations, adeterministic design is usually suitable (14).

Design procedures are generally derived for conditionsat the threshold of movement and, therefore, stable sizes aredetermined for very limited instability (e.g., a revetmentshould ensure that practically no damage will occur). Arecent design trend, however, has emerged that allows par-tial failure to occur. This can be suitable for situations whereconservative hydraulic loadings are adopted, monitoring isfrequent, a limited amount of damage is acceptable, or com-binations thereof (14).

Figure 1. Soil-water-structure interaction (SOWAS con-cept) (8, 22).

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2.1.3 Risk Analysis

Some form of risk analysis is used to determine thelevel of effort and investment in countermeasure design andinstallation in all countries visited. For example, Switzer-land uses a matrix to code flood hazard zones into increas-ing degrees of hazard (i.e., yellow, blue, and red) and infra-structure design and maintenance decisions are guided bythe degree of risk involved. The determination of protectionobjectives, and thus of the design flood discharge, is consid-ered a decision of major technical and economic conse-quence. Formerly, in Switzerland, the design for protectionworks was generally based on a flood with a return period of100 years (HQ100). Today, a differentiation of protectionobjectives is applied (Figure 2). According to the impor-tance or value of structures and land to be protected, therespective degree of safety can be chosen (23). For ex-ample, for infrastructure of national importance completeprotection would be provided up to a flood (Qa) that repre-sents the limit of damage (approximately a 50-year returnperiod). Limited damage (but not failure) would be acceptedup to a flood that represents the limit of danger (Qb). Be-yond that point, no protection would be provided in terms ofstructural or hydraulic design, but flood warning, evacua-tion, or bridge or roadway closure would be used to reducethe potential for loss of life.

As an example of the consideration of risk in design, inthe Netherlands it is recognized that absolute safety againststorm surges is nearly impossible to achieve in practice

(22). Therefore, the Dutch prefer to evaluate the probabil-ity of failure of a certain defense system. The ultimatepotential threat for Dutch sea defenses is derived from ex-treme storm surge levels with a very low probability ofexceedance (1 percent per century for sea-dikes) andequated with the average resistance of the dike. Underthese ultimate load conditions, probability of failure of thedike (seawall) should not exceed 10 percent. To apply thismethod, all possible causes of failure have to be analyzedand the consequences determined.

In Europe, the fault tree is considered a useful tool forintegrating the various failure mechanisms into a single ap-proach (8, 16, 22). For example, Figure 3 shows the faulttree for bed protection in which the foundation of the hy-draulic structure is the central point (8). The bed protectionhas to prevent or slow down a change in the geometry of thefoundation. A failure of the bed protection does not di-rectly imply the loss of the structure. However, when thesubsoil becomes unstable because of the existence of a well-developed scour hole, the resistance of the foundation isreduced.

A further advantage of fault tree analysis is that thismakes it possible to incorporate the failure of mechanical orelectrical components as well as human errors in the man-agement and maintenance of the structure. For instance, thesafety of a sluice can be dramatically improved by regularecho-sounding of the bed protection and by subsequentmaintenance if the initiation of a scour hole is discovered.The probability of instabilities affecting the foundation is

Figure 2. Differential safety concept—according to safety objectives a variable design flood canbe applied for flood control works (23).

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thus reduced to the coincidence of scour hole formation andfailings in inspection and maintenance (8).

More information about the design process, includingoutlines of main considerations relating to deterministic andprobabilistic design processes used in Europe, can be foundin CUR Report #169 (17).

2.1.4 Environmental Policy

Environmental impacts are considered for scour andstream instability countermeasure selection, design and in-stallation in all countries visited. In general, the approach isto emphasize environmental enhancement and sustainability,without creating an undue risk to lives and property inapplying environmental policy to structures in a riverinesystem.

In the United Kingdom, for example, the EnvironmentAgency is developing draft technical guidelines for erosionassessment and management in relation to waterway bankprotection (15). Selecting a strategy for controlling bankerosion depends on the following:

• Identifying the problem and its causes;• Determining the principles and objectives of control for

the specific site;• Prioritizing those objectives;• Assessing the risks associated with adoption of the strat-

egy, depending on the likelihood and the consequencesof failure; and

• Cost-effectiveness.

The strategies are classified into six types:

• Allowed natural adjustment,• Management,• Relocation,• Bioengineering,• Biotechnical engineering, and• Structural engineering.

The strategy chosen should be the most cost-effectiveone that addresses the cause and severity of the problem inthe most environmentally sensitive way.

In particular, the strategy should consider the conse-quences of bank failure. Where these are rated as severe,the risk associated with the failure of any strategy is high. Alow-risk strategy is, therefore, appropriate. For example,where flood defense is in question or navigation threatened,structural engineering is likely to be the only appropriatestrategy (Figure 4). Where the consequences of bank ero-sion are less significant, a riskier solution may be more ap-propriate because of its lower cost and, compared with struc-tural engineering, its greater benefit to ecological habitatand landscape (15).

The first three strategies are considered “ManagementSolutions,” and guidance recommends that a management

Figure 3. Fault tree for bed protection (8).

Figure 4. Property close to a river bank limits the choiceof strategies for erosion control. Structural solutions, suchas gabions, are often the only feasible measure (15).

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approach to dealing with bank erosion problems should al-ways be the first option considered. Management solutionsare preferred because they involve less interference with thenatural environment and can often be justified in terms ofenvironmental protection. Further, it is only by consideringmanagement options first and taking a positive decision to“rule them out” that a choice of any structural solution canbe properly justified (15).

A structural engineering strategy, sometimes termed“hard engineering,” includes the use of steel, concrete, andtimber piling. The approach also includes structures placedwithin the channel to control the flow. These includegroynes (spurs), vanes, and weirs (15).

Although structural engineering has the lowest risk offailure of the six strategies, it is suggested that it should notbe used simply because this is the case. Structural engineer-ing should be viewed as the “last resort,” appropriate onlywhere all other strategies have been purposely ruled out. Itis however, likely to be the only effective strategy whereverthe integrity of the entire channel is threatened. It is appro-priate wherever there is a risk of the following:

• Flooding of surrounding land;• Damage to structures;• Damage to property, towpaths, roads, railways, and so

forth; and• Damage to canal lining with consequent loss of water in

the channel through leakage.

Wherever structural engineering is chosen, it should bejustified. The purpose of the strategy should be defined, andit should be clear why it is essential (15).

The European approach to bioengineering and bio-technical engineering is discussed in a subsequent section.

2.1.5 River Geomorphology

In the United States, FHWA’s HEC-20 (3) stresses theneed to take a river system/geomorphic approach to channelinstability problems. All four countries in Europe recognizethe value of a geomorphic analysis in bridge and counter-measure design. At Delft Hydraulics, geomorphologists aswell as experts on remote sensing are employed in rivertraining studies. At the University of Nottingham in theUnited Kingdom, research in applying geomorphic recon-naissance techniques to river engineering problems has pro-duced useful and practical guidance for the hydraulic engi-neer. Such techniques support geomorphic classification ofa river system and permit a detailed investigation of formand process for critical reaches where instability could af-fect bridge design or countermeasure selection, design, andmaintenance (20, 21).

The Stream Reconnaissance Handbook (20) notes thatthe purpose of stream reconnaissance is as follows:

• To supply a methodological basis for field studies ofchannel form and process;

• To present a format for the collection of qualitative in-formation and quantitative data on the fluvial system;

• To provide a vehicle for progressive morphologicalstudies that start with a broadly focused catchmentbaseline study, continue through a fluvial audit of thechannel system, and culminate with a detailed investi-gation of geomorphological forms and processes in criti-cal reaches; and

• To supply the data and input information to supporttechniques of geomorphological classification, analy-sis, and prediction necessary to support sustainable riverengineering, conservation and management.

A set of field record sheets (checklists) is presented inthe Handbook to support a range of reconnaissance objec-tives, including the following: stream classification, anengineering-geomorphic analysis, field identification of chan-nel stability near structures, and supplying input for stablechannel design techniques and modeling of the river system.

In the United Kingdom, the Environment Agency,through the National Centre for Risk Analyses and OptionsAppraisal, has published a practical guide to river geomor-phology (19). The Guide notes that river geomorphologyprovides a practical basis for the assessment, protection, andenhancement of the physical environment in river channels.In this context, the Environment Agency can better achieveits objectives of protecting or enhancing the environment byadopting a geomorphological approach to river management.The geomorphological approach is also consistent with aholistic view of the environment, and the application of geo-morphologically aligned design and management can con-tribute toward achieving sustainable development and avoid-ing committing future generations to inflexible solutions orexpensive channel maintenance (19).

On the basis of large-scale hydraulic modeling experi-ments, H.R. Wallingford in the United Kingdom has devel-oped and published for the National Rivers Authority in-terim guidelines for design of straight and meanderingcompound channels. A hand calculation methodology ispresented, and implications for 1-dimensional river model-ing are discussed (18).

2.1.6 Scour Prediction

Although investigating improved scour prediction tech-niques was not the primary purpose of the scanning review,methods to calculate scour, particularly in complex flowsituations, were of interest to the scanning review team mem-bers. The problems of estimating scour at wide piers, thetime rate of scour in cohesive and non-cohesive materials,and the interaction of the various scour components are rec-ognized as being among the most pressing U.S. researchneeds in scour.

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Scour prediction in Europe relies heavily on pioneeringwork by Breusers, Raudkivi, Shields, Laursen, Neill, andothers. However, FHWA HEC-18, “Evaluating Scour atBridges, (2) is referenced in both of the recent scour refer-ences developed in the United Kingdom (9, 13) and is evalu-ated in some detail in the Dutch Scour Manual (8, p. 115).

The Dutch Scour Manual (8) provides the most com-prehensive treatment of scour prediction of the Europeanpublications encountered during the scanning review. Stan-dard practice for scour prediction in the United States willbenefit from careful consideration of the material presented(which extends the general introduction given by Breusersand Raudkivi in 1991) (24). The Scour Manual is viewedby its authors as a “revitalization” of Breusers’ equilibriummethod with the addition of laboratory and field experiencegained in the Netherlands and abroad.

