Monitoring Scour Critical Bridges (2009)

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SYNTHESIS OF PRACTICES The use of fixed instrumentation as a scour countermeasure is a process that begins with the evaluation of the scour counter- measure alternatives for a particular bridge site, includes the design and installation of the instrumentation and the devel- opment of a scour monitoring program, and can continue for many years with the scour monitoring program for the bridge. Owners and others that have completed the synthesis surveys have reported successes and failures at various steps of the process. This chapter presents a synthesis of the reported best practices and the lessons learned with the use of fixed scour monitoring instrumentation at bridges. Evaluation of Scour Countermeasure Alternatives Scour monitoring is often the preferred alternative for a variety of reasons. For bridges that are scheduled to be replaced, scour monitors can be selected because they can be less expensive than traditional structural or hydraulic countermeasures. The placement of armoring in a waterway can also result in envi- ronmental concerns and complicated permitting issues. In addition, armoring of the channel bottom can interfere with the construction of the new bridge. Fixed instrumentation is also being used on scour critical bridges where there are no bridge replacement plans. Scour monitors can be installed at these bridges as an interim counter- measure, before the installation of other hydraulic and/or structural countermeasures that can take longer to design and install. The fixed monitors can also be installed in conjunc- tion with other types of hydraulic and/or structural counter- measures, to confirm that they are functioning to protect the bridge. For example, if riprap is installed for pier protection, the 2001 FHWA Hydraulic Engineering Circular 23: Bridge Scour and Stream Instability Countermeasures guidance states that it should be monitored. The selection, location, and design are dependent on many factors. These include cost, environmental, construction, and maintenance considerations. Some advantages cited in the surveys include: • Provides safety for the traveling public • Allows for continuous monitoring of streambed eleva- tions and scour conditions • Can be quickly designed and installed • Is a cost-effective system relative to other hydraulic and structural scour countermeasures • Remote downloading of data reduces required visits to the bridge • Reduces the number of diving inspections and/or bathy- metric surveys owing to the information provided by the monitors • Increases the capability of measuring both scour and the refill processes • Allows for the development of a prescribed Plan of Action to guide decision making during a flood event • Is appropriate for large bridges and deep water conditions • Can be used to extend the life of a bridge • Can be used in combination with other scour counter- measures • Provides data useful for replacement bridges • Provides data for scour research. The various types of fixed instrument devices are summa- rized in Table 7. The best type of application, as well as some of the advantages and disadvantages of each type of device are listed. The scour monitoring system is custom designed for each bridge site. The type of monitoring instrument employed depends on the geometry of the bridge substructure and on the channel characteristics. Guidance on the selection of a scour monitoring system is provided in FHWA HEC-23. Factors such as the depth of the water, the size of the bridge, the geometry of the substructure unit, the frequency with which readings will be taken, and the extent of debris, ice, air entrainment, and/or turbidity in the channel need to be con- sidered in the selection of a scour monitoring system. The fixed instrumentation selection matrix, Table 8, was developed to compliment the countermeasure selection matrix in FHWA HEC-23. If fixed instrumentation is to be used to monitor a bridge, this table provides additional items to be considered in deciding between the various fixed instrument options. It was developed based on the results of the synthesis study state survey and literature search. Table 8 includes the following categories for suitable river environment for the various fixed instruments: • Type of waterway—riverine/tidal, • Flow habit, CHAPTER EIGHT CONCLUSIONS 53

54 • Water depth, • Bed material, and • Extreme conditions. The functional applications and bridge geometry include information on the characteristics of the bridges for the dif- ferent types of instruments: (1) Substructure monitored, and (2) Foundation type. The table includes additional items regarding the monitor- ing system capabilities that can be mandatory or desirable criteria for a particular bridge site: (1) Continuous monitoring, and (2) Remote technology. The last two columns include the installation experience by state for each type of fixed monitor for those that responded to the synthesis survey and also from the literature search. Design of Scour Monitoring System There are a variety of options to consider in the design of a fixed scour monitoring system for a particular bridge site. Careful evaluation of the bridge and site conditions can help ensure that the system will provide the necessary data and is robust