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<br />in an uncertain manner. In this section, describe the development of <br />meanders, bank erosion, channel braiding, head cutting, general scour <br />and deposition that is not related to the bridge, channel regulation, <br />and other factors. Channel straightening to improve the flow alinement <br />through the bridge can result in bank erosion in some soils and bank <br />materials. Proper bank protection may be needed. Evolving channel <br />conditions should be considered if there is a chance that the channel <br />will change significantly over the design life of the structure. <br /> <br />Soils Information.--Soils information should be collected by drilling <br />a system of holes to elevations well below probable bridge foundation. <br />Unusual foundation conditions can increase the initial cost of the <br />bridge. Potential severe scour conditions can cause the need for special <br />foundation designs to enhance the stability of the bridge. The scour <br />depth, and hence failure due to scour, is difficult to predict even <br />under conditions similar to those studied in laboratories. The objective <br />in this section is to identify conditions which should be considered and <br />to state the assumptions made. <br /> <br />Scour History.--Some bridges are replacements for existing obsolete <br />or failed structures. Others cross streams already crossed at other <br />locations. Hence, for some sites or nearby sites there is a scour <br />history. Although the data may not be quantitative or complete, this <br />experience should be summarized to provide a basis for design. Avoiding <br />certain conditions may be the best countermeasure for scour. Estimate <br />the parameters being used to determine where scour will occur. For <br />example, determine the maximum velocity and the velocity distribution. <br />Even though scour might never have caused a problem at an existing site, <br />the current centerline profile could be compared to the profile at the <br />time of the bridge construction. An obsolete bridge being replaced <br />could be on the verge of failure caused by scour. <br /> <br />Cost Data <br /> <br />Cost is the dependent variable in the analyses. one needs to know <br />the cost of construction, backwater damage, traffic interruption, and <br />embankment repairs if they were needed. Maintenance costs would be <br />needed if the cost of maintaining the various assumed bridge/embankment <br />combinations are significantly different. <br /> <br />Capital cost.--The biggest single cost item is normally the capital <br />cost of the project. Various procedures are used by highway agencies to <br />estimate these costs. For the purpose of this report, these costs were <br />obtained from the Highway Department. Capital costs increase with <br />bridge length and embankment height as shown in figure 3. <br />, <br /> <br />16 <br />