Several of the topics in the Dutch Scour Manual (8)that relate to high-priority research needs in the UnitedStates are discussed in more detail in Appendix D. Theseinclude time scale (characteristic time) for development ofscour and scour at wide piers. For example, the Dutchdivide the process of local scour around bridge piers intoseveral phases: initial phase, development phase, stabiliza-tion phase, and equilibrium phase. A “characteristic time”is defined as the time it takes for scour to reach a depthequal to the pier width for pier scour or as the time for scourto reach the initial flow depth for more general scour situa-tions (see Appendix D for potential applications in theUnited States).

At H.R. Wallingford in the United Kingdom, researchis ongoing for scour prediction under reversing (tidal) flowconditions. Again, the definition of a characteristic time isconsidered important. The Dutch Scour Manual (8) alsoconsiders scour under unsteady (tidal) flow conditions andoffers several methods for estimating scour depth.

For scour at wide piers, the approach in Europe is todetermine the point at which the process makes the transi-tion from scour on a “slender” pier (influenced largely bypier width) to scour on a wide pier (influenced primarily bywater depth). Appendix D contains discussion from DelftHydraulics on the wide pier problem.

The Dutch Scour Manual (8, pp. 19-22) presents a meth-odology to determine critical velocity in cohesive sedimentsbut observes that “the erosion characteristics of cohesivesediments are not yet fully understood.” At the H.R.Wallingford Laboratory, specialized apparatus has been de-veloped to investigate the time rate of scour in cohesivematerials.

In Europe, in general, the influence of turbulence inrelation to the structure (e.g., bridge and storm surge bar-rier) is considered a key factor for estimating scour poten-tial and designing scour countermeasures. Information onmodel testing, analysis, and design of bed protection for theRotterdam storm surge barrier is presented by Jorissen et al.(25).

In estimating scour at a bridge pier, researchers in the

Netherlands consider the interaction between the variousscour components (e.g., long-term degradation, general orcontraction scour, and local scour) when calculating totalscour. The interaction between scour holes on adjacent sub-structure elements is considered indeterminate. At Delft,research has been conducted on the combined effects of lat-eral channel migration and local scour, specifically the de-velopment of scour on groins in meander bends. The Dutchconsider the most pressing research needs in scour predic-tion to be as follows:

• Prediction of bed levels during floods in relation to thegeneral morphological behavior of the river,

• Determination of the relationship between the floodwave and the speed with which the riverbed responds(i.e., the relationship between scour development andflood duration), and

• Development of techniques to estimate the superposi-tion of general and local scour (e.g., scour at a pier in ariver bend or the interaction of contraction scour andlocal scour in a straight reach of river).

2.1.7 Modeling

Both physical hydraulic modeling in a laboratory andnumerical computer modeling are among the standard tech-niques available to analyze the scour problem and designcountermeasures. The scanning review team members vis-ited hydraulic modeling laboratories with exceptional facili-ties and capabilities at the laboratory of Hydraulics, Hydrol-ogy, and Glaciology of the Swiss Federal Institute (VAW/ETH) in Zurich, Switzerland; the Federal Waterways Engi-neering and Research Institute (BAW) in Karlsruhe, Ger-many; and the H.R. Wallingford Laboratory in Wallingford,United Kingdom. At Delft Hydraulics it was pointed outthat the laboratory had extensive experience abroad in bridgescour, stream stabilization, hydraulic studies, and, very of-ten, physical modeling in a local laboratory is carried out inconjunction with studies of these subjects.

In Europe, it is much more likely that physical model-ing, often in conjunction with computer modeling, will bean integral part of the hydraulic design process for bridgefoundations and countermeasures than is typical in theUnited States.

A major effort is underway in Europe to develop 1-, 2-,and 3-dimensional computer models with hydrodynamic,sediment transport, and in some cases, morphologic capa-bilities. For example, Delft Hydraulics, an independent,non-profit-distributing institute (privatized since 1991) hasestablished a goal to become a center of expertise in com-puter modeling. Delft also maintains extensive physicalmodeling capabilities for three reasons: validation of com-puter modeling, fundamental research with respect to physi-cal processes, and solving problems for which computerscannot presently be applied.

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The Danish Hydraulic Institute’s (DHI) computer modelMIKE 11, a dynamic, 1-dimensional modeling system forriver and channels, is widely used in Europe and abroad.The U.S. Army Corps of Engineers HEC models are alsoused in Europe. However, each of the laboratories visitedhad proprietary computer models for river-related analysesand had programs underway to develop enhanced softwarecapabilities.

At VAW in Switzerland, computer models and modeldevelopment efforts included the following:

• MORMO (MORphological MOdel) a 1-dimensionalquasi-steady-state model for simulating sediment trans-port in rivers and reservoirs;

• A 2-dimensional mobile-bed process model; and• FEMTool (Finite Element Method Tool Box) for 1-, 2-,

and 3-dimensional hydrodynamic problems, with a2-dimensional mesh generator.

At BAW in Germany, 1-dimensional unsteady flowmodeling was being used to evaluate river behavior and de-termine maintenance requirements. Data from both physi-cal models and field monitoring were being used to refineand calibrate the model.

At Delft Hydraulics in the Netherlands the followingsoftware is being used or under development for river engi-neering analyses:

• RIVCOM: a 2-dimensional computation of bed levelchanges, including spiral flow effects;

• SERES: a computation of sedimentation in reservoirs;• SUSTRA/SUTRENCH: a computation of morphologi-

cal effects resulting from changes in sediment trans-port, dredged trenches, and so forth;

• MIANDRAS: a computation of river meanders;• SOBEK-GRAD: a special 1-dimensional module for

dynamic simulation of the morphology of graded river-beds;

• DELFT2/3D-MOR: a special 2/3-dimensional modulefor dynamic simulation of the morphology of uniformsediment and graded riverbeds;

• SCOUR: local erosion at hydraulic structures;• DIPRO: a check of riprap and block revetment stability

subjected to flow and wave-induced forces caused bypassing ships; and

• SEDIM: sediment transport and management in hy-draulic structures.

As an example of the computational hydraulic soft-ware available in the United Kingdom, H.R. Wallingfordhas developed HYDROWORKS, an advanced hydraulicsimulator for stormwater, sewage, and combined wastewa-ter systems.

Although several specialized models being developedin Europe may be of interest, it appears that 1- and 2-dimen-sional hydrodynamic modeling capabilities to support scourpredictions and countermeasures design in the United Statesare comparable with what is currently available in Europe.

2.1.8 Inspection and Monitoring

Most of the countries visited have initiated efforts todevelop a bridge inspection or scour evaluation programcomparable to the National Bridge Inspection Standards(NBIS) in the United States. In Germany, the Federal High-way Research Institute (BAST) is investigating the use ofthe FHWA PONTIS bridge management system.

In Switzerland, specific guidance on the stability ofstructures in water has recently been published (1998) as amulti-agency guideline by the Federal Office for Highways(ASTRA), the Federal Office for Transport (BAV), the Fed-eral Office for Water Management (BWW), and the SwissFederal Railways (SBB). This document is presented as a“Recommendation for the preservation and maintenance ofexisting structures/Hints for the construction of new struc-tures.” (7). The guideline contains illustrations of typicalforms of damages and processes (e.g., scour) endangeringbridge structural components in water, procedures to assessthe safety and stability of those structural components (in-cluding risk assessment), and inspection methods and tech-niques. A flow chart (Figure 5) guides the process.

Figure 5 Flow chart for preservation of engineering struc-tures in water (7).

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In response to the Glanrhyd bridge failure in 1987, highwater marks were painted on most railroad bridges in theUnited Kingdom to guide inspectors’ decisions on cautionor closure, but this approach apparently met with only mixedsuccess as it resulted in unnecessary bridge closures.Railtrack Handbook 47 on scour was prepared to provideguidance on techniques to assess the failure risk to struc-tures subjected to hydraulic loading and techniques to imple-ment procedures to safeguard traffic and personnel underextreme flood conditions (9). An initial assessment is pre-scribed following the procedures of Appendix B to the Hand-book, which is a report by H.R. Wallingford on “HydraulicAspects of Bridges: Assessment of the Risk of Scour.”

Specifically, Appendix B to Railtrack Handbook 47 pre-sents advice and guidelines to enable British Rail to assesshydraulic aspects of bridges over water. Possible causes offailure are discussed and illustrated with a series of figures(Table 1). The Railtrack assessment method involves a pre-scriptive assessment procedure designed for use by non-specialist engineering staff. The method’s purpose is to pro-vide a preliminary scour risk assessment in order to identifythose structures that require further in-depth study. The as-sessment involves the user in defining a potential flooddepth. The calculation method then uses this information incombination with data gathered on site on the river channel,bridge dimensions and bed material, to estimate a potential

scour depth. The scour depth is then compared with thefoundation depth to derive a preliminary priority rating.Structures in the highest priority classes are recommendedas requiring further detailed analysis to confirm the risk ofundermining from scour (9, 11).

For the Highways Agency in the United Kingdom, anAdvice Note has been prepared to assist in the assessmentand analysis of scour at highway bridges (13). The method-ology proposed in the Advice Note (Figure 6) comprises thefollowing stages:

• An initial screening stage;• A second stage in which an estimate is made of the

potential depths of scour adjacent to the bridge basedon a site visit and estimated 200-year flood flow; and

• A simple method of prioritizing those bridges that maybe at some risk, as a function not only of the scourdepths, but of several other relevant param includingthe importance of the bridge.

If the second stage identifies a bridge to be at some riskfrom scour, then further consideration of that bridge may berequired, either in the form of more detailed studies andinvestigations with a view to carrying out such remedialworks as may be required or the implementation of suchworks if the costs are such that direct implementation would

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be the cheapest option. The prioritization provides a meansof identifying those bridges at which resources should firstbe concentrated. If further studies or works are considerednecessary, then specialist advice is recommended (13).