enough to function for the intended duration of the scour monitoring. The locations of the monitors on the bridge are selected in consideration of accessibility, protection against vandalism, and any potential debris or ice debris forces. The heightened security at the bridges in the past few years has made acces- sibility a major issue. Traffic safety, lane closures, and traffic detours for servicing the monitors also need to be considered. The increased use of cameras for bridge security can be employed to protect the scour monitors from vandalism. The location and number of the monitors will vary depend- ing on the extent of the existing and potential scour problem, the amount of risk the owner is willing to take, and the funding available for the scour monitors. The monitors are generally placed in locations where maximum scour is expected to occur. Accessibility is important to ensure access to the monitor- ing system when maintenance is required. It is necessary for servicing the system, inspection, and repairs. The daily data record produced by the system can also provide information on the health and operational status of the scour system. There are instances, however, where the data appear reasonable, yet one of the sensors is not functioning properly. Regularly scheduled routine maintenance and inspections help to ensure that the system is functioning properly and the streambed readings are accurate. The design of the monitoring instrument and the method with which it is attached to the bridge is site-specific. As-built plans and diving inspections can provide information on the geometry of the underwater portion of the pier or abutment. When there are uncertainties regarding underwater dimensions and clearances, adjustable arms can be designed for the mount- ing bracket. During installation, the contractor can then adjust these brackets so that a device such as the sonar projects out sufficiently to clear the footing and take streambed readings. Once the location of the device and the spot to be monitored are selected, the best approach would be for the design engineer to work with the structural and electrical engineers to detail the mounting and the conduit for the monitoring system. Items such as types of materials, bolts and their embedment depths, and conduit routing and attachments are best detailed by these specialists. Using robust, although often more expen- sive materials and methodologies, will most likely result in improved sensor integrity as well as significant savings in Type of Fixed Instrum entation Best Application Advantages Limitations Sonar Coastal regions Records infilling; time history; can be built with off- the-shelf components Debris, high sedi me nt loading, and air entrainm ent can interfere with readings Magnetic Sliding Collar Fine bed channels Sim ple, m echanical device Vulnerable to ice and debris impact; only measures ma xi mu m scour; unsupported length, binding Tilt Sensors All May be installed on the bridge structure and not in the stream bed and/or underwater Provides bridge m ovem ent data that may or may not be related to scour Float-Out Device Ephem eral channels Lower cost; ease of installation; buried portions are low main tenance and not affected by debris, ice, or vandalis m Does not provide continuous m onitoring of scour; battery life Sounding Rods Coarse bed channels Si mp le, mechanical device Unsupported length, binding, augering Ti me Do main Reflectom eters Riverine ice channels Robust; resistance to ice, debris, and high flows Lim it on maximum lengths for signal reliability of both cable and scour probe TABLE 7 FIXED INSTRUMENTATION SUMMARY

TABLE 8 FIXED INSTRUMENTATION SELECTION MATRIX (Countermeasure characteristics) Local Scour Contraction Flow Habit Water Depth Bed Material Extreme Conditions Foundation Type Maintenance Type of Fixed Abutments 1 Piers Scour Vertical Lateral Tidal Riverine E=Ephemeral A = < 3 ft F = Fine bed D=Debris P=Piles Continuous Remote H = High No. of No. of Installation Experience by Additional Installation Instrumentation Floodplain I=Intermittant B = 10-30 ft S = Sand bed T=Temperatures SF=Spread Ftg Monitoring Technology M = Moderate Bridge Instruments State from Surveys Experience by State and P=Perennial C = 31-50 ft C = Coarse bed S=Sediment loads DS=Drilled Shafts L = Low Sites (Note: States in bold have Other Sources Channel PF=Perennial/ D = 51-75 ft R = Riprap I=Ice flows U=Unknown indicated they plan to use fixed Flashy E = 76-100 ft V=High Velocity Flows instrumentation in the future) Sonar T, I, V Yes Yes M - H 48 164 AK, AR, CA, FL, GA, HI, IN, KS, MD, NC, NJ, NV, NY, TX, VA, WA CO, NM, OR, RI, WI Magnetic Sliding Collar A, B F, S, C Yes Yes M 8 22 CA, HI, IN, MN, NJ, NY CO, FL, ME, MI, NM, RI, TX, WI Tilt Sensors Yes Yes L 4 35 CA, WA Float Out Device E, I A, B F, S No Yes L 3 35 AL, CA, NV AZ Sounding Rods 1 A, B C T, S SF Yes No H 0 0 AR, IA, NY Time Domain Reflectometers 1 P, PF A, B F, S, C Yes Yes M 1 2 VT well suited/primary use 1 There were limited survey replies for monitoring of abutments, sounding rods and time domain reflectometers, therefore information from the literature was used for this table. possible application/secondary use 2 The following items listed in the FHWA HEC-23 countermeasure matrix are applicable to the full range of the characteristics for fixed instrumentation and were not included in the survey: unsuitable/rarely used River type: braided, meandering, straight N/A not applicable Stream size: wide, moderate, small suitable for the full range of the characteristics/conditions Bend radius: long, moderate, short Bank condition: vertical, steep, flat Floodplain: wide, moderate, narrow/none Installation Experience Stream Instability Waterway Type Capabilities Survey Respondents FUNCTIONAL APPLICATIONS SUITABLE RIVER ENVIRONMENT 2

