Effect of Sediment Size Scaling on Physical Modeling of Bridge Pier Scour

TLDR

In laboratory pier scour studies, using sediment sizes that differ from the prototype distorts the pier width‑to‑sediment‑size ratio, leading to artificially larger scour depths than observed in the field. The authors conducted scaled pier scour experiments with three uniform sediment sizes and three pier designs, then filtered the data by a Froude‑number criterion to isolate the influence of relative sediment size on dimensionless scour depth. They linked the sediment‑size distortion to the scaling of horseshoe‑vortex unsteadiness, showing that coherent vortex structures upstream of the pier, characterized by acoustic Doppler velocity measurements, govern the relationship between dimensionless scour depth and the pier width‑to‑sediment‑size ratio across physical scales.

Abstract

Local pier scour experiments were performed in the laboratory to investigate the effect of relative sediment size on pier scour depth using three uniform sediment sizes and three bridge pier designs at different geometric model scales. When the data from a large number of experimental and field investigations are filtered according to a Froude number criterion, the effect of relative sediment size on dimensionless pier scour depth is brought into focus. The choice of sediment size in the laboratory model distorts the value of the ratio of pier width to sediment size in comparison with the prototype which in turn causes larger values of scour depth in the laboratory than in the field. This model distortion due to sediment size is shown to be related to the scaling of the large-scale unsteadiness of the horseshoe vortex by studying the relevant time scales of its coherent structure upstream of a bridge pier using acoustic Doppler velocimeter measurements. Observations of sediment movement, probability distributions of velocity components, and phase-averaging of velocity measured upstream of a bridge pier reveal properties of coherent motions that are discussed in terms of their contribution to the relationship between dimensionless pier scour depth and the ratio of pier width to sediment size over a large range of physical scales.