Using 12 bridge sites in the field, a comparison of theBritish Railtrack and Highways Agency procedures wascompleted by Jeremy Benn and Associates (11) to identifyany theoretical differences between the two methods and tocheck on consistency between the priority rating scales.Among the general conclusions was the observation thatboth the methods use scour theory from the United States toestimate scour depths. This approach is considered ex-tremely conservative when used in United Kingdom prac-tice because of the following (11):

• “Many of the scour equations were derived from physi-cal model tests using unconsolidated, non-cohesivesediments whereas many United Kingdom bridges lieon consolidated material. In particular, given that manyUnited Kingdom bridges are more than 100 years old,the degree of consolidation of sediments beneath thebridge supports must be considerable (as this will en-hance the scour-resisting properties of the material).”

• “United Kingdom flood frequency curves are generallyless steep than those in less temperate climates such asthe United States. In other words, the relative differ-ences between a 100- and 200-year design flood are lessthan that in other countries and hence the sensitivity ofpriority rating to design flood is less marked.”

Figure 6. Overall scour assessment methodology (13).

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An effort comparable to NCHRP Project 21-3 has beenmade in the United Kingdom to develop fixed instrumenta-tion for measuring and monitoring scour at bridge piers andabutments. This resulted in the patented Wallingford “Tell-Tail” device which was installed on several railroad bridgesfollowing the Glanrhyd bridge failure.

In none of the four countries visited was technologyavailable to determine the characteristics of unknown bridgefoundations (i.e., foundations for which design or as-builtdrawings do not exist). In the United Kingdom, there arenumerous unknown foundation bridges and the problem isconsidered as serious as it is in the United States.

2.2 OBSERVATIONS ON SPECIFICCOUNTERMEASURES

2.2.1 Riprap

The use of riprap (i.e., armor stone in combination witha geotextile or granular filter) is by far the most commonscour and stream instability countermeasure in all countriesvisited in Europe. Its availability, economy, ease of installa-tion, and flexibility are considered highly desirable charac-teristics in all four countries visited. As a result, consider-able effort has been devoted to techniques for determiningsize, gradation, layer thickness and horizontal extent, filters,and placement techniques and equipment for revetment andcoastal applications. In Europe, riprap is considered an ef-fective and permanent countermeasure against channel in-stability and scour, including local scour at bridge piers.

Generally, riprap is sized using the Hudson formula(coastal applications), Shields diagram, or methods devel-oped in New Zealand, the Netherlands, the United King-dom, or the United States. The need for designing the riprapfor a specific site was emphasized. Great care is taken inplacing the riprap at critical locations, and, in many cases,stones are placed individually in the riprap matrix. Highlyspecialized equipment has been developed by constructioncontractors in Europe for placing riprap, particularly forcoastal installations. The use of bottom-dump or side-dumppontoons (barges) is common in both Germany and the Neth-erlands. By loading pontoon “bins” selectively with differ-ent sizes of rock, a design gradation in the riprap can beachieved. For large installations, vessels for placing riprapare equipped with dynamic positioning systems using Dif-ferential Global Positioning System technology and thrust-ers to maintain position and echo sounders (or divers) toverify the coverage of the riprap layer. Some of the smallerpontoon systems, particularly the bottom-dump pontoonsdeveloped in Germany, could be used to place riprap in wa-ter at larger bridges.

The scanning review team members’ visit to the East-ern Scheldt Barrier in the Netherlands provided an introduc-tion to riprap design techniques and specialized construction(placement) capabilities used in Europe. Following the cata-

strophic floods of 1953, which inundated large areas of theDelta (Figure 7) and claimed 1,853 lives, the Delta projectwas undertaken to close the main tidal estuaries and inlets inthe southwestern part of the Netherlands, except for thosegiving access to the ports of Rotterdam and Antwerp. TheEastern Scheldt Barrier (Figure 8) was the final part of theproject (completed in 1986) and consisted of a series of18,000-ton concrete piers creating a dam with moveablegates.

To increase the stability of the piers once they wereinstalled, a sill built up of graded layers of stone was con-structed under water around the base of the piers. The outerlayer of 6- to 10-ton stone was designed to withstand cur-rents expected should one of the gates not close during astorm. The largest stones could not be dropped into posi-tion, as the risk of their damaging the piers was too great. Aspecialized vessel, the Trias, was designed to lay the toplayer of stone. This vessel was equipped with a large cranewith a long extendible arm that was used to place the heavystones accurately. Five million tons of stone were used inthe construction of the sill. Figure 9 shows the stone deposi-tion barge, a stone dumping pontoon, and a schematic of theplacement process (26).

At the BAW in Germany, the scanning review teammembers observed wave tank testing of prototype scale par-tially grouted riprap (Figure 10). In general, the objective isto increase the stability of the riprap without sacrificing allof the flexibility. Contractors in Germany have developed

Figure 7. Map of the Netherlands and the Delta Project(26).

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Figure 8. The Eastern Scheldt Barrier, the Netherlands.

Figure 9. Armor stone and specialized stone placing equipment—Eastern Scheldt Barrier, the Netherlands (26).

Figure 10. Wave tank test of partially grouted riprap—BAW, Karlsruhe, Germany.

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techniques and equipment to achieve the desired grout cov-erage (i.e., filling about 40 percent of the voids at the sur-face) and the right penetration (i.e., decreasing grout fillwith depth into the riprap matrix and no grout in contactwith the geotextile filter). With the correct slurry mix(recipe) partial grouting can be achieved underwater withminimal environmental impact. Although current guidancein the United States tends to discourage the use of groutedriprap, BAW engineers believe that partial grouting, if donecorrectly, will ensure that the riprap retains sufficient flex-ibility while enhancing stability. Partial grouting of riprapmay be well suited for areas where rock of sufficient size isnot available to construct a loose riprap revetment. Partialgrouting of riprap is presented as one of several standarddesign forms for permeable revetments in a discussion ofconsiderations regarding the experience and design of Ger-man inland waterways (27).

The river and channel revetments design manual re-cently published by H.R. Wallingford in the United King-dom (14) provides design guidance for grouting “hand-pitched stone” with both bituminous and cement grout. Forgrouting riprap in the United Kingdom, bitumen is the mate-rial most commonly used. Although various degrees ofgrouting are possible, effective solutions are usually pro-duced when the bituminous mortar envelopes the loose stoneand leaves relatively large voids between rock particles. Thedegrees of grouting available are as follows:

• Surface grouting (which does not penetrate the wholethickness of the revetment and corresponds to aboutone-third of the voids filled),

• Various forms of pattern grouting (where only some ofthe surface area of the revetment is filled, between 50 to80 percent of voids), and

• Full grouting (an impermeable type of revetment).

Cement mortar is also used in conjunction with riprap,particularly to increase its stability at transitions with hydrau-lic structures or other types of revetment and is usually con-fined to small areas. Hand-pitched stone is normally groutedwith cement mortar where it is necessary to provide increasedstability, such as near the confluence of streams or at inlet oroutlet structures. The workability of the mortar generallyneeds to be increased by appropriate additives (14).

2.2.2 Filters

In Europe, as in the United States, a properly designedgeotextile or granular filter is considered essential to thesuccess of riprap and most other countermeasures on sand orfine-grained material. In Germany and the Netherlands, asignificant investment has been made in the developmentand testing of geosynthetic materials, and innovative instal-lation techniques have been developed that could find appli-cation for bridge pier and abutment countermeasures in theUnited States.

At the BAW in Karlsruhe, Germany, a highly special-ized laboratory is available for testing a wide range ofgeotextile characteristics, including the following (1) im-pact testing performed to determine punching resistance(e.g., when large stone is dropped on the geotextile (Figure11); (2) abrasion test (Figure 12); (3) permeability, clay clog-ging, and sand clogging tests; and (4) tests of material char-acteristics such as elongation and strength. Testing appara-tus has been devised to test performance under typicalconditions that might lead to failure when geotextiles areused with scour countermeasures. The scanning review teammembers are not aware of any similar test facilities in theUnited States. Through this testing program, geotextile ma-terials have been developed for use in innovative approachesto filter placement for riprap and other countermeasures.

Geotextile containers (large sand bags) made of me-chanically bonded non-woven fabrics up to 1.25 cubic m involume have been used to provide a filter layer for riprapinstallation at several large projects in Germany. The con-tainers are sewn on three sides at a factory and filled on siteto approximately 80 percent of capacity with sand/gravelfilter material using a hopper system. The final seam issewn on site. The containers are placed in layers using aside-dump pontoon. The elongation capabilities of the fab-ric and partial filling allow the containers to adjust to irregu-larities of the substrate at the installation site. Riprap is thenplaced over the layer of geotextile containers (28).

At the Eidersperrwerk storm surge barrier on the Eiderestuary in Germany, a filter layer of more than 48,000geotextile containers was used to repair a 30-m-deep scourhole at the barrier (Figure 13). An armor layer of 1- to 6-tonstone and toe stabilization using a fascine mat with smallerstone completed the installation. Similarly, a geotextile bagfilter and riprap protection were used as a countermeasureagainst pier scour at a new bridge on the Peena River inGermany. The Dutch used a similar concept to place a filterat the Eastern Scheldt storm surge barrier (Figures 7 and 8).Instead of individual sand bags, large sand mats or mat-

Figure 11. Impact test apparatus—BAW, Karlsruhe,Germany.

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tresses (consisting of two layers of non-woven geotextilewith granular material in between) were fabricated on landand placed with large barge-mounted rollers as a foundationfor individual precast dam components and as a filter forriprap placed for scour protection (26).

The Dutch have investigated the use of granular filterswith large ratios for top layer and filter/base material insteadof geometric tightness. Design rules for these “hydrody-namically sandtight” filters or geometrically open filters arepresented by Bakker et al. (29). The concept can be appliedto geotextile filters and design rules for hydrodynamicallysandtight geotextiles were developed at Delft Hydraulics.Erosion control by hydrodynamically sandtight geotextilesis discussed by Klein and Verheij (30).