future repair costs, especially on bridges over deep waters. This is the result of the high costs associated with underwater installations, maintenance, and repairs. Severe environmental conditions that can interfere with the functioning of the monitors, such as debris, ice, and tidal waters, need to be considered when choosing the materials and type of mountings for the fixed instruments. Many fixed monitors will not operate under frozen water conditions. Owing to the cold weather and tidal waterways in the northeast installations, AISI Grade 316 stainless steel has been used. A lower grade of stainless steel (AISI Grade 304) was employed during an emergency installation in New York, and a few years later the mountings had extensive corrosion. On one Alaska bridge installation there were instances where float- ing debris ripped the sonar sensor from the substructure. In Alaska, they have developed a “retractable arm” that lowers the sonar into the water at designated times to take readings, and then retracts back to a designated location under the bridge. The power source will vary depending on what is available and most reliable for a particular bridge site. The monitoring system can be solar powered or connected to electrical power at the bridge, if available. The monitoring systems require low power; therefore, solar power is adequate and in more recent installations, the preferred power source. Initially in the early installations there was concern regarding the use of solar panels owing to potential vandalism. Numerous panels have been installed when there was no other power source, and these have performed better than the locations using traditional electrical power. The locations powered by alter- nating current have required replacement float chargers, most likely the result of power surges. Remote monitoring has been installed using cellular tele- phone, telephone landline, or satellite technology. The tele- phone lines have proved to be the most reliable. They do not require power and are continuously available. Cellular telephones are also reliable, but they are not continuous, and need to be turned on and off at regular intervals using solar panels. Satellite service has been used when the other two options were not available. Satellite service, although less expensive than cellular systems, has a disadvantage—it can provide only one-way communication from the bridge. The system can send data from the bridge; however, incoming commands to examine, modify, or repair the system cannot be transmitted to the bridge, as is done with the other methods. More recent monitoring systems transmit data to a server and it is posted on the Internet so that those with authorized pass- words can access the data. This provides greater flexibility because the data can be retrieved and analyzed from any location with a computer and Internet access. The mechanism for the design and installation of the scour monitoring instrumentation and the program can be accomplished under numerous types of contracts. The plans and specifications can be developed as part of a larger bridge rehabilitation program. In this case, careful attention is required for the timing of the installation of the scour monitors, as well as the protection of the monitors during the construction. The scour monitors can be installed as a stand-alone contract, accomplished under emergency conditions, or if funding is available for this type of scour countermeasure system. Numerous monitoring systems have been installed as part of research projects. These often include devices that measure scour and other hydraulic variables, which can provide data useful for scour research. One problem with the research installations is that they are often limited by the duration of the project, which is often two to four years. Provisions for funding the continued operation of the scour monitoring sys- tem can be made so that the bridge owner is able to continue to retrieve the data and maintain the monitoring system upon the completion of the research. The data from the monitors can be taken at programmed intervals and downloaded at any time. The data can be set up to automatically alert the owner or designated others of emer- gency situations. The systems can provide round-the-clock monitoring, even during storms. Installation of the Monitoring System Scour monitoring systems are a relatively new technology. Electrical and underwater contractors most often install the system. It should be noted that on larger bridges in deep waters, the contractor installation costs often equal or exceed the cost of the manufacture of the scour monitoring system. Most likely, the contractor has not performed this type of work, so it is necessary that the plans and specifications be very detailed to ensure the successful installation of the system. The