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41 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATIONS INTRODUCTION length would constrain its use to bridges relatively close to the water. Any type of float deployment with a tether would cre- This chapter presents interpretation, appraisal and applica- ate the typical cable management problems encountered with tions for the portable scour monitoring instruments tested deployment of any cabled instrument. The instrument has no under this project. Although all aspects of the research com- data storage capability and no way to output the data, so the pleted are discussed, Chapter 3 concentrates on the articulated inspector would have to make notes on conditions displayed arm truck. Application of this device should improve the abil- on the monitor. Reasonable pictures of the display monitor ity of inspectors to make portable scour measurements during were possible using a digital camera and such pictures could flood conditions, when such measurements can be difficult be part of the documentation record. because of flow conditions. 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 INTERPRETATION AND APPRAISAL should be considered as a useful tool to include in a compre- The primary product of the research was the articulated arm hensive portable instrumentation package. 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 Low-Cost GPS to the objectives of the research, are presented below. Prior to Low-cost GPS was included in the research plan to evalu- that discussion, a general evaluation and appraisal of the sec- ate the application of this technology for positioning scour ondary components of the research, specifically the Interphase instrumentation. Potential applications include locating the Scanning Sonar and the low-cost GPS systems, are presented. inspection truck on the bridge deck, directly tracking the loca- tion of the end of the articulated crane, and/or locating a boat Interphase Scanning Sonar being used either as a manned or unmanned monitoring ves- sel. A limitation of GPS application in scour monitoring has The Interphase Scanning Sonar was included in the research been cost, so this task was designed to evaluate a specific class plan to evaluate its potential application as a scour monitoring of GPS receivers in the $5,000 to $8,000 range. device. The expectation was that it could provide a relatively The design of the Phase II test program only allocated a low-cost method to get a three-dimensional perspective on small effort for this task; hence the testing should not be con- conditions at a pier. The design of the test program only allo- sidered comprehensive. The test program included only two cated a small effort for this task; hence, the testing should not GPS receivers, although the design of the test program was be considered comprehensive. Furthermore, none of the field reasonably complete and comprehensive. The results of the sites visited provided ideal testing conditions. One of the find- testing found that this class of GPS receivers is not currently ings was that this instrument requires at least 10 ft (3 m) of capable of providing the accuracy defined by the objectives of water, which given the drought conditions did not exist at this research (1 ft or 30 cm). many of the bridges visited. The testing did not include evalu- These results limit the application of GPS positioning, using ation in high-velocity, suspended sediment, or air entrainment this grade of equipment, with the articulated arm or in other conditions. scour measurement applications. Scour hole geometry is often Based on the limited testing completed, this instrument confined to a relatively small area around a pier or abutment, could be a valuable addition to the "toolbox" of devices used and the ability to map the scour hole or to compare measure- for scour monitoring. Field testing was completed with the ments on successive measurements requires higher accuracy articulated arm, and laboratory testing had the transducer and more consistent performance than these units can provide. mounted in a pontoon-style float for deployment. Given the A higher-priced GPS receiver could be used and/or a local cables necessary from the display to the transducer, the cable base station could be setup on the bridge itself, but cost and
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42 ease of use issues limit these approaches as they relate to scour · Easily used and affordable by state and local bridge monitoring applications. owners; However, as with many new technologies, GPS equipment · Transportable by pickup, van, or similar vehicle; and continues to improve while costs decrease. Therefore, although · Accuracy of +/- 12 in. (30 cm). the results obtained under this testing were not positive, future improvements in equipment and technology may provide the necessary accuracy at a reasonable cost. High-Velocity Conditions To be applicable in flood conditions, the articulated arm Articulated Arm Truck needed to deploy sensors in high-velocity flow conditions. At the conclusion of Phase I, one of the stated research needs was After completion of Phase I, the development of an articu- to develop a hydrodynamic probe that could be used with the lated arm for deployment of scour instrumentation became the crane, given the concern for vortex shedding that might occur primary focus of the research and most of the Phase II research with a simple rod-shaped probe. This resulted in the use of a was dedicated to this task. The objective was to develop a section of helicopter blade that could swivel to follow the cur- mechanical arm that could be used for deploying various scour rent no matter how the crane was positioned when deploying monitoring sensors. Given the availability of knuckle booms the wireless sonar. The Colorado site provided the opportunity or folding cranes in the construction industry, the research was to test the articulated arm in such conditions. The arm proved designed around modifying and instrumenting this type of to be very stable in the fast, turbulent current under this bridge. articulated crane for use in scour monitoring research. The combination of a strong, stable mechanical arm and a The use of a crane for scour monitoring was expected to streamlined probe provided very successful results in high- provide a solid platform for deployment, even under flood flow velocity conditions. 