National Academies Press: OpenBook

Portable Scour Monitoring Equipment (2004)

Chapter: Chapter 3 - Interpretation, Appraisal, and Applications

« Previous: Chapter 2 - Findings
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2004. Portable Scour Monitoring Equipment. Washington, DC: The National Academies Press. doi: 10.17226/13719.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2004. Portable Scour Monitoring Equipment. Washington, DC: The National Academies Press. doi: 10.17226/13719.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2004. Portable Scour Monitoring Equipment. Washington, DC: The National Academies Press. doi: 10.17226/13719.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2004. Portable Scour Monitoring Equipment. Washington, DC: The National Academies Press. doi: 10.17226/13719.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2004. Portable Scour Monitoring Equipment. Washington, DC: The National Academies Press. doi: 10.17226/13719.
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2004. Portable Scour Monitoring Equipment. Washington, DC: The National Academies Press. doi: 10.17226/13719.
×
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Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Applications." National Academies of Sciences, Engineering, and Medicine. 2004. Portable Scour Monitoring Equipment. Washington, DC: The National Academies Press. doi: 10.17226/13719.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

41 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATIONS INTRODUCTION This chapter presents interpretation, appraisal and applica- tions for the portable scour monitoring instruments tested under this project. Although all aspects of the research com- pleted are discussed, Chapter 3 concentrates on the articulated arm truck. Application of this device should improve the abil- ity of inspectors to make portable scour measurements during flood conditions, when such measurements can be difficult because of flow conditions. INTERPRETATION AND APPRAISAL The primary product of the research was the articulated arm truck, which also included a modified Minnesota-style winch system and data collection software. A detailed analysis and evaluation of this integrated device, particularly as they relate to the objectives of the research, are presented below. Prior to that discussion, a general evaluation and appraisal of the sec- ondary components of the research, specifically the Interphase Scanning Sonar and the low-cost GPS systems, are presented. Interphase Scanning Sonar The Interphase Scanning Sonar was included in the research plan to evaluate its potential application as a scour monitoring device. The expectation was that it could provide a relatively low-cost method to get a three-dimensional perspective on conditions at a pier. The design of the test program only allo- cated a small effort for this task; hence, the testing should not be considered comprehensive. Furthermore, none of the field sites visited provided ideal testing conditions. One of the find- ings was that this instrument requires at least 10 ft (3 m) of water, which given the drought conditions did not exist at many of the bridges visited. The testing did not include evalu- ation in high-velocity, suspended sediment, or air entrainment conditions. Based on the limited testing completed, this instrument could be a valuable addition to the “toolbox” of devices used for scour monitoring. Field testing was completed with the articulated arm, and laboratory testing had the transducer mounted in a pontoon-style float for deployment. Given the cables necessary from the display to the transducer, the cable length would constrain its use to bridges relatively close to the water. Any type of float deployment with a tether would cre- ate the typical cable management problems encountered with deployment of any cabled instrument. The instrument has no data storage capability and no way to output the data, so the inspector would have to make notes on conditions displayed on the monitor. Reasonable pictures of the display monitor were possible using a digital camera and such pictures could be part of the documentation record. Therefore, although the instrument is thought to have lim- ited application and would not be considered the primary sen- sor for scour monitoring during floods, it does have value and should be considered as a useful tool to include in a compre- hensive portable instrumentation package. Low-Cost GPS Low-cost GPS was included in the research plan to evalu- ate the application of this technology for positioning scour instrumentation. Potential applications include locating the inspection truck on the bridge deck, directly tracking the loca- tion of the end of the articulated crane, and/or locating a boat being used either as a manned or unmanned monitoring ves- sel. A limitation of GPS application in scour monitoring has been cost, so this task was designed to evaluate a specific class of GPS receivers in the $5,000 to $8,000 range. The design of the Phase II test program only allocated a small effort for this task; hence the testing should not be con- sidered comprehensive. The test program included only two GPS receivers, although the design of the test program was reasonably complete and comprehensive. The results of the testing found that this class of GPS receivers is not currently capable of providing the accuracy defined by the objectives of this research (1 ft or 30 cm). These results limit the application of GPS positioning, using this grade of equipment, with the articulated arm or in other scour measurement applications. Scour hole geometry is often confined to a relatively small area around a pier or abutment, and the ability to map the scour hole or to compare measure- ments on successive measurements requires higher accuracy and more consistent performance than these units can provide. A higher-priced GPS receiver could be used and/or a local base station could be setup on the bridge itself, but cost and