inclusion of good details can also aide in keeping the bid prices reasonable because the contractor will better under- stand the extent of the work. It is also advisable to have one of the designers of the monitoring system on-site or in close contact with the contractor throughout the installation. There are often many unknowns both in the underwater conditions and in the as-built geometry of the substructure unit. New site information on existing scour can result in changes to the location of the scour monitors. Having the system designer available during the installation ensures that the proper changes are made in the field. There can be numerous unknowns for underwater instal- lations. If the underwater contractor is not receiving a lump sum payment, but the work is based on the time to install (time and materials), the designer can specify the means and method of installation. For example, installation equipment such as the type of drill the contractor uses to install the under- water components can be specified. A pneumatic drill has been used effectively to minimize the time it takes for the installation of anchor bolts into concrete substructure units. There could be extensive time delays when the contractor uses drills that are not appropriate for underwater construction. 56

Because the construction inspector cannot view the under- water components, it is advisable to have these components of the installation inspected by an independent contractor before completion of the contract. This will ensure that all bolts and attachments are in place and that the mounting is prop- erly secured to the substructure unit. Underwater installation photographs by the contractors ensure the proper installation and also provide as-built information for future inspections, maintenance, and repairs. In smaller waterways, and in areas of installation that are less complicated, there have been cases where the department of transportation (DOT) maintenance group or others have installed the scour monitoring system. Here also, it is sug- gested that a member of the monitoring design team work with these groups. As with all bridge reconstruction projects, it is good practice to develop a set of as-built plans following the installation of the system. This is particularly true for the underwater com- ponents of the system. This will aid in future maintenance, inspections, and repairs to the system. Plan of Action The federal requirements for bridge inspection are set forth in the National Bridge Inspection Standards (NBIS). The NBIS require bridge owners to maintain a bridge inspection program that includes procedures for underwater inspection. This information can be found in the FHWA Federal Register, Title 23, Code of Federal Regulations, Highways, Part 650, Bridges, Structures, and Hydraulics, Subpart C, National Bridge Inspection Standards (23 CFR 650, Subpart C). The most recent ruling was enacted on January 13, 2005. The revisions underscore actions required for bridges that are determined to be scour critical. These include the preparation of a Plan of Action to monitor known and potential deficien- cies and to address critical findings and monitoring of bridges in accordance with the plan for bridges that are scour critical (23 CFR 650.313). FHWA HEC-23 contains guidance on the development of a Plan of Action. The two primary components of the Plan of Action are instructions regarding the type and frequency of inspections to be made at the bridge and a schedule for the timely design and construction of scour countermeasures. A Plan of Action includes the following: (1) management strategies, (2) inspection strategies, (3) bridge closure instruc- tions, (4) countermeasure alternatives and schedule, and (5) miscellaneous information. Scour monitoring programs with flood, portable, and/or fixed monitoring are important components of a Plan of Action. In 2006, the FHWA posted a revised Plan of Action standard template on their website. The section on Monitoring Programs includes items for detailed documentation of regular/increased inspections, fixed scour devices, and flood monitoring. In 2007, a new National Highway Institute (NHI) course (FHWA-NHI-135085) enti- tled “Plan of Action (POA) for Scour Critical Bridges” was developed. The course provides guidance on developing a POA and case studies for the development of a POA. One case study uses fixed instrumentation for monitoring. The course and Standard Template can be downloaded from the FHWA website. Implementation of Scour Monitoring Program The implementation of the scour monitoring program is a critical aspect of the program. Owing to the interdisciplinary nature of scour monitoring, and perhaps the result of in part the newness of the FHWA bridge scour program and of these devices, it is not always obvious which division of the owner will be responsible for the scour monitoring program. It is important during the design process for the owner to identify the group(s) that will be responsible for the scour monitoring program. This could be the owner or it can be outsourced. The process includes the design of the system protocol; routine and emergency monitoring; analysis of the data and determi- nation of the safety of the bridge; the chain of command to make decisions during an emergency situation; maintenance, inspection, and