Potential-scour assessments and estimates of maximum scour at selected bridges in Iowa

Water-Resources Investigations Report 95-4051




The results of potential-scour assessment at 130 bridges in Iowa, an evaluation of the potential- scour technique, and the results of estimates of maximum scour at 10 bridges are described. The potential-scour assessments were made using a potential-scour index developed by the U.S. Geological Survey for a study in western Tennessee. Higher values of the index suggest a greater likelihood of scour-related problems occurring at a bridge. For the Iowa assessments, the maximum index value was 24.5, the minimum was 3, and the median of the indices was 11.5. The two components of the potential-scour index that affected the indices the most in this study were the bed-material component, which accounted for 27 percent of the overall total of the indices, and bank erosion at the bridge, which accounted for 18.3 percent of the overall total. The usefulness of potential-scour assessments is dependent upon regular assessments if the index is used to help monitor potential-scour conditions; however, few of the components of the index considered in this study are likely to change between assessments. The estimates of maximum scour at 10 bridges were made using scour equations recommended by the Federal Highway Administration. In this study, the long-term aggradation or degradation that occurred during the period of streamflow data collection at each site was evaluated. The streambed appeared to be stable at 6 of the 10 sites, was degradation at 3 sites, and was aggrading at 1 site. Most of the estimates of maximum scour were made using 100-year and 500-year flood discharges. With respect to contraction scour, channel cross sections measured during floods show parts of the streambed to be scoured lower than the computed maximum contraction-scour depths at 4 of the 10 sites. The measured discharges at 3 of the sites were less than 100-year floods used to compute scour. No pier-scour measurements were obtained except for about 4 feet of local pier at the time the measurement was made. Cross sections measured at two bridges with piers that are not supported by piling show the streambed to be scoured between the piers below the elevation of the bases of the piers. Additional investigation may be warranted at these sites to determine whether the streambed has scoured in front of the piers.

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Potential-scour assessments and estimates of maximum scour at selected bridges in Iowa
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Water-Resources Investigations Report
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U.S. Geological Survey; Earth Science Information Center, Open-File Reports Section [distributor],
iv, 74 p. :ill., maps ;28 cm. [PGS - 75 p.]