42 ease of use issues limit these approaches as they relate to scour monitoring applications. However, as with many new technologies, GPS equipment continues to improve while costs decrease. Therefore, although the results obtained under this testing were not positive, future improvements in equipment and technology may provide the necessary accuracy at a reasonable cost. Articulated Arm Truck After completion of Phase I, the development of an articu- lated arm for deployment of scour instrumentation became the primary focus of the research and most of the Phase II research was dedicated to this task. The objective was to develop a mechanical arm that could be used for deploying various scour monitoring sensors. Given the availability of knuckle booms or folding cranes in the construction industry, the research was designed around modifying and instrumenting this type of articulated crane for use in scour monitoring research. The use of a crane for scour monitoring was expected to provide a solid platform for deployment, even under flood flow conditions, that could be instrumented to allow precise mea- surement of the movement of the crane. Unlike the cranes com- monly used in the construction industry, which are designed to handle large weight but with limited extension, the crane nec- essary for scour monitoring needed the ability to reach long dis- tances, without having to manage much weight or force. Ultimately, the selected crane with the hydrodynamic ex- tension for the sonar could reach directly into the water nearly 30 ft (9.1 m) below the bridge deck. Working off higher bridges required cable suspension methods, provided by the Minnesota-style winch included in the Phase II research. Mod- ified deployment methods were developed to allow working off of various bridge configurations, including the wireless sonar in a sounding weight and in a kneeboard deployment with a rigid frame. A comprehensive data collection software pack- age was developed that facilitated the use of the articulated arm, providing the inspector with immediate access to the data collected. The following paragraphs discuss the evaluation and ap- praisal of the articulated arm, as developed under this research, relative to the defined research objectives. As defined in Chap- ter 1, the equipment and techniques developed under the research should be operational under the following conditions: • Flow velocities exceeding 11 fps (3.5 m/s); • High sediment concentrations; • Floating debris; • Ice accumulation; • Limited clearance; • Pressure flow; • Overhanging or projecting bridge geometries; • Bridges with decks more than 50 ft (15 m) above the water; • Air entrainment; • Easily used and affordable by state and local bridge owners; • Transportable by pickup, van, or similar vehicle; and • Accuracy of +/− 12 in. (30 cm). High-Velocity Conditions To be applicable in flood conditions, the articulated arm needed to deploy sensors in high-velocity flow conditions. At the conclusion of Phase I, one of the stated research needs was to develop a hydrodynamic probe that could be used with the crane, given the concern for vortex shedding that might occur with a simple rod-shaped probe. This resulted in the use of a section of helicopter blade that could swivel to follow the cur- rent no matter how the crane was positioned when deploying the wireless sonar. The Colorado site provided the opportunity to test the articulated arm in such conditions. The arm proved to be very stable in the fast, turbulent current under this bridge. The combination of a strong, stable mechanical arm and a streamlined probe provided very successful results in high- velocity conditions. High Sediment Conditions Flood conditions often produce large suspended sediment loading, which can complicate measurements with some sen- sors, particularly sonar. High flow conditions were encoun- tered in Indiana and Missouri, with what could be described as typical flood-level suspended sediment conditions for these regions. No gage data were available to quantify the suspended sediment loading, and yet, given that the rivers were at flood stage and data were successfully collected, it is reasonable to conclude that the equipment can perform in higher sedi- ment conditions typical of a midcontinent river at flood stage. Extreme suspended sediment conditions, such as might exist in a sand bed channel in the southwest, were not encountered during field testing and it is unknown whether or not the equip- ment would have worked under these conditions. Part of the success in high sediment conditions is minimiz- ing other factors that can complicate a sonar measurement, including separation zones and high air entrainment. The streamlined probe that was developed minimized these effects by reducing turbulence induced by the probe and streamlining the flow over the transducer. The streamlined probe also posi- tioned the transducer about 12 in. (30 cm) under water for the measurement, which eliminates surface interference issues typical of a floating deployment where the transducer is skim- ming the surface. Floating Debris The accumulation of floating debris is a common problem on the upstream side of a bridge. Trees, logs, and branches are