repairs to the system; and the funding for the continued operation of the scour monitors. This information should be documented in the scour monitoring program man- ual and Plan of Action for the bridge. The manual needs to be updated on a regular basis to reflect any changes in the pro- gram. The responsibility for the monitoring system has been the most difficult aspect in the implementation of the scour monitoring programs reported in the synthesis surveys. If a clear protocol detailing responsibilities is in place, this can help to provide proper maintenance to prevent a sensor or system failure. If the person(s) responsible for monitoring is transferred to another position, or if they retire, a new person(s) needs to be given the responsibility and training for the system. There have been instances where the telephone service has been interrupted owing to non-payment of the telephone bill. This was the result of job transfers and, in one case, the invoice was being sent to someone not involved in the scour program. In one situation, the area code in a city changed and the data could not be accessed because the new area code needed to be programmed into the new monitoring system. Routine and Emergency Monitoring and Data Analysis The development of a clear set of detailed instructions for those responsible for the routine and emergency monitoring of the bridge is essential. There could be a chain of command so that responsibility is transferred when those who are responsible are on vacation, ill, unable to monitor, or are no longer in their particular position. The routine and emergency procedures are very site specific. Often an owner will start with a conservative program with high frequencies for routine and emergency monitoring. After a period, the records will be reviewed and the frequency of monitoring can be adjusted. 57

58 A clear chain of command of those responsible for emer- gency situations needs to be in place. Those responsible for analyzing the data should have instructions as to who they should contact “round-the-clock” should the scour readings indicate a problem. The Plan of Action would indicate possi- ble procedures to follow, which can include closure of the bridge, land monitoring, underwater inspections, the emer- gency installation of contingency countermeasures such as riprap, etc. The scour monitoring systems that are continuous are capable of producing a large amount of data. Consideration needs to be given to the intervals at which the data would be recorded and collected. Data reduction methods using com- puter spreadsheet programs provide valuable assistance for analyzing and storing the data. They help identify trends and can be useful when comparing data with other bridge sites. Changes in the watershed can also affect the data. It is important that those responsible for analyzing and interpret- ing the data be kept informed about new developments, con- struction, dredging, mining, or other situations that might cause scour or siltation at the bridge. Maintenance, Inspection, and Repairs It is important to develop a regular maintenance and inspection program. The maintenance crews for the owner can be respon- sible for routine, above-water maintenance. The frequency of underwater and structural inspections and fathometer surveys at each bridge will vary. The owner can add inspection and maintenance requirements for the scour monitoring system to the underwater and structural inspection contracts. If the bridge is a movable bridge and there are also electrical inspectors these can aide in the above-water inspection of the electrical components of the system. The inspection guidelines and requirements could include detailed checklists and sketches to guide the inspectors, and to ensure that the scour monitor- ing system is examined periodically. Provisions can be made in these contracts for minor repairs as well. During the inspec- tions, it is advisable that a member of the scour monitoring team coordinate with the inspection crew to ensure that all important components are inspected, and to help interpret their findings. If possible, this person would be on-site dur- ing the inspection. The streambed elevations recorded during diving inspections and fathometer surveys can also be used as ground truth measurements to check the accuracy of the scour monitoring devices. RESEARCH NEEDS Scour Monitoring Instruments The advancements that bridge owners would like to see for future fixed scour monitoring technology include the devel- opment of durable instrumentation, with increased reliability and longevity, decreased costs, and minimum or no mainte- nance. This equipment would include instrumentation that measures scour, and also water elevations and velocities. A discussion of states that are currently sponsoring research on the development of scour monitoring devices can be found in chapter seven. A pooled fund project for the development of scour monitoring devices can be considered because these instruments can be used under similar conditions in numerous states. The owners reported mostly the same problems with respect to the existing scour monitoring devices and they are attempting to address comparable challenges. A pooled fund project would provide in-depth testing and analysis of scour monitoring devices technology. One bridge owner