Three countries (i.e., Germany, the Netherlands, andthe United Kingdom) use fascine mats, a very old, tradi-

tional approach for scour protection, as a means of placing ageotextile filter in deep water. The fascines consist of amatrix of willow or other natural material woven in longbundles (15 to 20 cm in diameter) to form a matrix which isassembled over a layer of woven geotextile. The geotextilehas ties which permit fastening it to the fascine mat. Thefascine mattress or “sinker mat” is floated into position andsunk into place by dropping riprap-size stone on it from abarge. Fascine sinker mats and riprap have been used toprotect the toe of the geotextile container/riprap protectionat the Eider estuary storm surge barrier in Germany (Figure13) and for coastal applications in the Netherlands. Figure14 shows a scanning review team member investigating thecharacteristics of a fascine mat at the New Waterway stormsurge barrier near Rotterdam, the Netherlands.

The BAW Code of Practice for the use of geotextilefilters on waterways (31) covers various filter applications.Other relevant publications are DVWK Guideline 306 (32)for Application of Geotextiles in Hydraulic Engineering andseveral Permanent International Association of NavigationCongresses (PIANC) guidelines (33, 34). Many of the tech-niques referenced in this section are summarized in a 1996paper by BAW staff on installation of geosynthetics in wa-terways (35). Additional discussion is presented byPilarczyk (16) and Kohlhase (36).

2.2.3 River Training

River training and stabilization techniques against lat-eral channel migration in the major navigable waterways ofEurope are similar to those employed by the U.S. ArmyCorps of Engineers on navigable waterways in the UnitedStates (e.g., the Mississippi, Ohio, and Missouri river sys-tems). Groins and jetties projecting roughly perpendicular

Figure 12. Abrasion test apparatus—BAW, Karlsruhe,Germany.

Figure 13. Eidersperrwerk on the Eiden estuary, Germany. Geotextile containers used to repair 3-m-deep scour hole andfascine sinker mat to stabilize the toe (28).

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to the river bank, dikes placed parallel to the river bank, orrevetment placed on the river bank are the most commonriver training works in Europe. Generally, riprap is the pre-ferred construction material. Scour at the noses of groinsand jetties, at the heads of dikes, and at the toes of revet-ments are the most commonly cited problems.

River training has been an ongoing process on Europe’snavigable rivers (or on canals in the Netherlands) for hun-dreds of years, and there are few unprotected reaches ofriver. Thus, lateral instability because of river meander is arare occurrence and is not considered a threat to bridges. Onthe Rhine River, from Koblenz to Mainz (the only reach ofriver that scanning review team members were able to ob-serve in detail [Figure 15]), long parallel dikes (placedroughly one third of the channel width from the river bankused to constrict the flow) are much more common than inthe United States, where a groin field would be used for thesame purpose. Flow is allowed to pass through the area

between the dike and the river’s bank, sometimes over asubmerged groin or weir (Figure 16).

To protect the toe of river bank (or canal bank) revet-ment, two approaches are usually employed. Either a toetrench is excavated and riprap is placed in the trench, or a“falling apron” approach is used. The falling apron or self-launching of riprap revetment was mentioned in all fourcountries. With this approach, stone is placed in a windrowalong a bankline or at the toe to be protected, and, as theriver erodes into the bankline or toe, it launches the materialalong the face of the slope and onto the toe. Methods areavailable to estimate the amount of extra material requiredto protect the revetment toe and to compensate for not hav-ing a filter. A range of toe protection alternatives is illus-trated in the H.R. Wallingford river and channel revetmentdesign manual from the United Kingdom (Figure 17).

2.2.4 Riverbed Degradation

Sills, grade control structures, low check dams, or weirsconstructed of various materials are commonly used in Eu-rope to protect against vertical channel instability (degrada-tion) as they are in the United States. However, innovativeapproaches to the problem that justify further considerationwere presented in Switzerland and Germany.

In Switzerland, an experiment was undertaken in thefield with local channel widening in lieu of replacing dete-riorating check dams as a means of grade control on theEmme River near Berne (23, 37). Enhanced environmentaldiversity on a narrowly channelized river is seen as a ben-efit, but some local instability and the need to protect theshoulders of the widened section may be a detriment.

In Germany, the approach to the problem of degrada-tion on the Rhine River has involved sediment managementon a large scale. Here, it is recognized that long-term degra-dation problems are generally related to a deficiency in thesupply of sediment to a river reach or river system. As aresult, a systemwide sediment management program has

Figure 14. Fascine mat at new waterway storm surge bar-rier near Rotterdam, the Netherlands (Scanning review teammember: Mr. Jorge Pagan—FHWA).

Figure 15. Rhine River bank protection near SchlossStolzenfels, Germany—revetment wall with riprap toe. Figure 16. Low rock groin on the Rhine River, Germany.

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evolved—one component of which is an attempt to replen-ish the sediment supply by “feeding the Rhine.”

In the Rhine, the natural supply of bed-load materialfrom the upriver reaches has been totally stopped by theimpoundment system in the headwaters down to theIffezheim dam at kilom 334 (38). In order to avoid theformation of an “erosion wedge,” an artificial supply of ma-terial has been provided down river through the dumping ofgravel and sand from barges.

Between the last impoundment at Iffezheim and theGerman-Dutch border, research and field measurementshave established the bed-load transport balance and identi-fied nine river reaches with alternating aggradation or deg-radation regimes. In this reach of the Rhine, a sedimentdeficit of about 350,000 tons per year has been identified,and some 260,000 tons per year of “artificial” bed materialhas been supplied since 1991. This material has been de-rived from off-river sources and techniques such as dredg-ing a transverse trench in the Rhine River bottom in anaggradational reach to trap sediment and transporting thematerial by barge to a sediment deficient reach (39).

The Swiss also are concerned with the effects of sedi-ment deficiency on river system stability. Before 1970,gravel mining (or harvesting) from rivers was allowed inSwitzerland, but when scour problems were noted in adja-cent reaches, the practice was restricted.

2.2.5 Alternative Countermeasures

Among the areas of particular interest to the scanningreview team members during the scanning review were al-ternative countermeasures such as flow-altering devices oralternatives to riprap (particularly for the pier scour prob-lem). The following paragraphs outline some of the scan-ning review team members’ observations on alternativecountermeasures in relation to U.S. practice.

Precast Armor Units

The floods of August 24-25, 1987, caused considerabledamage in the Reuss River valley near Wassen, in the Can-ton of Uri, Switzerland. For example, the Swiss experi-enced a near catastrophic failure of a major national high-way bridge when the Reuss River migrated laterally andundermined the foundation of a bridge pier (Figures 18 and19). The countermeasure system developed by the VAW/ETH laboratory included a pile wall in front of the bridgepiers, five concrete spurs, large concrete groins, and theplacement of about 175 concrete prisms to correct and pre-vent further channel migration or lateral erosion.

The riverbank between the groins was protected by theprecast concrete prisms, triangular in cross section, placedindividually as revetment. In lieu of smaller interlockingarmor units that would be costly to fabricate, the decisionwas made to cast much larger prisms with a simple shapeand use the mass of the prisms to protect against river bankscour. The precast, hollow prisms were filled with concreteafter they were placed in their final position. The groin fieldand prism revetment were then covered with a layer of natu-ral stone for aesthetic and environmental reasons. The eco-nomics of the tradeoffs between smaller, high cost inter-locking shapes for artificial riprap and simpler shapes withmore mass are worth further consideration.

Figure 17. Examples of toe details (ds is anticipated scourdepth) (14).

Figure 18. Reuss River bridge failure near Wassen, UriCanton, Switzerland, August 1987.

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Proprietary Products

In general, proprietary products such as interlockingblock, articulating cable-tied block, and articulating grout-filled mattresses for revetment and channel bed protectionare not considered as effective as riprap in Europe. Theneed for adequate toe protection and anchoring was empha-sized. Block and mattress manufacturers in the United Statesand Europe are developing design criteria based on full-scale laboratory testing of specific products. Such testsshould provide the necessary guidance for the successfuldesign and installation of proprietary products for revetmentand channel protection.

Recent laboratory testing by NCHRP, FHWA, and oth-ers in the United States indicates that when articulating matproducts are used as a pier scour countermeasure, the jointbetween the mattress and the pier must be protected to pre-vent scour under the mat. The scanning review team mem-bers encountered two approaches to solving this problemthat justify further evaluation. In Germany, reference wasmade (Dr. S. Kohlhase, University of Rostock) to a propri-etary system for installing a collar and tying the geotextilefilter underlying a mattress to the bridge pier using a pneu-matic tie (Figure 20). This approach appears feasible forcircular piers. Considering possible settlement of the mat-tress relative to the structure (pile), a steel sleeve and a “tophat” of filter fabric were proposed with a collar of fabriformlaid on top of the mattress and tight to the sleeve as indicatedin Figure 20. As relative settlement occurs, the sleeve isexpected to slide down the pile and the top hat to expand,bellow fashion, with a collar for protection. This approachmay be limited in areas where the top hat could be damagedby abrasion.

In the Netherlands, the recommended approach to theproblem of sealing the joint between a mattress and a bridgepier is to place granular filter material to a depth of about 1m below the streambed for about 5 m around the pier. Thegeotextile filter and block mat placed on the streambed over-

lap this granular filter layer and the remaining gap betweenthe mat and the pier is filled with riprap. Successful fieldinstallations have apparently been made using this technique.

