Updating HEC-18 pier scour equations for noncohesive soils.
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Updating HEC-18 pier scour equations for noncohesive soils.

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      A dataset of 594 bridge pier scour observations from two laboratory and three field studies was compiled. The dataset served as the testing ground for evaluating potential enhancements to the pier scour tools for noncohesive soils in Hydraulic Engineering Circular 18 (HEC-18). In the current (fifth) edition of HEC-18, there are two primary equations for pier scour in noncohesive soils. One is the general equation applicable to most situations, including clear water and live bed conditions. The second is a coarse bed material equation recommended only for use under clear water conditions with coarse bed materials. The objective of this research was to determine if the coarse bed materials equation could be used for conditions beyond those under which it is currently limited. A framework for evaluating the two equations was developed using qualitative and quantitative tools. The coarse bed equation is referred to as the Hager number/gradation coefficient (HN/GC) equation because itreferences the use of both in the equation formulation. After adjusting the HN/GC equation to a target reliability index of 2.0, it was evaluated on its ability to predict scour for a wide range of conditions in noncohesive soils. Partitioned subsets of the data based on key conditions—including the HEC-18 coarse bed criteria, clear water versus live bed transport conditions, gradation, and median grain size—were used for the evaluation. The equation performed reasonably consistently in all partitioned datasets, leading to the conclusion that it may be used for a broader range of conditions. A subgroup of pier group scour observations was assessed to determine if the equation could also be used for pier groups. The equation performed better for single piers but offered a basis for predicting local scour at pier groups.Considering these findings, the modified HN/GC equation is recommended for use on a broader range of noncohesive soil conditions for pier scour. Recommended limits for application of the equation are as follows: (1) clear water or live bed conditions (V1/Vc,50 < 5.2), (2) sands, gravels, and cobbles (0.0079 inches (0.2 mm) < D50 < 5 inches (127 mm)), (3) gradation coefficients ( ) less than 7.5, (4) Froude number less than 1.7, and (5) single piers.
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