noted that the current fixed scour mon- itors will take a measurement in one location, and this point measurement can or cannot be the deepest point. The deepest point of a scour hole can also change from one event to another. They recommended the development of an instrument that measures the depth and location of the deepest point of a scour hole, or one that would map an entire scour hole. The multi-beam sonar technology that is currently being employed for fathometric surveys can be an option, although expensive, if this type of measurement is required. Scour Monitoring Protocols As discussed in chapter four, the problems with maintenance of the scour monitoring system and program were the main concern expressed by the bridge owners with systems, and also by some who have not used them. The development of a detailed handbook on the implementation of a scour moni- toring program would help owners anticipate both the advan- tages and responsibilities of a successful scour monitoring system. The focus of the scour monitoring technology has been on the development and improvement of the devices. The recently published FHWA guidance on the Plan of Action discussed earlier in this chapter could be useful in the devel- opment of a detailed, hands-on protocol for emergency actions for scour monitoring programs. An additional, more practical manual with guidance to ensure that the scour monitoring system remains active would help DOTs and other bridge owners that can be considering the use of fixed scour moni- toring systems. Bridges with Tidal Influences Although the 1997 NCHRP study on fixed monitors (NCHRP Report 396: Instrumentation for Measuring Scour at Bridge Piers and Abutments) tested only two tidal bridge sites, since that time, many bridges over tidal waterways have been instrumented with fixed scour monitors. Some of the same devices that are employed in riverine bridges are being used on tidal bridges. Twelve of the 56 sample sites that replied to the survey reported that their bridges with fixed monitors were over tidal waterways. All of the sites used sonar monitors with one exception. One site used magnetic sliding collars

59 and one site had both sonar and collar installations. In the case of bridges over tidal waterways, the worst scour can be on the ebb or the flood tide of the bridge. Scour monitors were installed on one or both sides, depending on where the scour had or was expected to occur. The survey respondents reported on the use of robust materials to protect the under- water components of fixed scour devices in tidal installations. These included AISI 316 stainless steel and protective shields. Materials and techniques need to be developed to protect monitoring devices from corrosion and marine growth in the harsh tidal environment. Unknown Foundations Guidance on bridges with unknown foundations can be found in FHWA HEC-18 (Richardson and Davis, Evaluating Scour at Bridges) and on the FHWA bridge scour technology website. A bridge with unknown foundations is one where the type and/or condition of the substructure is not known. These bridges are classified as “U” in the scour critical code (Item 113) of the Coding Guide. The screening program in the National Bridge Scour Evaluation Program has identi- fied more than 67,240 bridges with unknown foundations. The bridge information necessary to analyze the stability and determine if it is scour critical includes the type (spread foot- ing, piles, or columns), material (steel, concrete, or timber), dimensions (length, width, or thickness), reinforcing, and/or elevation of the foundation. The FHWA is taking action toward enhancing the current guidance to address bridges in the unknown foundations category of the National Bridge Scour Evaluation Program. Unknown foundation bridges, with the exception of Inter- state bridges, have been exempted from evaluation for scour by the FHWA. They do suggest, however, that until this guidance is available, that DOT management officials con- sider monitoring these bridges during and after a flood event as they can deem it necessary. The monitoring can be using flood, portable, and/or fixed instrumentation methodologies. The FHWA guidance states that a Plan of Action should be developed for bridges with unknown foundations. This Plan of Action includes a plan for the timely installation of countermeasures to reduce the risk from scour and also the development and implementation of a scour monitoring and/or inspection program. The FHWA is currently sponsoring a synthesis on unknown foundations, which should provide a better perspective of technologies, methods, and managerial practices being used in this area. In November 2005, the FHWA sponsored an Unknown Foundations Summit. This summit served to share knowledge on current technologies available through the industry and management strategies that have been used by DOTs to deal with bridges with unknown foundations. During a follow-up meeting to the summit, four teams were established to work on developing policy and guidance and training and research needs on the subject of unknown foundations. Suggestions for a National Scour Database There are two existing databases, the National Bridge Scour Database and the National Bridge Inventory (NBI) that can be modified for a national scour database to include data from the