43 trapped by the piers and gradually build out to create a poten- tially large debris jam. This debris not only complicates the measurement process, but can also increase the scour that oc- curs. The use of the articulated arm could increase the oppor- tunity for success under these conditions when it is possible to position the end of the crane upstream of the debris pile and point the sonar under the debris. However, debris piles often have substantial depth, sometimes accumulating down to the channel bed, that would limit success even if the crane could be positioned at the upstream edge of the debris pile. Addi- tionally, as the tilt angle increases, the strength of the sonar return signal gets weaker and any angled measurement would need to be corrected to vertical. Alternatively, the physical probe that was developed, con- sisting of a 2 in. (5 cm) stainless steel pipe, might be forced through a debris pile using the crane hydraulics. However, once through the debris, the same crane hydraulics would make it difficult to detect the channel bottom. Without devel- oping some type of sensor at the end of the physical probe to detect the channel bottom, this is not a practical solution. Therefore, debris has been and continues to be a serious problem complicating scour measurements. The articulated arm may improve the potential for a successful measurement in a few cases, but overall, the research completed has not resolved this problem. Ice Accumulation Ice accumulation creates problems similar to debris accu- mulation by creating a physical obstacle to measurement and potentially increasing the scour that occurs. Similar to the con- clusions stated in the debris discussion, the research completed has not resolved this problem. Limited Clearance Limited clearance conditions often exist during floods because of high stage conditions. This reduces the clearance under the bridge, which can complicate scour measurements. For example, the original articulated crane developed by FHWA had difficulty working in limited clearance situations because the crane could not be articulated into a position that would allow a measurement without submersing the boom. The crane developed under this research can be articulated such that direct measurements can be made from a water level just below the bridge deck downward to about 30 ft (9 m). Pressure Flow When flow is so high that the low-chord of the bridge is underwater, the use of floating deployments is virtually elimi- nated. However, under these conditions, the crane is still feasi- ble and within certain physical limits could be articulated into position under water. Another concern is that under pressure flow, the velocity is typically accelerated from free surface con- ditions, and more turbulence exists. Although these conditions were not tested, the overall stability of the articulated crane and the strength of the crane hydraulics would facilitate making measurements in such adverse flow conditions. Overhanging Bridge Geometry Overhanging bridge geometry is a common problem, and many of the bridges tested during this research had such con- ditions. In fact, the only bridges that did not have any type of overhang were in Idaho. The bridges in Alabama had a slight overhang, but also had a battered pile such that the crane was at the correct location when the arm was vertical over the rail. However, most of the other bridges were hammerhead designs with significant deck overhang. With the ability to tilt the rotator, the crane could be articu- lated slightly to allow some positioning under the bridge deck. However, this ability is limited; and, without an alternate solu- tion, such bridge geometries would continue to create mea- surement difficulties. This led to the development of a rigid frame for the kneeboard. The framework allowed pushing the kneeboard under the bridge deck up to 10 ft (3 m) and was attached to the rotator to allow a side-to-side movement under the bridge. Field testing found that it could be difficult to get the knee- board positioned on the water and ready to push under the bridge. The original version of the framework was aluminum and was bent during testing in Wisconsin. A second version was made of steel and, although stronger, was still difficult to get into position. One of the problems with a kneeboard on a rigid frame was the tendency for the kneeboard to submarine when a side edge caught a wave. Once on the water, the knee- board provided good data at overhanging bridge geometries, but this measurement was considerably more problematic than a direct measurement with the streamlined probe. High Bridges High bridges, where the water surface is well below the bridge deck, can create difficult measuring situations. Not only is the height of the bridge an issue, but at such locations there can often be significant wind blowing through the bridge opening. The limit of the articulated crane under a direct mea- surement is 30 ft (9 m), and therefore, high bridges typically require a cable-suspended approach. The Minnesota-style winch was built and added to the artic- ulated arm truck to allow measurement on high bridges. The development of the two-winch concept on a post mount- ing (Figure 9) provided more versatility in the measurement approach. Problems of a single-winch deployment are the drift under the bridge and the effects of wind on the cable or the deployment platform. The original Minnesota application