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 High Sediment Conditions handle large weight but with limited extension, the crane nec- essary for scour monitoring needed the ability to reach long dis- Flood conditions often produce large suspended sediment tances, without having to manage much weight or force. loading, which can complicate measurements with some sen- Ultimately, the selected crane with the hydrodynamic ex- sors, particularly sonar. High flow conditions were encoun- tension for the sonar could reach directly into the water nearly tered in Indiana and Missouri, with what could be described as 30 ft (9.1 m) below the bridge deck. Working off higher typical flood-level suspended sediment conditions for these bridges required cable suspension methods, provided by the regions. No gage data were available to quantify the suspended Minnesota-style winch included in the Phase II research. Mod- sediment loading, and yet, given that the rivers were at flood ified deployment methods were developed to allow working stage and data were successfully collected, it is reasonable off of various bridge configurations, including the wireless to conclude that the equipment can perform in higher sedi- sonar in a sounding weight and in a kneeboard deployment with ment conditions typical of a midcontinent river at flood stage. a rigid frame. A comprehensive data collection software pack- Extreme suspended sediment conditions, such as might exist age was developed that facilitated the use of the articulated in a sand bed channel in the southwest, were not encountered arm, providing the inspector with immediate access to the data during field testing and it is unknown whether or not the equip- collected. ment would have worked under these conditions. The following paragraphs discuss the evaluation and ap- Part of the success in high sediment conditions is minimiz- praisal of the articulated arm, as developed under this research, ing other factors that can complicate a sonar measurement, relative to the defined research objectives. As defined in Chap- including separation zones and high air entrainment. The ter 1, the equipment and techniques developed under the streamlined probe that was developed minimized these effects research should be operational under the following conditions: by reducing turbulence induced by the probe and streamlining the flow over the transducer. The streamlined probe also posi- · Flow velocities exceeding 11 fps (3.5 m/s); tioned the transducer about 12 in. (30 cm) under water for the · High sediment concentrations; measurement, which eliminates surface interference issues · Floating debris; typical of a floating deployment where the transducer is skim- · Ice accumulation; ming the surface. · Limited clearance; · Pressure flow; · Overhanging or projecting bridge geometries; Floating Debris · Bridges with decks more than 50 ft (15 m) above the water; The accumulation of floating debris is a common problem · Air entrainment; on the upstream side of a bridge. Trees, logs, and branches are
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43 trapped by the piers and gradually build out to create a poten- position under water. Another concern is that under pressure tially large debris jam. This debris not only complicates the flow, the velocity is typically accelerated from free surface con- measurement process, but can also increase the scour that oc- ditions, and more turbulence exists. Although these conditions curs. The use of the articulated arm could increase the oppor- were not tested, the overall stability of the articulated crane and tunity for success under these conditions when it is possible the strength of the crane hydraulics would facilitate making to position the end of the crane upstream of the debris pile and measurements in such adverse flow conditions. 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 Overhanging Bridge Geometry be positioned at the upstream edge of the debris pile. Addi- Overhanging bridge geometry is a common problem, and tionally, as the tilt angle increases, the strength of the sonar many of the bridges tested during this research had such con- return signal gets weaker and any angled measurement would ditions. In fact, the only bridges that did not have any type of need to be corrected to vertical. overhang were in Idaho. The bridges in Alabama had a slight Alternatively, the physical probe that was developed, con- overhang, but also had a battered pile such that the crane was sisting of a 2 in. (5 cm) stainless steel pipe, might be forced at the correct location when the arm was vertical over the rail. through a debris pile using the crane hydraulics. However, However, most of the other bridges were hammerhead designs once through the debris, the same crane hydraulics would with significant deck overhang. make it difficult to detect the channel bottom. Without devel- With the ability to tilt the rotator, the crane could be articu- oping some type of sensor at the end of the physical probe to lated slightly to allow some positioning under the bridge deck. detect the channel bottom, this is not a practical solution. However, this ability is limited; and, without an alternate solu- Therefore, debris has been and continues to be a serious tion, such bridge geometries would continue to create mea- problem complicating scour measurements. The articulated surement difficulties. This led to the development of a rigid arm may improve the potential for a successful measurement frame for the kneeboard. The framework allowed pushing in a few cases, but overall, the research completed has not the kneeboard under the bridge deck up to 10 ft (3 m) and was resolved this problem. attached to the rotator to allow a side-to-side movement under the bridge. Ice Accumulation Field testing found that it could be difficult to get the knee- board positioned on the water and ready to push under the Ice accumulation creates problems similar to debris accu- bridge. The original version of the framework was aluminum mulation by creating a physical obstacle to measurement and and was bent during testing in Wisconsin. A second version potentially increasing the scour that occurs. Similar to the con- was made of steel and, although stronger, was still difficult to clusions stated in the debris discussion, the research completed get into position. One of the problems with a kneeboard on a has not resolved this problem. 