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45 relates to the articulated arm truck, this criterion was more crit- scour monitoring (as is the case with fixed instrument moni- ical for the positioning system. After concluding that GPS was toring). However, the articulated arm did improve the ability not a viable approach for positioning (either for the truck or to make measurements in high-velocity flow during flood con- the end of the crane), an alternate method was developed. ditions substantially. These measurements can be completed This method was based on a surveyor's wheel for position of from various bridge geometries (limited clearance, overhang- the truck and several tilt sensors and draw wires for the angle ing geometry, and high bridges) using a truck that is afford- and extension, respectively, of the crane. able and maneuverable. The data collection process has been Individually, the accuracy of each sensor is much better than automated, and the scour data are presented in the bridge the required accuracy. The combined accuracy of the entire coordinate system, thereby allowing rapid evaluation of scour system, including the software calculations to reduce the data, criticality. Overall, the ability to make portable scour mea- was verified by ground truthing with a grid on the pavement. surements during flood flow conditions has been substantially Using the grid, the crane was moved from one point to the improved. next and the output from the computer was compared with the known coordinates. The results of this test found the measure- ments to be well within +/-3 inches. APPLICATIONS Ultimately, what controlled the accuracy of the system was probably the deflection in the crane. At full extension and in Scour Monitoring Concepts high flow conditions, some deflection was observed in the crane. Additionally, movement of the crane, either during an Approximately 584,000 bridges in the National Bridge arc measurement or while driving across the bridge deck when Inventory (NBI) are built over streams. Many of these bridges making a cross-section measurement, caused some measure- span alluvial streams that are continually adjusting their beds ments to be in error by more than +/-12 inches. During the and banks. Many of these bridges will experience problems Idaho measurements, considerable variation was noted in the with scour and stream instability during their useful lives. In y-direction (streamwise direction) during a cross-section mea- fact, the most common cause of bridge failure is scouring of surement. This resulted from driving the truck too fast and bed material from around bridge foundations during flood- developing some bounce in the crane that caused the tilt sen- ing (13). The magnitude of scour and stream instability prob- sors to overreact. Therefore, the conclusion is that the articu- lems is demonstrated by annual flood damage repair costs of lated arm truck can provide the desired accuracy, however, approximately $50 million for highways on the Federal-aid these limits may be exceeded if proper and careful application system (14). procedures are not followed. Scour and stream instability problems have always threat- ened the safety of the U.S. highway system. The National Bridge Inspection Standards (NBIS) requires bridge owners Research Criteria Summary to maintain a bridge inspection program (23 CFR 650, Sub- part C) that includes procedures for underwater inspection. A Based on all of the above information, Table 8 summarizes national scour evaluation program as an integral part of the how well the articulated arm satisfied the research criteria. NBIS was established in 1988 by FHWA Technical Advisory The articulated arm met most, but not all, of the criteria T5140.20, which was superseded in 1991 by Technical Advi- defined for this research. The research did not solve the mea- sory T5140.23. surement problems associated with debris and ice, which have Technical Advisory T 5140.23 specifies that a plan of action been and probably will continue to be the nemesis of portable should be developed for each bridge identified as scour criti- TABLE 8 Summary of success in meeting the research criteria by the articulated arm* Research Objectives Articulated Arm Device 1. Flow velocities > 3.5 m/s Excellent 2. High sediment concentrations Fair 3. Floating debris Poor 4. Ice accumulation Poor 5. Limited clearance Good 6. Pressure flow Good 7. Overhanging geometries Good 8. Higher than 15 m Fair 9. Air entrainment Good 10. Easily used and affordable Excellent 11. Transportable by pickup, van or similar Excellent 12. Accuracy to 30 cm Good *Including Minnesota-style winch and other deployment methods developed

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46 cal in Item 113 of the NBIS. The two primary components of Application of the Articulated Arm Truck the plan of action are (1) instructions regarding the type and frequency of inspections to be made at the bridges and (2) a The type and frequency of inspection work called for in the schedule for the timely design and construction of scour coun- plan of action can vary dramatically depending on the severity termeasures. A scour countermeasure is something incor- of the scour problem and the risk involved to the traveling pub- porated into a highway-stream crossing to monitor, control, lic. For example, a bridge rated scour critical by calculations, inhibit, change, delay, or minimize stream instability or scour but which has relatively deep piles in an erosion-resistant mate- problems. The primary categories of countermeasures are rial and has been in place for many years with no sign of scour, hydraulic, structural, and monitoring (15). might be adequately addressed through the regular inspec- Hydraulic countermeasures are designed primarily to mod- tion cycle and after major flood events. Alternatively, a bridge ify the stream flow or resist erosive forces. Examples of hy- found to be scour critical by inspection, such as during an draulic countermeasures include the installation of river underwater inspection that finds a substantial scour hole under- training structures and the placement of riprap at piers or mining the foundation, would obviously be a greater concern abutments. Structural countermeasures usually involve mod- and would require a more aggressive inspection plan. ification of the bridge substructure to increase