Flow-Altering Devices

There is considerable interest in developing flow-alter-ing devices such as hydrofoils, collars, and other bridge pierappurtenances as local scour countermeasures. Recent labo-ratory tests in the United States sponsored by NCHRP haveshown that several of these types of devices, scour collars,sacrificial piles, and guide vanes, are only marginally effec-tive. The scanning review team members did not encounterany successful applications of flow-altering devices as a pierscour countermeasure in any of the countries visited. How-ever, researchers at the VAW hydraulic laboratory in Swit-zerland studied pressure flow at a bridge using devices tomodify the flow. Pressure flow occurs when flood watersare high enough to submerge bridge superstructure elementsor overtop the bridge deck. One of the devices studied was acurved plate, called a pressure flow shield, that is placed onthe upstream side of a bridge (40). The study concluded thatthe pressure flow shield could prevent overtopping and im-prove flow conditions through the bridge opening by scour-ing accumulated sediment from beneath the bridge. In an-other experiment at VAW the upstream, bottom edge of eachbridge girder was modified by the addition of a rounded“nose.” This improved flow conditions through the bridgeunder pressure flow and reduced backwater upstream of thebridge. This approach also appeared to decrease scour un-der the bridge and improve the passage of debris (trees andother vegetation) through the bridge opening.

Figure 19. Close-up of Reuss River bridge, Switzerland,August 1987.

Figure 20. Flexible collar arrangement at a pile to sealthe joint with a mattress.

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Debris

Protecting bridges from the accumulation of debris andpredicting the increase in scour caused by debris at a bridgeis a problem worldwide. The scanning review team mem-bers did not encounter any applications of “debris deflec-tors” or other devices at a bridge during the scanning re-view. The Swiss were, however, experimenting with thedesign of large “trash racks” at sedimentation basins to catchvegetative debris before it moves downstream to a bridge.At the University of Nottingham in the United Kingdom, aneffort has been made to develop software to aid in the pre-diction of scour when debris accumulate at a bridge.

2.2.6 Bioengineering

Bioengineering techniques are integrated with tradi-tional engineering countermeasures for river system man-agement in Europe; however, hydraulic engineers in all fourcountries visited do not recommend relying on bioengineer-ing countermeasures as the only countermeasure techniqueif damage to property or to a structure or loss of life arepossible. The primary concern expressed was a lack ofknowledge about the properties of the materials being usedin relation to force and stress generated by flowing waterand the difficulties in obtaining consistent performance fromcountermeasures relying on living materials.

As discussed in the section on Environmental Policy,bioengineering and biotechnical engineering approaches areamong the strategies considered when selecting techniquesfor controlling bankline erosion in the United Kingdom (15).Accepted definitions of these terms are as follows (14):

• Bioengineering—corresponding to the traditionallytermed “soft revetments” using living plant materials,or plant products, as the primary means of protection;

• Biotechnical revetments—those revetments that incor-porate some form of vegetative protection but also relyon the technical ability of harder materials (typical ex-amples are grassed concrete blocks); and

• Structural revetments—revetments formed exclusivelyby non-live materials (examples include concrete liningand riprap).

Bioengineering is considered a suitable strategy in theUnited Kingdom under the following circumstances (15):

• Conditions for the growth of vegetation species withengineering value are not limiting.

• Vegetation alone is able to protect the bank againstscour (i.e., the flow velocity in the channel is less thanthe maximum “safe velocity” for the vegetation-linedchannel).

• Plant roots can develop below the depth of any poten-tial slide plane and thereby anchor the bank material tothe underlying substrate.

• A long-term monitoring program can be designed andimplemented.

The objective of biotechnical engineering in the UnitedKingdom is to combine the advantages of engineering struc-tures with the engineering and environmental benefits ofvegetation. In one view, the strategy combines the greatercertainty associated with the design and performance of en-gineering materials with the uncertainty of the vegetationcover, providing a “back-up” should the vegetation, for anyreason, fail. An alternative view is that it adds the greaterresilience and indefinite life span of the vegetation cover toan engineering structure, resulting in an increase in the over-all factor of safety (15).

A PIANC document, “From Sheet Piling to VegetatedEmbankments—Conventional and Biological EngineeringWorks for Bank Protection On Waterways” (41), also ap-pears representative of the general European approach tobioengineering and biotechnical engineering from a hydrau-lic engineering perspective. Here, a range of constructiontechniques—from traditional engineering to bioengineer-ing—is reviewed. It is recognized that a correct approach is“to keep interference with nature as low as possible,” and“in the field of hydraulic engineering there are many oppor-tunities” to promote a natural balance (e.g., “by choosingnatural construction materials and by suitable designs”).

Several well-tested biological methods for bank protec-tion have been used in Europe including the following:fascines, brushwood set in horizontal strips, brush layers/hedges, brushwood mats, vegetation mats, and wattle. ThePIANC report (41) concludes that “if designed, planned andimplemented properly, biological engineering works canmeet both technical and ecological requirements.” How-ever, safety issues must generally be assessed by “purelytechnical aspects” and certain fundamental hydraulic andgeotechnical requirements “have to be accepted as guide-lines for river engineering and for the construction of safewaterways.”

CHAPTER 3

APPLICATIONS TO U.S. PRACTICE

The scanning review team members were able to visitfour European countries where they observed scour predic-tion techniques, inspection and monitoring practices, andnumerous specific countermeasures for bridge scour andstream instability problems. Team members also were ableto discuss design philosophy as well as these techniques,practices, and specific countermeasures with their Europeancounterparts. The following sections (which summarizewhat the team members learned) discuss how Europeanbridge scour techniques could be used to improve U.S. prac-tice. These techniques should be considered further by ap-propriate research funding agencies (e.g., TRB, NCHRP,FHWA, and state DOTs and other bridge owners) or agen-

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cies, such as FHWA and AASHTO, that establish transpor-tation policy, code, guidelines, and specifications.

3.1 TECHNIQUES TO REDUCE BRIDGE SCOURAND TO ENHANCE STREAM STABILITY

• Conduct a thorough review of European literature onbridge scour and stream instability technology, particu-larly the comprehensive scour manuals obtained duringthe scanning review. The references in Appendix C,available in English, provide a starting point, but nu-merous potentially useful references, not necessarily inEnglish, remain to be identified and reviewed. Thescope and potential value of the literature identified dur-ing the scanning review underscores the need to increasecommunications between researchers and practitionersin the United States and overseas.

• Re-evaluate bridge scour design philosophy regardingthe role of countermeasures in new bridge design andconstruction. Consider techniques to move scour awayfrom the structure during initial design and constructionof bridges.

• Encourage increased use of risk analysis in the designof new bridges and evaluation of existing bridges. Con-sider accepting a variable degree of protection depend-ing on the importance of the structure. Suggestions forapplying risk analysis techniques to the bridge failureproblem are discussed by Annandale (42).

• Adapt stream reconnaissance techniques to the evalua-tion of stream stability in the vicinity of highway struc-tures, and continue to encourage a geomorphic approachfor stream system analysis, bridge design, and counter-measure selection.

• Improve techniques to analyze and predict scour, par-ticularly for complex flow situations such as wide piers(see Appendix D), pressure flow, debris, and the inter-action of general and local scour components by a moredetailed evaluation of European practice.

• Investigate the role of turbulence intensity and its influenceon scour prediction and countermeasure location and design.

• Investigate the characteristics of time rate of scour innon-cohesive and cohesive materials (see Appendix D).

• Increase the use of physical hydraulic models and com-puter models to evaluate scour in complex flow situa-tions and for the design of countermeasures.

• Consider the applicability of sediment management as astrategy to counteract long-term riverbed degradationproblems.

3.2 COUNTERMEASURE TECHNIQUES FORBRIDGE FOUNDATIONS

• Evaluate fault tree analysis techniques for the selectionand design of bridge scour and stream instability coun-

termeasures. Suggestions for adapting fault tree analy-sis to analysis of a bridge failure resulting from scourand channel instability are provided by Johnson (43).

• Review European inspection and monitoring programsand manuals in relation to the National Bridge Inspec-tion Standards (NBIS).

• Evaluate the economics of including scour and streaminstability countermeasures in the initial construction ofa bridge.

• Evaluate the use of risk analysis in countermeasuredesign, particularly in the selection and design of coun-termeasures for existing scour-critical or unknown foun-dation bridges to ensure that the cost of the recom-mended solution is commensurate with the risk to thestructure.

• Apply geomorphic reconnaissance and analysis tech-niques in the selection and design of countermeasures.

• Re-evaluate design and installation techniques for riprapand reconsider its viability as a permanent countermea-sure against pier scour.

• Evaluate and test European techniques for the designand installation of partially grouted riprap and re-evalu-ate its applicability to U.S. practice.

• Evaluate and test the use of innovative techniques forplacing filters under riprap and other countermeasures,including geotextile containers, geotextile mattresses,the use of fascine mats, and hydrodynamically sand tightfilters.

• Investigate the economics of tradeoffs between smaller,high-cost interlocking shapes for artificial riprap (e.g.,Toskanes) and simpler shapes with more mass to resisthydraulic stress (e.g., precast concrete prisms).

• Consider the relative merits of proprietary products(e.g., interlocking block, cable-tied block, articulatingblock, and mattresses) in relation to the use of riprap forchannel protection and as local scour countermeasures,and encourage field and laboratory testing of these prod-ucts to develop appropriate design guidance.

• Evaluate and test European techniques to prevent scourat the “joint” between articulating mattresses and abridge pier when these products are used as a pier scourcountermeasure.

3.3 TECHNIQUES TO ADDRESSENVIRONMENTAL ISSUES

• Consider risk to the structure, lives, or property in ap-plying environmental policy to bridge scour protectionand countermeasures.

• Integrate the consideration of management strategiessuch as allowing natural adjustment and relocation intothe scour and channel instability engineering designprocess.

• Evaluate and test bioengineering and biotechnical engi-neering techniques as bridge scour countermeasures for

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situations where public safety considerations would notpreclude their use.

CHAPTER 4

RECOMMENDATIONS ANDIMPLEMENTATION PLAN

This chapter provides recommendations for adaptingseveral elements of European practice to improve U.S. ca-pabilities to deal with stream instability and bridge scourproblems on a high priority basis. An implementation planis also suggested to ensure that the technology acquisitionactivities initiated by the scanning review will continue andwill be disseminated to bridge owners and their engineeringstaff.

