A Reassessment of Contraction Scour at Bridge Waterways

Abstract

This technical note reassesses the prevailing understanding of lateral-contraction scour at bridge waterways. Herein, lateral contraction (or simply contraction) implies a reduction of flow width. Nonuniform flow to and along a channel contraction, the formation of a vena contracta within a contraction’s entrance, and complexities in alluvial bed dynamics to and along a contraction serve to complicate estimation of contraction scour depth. The reassessment uses data from flume experiments and explains that the aforementioned complexities invalidate the commonly assumed concept of long-contraction scour. These considerations cause trends in the depth of contraction scour, especially live-bed scour, to deviate from the trends suggested by common guidelines. Consequently, this note suggests adjustments to the standard equations, but it also suggests flow in alluvial, open-channel contractions requires further investigation. Also, this note briefly discusses the practical implications of the reassessment.