44 used an articulated crane, with the pulley at the end of the crane, allowing an extension up river to 40 ft (12 m). This min- imized some of the drift issues; however, the dual-winch approach allows a second cable and better control of the loca- tion of the deployment platform, as illustrated in Figure 7. With any cable-suspended operation, particularly on high bridges, the ability to track the position of the sensor accurately is lost. Therefore, the positional accuracy will always be bet- ter with the direct measurements made with the streamlined probe or the physical probe. Air Entrainment The problems with air entrainment are particularly sig- nificant when making sonar measurements. Similar to high suspended sediment conditions, high air entrainment can complicate a sonar measurement. The streamlined probe that was developed minimized these effects by reducing tur- bulence induced by the probe and streamlining the flow over the transducer. The streamlined probe also positioned the transducer about 12 in. (30 cm) under water for the measurement, which elimi- nates surface interference issues typical of a floating deploy- ment where the transducer is skimming the surface. Therefore, although no testing was completed in conditions with exces- sive air entrainment, the streamlined probe deployment of the sonar should provide the best opportunity for success when using sonar in high air entrainment conditions. Easily Used and Affordable To facilitate future implementation and use, the cost of any device developed under this research was important. The products from this research needed to be affordable by state and local transportation agencies to better ensure widespread application of the research results. For example, based in part on cost and the specialized training necessary, many states only have one underbridge inspection truck. This vehicle and its crew are often on the road year round and are scheduled so far in advance that they have little or no flexibility for break- down or complications. By analogy, if the cost of scour mon- itoring technology was so high that states could only afford one or two devices, they might not be able to respond ade- quately to, or cover, widespread flood conditions. One way to control cost is to use commercially available, “off-the-shelf” products whenever possible and thereby avoid special fabrication requirements. Therefore, the articulated arm truck was designed using readily available components whenever possible. These components and pieces were also designed to be a bolt-on installation, so that the articulated arm truck could be readily used for other purposes outside of the flood season. In fact, many transportation agencies already have articulated arm trucks that could be retrofitted for scour monitoring work based on the design concepts developed through this research. The smaller truck chassis used in the research did minimize the cost of the vehicle itself, however, a larger truck also offers advantages. The F-450 used in the research was not large enough to handle even the lightweight crane selected without hydraulic stabilizers. This lead to development of the castor system to allow driving the truck with the crane deployed for measuring cross sections. As an alternative, a larger truck chas- sis might be able to handle the lightweight crane selected for the research without the outriggers and the additional cost and complication of the castor system. The cost of the truck and crane were about $50,000 prior to adding instrumentation. Allowing $25,000 for instrumentation and fabrication, the total cost for the scour monitoring truck is about $75,000. Discussion of cost with the states visited dur- ing the testing program suggested that a cost in this range for a scour monitoring inspection vehicle was not viewed as prohibitive. To make the system easy to use, a comprehensive software package was developed to collect the data. The program was written in Visual Basic to provide an easy-to-use Windows- style environment, and the menus and layout of the program were designed to provide an intuitive operating process. The data are reported in station-elevation format to allow imme- diate comparison of the results with information on the bridge plans. The sensors selected for monitoring the position of the truck and crane movement were fairly simple, robust, and easy to replace. The only exception is the sensor setup to measure the angle of the rotator at the end of the crane. The computer program was designed with a calibration menu to allow easy zeroing of any new sensor, should one need to be replaced. However, as with any automated system that relies heavily on sensors, data loggers, and computers, the system does require an aptitude for electronics and computers and would require some training for users to be able to understand and operate the system. Easily Transportable The articulated arm truck was designed around the smallest truck chassis possible, so as to control cost, provide maneu- verability, and minimize the traffic control requirements. The latter issues are important when operating in a flood response mode, because the inspection crew needs to collect data as effi- ciently as possible and get to as many bridges as it can in a short time. An F-450 truck chassis is not much bigger than a full-sized pickup, and therefore, the articulated arm truck as developed was easily transportable and maneuverable. Accuracy The desired accuracy of the measurement was +/−12 in. (30 cm). Most sonar devices meet this criterion, and so as it