4.1 HIGH-PRIORITY RECOMMENDATIONS

4.1.1 Riprap and Filters

The use of riprap (i.e., armor stone in combination with ageotextile or granular filter) is by far the most common scourand stream instability countermeasure in all countries visitedin Europe. Current policy in the United States considers riprapplaced at bridge piers to be only a temporary countermeasureagainst pier scour, and guidance dictates that riprap placed atbridge piers must be monitored by periodic inspection or withfixed instruments. This policy derives from experience withthe difficulty of adequately sizing riprap to withstand the tur-bulence and hydraulic stress generated in the vicinity of abridge pier, particularly under flood-flow conditions. Thefailure of the Schoharie Creek bridge in 1987 (attributed tothe cumulative loss of riprap around a spread footing founda-tion) and numerous instances on sandbed channels (wherelarge pier riprap has been swept downstream or loses its ef-fectiveness as it is buried in the sandbed) have substantiatedthe need for a conservative policy when considering riprap asa pier scour countermeasure.

During the scanning review, it was apparent that Euro-pean counterparts in the countries visited consider riprap asa permanent pier scour countermeasure. The difference be-tween U.S. and European practice is not necessarily derivedfrom the availability of better techniques for sizing riprap(although consideration of turbulence intensity could lead tomore refined riprap design), but rather from the higher stan-dard of care and quality control in placing the stone andproviding an appropriate filter on sandbed channels. In ad-dition, European practice includes inspection and monitor-ing to verify that riprap is performing properly. Contractorsin Europe have developed specialized pontoons (barges) forplacing riprap accurately and in the appropriate thickness(Figure 9), and, if necessary, each stone is placed individu-

ally to optimize performance in critical locations (e.g., theEastern Scheldt Barrier in the Netherlands, Figure 8).

Equally important for the confidence that European hy-draulic engineers have in the use of riprap as a permanentlocal scour countermeasure is their use of innovative tech-niques for placing an effective filter beneath the riprap inflowing or deep water. The use of large geotextile sandcontainers at the Eidersperrwerk in Germany (Figure 13),the use of a geotextile mattress filled with granular filtermaterial at the Eastern Scheldt Barrier in the Netherlands,and the use of fascine sinker mats (Figure 14) at both loca-tions are examples of these techniques. The availability oftesting apparatus to ensure that geotextiles will perform asrequired (Figures 11 and 12) and development of specificcodes to guide the design and installation of geotextiles (31,32, 33, 34) contribute to the success of these installations.

As state DOTs in the United States develop Plans ofAction for their scour-critical bridges, improved techniquesto use riprap effectively as a pier scour countermeasure couldresult in significant savings, particularly where the onlyalternative may be rehabilitation or replacement of theaffected bridge. A high-priority evaluation of Europeanpractice for the design and installation of riprap with anappropriate filter as a permanent pier scour countermeasureis warranted.

4.1.2 Partially Grouted Riprap

Current practice in the United States discourages theuse of grouted riprap, primarily because grouting converts aflexible revetment material into a rigid mass susceptible toundermining and failure. The scanning review team mem-bers are aware of only a few instances in the United States(e.g., an installation by CALTRANS) where anything otherthan total grouting of the riprap layer has been attempted.Ongoing tests in Germany at BAW, experience on Germaninland waterways (27), and development of design guidancefor partial bituminous and cement grouted riprap (14) indi-cate that design guidelines and installation experience areavailable or are being developed in Europe. These Euro-pean design guidelines, specifications, and installation tech-niques for partially grouted riprap should be investigated ona high-priority basis.

4.1.3 Risk Analysis

The scanning review team members found that someform of risk analysis is used to determine the level of effortand investment in countermeasure design and installation inall countries visited. In Switzerland, for example, a differ-entiation of protection objectives is applied (Figure 2), andan appropriate degree of safety is selected according to theimportance of the structure to be protected (23). This con-trasts with the general approach in the United States of usinga 100-year design flood and 500-year check flood for allstructures. However, the use of a super flood, such as the

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200-year flood in the United Kingdom, or the 1,000 yearflood for sea defenses in the Netherlands, for scour evalua-tions appears to be standard practice in Europe. Annandale(42) has outlined techniques for applying risk analysis to thebridge failure problem.

The use of fault tree analysis (Figure 3) was recom-mended in several countries visited. Johnson (43) has sug-gested techniques to apply fault tree analysis techniques tothe analysis of a bridge failure resulting from scour andchannel instability.

The increased use of risk analysis in countermeasureselection and design and the use of techniques such as faulttree analysis could result in more economical design ofbridge scour countermeasures as state DOTs develop Plansof Action for scour-critical bridges. These concepts shouldbe evaluated and disseminated, as appropriate, to bridgeowners in the United States.

4.1.4 Scour Prediction

In the United States, the problems of estimating scour atwide piers, the time rate of scour in cohesive and non-cohe-sive materials, and the interaction of the various scour com-ponents are among the most pressing U.S. research needs inscour. The Dutch Scour Manual (8), in particular, providesa comprehensive treatment that builds on earlier Europeanliterature on scour. A detailed review of the Dutch ScourManual and other comprehensive treatments of the scourprocess (9, 13), is warranted. Appendix D presents insightson two high-priority research needs: the time scale (charac-teristic time) for development of scour, and scour at widepiers as an example of the potential benefits of a more thor-ough review of the European literature on scour.

4.1.5 Update of the FHWA HECs


• HEC-18 Evaluating Scour at Bridges,• HEC-20 Stream Stability at Highway Structures, and• HEC-23 Bridge Scour and Stream Instability Counter-

measures.

FHWA is revising and updating these manuals, with draftrevisions scheduled for completion in October 1999. Thescope of work for these revisions includes reviewing andevaluating the European literature on bridge scour and streaminstability obtained by the scanning review team membersduring the scanning review. It is anticipated that the update ofthe three HECs will be followed by revisions to the two Na-tional Highway Institute (NHI) training courses on bridgescour: NHI Course No. 13046—Stream Stability and Scour atHighway Bridges, and NHI Course No. 13047—Stream Sta-bility and Scour for Bridge Inspectors.

The FHWA HECs and NHI training courses represent

the most efficient means of disseminating new technologyto state DOTs and other bridge owners. Information gainedfrom the countermeasures scanning review on Europeanpractice that does not require further research or laboratoryor field testing should be incorporated into the current revi-sions of the HECs and training course materials.

Other activities to continue the technical contacts withcounterparts in Europe and disseminate information gainedduring the scanning review are outlined in the Implementa-tion Plan that follows.

4.2 IMPLEMENTATION PLAN

During the final scanning review team meeting, initial stepswere taken to develop an implementation plan. Since returningfrom the scanning review, several implementation activities havebeen completed and others are being planned. This sectionsummarizes these activities and suggests other implementationactions that should be considered for the future.

4.2.1 Implementation Activities Accomplished

• In November 1998, shortly after returning from thescanning review, Mr. William Hulbert, South CarolinaDOT Scanning review team member, made a presenta-tion on initial findings at the AASHTO meeting inBoston.

• Mr. Jorge Pagan of FHWA and Dr. Peter Lagasse ofAyres Associates submitted an abstract for a paper onscanning review results to the American Society of CivilEngineers Water Resources Division Specialty Confer-ence scheduled for August 1999 in Seattle, Washing-ton. The paper has been accepted for presentation andpublication in the conference proceedings.

• In January 1999, during the annual TRB meeting inWashington D.C., Dr. Peter Lagasse of Ayres Associ-ates presented a short overview of initial findings fromthe scanning review to the Hydraulics, Hydrology, andWater Quality (A2A03) committee. A more detailedpresentation of findings is scheduled for the commit-tee’s mid-year meeting in June 1999.

• In March 1999, Mr. Sterling Jones of FHWA made apresentation on the initial findings to the FHWA’s In-ternational Coordination Group in Washington, D.C.

• FHWA and NHI authorized presentation of NHI CourseNo. 13046 in Wallingford, United Kingdom, from April28 through April 30, 1999.

• A Stream Stability and Scour course was held in coop-eration with the H.R. Wallingford Laboratories in theUnited Kingdom. Its primary purpose was to build onthe rapport established with scour researchers and prac-titioners during the visit to the United Kingdom. Thecourse was staffed with FHWA and Ayres instructors toincrease the opportunities for productive exchange.

29

4.2.2 Implementation Activities Planned

Among the implementation suggestions at the finalscanning review team meeting were the following actions:

• Incorporate new technology into the FHWA plannedupdate of HEC-18, 20, and 23 and in NHI planned revi-sions to Stream Stability and Scour at Bridges courses(NHI No. 13046 and 13047) and Highways in the RiverEnvironment course (NHI No. 13010).

• Present findings from the scanning review at the fol-lowing meetings or conferences:

— AASHTO Bridge Conference, San Diego, May1999 (Pagan)

— AASHTO Special Committee on Activity Coordi-nation meeting, Washington, D.C., May 1999(Pagan)

— Western Regional Hydraulic Engineers Confer-ence, Lake Tahoe, May 1999 (Lagasse)

— AASHTO Bridge Conference, San Diego, Califor-nia, May 1999

— TRB 5th Bridge Conference, Tampa, Florida, April2000 (to include Countermeasures Scanning Re-view Overview (Hulbert, Ghere, and Bryson), WidePiers (Jones et al.), Risk Analysis (Jones et al.),Comprehensive Scour Evaluation Methodology(Lagasse et al.)

— International Society for Soil Mechanics andGeotechnical Engineering (ISSMGE) Year 2000Conference, Melbourne, Australia

4.2.3 Implementation Activities Suggested

• Explore opportunities to reach the Association of Gen-eral Contractors regarding techniques, equipment, qual-ity control, specifications, and so forth for riprap andother countermeasure placement.

• Seek support for a study of AASHTO riprap speci-fications and work with AASHTO on recommendationsfor improvement of installation and quality controltechniques.

• Evaluate European time rate of scour concepts to estab-lish limits on the amount of scour that can reasonablybe expected to occur when hydraulic stresses are of shortduration (e.g., a coastal estuary bridge during a hurri-cane storm surge). The procedure suggested in Appen-dix D could result in significant savings for the plannedwidening of I-95 crossings in Georgia and should beevaluated further. If appropriate, results should be in-cluded in the next edition of HEC-18.

