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3 CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH PROBLEM STATEMENT failures. Yet, although the decision to close 1,000 bridges probably saved lives, it also crippled the transportation Scour monitoring can be completed by either fixed or system in the flooded area (2). It is possible that, because portable instruments. Fixed instruments are those that are of limitations of existing portable scour monitoring equip- installed and left at the bridge and typically involve a sensor ment, some bridges were closed unnecessarily, while others for making the scour measurement, a power supply, and a data perhaps should have been closed to reduce the risk to the logger. More recently, telemetry has become a common com- traveling public and/or to minimize structural damage to ponent of many fixed instrument systems. During the last the bridge. 10 years, significant research and progress has been made Another indication of the need for better portable scour mon- with fixed instrumentation, through research activity, itoring technology was the widespread interest in the portable commercial development, and field installations, often com- instrumentation lecture and demonstration presented during pleted by state transportation agencies. However, fixed instru- DP-97. In some cases, this might have been because portable mentation is not suitable, practical, or cost-effective for all instrumentation was perceived as a quick, low-cost entry into bridges. In many cases, portable monitoring during a flood is scour monitoring activities. However, more often it was related a better solution, and yet not much research has been com- to a real desire to learn more about portable instrumentation pleted to improve this type of technology. and application techniques because many of the participants Physical probing has been used for many years as the had faced the real-world problems presented by attempts at primary method for portable scour monitoring by many state making portable instrument measurements during flood flow transportation agencies. More recently, sonar has seen conditions. 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 RESEARCH OBJECTIVE efforts of the U.S. Geological Survey (USGS) in supporting state scour programs. However, probing and sonar techniques The objective of this research was to develop improve- both have limitations during flood events when the flow depth ments and/or alternatives to existing portable scour monitor- and/or velocity are high. Low flow monitoring during the ing equipment and techniques for measuring stream bed 2-year inspection cycle with this type of technology has been elevations at bridge foundations during flood conditions. The effective and is used by many state transportation agencies; equipment and techniques developed should be operational however, critical decisions on bridge safety during flood flow under the following conditions: conditions have been hampered by the limitations of the exist- ing equipment and its application. As a result, some bridges Flow velocities exceeding 11 fps (3.5 m/s) have been closed unnecessarily, causing traffic delays and High sediment concentrations increased expenses, while other bridges that should have been Floating debris closed were not, resulting in increased risk and liability. Ice accumulation For example, during the 1994 flooding in Georgia, more Limited clearance than 2,100 bridges were checked during the flood and approx- Pressure flow imately 1,000 of these were closed (2). Many of these bridges Overhanging or projecting bridge geometries were checked by an experienced inspection crew who had Bridges with decks more than 50 ft (15 m) above the good communication during the measurements with bridge water maintenance engineers, who were making closure decisions. Air entrainment The inspection crew used a fathometer deployed from the Easily used and affordable by state and local bridge bridge deck, typically using a transducer mounted on a range owners pole. Georgia is to be commended for the prompt action, and Transportable by pickup, van, or similar vehicle through these efforts no lives were lost as a result of bridge Accuracy of +/- 12 in. (30 cm)