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<br />r <br />I <br />l <br />f <br />~ <br />, <br />! <br /> <br />countermeasure after scour has occurred or when a bridge is assessed as scour <br />critical. <br /> <br />Pier geometry modifications are used to either reduce local scour at bridge piers or <br />to transfer scour to another location. These modifications are used primarily to <br />minimize local scour. <br /> <br />3.2.3 Group 3. Monitoring <br /> <br />Monitoring describes activities used to facilitate early identification of potential scour <br />problems. Monitoring could also serve as a continuous survey of the scour progress around <br />the bridge foundations. Monitoring allows for action to be taken before the safety of the <br />public is threatened by the potential failure of a bridge. Monitoring can be accomplished with <br />instrumentation or visual inspection. Two types of instrumentation are used to monitor <br />bridge scour: fixed instruments and portable instruments. <br /> <br />Fixed instrumentation describes monitoring devices which are attached to the <br />bridge structure to detect scour at a particular location. Typically, fixed monitors are <br />located at piers and abutments. The number and location of piers to be instrumented <br />should be defined, as it may be impractical to place a fixed instrument at every pier <br />and abutment on a bridge. Instruments such as sonar monitors can be used to <br />provide a timeline of scour, whereas instruments such as magnetic sliding collars can <br />only be used to monitor the maximum scour depth. Data from fixed instruments can <br />be downloaded manually at the site or it can be telemetered to another location. <br /> <br />Portable Instrumentation describes monitoring devices that can be manually <br />carried and used along a bridge and transported from one bridge to another. <br />Portable instruments are more cost effective in monitoring an entire bridge than fixed <br />instruments; however, they do not offer a continuous watch over the structure. The <br />allowable level of risk will affect the frequency of data collection using portable <br />instruments. <br /> <br />Visual inspection describes standard monitoring practices of inspecting the bridge <br />on a regular interval and increasing monitoring efforts during high flow events (flood <br />watch). Typically, bridges are inspected on a biennial schedule where channel bed <br />elevations at each pier location are taken. The channel bed elevations should be <br />compared with historical cross sections to identify changes due to scour. Channel <br />elevations should also be taken during and after high flow events. If measurements <br />cannot be safely collected during a high flow event, the bridge owner should <br />determine if the bridge is at risk and if closure is necessary. Underwater inspections <br />of the foundations could be used as part of the visual inspection after a flood. <br /> <br />A well designed monitoring program can be a very cost-effective countermeasure. It should <br />be noted that a Plan of Action for a scour-critical bridge should include: <br /> <br />. Timely installation of temporary scour countermeasures, such as monitoring or <br />riprap with monitoring. <br /> <br />. Development of a monitoring program which includes both scour measurements <br />and detailed bridge closure instructions. <br /> <br />5 <br />