Portable Scour Monitoring Equipment (2004)

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3CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH PROBLEM STATEMENT Scour monitoring can be completed by either fixed or portable instruments. Fixed instruments are those that are installed and left at the bridge and typically involve a sensor for making the scour measurement, a power supply, and a data logger. More recently, telemetry has become a common com- ponent of many fixed instrument systems. During the last 10 years, significant research and progress has been made with fixed instrumentation, through research activity, commercial development, and field installations, often com- pleted by state transportation agencies. However, fixed instru- mentation is not suitable, practical, or cost-effective for all bridges. In many cases, portable monitoring during a flood is a better solution, and yet not much research has been com- pleted to improve this type of technology. Physical probing has been used for many years as the primary method for portable scour monitoring by many state transportation agencies. More recently, sonar has seen increased use, in part because of the technology transfer pro- vided through the FHWA Demonstration Project 97 (DP-97), “Scour Monitoring and Instrumentation” (1) and through the efforts of the U.S. Geological Survey (USGS) in supporting state scour programs. However, probing and sonar techniques both have limitations during flood events when the flow depth and/or velocity are high. Low flow monitoring during the 2-year inspection cycle with this type of technology has been effective and is used by many state transportation agencies; however, critical decisions on bridge safety during flood flow conditions have been hampered by the limitations of the exist- ing equipment and its application. As a result, some bridges have been closed unnecessarily, causing traffic delays and increased expenses, while other bridges that should have been closed were not, resulting in increased risk and liability. For example, during the 1994 flooding in Georgia, more than 2,100 bridges were checked during the flood and approx- imately 1,000 of these were closed (2). Many of these bridges were checked by an experienced inspection crew who had good communication during the measurements with bridge maintenance engineers, who were making closure decisions. The inspection crew used a fathometer deployed from the bridge deck, typically using a transducer mounted on a range pole. Georgia is to be commended for the prompt action, and through these efforts no lives were lost as a result of bridge failures. Yet, although the decision to close 1,000 bridges probably saved lives, it also crippled the transportation system in the flooded area (2). It is possible that, because of limitations of existing portable scour monitoring equip- ment, some bridges were closed unnecessarily, while others perhaps should have been closed to reduce the risk to the traveling public and/or to minimize structural damage to the bridge. Another indication of the need for better portable scour mon- itoring technology was the widespread interest in the portable instrumentation lecture and demonstration presented during DP-97. In some cases, this might have been because portable instrumentation was perceived as a quick, low-cost entry into scour monitoring activities. However, more often it was related to a real desire to learn more about portable instrumentation and application techniques because many of the participants had faced the real-world problems presented by attempts at making portable instrument measurements during flood flow conditions. RESEARCH OBJECTIVE The objective of this research was to develop improve- ments and/or alternatives to existing portable scour monitor- ing equipment and techniques for measuring stream bed elevations at bridge foundations during flood conditions. The equipment and techniques developed 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 • Accuracy of +/− 12 in. (30 cm)

RESEARCH TASKS The research project was conducted in two phases. The first phase was designed to identify the most promising alterna- tives and to develop a work plan for testing under Phase II. Phase II consisted of prototype development, testing, and doc- umentation. The specific research tasks defined to achieve the research objective were as follows: • Phase I — Task 1. Literature Review — Task 2. Identify Alternative Technologies — Task 3. Work Plan — Task 4. Interim Report • Phase II — Task 5. Prototype Development and Limited Testing — Task 6. Detailed Field Testing — Task 7. User Manuals — Task 8. Final Report RESEARCH APPROACH The purpose of Task 1 was to identify a wide range of tech- nologies and procedures with either a direct or indirect rela- tionship to portable scour monitoring. The next step was to sort through that information to identify the most promising technologies that should be considered as part of this research 4 effort. This was a comprehensive analysis that considered advantages, limitations, purchase and operational costs, poten- tial problems, and other important features or considerations. The identification of the most promising technologies included consideration of the wide range of instrumentation capability and proficiency within different transportation agencies throughout the United States. While some states would welcome very high-technology solutions and proce- dures, others may prefer a simpler approach. It was consid- ered unlikely that only one technique or procedure would be developed. Based on the findings from Tasks 1 and 2, the work plan for the Phase II research was refined and improved to better ensure a successful research effort (Task 3). The interim report presented the results of Tasks 1, 2, and 3, and provided the basis for discussion of the Phase II work effort with the NCHRP project panel. The objective of the Phase II research was to develop prototype devices and conduct laboratory and field testing. Development of prototype devices was necessary to provide instrumentation for the testing program, which included lim- ited testing during development (Task 5), and then more wide- spread testing in cooperation with state transportation agencies across the country (Task 6). The final results of the research were documented in two reports, a users manual intended to provide enough information to replicate the preferred proto- type devices (Task 7), and the final report documenting the entire research effort (Task 8).