fixed scour monitoring instrumentation. The proposed new 20-year program, Long-Term Bridge Performance (LTBP) can also be considered for use as a national database for scour monitoring data. A discussion of information that could be assembled in a national scour database that includes the scour monitoring data is outlined in Appendix H. Following is a discussion of the three databases and observations regarding the inclusion of scour monitoring data. National Bridge Scour Database The National Bridge Scour Database is a cooperative effort of the U.S. Geological Survey (USGS), FHWA, NCHRP, and the University of Louisville. The database is posted on the USGS website: http://water.usgs.gov/osw/techniques/bs/ BSDMS/BSDMS_1.html. It contains scour data for 93 bridges in 20 states. The database is comprised of detailed data tables containing site, pier scour, contraction scour, and abutment scour information, as well as miscellaneous supporting files. This database was developed to assist in the documentation, compilation, and analysis of observed scour. It was hoped that this would provide the data needed to improve the under- standing and prediction of the scour processes. Currently, there is no funding to modify, maintain, and update the National Bridge Scour Database. National Bridge Inventory The National Bridge Inspection Program was initiated in 1969, requiring regular and periodic inspections of all highway bridges. In 1971, the NBIS came into being. The primary purpose of NBIS is to locate and evaluate existing bridge deficiencies to ensure the safety of the traveling public. The NBIS sets national policy regarding bridge inspection and rating procedures, frequency of inspections, inspector quali- fications, report formats, and the preparation and maintenance of a state bridge inventory. Each state or federal agency must prepare and maintain an inventory of all bridges subject to the NBIS. Certain Structure Inventory and Appraisal (SI&A) data must be collected and retained by the state or federal agency for collection by the FHWA as requested. A tabulation of these data is contained in the SI&A sheet, which can be found in the FHWA’s “Recording and Coding Guide for the Structure Inventory and Appraisal of the National’s Bridges” (December 1995). The NBI is the aggregation of SI&A data collected to fulfill the requirements of the NBIS. The organi- zation of the NBI database could be used for the elements that both share in common and modified for additional elements relative to hydraulics and scour.

60 FHWA LTBP The FHWA initiated a major program in early 2006 with the objective of improving knowledge regarding bridge perfor- mance over a long period of time. The FHWA LTBP program will instrument, monitor, and evaluate a large number of bridges throughout the United States to capture performance data over a 20-year period of time and, on the basis of the information collected from these structures, provide sig- nificantly improved life-cycle cost and performance and predictive models that can be used for bridge and asset- management decision making. The LTBP program will also conduct forensic investigations on decommissioned bridges, as the opportunity arises. The report notes that the NBI database is one of the most comprehensive sources of long-term bridge information in the world. In recent years, a majority of the states have imple- mented element-level inspection programs to support state and local level bridge management programs. They note that a basic limitation of both the NBI and element level approach is that the data collected relies on visual inspection techniques. With visual inspection, hidden or otherwise invisible, deteri- oration damage is missed. The LTBP program will include detailed inspection, periodic evaluation and testing, contin- uous monitoring, and forensic investigation of representative samples of bridges throughout the United States to capture and document their performance. The report concludes that there is a need for quantitative performance databases, which include relevant data to implement true life-cycle-cost analysis. The same data are necessary to implement performance-based specifications. It is anticipated that the LTBP program will create such databases by collecting high-quality, quantitative performance data on bridges, which can then be integrated into the bridge management processes of the future. The con- tinuous monitoring portion of this project will provide useful hydraulic and scour data. The database will include the collec- tion of data on bridge scour, movement, and settlement. USGS Recommendations on Scour Data The USGS, in a memorandum on guidance for bridge scour studies (2003), notes that although the primary objective of scour monitoring is to provide for the safety of the pub- lic without closing bridges during high flows and without installing expensive countermeasures, it provides an excel- lent opportunity to meet the operational needs of the bridge while collecting much needed data on scour. Real-time mon- itoring using fixed or portable instrumentation for the DOT only requires the elevation of the streambed to evaluate the stability of the bridge foundations. The USGS notes that when the streambed elevation measurements are combined with hydraulic measurements, the data becomes valuable for bridge scour