Observations from Contraction–Scour Experiments Conducted with a Large Rectangular Channel

Abstract

This paper presents observations on clear-water and live-bed contraction scour caused by subcritical flow along a large rectangular channel. Three contraction ratios (contraction width/approach–channel width) were used: 0.25, 0.50, and 0.75. Each ratio included a 45° transition linking the contraction to the approach channel. Contractions scour relates to three categories of contraction length: length shorter than flow-separation length at contraction entrance, length exceeds flow-separation length, and length is sufficiently long to develop uniform flow in contraction. Flow-choking was avoided. The experiments focused on the second category, herein termed intermediate-length contractions. Light detection and ranging (LiDAR) scans showed that maximum scour depths occurred at a flow vena contracta (formed within the contraction entrance), the entrance corners, and the contraction exit. The observed sour depth in the vena contracta exceeded the depths estimated using the existing HEC-18 equations for contraction-scour. The deepest scour occurred at entrance corners except for the smallest ratio, which produced a step scour.