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10 deployed easily; operate with one person; more idiot proof; encountered by the inspector attempting to make these mea- rapid deployment without traffic control; easily deployed; surements. Given these conditions, which represent the actual easy deployment; small hand-held point and shoot device; conditions that bridge inspectors must often work under, it is and, Simple system that . . . can be used in high velocity flood unlikely that one instrument or device will meet all the desir- water. Five comments referred to the need for data logging, able criteria and several different types of equipment will with phrases such as better data recording; digital recording; probably need to be developed. downloaded to a PC; . . . keep track of the information; and, Based on the literature review, various observations and data logging capability. Four also commented specifically on findings concerning portable scour monitoring techniques can the need for durability with descriptors such as more bullet- be made. In particular, the combined results and conclusions proof; durable enough . . . ; rugged and dependable; and, . . . from the two survey/questionnaire efforts provide significant take a beating. Four commented on cost through words/phases insight on what people in the field using portable equipment such as inexpensive; price . . . won't sell otherwise; cost . . . would like to have, and this defined a fairly clear mandate for such that each district has ready access; and, . . . low cost. the proposed research. Specifically, devices that are relatively These conclusions are consistent with results reported by simple to operate and deploy, provide data logging capability, Mueller and Landers (4) from a similar survey effort. From and that are durable and reasonably priced, seem to be high their survey, they concluded the following: priorities. These characteristics are consistent with low-cost and easily transportable items in the criteria list defined as part Transportation agency personnel use various techniques of the research objective and suggest that complex, high-tech- and equipment, including sounding weights and sonar nology approaches are not desirable. However, in certain devices. cases, this may complicate meeting some of the other sug- Most responses indicated that simplicity of operation is gested criteria. the most important functional characteristic. The following sections discuss specific findings for each Other important features were portability and size, dura- component of a scour measuring system. As stated above, the bility, cost, and the ability to provide permanent records. four components of a portable scour measurement system are An important operational criterion was the need to limit as follows: personnel requirements and supporting equipment. Most respondents considered $1,000 a reasonable price 1. An instrument for making the measurement, per unit. 2. A system for deploying the instrument(s), 3. A method to identify and record the horizontal position From this and other information received from the ques- of the measurement, and tionnaire, the researchers concluded that a bridge inspection 4. A data-storage device. system should do the following: The use of different measurement technology (Item 1) from Measure streambed elevation. different deployment platforms (Item 2) could produce vari- Be easily transportable and durable. ous alternative approaches. For example, it may be that the Operate in water velocities of at least 13 fps (4 m/s). measurement technology is not dramatically new (e.g., phys- Be hand deployable. ical probing or sonar), but the deployment techniques are Be operable by one or two persons (preferably one). improved to facilitate flood flow monitoring and application Be able to measure under the bridge and along the sides from high bridges or bridges with low clearance. of piers and abutments. Alternatively, the measurement device might be something Provide a graphical or numerical display (permanent new or not currently used for scour monitoring deployed in a record is optional). more traditional manner from the bridge deck or from a bridge Provide depth measurement accuracy of about 1 ft inspection boat. Ideas include some of the offshore industry (0.3 m). technology using side scan or multi-beam sonar, towfish, auto- Be powered by small portable batteries. nomous underwater vehicles (AUVs), and/or the use of under- water video cameras and flood lights. As will be described below, the research plan was directed more toward improving FEASIBLE ALTERNATIVES deployment techniques. The objective of this research was to develop improve- ments and/or alternatives to existing portable scour moni- Instrument for Making the Measurement toring equipment and techniques for measuring stream bed elevations at bridge foundations during flood conditions. Results from the literature review and questionnaire suggest Under flood conditions, the velocity of flow, sediment trans- that there are no innovative approaches currently available for port, and air entrainment can be high; debris loading can be measuring scour depth. The idea of an underwater camera and a problem; and various different bridge geometries will be the application of a green laser were introduced as possible