research. Sites with fixed scour monitoring equipment, if supplemented with a continuous-record streamgaging station, can provide valuable data on the initiation and rate of scour, as well as, under what conditions scour holes refill (if the installed technology allows measuring the refilling process). In addition, mobile field teams making measurements at bridge sites can supplement the streambed elevation measurement with a discharge measurement and other hydraulic observations to complete a limited-detailed data set. The USGS states that scour monitoring projects can represent a significant opportu- nity to collect field data that can be used for scientific research, while meeting a fundamental need of many highway depart- ments. The extension of scour monitoring to include hydraulic measurements for research purposes is an ideal application for federal–state cooperative funds. The USGS concludes that there is also potential for projects that develop and test equipment that can be used for scour monitoring. They note that instruments that work effectively in steep mountain streams and in streams with ice are needed. Potential Sites for Future Monitoring Potential sites for future in-depth monitoring case studies were examined. These can include sites that have a large amount of information available, sites that have experienced or are likely to experience scour depths, and sites where there can be funding to install scour monitors. From the survey responses, extensive testing and analy- ses had been performed for the bridge sites in Maryland, Florida, Alabama, and Long Island, New York. This infor- mation includes hydraulic computer modeling, hydraulic and scour analyses, borings, pier stability tests, and/or flume tests. There are a number of new bridges under construction that can also be considered possible candidates for fixed scour monitoring systems. The Maryland bridge is the new Woodrow Wilson Memo- rial Bridge over the Potomac River. The existing bridge was monitored with sonar scour monitors from 1999 until it was demolished in 2006. The Florida bridge, St. John’s Pass, is also scheduled to be replaced. It was one of the test sites for the NCHRP scour monitoring project. The two Wantagh Parkway bridges in Long Island that were discussed in this report under case studies are going to be replaced. Extensive information is available for all these sites and the installation of a scour monitoring system during construction often reduces the cost and can provide a better, more secure installation. Other sites that can be considered for instrumentation include the new bridge over Indian River Inlet in Delaware. Historically, this site has had extensive scour with as much as 30.5 m (100 ft) of scour in certain locations. The existing bridge has two piers in the channel and they are protected by riprap. The new bridge will not have piers in the inlet, but a system could be designed to monitor the bulkhead. Also, there has been discussion that the inlet can be widened at a future date. Other new bridges that could be scour monitoring case studies include crossings of the Mississippi and Missouri

61 Rivers, and the new Tacoma Narrows Bridge in Tacoma, Washington. Consideration could also be given to structural bridge health monitoring. These systems have many similarities to the fixed scour monitoring systems, including the data loggers. The possibility of integrating these two systems on a bridge can be beneficial, particularly in terms of cost reduction and maintenance concerns. CONCLUSIONS Scour monitoring with fixed instrumentation has been used in 32 states and the District of Columbia. A scour monitoring program can be an efficient, cost-effective alternative or com- plement to traditional scour countermeasures. The system and program are custom-designed for each bridge and site. There have been many innovations in scour monitoring tech- nology and this report outlines some of the lessons learned in installations in a wide variety of locations. The systems can provide round-the-clock monitoring, even during storms; scour data for bridge scour research, velocity, and water stage records, and the integration of the newest scour prediction techniques with physical data collection. The data traditionally collected by the majority of scour monitoring systems is in the form of streambed elevations. More recent installations include tilt sensors that measure movement of the bridge as a result of scour or other causes. Instrumentation that measures additional hydraulic variables was reported in a small number of installations. The develop- ment of monitoring systems that also measure water velocity and stage will provide data that can be used for the improve- ment of current scour prediction methodologies. These data can be stored in one of the existing national databases, or in the new FHWA Long-Term Bridge Performance Program database that is currently under development. The main problems reported by the states in the use of fixed scour instrumentation include the maintenance and repairs to the systems and the funding to continue the operation and scour monitoring program. A thorough and systematic plan developed before the installation of the scour monitoring sys- tem can result in a program that is successful to ensure the safety of the bridge and of the traveling public.