bridge stabil- In either case, the application of portable scour monitoring ity. Typical structural countermeasures are underpinning and devices during and after a flood, such as the articulated arm pier modification. truck developed under this research, could be a key element of The purpose of the plan of action is to provide for the safety a scour monitoring program developed as part of the plan of of the traveling public and to minimize the potential for bridge action for a scour-critical bridge. The articulated arm truck failure by prescribing site-specific actions that will be taken provides a stable platform for deploying various scour instru- at the bridge to correct the scour problem. A defined moni- ments. The size of the truck and the automated data collection toring program is an important aspect of the plan of action and system facilitate flood measurements by allowing detailed data can incorporate various fixed and portable scour instrumenta- to be collected in a short time. tion devices. A properly designed scour monitoring program During field testing, some state inspectors questioned the includes two primary components (15): complexity of the truck, as compared with their conventional approach using a lead line measurement from fixed locations across the bridge. The lead line approach is simple and can 1. The frequency and type of measurements to facilitate provide fast results without the complexity of the articulated early identification of potential scour problems, and arm. However, it is important to recognize the complications 2. Specific instructions describing precisely what must be presented by flood flow conditions, as highlighted by the 12 cri- done if a bridge is at risk because of scour. teria that the research addressed. The conditions described were typical of large flood events, such as might be produced In summary, any bridge categorized as scour critical should during the 100-year storm. The application of conventional have a plan of action describing what will be done to correct the methods in use by many bridge inspectors would be extremely scour-related deficiency. The plan of action details the counter- difficult under such severe conditions. measures that will be implemented to correct the scour prob- Another difference between the truck and more conven- lem, which could include the use of instrumentation as part of tional methods in widespread use is the large amount of data a monitoring plan. The monitoring plan may be a short-term that can be collected with the truck in a relatively short time. countermeasure, implemented while the design and construc- Conventional methods generally produce point measurements tion of hydraulic and/or structural countermeasures occurs, or at defined locations across the bridge, which under many con- it alone may be the selected long-term countermeasure. Note ditions may be adequate to evaluate scour criticality. Although that when monitoring is selected, without other structural or the truck can provide the same data, its real benefit and value hydraulic countermeasures, the bridge retains its scour critical occurs when more data are necessary or desirable to define rating because monitoring alone does not fix the scour problem. the scour problem, and these data must be collected under the As of 2002, scour evaluations have been completed for adverse conditions of an extreme flood event. For example, about 93 percent of the bridges in the NBI. Based on these positioning the truck at a pier and sweeping arcs with the crane scour evaluations, approximately 26,000 bridges were rated can provide enough points to map the approach conditions and as scour critical. Having completed, or nearly completed, the scour hole itself. Once in position on the bridge, this measure- scour evaluation process, many states are now considering the ment can typically be completed in 15 minutes or less and can plan of action requirements for their scour-critical bridges. be done equally well during low flow or at flood flow con- Given that a monitoring program could be an integral part ditions. The truck can also provide continuous cross section in of the plan of action for many of these 26,000 scour-critical the time its takes to drive across the bridge at a slow speed bridges, the potential application of scour instrumentation in (typically 10 to 20 minutes). This allows identification of prob- the near future is tremendous as states begin to address the lems between piers that might be missed by simple point mea- question of scour-critical bridges. surements at the centerline of each pier.

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47 Therefore, it is important to recognize that the articulated and after floods, several trucks and trained crews might be nec- arm truck was designed for a specific application, that being essary. These same crews might also be doing 2-year inspec- flood flow conditions, and it may not be the best tool for tions or lower flood event monitoring with more conventional all situations. At lower flow conditions, or when fewer data methods; but, when a big flood occurs, they are the only ones can adequately address the problem, other methods may be who are trained and ready to operate the truck. Ultimately, preferable. With the number of sensors, the data loggers, and once trained and comfortable with the truck, these same crews the computer data collection methods, the truck is a more might find using the system for more regularly occurring bridge complicated device than most conventional scour monitor- inspections and surveys would be beneficial. ing methods. Proper use of the articulated arm truck will As with all scour monitoring methods, the truck has advan- require some training and a certain aptitude to operate and tages and limitations. Recognizing and remembering what maintain. these are will facilitate successful application of the articulated Therefore, the integration of the articulated arm truck into a arm truck in a scour monitoring program. The articulated arm state scour inspection program might be based on a single truck should be viewed as another tool in the inspector's tool- truck and a crew specifically trained in its use. In larger states, box for scour monitoring and, for any given job, the right tool or states with more scour-critical bridges to monitor during or combination of tools must be applied.