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, Limited Clearance but this measurement was considerably more problematic than a direct measurement with the streamlined probe. Limited clearance conditions often exist during floods because of high stage conditions. This reduces the clearance under the bridge, which can complicate scour measurements. High Bridges For example, the original articulated crane developed by FHWA had difficulty working in limited clearance situations High bridges, where the water surface is well below the because the crane could not be articulated into a position that bridge deck, can create difficult measuring situations. Not would allow a measurement without submersing the boom. only is the height of the bridge an issue, but at such locations The crane developed under this research can be articulated there can often be significant wind blowing through the bridge such that direct measurements can be made from a water level opening. The limit of the articulated crane under a direct mea- just below the bridge deck downward to about 30 ft (9 m). 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- Pressure Flow ulated arm truck to allow measurement on high bridges. The development of the two-winch concept on a post mount- When flow is so high that the low-chord of the bridge is ing (Figure 9) provided more versatility in the measurement underwater, the use of floating deployments is virtually elimi- approach. Problems of a single-winch deployment are the drift nated. However, under these conditions, the crane is still feasi- under the bridge and the effects of wind on the cable or the ble and within certain physical limits could be articulated into deployment platform. The original Minnesota application
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44 used an articulated crane, with the pulley at the end of the monitoring work based on the design concepts developed crane, allowing an extension up river to 40 ft (12 m). This min- through this research. imized some of the drift issues; however, the dual-winch The smaller truck chassis used in the research did minimize approach allows a second cable and better control of the loca- the cost of the vehicle itself, however, a larger truck also offers tion of the deployment platform, as illustrated in Figure 7. advantages. The F-450 used in the research was not large With any cable-suspended operation, particularly on high enough to handle even the lightweight crane selected without bridges, the ability to track the position of the sensor accurately hydraulic stabilizers. This lead to development of the castor is lost. Therefore, the positional accuracy will always be bet- system to allow driving the truck with the crane deployed for ter with the direct measurements made with the streamlined measuring cross sections. As an alternative, a larger truck chas- probe or the physical probe. 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. Air Entrainment The cost of the truck and crane were about $50,000 prior to adding instrumentation. Allowing $25,000 for instrumentation The problems with air entrainment are particularly sig- and fabrication, the total cost for the scour monitoring truck is nificant when making sonar measurements. Similar to high about $75,000. Discussion of cost with the states visited dur- suspended sediment conditions, high air entrainment can ing the testing program suggested that a cost in this range complicate a sonar measurement. The streamlined probe for a scour monitoring inspection vehicle was not viewed that was developed minimized these effects by reducing tur- as prohibitive. bulence induced by the probe and streamlining the flow over To make the system easy to use, a comprehensive software the transducer. package was developed to collect the data. The program was The streamlined probe also positioned the transducer about written in Visual Basic to provide an easy-to-use Windows- style environment, and the menus and layout of the program 12 in. (30 cm) under water for the measurement, which elimi- were designed to provide an intuitive operating process. The nates surface interference issues typical of a floating deploy- data are reported in station-elevation format to allow imme- ment where the transducer is skimming the surface. Therefore, diate comparison of the results with information on the bridge although no testing was completed in conditions with exces- plans. sive air entrainment, the streamlined probe deployment of the The sensors selected for monitoring the position of the truck sonar should provide the best opportunity for success when and crane movement were fairly simple, robust, and easy to using sonar in high air entrainment conditions. 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 Easily Used and Affordable zeroing of any new sensor, should one need to be replaced. However, as with any automated system that relies heavily on To facilitate future implementation and use, the cost of any sensors, data loggers, and computers, the system does require device developed under this research was important. The an aptitude for electronics and computers and would require products from this research needed to be affordable by state some training for users to be able to understand and operate the and local transportation agencies to better ensure widespread system. 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 Easily Transportable 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- The articulated arm truck was designed around the smallest down or complications. By analogy, if the cost of scour mon- truck chassis possible, so as to control cost, provide maneu- itoring technology was so high that states could only afford verability, and minimize the traffic control requirements. The one or two devices, they might not be able to respond ade- latter issues are important when operating in a flood response quately to, or cover, widespread flood conditions. 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 One way to control cost is to use commercially available, short time. An F-450 truck chassis is not much bigger than a "off-the-shelf" products whenever possible and thereby avoid full-sized pickup, and therefore, the articulated arm truck as special fabrication requirements. Therefore, the articulated developed was easily transportable and maneuverable. 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 Accuracy arm truck could be readily used for other purposes outside of the flood season. In fact, many transportation agencies already The desired accuracy of the measurement was +/-12 in. have articulated arm trucks that could be retrofitted for scour (30 cm). Most sonar devices meet this criterion, and so as it