45 relates to the articulated arm truck, this criterion was more crit- ical for the positioning system. After concluding that GPS was not a viable approach for positioning (either for the truck or the end of the crane), an alternate method was developed. This method was based on a surveyor’s wheel for position of the truck and several tilt sensors and draw wires for the angle and extension, respectively, of the crane. Individually, the accuracy of each sensor is much better than the required accuracy. The combined accuracy of the entire system, including the software calculations to reduce the data, was verified by ground truthing with a grid on the pavement. Using the grid, the crane was moved from one point to the 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. Ultimately, what controlled the accuracy of the system was probably the deflection in the crane. At full extension and in high flow conditions, some deflection was observed in the crane. Additionally, movement of the crane, either during an arc measurement or while driving across the bridge deck when making a cross-section measurement, caused some measure- ments to be in error by more than +/−12 inches. During the Idaho measurements, considerable variation was noted in the y-direction (streamwise direction) during a cross-section mea- surement. This resulted from driving the truck too fast and developing some bounce in the crane that caused the tilt sen- sors to overreact. Therefore, the conclusion is that the articu- lated arm truck can provide the desired accuracy, however, these limits may be exceeded if proper and careful application procedures are not followed. Research Criteria Summary Based on all of the above information, Table 8 summarizes how well the articulated arm satisfied the research criteria. The articulated arm met most, but not all, of the criteria defined for this research. The research did not solve the mea- surement problems associated with debris and ice, which have been and probably will continue to be the nemesis of portable scour monitoring (as is the case with fixed instrument moni- toring). However, the articulated arm did improve the ability to make measurements in high-velocity flow during flood con- ditions substantially. These measurements can be completed from various bridge geometries (limited clearance, overhang- ing geometry, and high bridges) using a truck that is afford- able and maneuverable. The data collection process has been automated, and the scour data are presented in the bridge coordinate system, thereby allowing rapid evaluation of scour criticality. Overall, the ability to make portable scour mea- surements during flood flow conditions has been substantially improved. APPLICATIONS Scour Monitoring Concepts Approximately 584,000 bridges in the National Bridge Inventory (NBI) are built over streams. Many of these bridges span alluvial streams that are continually adjusting their beds and banks. Many of these bridges will experience problems with scour and stream instability during their useful lives. In fact, the most common cause of bridge failure is scouring of bed material from around bridge foundations during flood- ing (13). The magnitude of scour and stream instability prob- lems is demonstrated by annual flood damage repair costs of approximately $50 million for highways on the Federal-aid system (14). 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 to maintain a bridge inspection program (23 CFR 650, Sub- part C) that includes procedures for underwater inspection. A national scour evaluation program as an integral part of the NBIS was established in 1988 by FHWA Technical Advisory T5140.20, which was superseded in 1991 by Technical Advi- sory T5140.23. Technical Advisory T 5140.23 specifies that a plan of action 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

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

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

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TRB's National Cooperative Highway Research Program (NCHRP) Report 515: Portable Scour Monitoring Equipment presents the findings of a research project undertaken to develop portable scour monitoring equipment for measuring streambed elevations at bridge foundations during flood conditions. The report provides specific fabrication and operation guidance for a portable scour monitoring device.

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