APPENDIX A

ORGANIZATIONS AND CONTACTS

Zürich, Switzerland

Swiss/Austrian Participants

Laboratory of Hydraulics, Hydrology, and Glaciology(VAW), ETH-Zentrum, Zürich

Bezzola, Gian-Reto, Dipl.Ing.Fäh, Roland, Dr., Dipl.Ing.Hager, Willi H., PD Dr.Hermann, Felix, Dr., Dipl.Ing.Hunzinger, Lukas, Dr., Dipl.Ing.Minor, Prof. Dr., DirectorRaemy, Félix, Dr., Dipl.Ing.Roth, Marcel, Dipl.Ing.Rutschmann, Peter, Dr., Dipl.Ing.Schatzmann, Markus, Dipl.Ing.Schram, Karin, Dr., Dipl.Ing.Speerli, Jürg, Dr., Dipl.Ing.Sulzer, Sabine, Dipl.Ing.Tognacca, Christian, Dipl.Ing.Volkart, Peter, Dr., Dipl.Ing.Weber, Monika, Dipl.Ing.

Departement Bau und Umwelt, ETH ZürichMüller, Andreas, Dr., Dipl.Phys.

Bundesamt für Strassenbau, Abt. Projektierung, Umwelt,Verkehr/Brücken, BernDonzel, Michel

Bundessamt für Wasserwirtschaft, BielGötz, Andreas, Dipl.Ing.

Ingenieurbüro Basler & Hofmann AG, ZürichKurmann, P., Dipl.Ing.

Institut für konstruktiven Wasserbau und Tunnelbau,Universität Innsbruck, InnsbruckSchöberl, Friedrich, Prof. Dr.

Ingenieurbüro Staubli, Kurath & Partner AG, ZürichStaubli, R., Dipl.Ing.

Karlsruhe, Germany

Bundesanstalt für Wasserbau (BAW)Eisenhauer, N. Dr.-Ing.

30

Heibaum, M.H., Dr.-Ing.Pietsch, M.Rossbach, B., Dr.-Ing.Grath, S.

Bergisch-Gladbach, Germany

Bundesanstalt für Strassenwesen (BAST)Thamm, B., Dr.-Ing.

Rostock UniversityKohlhase, S., Prof. Dr.-Ing.

Heinrich Hirdes Gmbh, RostockSchlie, S., Dipl.-Ing.

The Netherlands

Delft Hydraulicsvan Meerendonk, E., M.Sc.Verheij, H.J., M.Sc.

Ministry of Transport, Public Works and Water ManagementBerendsen, E., M.Sc.de Wilde, D.P., B.Sc.Hoffmans, G.J.C.M., Dr.Pilarczyk, K.W., M.Sc.

United Kingdom

H.R. WallingfordBettess, R., Dr.May, R.W.P., Dr.Escarameia, M.

University of StrathclydeRiddell, J.F., Dr.

Highways AgencyHalliday, J.

RailtrackFawcett, S.

ATPEC River Engineering ConsultancyPepper, A.

Jeremy Benn and AssociatesBenn, J.

Binnie Black & VeatchClark, P.B.

University of NottinghamDowns, P.W.Skinner, K.

Soar, P.Thorne, C.R., Dr.Wallerstein, N.Wood, A.Wright, N., Dr.

Poland

Roads Bridge Research Institute, ZmigrodWysokowski, A., Dr.Ing.

Dr. Wysokowski joined the Panel in Karlsruhe and partici-pated in the scanning review during our three days in Ger-many. He made a presentation on the floods of July 1997and July 1998 on the Oder River, which forms the borderbetween Germany and Poland, in which hundreds of bridgeswere damaged and thousands declared unsafe as a result ofsubsequent scour evaluations. He provided two detailed re-ports, in Polish, showing damages and recovery efforts.

APPENDIX B

GLOSSARY OF ACRONYMSAND ABBREVIATIONS

Swiss Organizations

ASTRA Federal Office for HighwaysBAV Federal Office for TransportBWW Federal Office for Water ManagementETH Swiss Federal Institute of TechnologySBB Swiss Federal RailwaysVAW Laboratory of Hydraulics, Hydrology, and

Glaciology

German Organizations

BAST Federal Highway Research InstituteBAW Federal Waterways Engineering and

Research InstituteDVWK German Association for Water Resources

and Land Improvement

British Organizations

HA Highways AgencyNRA National Rivers Authority

United States Organizations

AASHTO American Association of State Highwayand Transportation Officials

DOT Department of TransportationFHWA Federal Highway AdministrationNCHRP National Cooperative Highway Research

Program

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NHI National Highway InstituteSHA State Highway AgencyTRB Transportation Research Board

General Terminology

HEC Hydraulic Engineering Circular (FHWA)HEC Hydrologic Engineering Center (U.S. Army

Corps of Engineers)NBIS National Bridge Inspection Standards (US)PIANC Permanent International Association of

Navigation CongressesPONTIS Bridge Management System Software

(U.S.)SOWAS Soil, Water, Structure

APPENDIX C

REFERENCES

1. Lagasse, P.F., M.S. Byars, L.W. Zevenbergen, and P.E.Clopper. “Bridge Scour and Stream Instability Coun-termeasures: Experience, Selection and Design Guid-ance.” Hydraulic Engineering Circular 23, FHWA HI-97-030, Washington, D.C. (1997).

2. Richardson, E.V. and S.R. Davis. Evaluating Scour atBridges.” Hydraulic Engineering Circular 18, ThirdEdition, FHWA HI-96-031, Washington, D.C. (1995).

3. Lagasse, P.F., J.D. Schall, F. Johnson, E.V. Richardson,and F. Chang. “Stream Stability at Highway Structures.”Hydraulic Engineering Circular 20, Second Edition,FHWA HI-96-032, FHWA, Washington, D.C. (1995).

4. Lagasse, P.F., E.V. Richardson, J.D. Schall, and G.R.Price. NCHRP Report 396: “Instrumentation for Mea-suring Scour at Bridge Piers and Abutments.” TRB,National Research Council, Washington, D.C. (1997).

5. Schall, J.D., G.R. Price, G.A. Fisher, P.F. Lagasse, andE.V. Richardson. NCHRP Report 397B, “MagneticSliding Collar Scour Monitor: Installation, Operation,and Fabrication Manual.” TRB, National ResearchCouncil, Washington, D.C. (1997).

6. Schall, J.D., G.R. Price, G.A. Fisher, P.F. Lagasse, andE.V. Richardson. NCHRP Report 397A “Sonar ScourMonitor: Installation, Operation, and FabricationManual.” TRB, National Research Council, Washing-ton, D.C. (1997).

7. Federal Office for Highways (ASTRA), Federal Officefor Transport (BAV), Federal Office for Water Man-agement (BWW), Swiss Federal Railways (SBB). Sta-bility of Structures in Water: Recommendation for thepreservation and maintenance of existing structures/Hints for the construction of new structures. FederalPublications and Supplies Office, CH 3000 Berne,Switzerland (1998).

8. Hoffmans, G.J.C.M. and H.J. Verheij. Scour Manual.A.A. Balkema: Rotterdam, Brookfield (1997).

9. British Railways Board. Precautions Against ScourAction on Structures. Handbook 47, Revision A, GroupStandards, MacMillan House, Paddington, U.K. (1993).

10. Railtrack Southern. Managing the Danger to the Rail-way from Flooding and Tidal Action. Standards andProcedures Manual, Waterloo, U.K. (1998).

11. Jeremy Benn and Associates. Scour Assessment Com-parison. Railtrack Pic, Project Delivery Services, Mer-cury House, London, U.K. (1997).

12. Jeremy Benn and Associates. Scour Risk Assessmentand Protection Design. Course Notes, Railtrack ProjectDelivery Services—Southern, Mercury House, London,U.K. (1997).

13. Binnie Black & Veatch. Assessment of Scour at High-way Bridges (Draft). Advice Note, Revision D, High-ways Agency, London, U.K. (1998).

14. Escarameia, M. River and Channel Revetments, A De-sign Manual. H.R. Wallingford, DETR EnvironmentTransport Regions, Environment Agency, Published byThomas Telford Publications, Thomas Telford Ltd.,U.K. (1998).

15. Morgan, R.P.C., A.J. Collins, M.J. Hann, J. Morris,J.A.L. Dunderdale, and D.J.G. Gowing, Waterway bankprotection: A Guide to Erosion Assessment andManangement. R & D Draft Technical Report W5/i635/3, Environment Agency, Almondsbury, Bristol,U.K., date unknown.

16. Pilarczyk, K.W. (ed.). Dikes and Revetments: Design,Maintenance, and Safety Assessment. A.A. Balkema,Rotterdam, Brookfield (1998).

17. CUR Report 169. “Manual on the Use of Rock in Hy-draulic Engineering.” Prepared by the Netherlands Cen-tre for Civil Engineering Research and Codes (CUR),A.A. Balkema Publishers, Old Post Road, Brookfield,VT (1995).

18. Wark, J.B., C.S. James, and P. Ackers. Design ofStraight and Meandering Compound Channels: InterimGuidelines on Hand Calculation Methodology. H.R.Wallingford, R&D Report 13, National Rivers Author-ity, Bristol, U.K. (1994).

19. Environment Agency. River Geomorphology: A Prac-tical Guide. Guidance Note 18, National Centre forRisk Analysis and Options Appraisal, prepared by Uni-versities of Nottingham, Newcastle, and Southampton,Almondsbury, U.K. (1998).

20. Thorne, C.R. Stream Reconnaissance Handbook: Geo-morphological Investigation and Analysis of RiverChannels. Department of Geography, University ofNottingham, John Wiley & Sons, Chichester, U.K.(1998).

21. Thorne, C.R., R.D. Hey, and M.D. Newson (eds.). Ap-plied Fluvial Geomorphology for River Engineeringand Management. John Wiley & Sons, Chichester,U.K. (1997).

