Evaluation of Design Methods to Determine Scour Depths for Bridge Structures

Scour of bridge foundations is the most common cause of bridge failures. The overall goal of this project was to evaluate the applicability of the existing Hydraulic Engineering Circular (HEC-18) documents method to Louisiana bridges that are mostly situated on cohesive soils and hence develop a more reliable design method for scour depth and scour rate prediction. The errors of scour depth prediction of the HEC-18 method are mainly from three sources: (1) the driving force of scour, i.e., the hydrologic and hydraulic properties of flood flow causing scour development; (2) the resistive force of scour, i.e., the geotechnical properties of streambed soils or sediments that are removed by stream flow; and (3) the geometry, size, and shape of the obstacles (e.g., piers and pile caps). The third error source is not a focus of this study. Due to the availability of the geotechnical data on streambed soils, the second error source was investigated at a secondary priority, and the primary priority of this study was to evaluate the existing method’s applicability to cohesive soils in Louisiana using real hydrological data derived from archived satellite remote sensing data. A total of seven bridges situated on clays, silts, and sands were selected as case studies for scour analysis over a 10- to 15- year period. The hydraulic properties were determined by analyzing satellite sensing data, which were then used as inputs to the HEC-18 method via a software program WASPRO. The recorded scour survey data were also analyzed and compared with data predicted by the HEC-18 using the real flood data. Significant discrepancy existed among the HEC-18 prediction and surveyed scour depth, and the predicted values were always greater than the surveyed depth. Therefore, for cohesive soils, the HEC-18 method usually provides a more conservative design. Although the bridges were safe for the final scour depth, the method typically yields a more costly design.

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