Author(s): D. C. Froehlich

Linked Author(s): David Froehlich

Keywords: Bridge; pier; scour; coarse-bed; empirical model; quantile regression; safety margin

Abstract: Streams can be divided into two broad classes based on the predominant composition of the alluvial material in which they are formed: sand-bed and coarse-bed. Coarse-bed streams are formed primarily in gravel and larger sediments. They are common in mountain and foothill regions, mainly where igneous and metamorphic bedrock exists, and in areas of glacial-fluvial deposits. A feature that characterizes nearly all coarse-bed streams is a surface covered by large sediment particles compared to those forming the substratum –an armor layer. The presence of an armor layer has a considerable influence on the physical processes that scour material from around a bridge pier. Several methods exist for estimating local scour depth at bridge piers. Data collected in small-scale laboratory experiments in sand-filled flumes, which hardly resemble conditions in coarse-bed streams, are the basis for most. Fortunately, recent on-site measurements of local pier scour in coarse-bed channels supply a collective data set used in this analysis to develop a scour prediction relation for application in streams where armor layers are present. Nonlinear quantile regression provides prediction models for quantile levels of 0.5 (which gives the median value), 0.95, and 0.99. Estimates for the larger quantiles (corresponding to the 95 and 99 percentiles) offer statistically sound safety margins needed for a reliable bridge design that is not overly excessive. The predictive ability of this study’s local pier-scour relations for coarse-bed streams provides significant improvements over other commonly used empirical equations.

DOI: https://doi.org/10.26077/6945-08ed

Year: 2022