22. Pilarczyk, K.W. “Design Tools Related to RevetmentsIncluding Riprap.” In River, Coastal and Shoreline

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Protection. Erosion Control Using Riprap andArmourstone, edited by C.R. Thorne, S.R. Abt, F.B.J.Barends, S.T. Maynord, K.W. Pilarczyk, John Wiley &Sons, Chichester, U.K. (1995) pp. 17-38.

23. Federal Office for Water Management. Flood Pro-tection: A Common Goal for Federal, Cantonaland Municipal Authorities. Bienne, Switzerland,undated.

24. Breusers, H.N.C. and A.J. Raudkivi. Scouring. A.A.Balkema Publishers, Rotterdam, the Netherlands(1991).

25. Jorissen, R.E., E. Berendsen, and D.P. deWilde. “De-sign of the Bed Protection of the Rotterdam Storm SurgeBarrier.” In River, Coastal and Shoreline Protection.Erosion Control Using Riprap and Armourstone, editedby C.R. Thorne, S.R. Abt, F.B.J. Barends, S.T.Maynord, K.W. Pilarczyk, John Wiley & Sons,Chichester, U.K. (1995) pp. 718-724.

26. Rijkswaterstaat, De Oosterschelde Stormvloedker-ing Bouwcombinate, and Ostem (A Joint Publication).The Storm Surge Barrier in the Eastern Scheldt. TheNetherlands (1987).

27. Schulz, H. “Considerations Regarding the Experienceand Design of German Inland Waterways.” In River,Coastal and Shoreline Protection. Erosion ControlUsing Riprap and Armourstone, edited by C.R. Thorne,S.R. Abt, F.B.J. Barends, S.T. Maynord, K.W.Pilarczyk, John Wiley & Sons, Chichester, U.K. (1995)pp. 414-437.

28. Naue Fasertechnik GmbH & Co. NAUE Sand Contain-ers. Lemförde, Germany, undated.

29. Bakker, K.J., H.J. Verheij, and M.B. deGroot. DesignRelationship for Filters in Bed Protection. TechnicalNote, ASCE, Journal of Hydraulic Engineering, Vol.120, No. 9 (September 1994) pp. 1082-1688.

30. Klein Breteler, M. and H.J. Verheij. Geotextiles,Geomembranes and Related Products. 4th InternationalConference on Geotextiles, the Hague (1990).

31. Federal Waterways Engineering and Research Institute(BAW). Code of Practice: Use of Geotextile Filters onWaterways (MAG). Karlsruhe, Germany (1993).

32. Deutscher Verband für Wasserwirtschaft und Kulturbaue.V. (DVWK). Application of Geotextiles in HydraulicEngineering. Guidelines for Water Management 306,Issued in cooperation with International Commissionon Irrigation and Drainage (ICID), published byWirtschafts-und Verlagsgesellschaft Gas and WassermbH, Bonn, Germany (1993).

33. Permanent International Association of NavigationCongresses (PIANC). Guidelines for the Design andConstruction of Flexible Revetments IncorporatingGeotextiles for Inland Waterways. Report of WorkingGroup 4 of the Permanent Technical Committee I,Supplement to Bulletin No. 57, Brussels, Belgium(1987).

34. Permanent International Association of NavigationCongresses (PIANC). Guidelines for the Design andConstruction of Flexible Revetments IncorporatingGeotextiles in Marine Environment. Supplement toBulletins Nos. 78/79, Brussels, Belgium (1992).

35. Abromeit, H.U. and M.H. Heibaum. Stressing ofGeosynthetics during Installation and Construction onSite: Installation of Geosynthetics in Waterways. InGeosynthetics: Applications, Design and Construction,DeGroot, Den Hoedt & Termaat (eds.), Balkema,Rotterdam (1996).

36. Kohlhase, S. Some Aspects of the use of Geotextiles inthe field of Coastal Engineering. In First German-Chinese Joint Seminar on Recent Developments inCoastal Engineering, Hasenwinkel, September 8th to10th 1997, published by Shaker Verlag, Aachen, Ger-many (1998) pp. 235-265.

37. Hunzinger, L. and B. Zarn. Morphological Changes atEnlargements and Constrictions of Gravel Bed Rivers.Third International Conference on River Flood Hydrau-lics, Stellenbosch, South Africa (November 1997) pp.227-236 and 5-7.

38. Dröge, B. Change of River Morphology by ControlledErosion and Deposition—Bed Load Budget of the RhineRiver. In 5th International Symposium on River Sedi-mentation, Karlsruhe, Germany (1992) pp. 85-94.

39. Gölz, E. Recent Morphologic Development of the RhineRiver. In 5th International Symposium on River Sedi-mentation Karlsruhe, Germany (1992) pp. 95-103.

40. Schilling, M. Increase in Flood Risk due to BedloadExamples from the Swiss Alps. International Journal ofSediment Research, Vol. 12, No. 3 (December 1997)pp. 160-169.

41. Bestmann, L., M. Heibaum, and S. Roth. From SheetPiling to Vegetated Embankments—Conventional andBiological Engineering Works for Bank Protection onWaterways. Inland Waterways and Ports (for Commer-cial and Pleasure Navigation), Subject 4, Compatibilitybetween nature preservation and waterways, PermanentInternational Association of Navigation Congresses(PIANC), Seville (1994).

42. Annandale, G.W. “Risk Analysis of River Bridge Fail-ure.” In Stream Stability and Scour at HighwayBridges: A Compendium of Papers from ASCE WaterResources Engineering Conferences 1991 to 1998,Richardson, E.V. and Lagasse, P.F. (eds.), ASCE,Reston, VA (1999) pp. 1003-1012.

43. Johnson, P. Fault Tree Analysis of Bridge Failure dueto Scour and Channel Instability. ASCE Water Re-sources Engineering ’98, Proceedings of the Interna-tional Water Resources Engineering Conference, Mem-phis, TN (August 3-7, 1998) pp. 110-114.

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APPENDIX D

DESIGN METHODOLOGY

TIME RATE OF SCOUR

An area of European research that could be extremelyuseful to U.S. hydraulic engineers is time dependent scour.The Dutch Scour Manual (8) includes methods for predict-ing the rate of scour development. The presentation atWallingford, England also included the topic of rate of pierscour. In each case, a characteristic time was defined thatwas related to a characteristic depth of scour. Although thecharacteristic depths and times were defined differently,each is related to the critical (incipient motion) velocity, theapproach velocity and a coefficient related to turbulence in-tensity. The British define the coefficient as the ratio ofmaximum velocity around a pier to the approach velocityand the Dutch (8) define the coefficient as the relative turbu-lence intensity. These concepts may be useful to scour prac-tice and scour research in the U.S.

In tidal areas in the U.S., hurricane storm surges oftenproduce extreme hydraulic conditions. Computing ultimatecontraction scour amounts for these conditions may not bereasonable based on the short duration (approximately 3hours) of the surge. Based on equations in the Dutch ScourManual (8), the time development of a contraction scour hole

was estimated for a bridge in the southeastern U.S. (Georgiacoast) for a 500-year storm surge. To provide confirmation ofthese results, the Yang sediment transport equation was usedto compute contraction scour hole development based on theerosion of the scour hole equal to the transport capacity in thecontracted bridge opening. The scour rates for this situationare shown in Figures D1 and D2. Figure D1 shows the fulldevelopment of the scour with time plotted on a logarithmicaxis, and Figure D2 shows the first 100 hours of developmentwith time plotted on an arithmetic axis. The scour rates pre-dicted by the two methods are extremely similar and indicatethat the scour that could be generated in the few hours avail-able during a storm surge is significantly less than the ulti-mate contraction scour condition.

Also shown in Figures D1 and D2 is the development ofa pier scour hole for the same hydraulic conditions. FigureD2 shows that the pier scour hole reaches 90 percent ofultimate scour in the first 20 hours while the clear watercontraction scour reaches only about 30 percent of ultimatescour.

The Dutch methods are based on clear water scour andthe conditions used to test the Yang equation were close toclear water. The Scour Manual (8) indicates that under livebed conditions, scour reaches ultimate conditions more rap-idly and that the ultimate scour is less than the equivalentclear water case which is consistent with current U.S. guid-ance. Figure D3 shows the development of contraction scour

Figure D1. Time development of scour.

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Figure D2. Initial scour development.

Figure D3. Contraction scour development with sediment supply.

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(using the Yang equation) under varying amounts of up-stream sediment supply relative to the transport capacity inthe bridge opening. For the case shown, if the upstreamchannel is supplying 50 percent of the contracted sectiontransport capacity, the scour hole reaches its ultimate depthin approximately 1 hour. Based on this review, it appearsthat contraction scour should be reviewed on a case-by-casebasis to assess the level of scour that could occur over ashort time.

The time-dependent scour information obtained duringthe scanning tour has been extremely useful on several on-going tidal scour studies for bridge rehabilitation. Signifi-cant cost savings are expected for bridge rehabilitation andnew bridge design based on this topic alone.

SCOUR AT WIDE PIERS

In commenting on the initial Summary Report for thescanning review, Mr. Henk Verheij of Delft Hydraulics inthe Netherlands made the following observations.

In the past, Delft Hydraulics developed a formula forcalculating scour at wide piers (and slender piers), namely.the Breusers formula (Breusers, Nicollet, and Shen, “LocalScour Around Cylindrical Piers,” Journal of Hydraulic Re-search, Vol. 15, no. 3 [1977] pp. 211-252):

ys = 1.5btanh(ho/b)

with ys = scour depth, b = pier width, and ho = water depth.

This formula is not mentioned by Breusers and Raudkiviin their 1991 manual, but it is in the Scour Manual byHoffmans and Verheij on page 114 (1997).

Depending on the ratio ho/b this formula predicts scourfor slender piers (ho/b > 1) or wide piers (ho/b < 1):

wide piers: b/ho > 1 resulting in: ys = 1.5 hslender piers: b/ho < 1 resulting in: ys = 1.5 b

Bridge Scour-Europe Scour

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