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11 scour monitoring devices, but both have serious limitations. With additional research and development, the articulated The conventional methods (physical probing, sonar, and geo- arm could solve some difficult deployment problems, but the physical), have all been used with varying degrees of success FHWA prototype that was developed may be limited in poten- in flood monitoring work. Because of the drag forces created tial application by cost and complexity. The research report for by high velocity and flow depth at flood flow conditions, phys- this prototype (11) indicated that the original plan was to ical probing is generally not a viable approach. Large sound- develop an inexpensive, basic, and minimal system for deploy- ing weights have been used successfully at flood flow, but this ing a depth sounding transducer from a pickup truck on bridges approach can be somewhat slow and tedious. The technology that were within 20 ft (6 m) of the high-water surface. As the for this type of measurement is already well developed and project progressed, it was decided to build a larger prototype there is no apparent need to research or develop this method that would have greater flexibility and wider application for beyond current practice. field testing. The resulting product was a custom-fabricated Sonar technology, particularly single-beam methods often articulated arm that was never widely adopted or implemented. completed with fish-finder type devices, are widely used for If a commercially available articulated crane from another scour monitoring. Although sonar has been used successfully industry, such as the construction materials industry, could for portable scour measurements, getting a good reading in be adapted for scour monitoring applications, a lower cost shallow flow, high suspended sediment, and high-velocity device might be possible. conditions can be difficult. More complex sonar equipment, A towed acquisition vehicle, similar to that used in the off- including side scan and scanning systems, have been used shore industry might be a viable approach for positioning a only on a limited basis and may be too complex and expen- sensor when the device can be controlled from a boat upstream sive for widespread use. One advantage of the more complex of the bridge or possibly using a crane off the bridge deck. This sonar techniques, particularly when implemented with a tow- idea is similar to the towfish approach often used for side-scan fish (as used in the offshore industry), might be the ability sonar, but could be used with other measurement approaches, to complete a measurement at a debris-laden pier by allow- including simpler sonar devices. Boat-based application would ing the current to carry the towfish to the edge of, and per- limit this approach to larger rivers with boat ramp access and haps even under, the debris pile. However, this could be a bridges without clearance problems. Although this idea has hazardous operation and could result in the loss of the tow- merit and might allow measurement under a debris pile, this fish if it became entangled in the debris. An alternative is the operation is considered inherently dangerous. low-cost scanning sonar sold by Interphase, which would Water surface deployment by unmanned boats offers sev- reduce the economic risk of this approach, but not the haz- eral advantages for flood monitoring work, including safety, ards involved in implementing it. access issues, and the ability to work at both low and high Although geophysical technology continues to improve, clearance bridges. Development of this concept will require with reduced cost and easier operation, the widespread appli- research on boat hulls and propulsion systems. cation of these devices for scour monitoring may still be limited by cost and complexity. If the radar antenna could successfully be placed above the water surface for measure- Method to Identify and Record ment of the channel bottom, this could be a very useful and the Horizontal Position powerful methodology. The ability to measure and then record position data appears to be a high-priority criterion for any proposed system. The System for Deploying the Instrument results of the survey/questionnaire suggested that those who have been using portable scour instruments would like the effi- Bridge deck deployment methods are generally well devel- ciency of a fully automated system. Specifically, not having to oped and include standard sounding reels, cranes, and write data in a field book, as is often done with approximate sounding weight technology from the USGS and custom- methods (e.g., . . . scour depth of 12.5 ft at a location 3 ft in fabricated boom devices, such as those developed by front of pier 8 . . .) or trying to merge land survey based mea- Minnesota DOT. Another option is the use of a commercially surements of position with independent scour depth measure- available boom (often called a non-articulating crane). These ments that then require additional office time to merge data deployment approaches can be used for a direct physical sets and produce a final product (such as a plot of the cross sec- probing (sounding weight measurement) or for positioning a tion with the bridge substructure shown or a bathymetric map). sensor, such as a sonar transducer. A floating platform (e.g., Therefore, although these approaches can be used and may be kneeboard or pontoon float) is typically deployed directly expedient for certain situations, a valuable research contri- from the boom cable and allowed to drift into position. This bution toward improving applied practice will be developing method generally works well, but can be more difficult on more automated positioning technology. bridges that are high above the water surface or when the cur- The most attractive approach to automating the position rent is quite fast. measurement is to use GPS-based technology, recognizing