Critical incipient failure conditions for angular riprap on steep trapezoidal channels using the movability number

Abstract

A 1:15 scale physical hydraulic model was designed and constructed to investigate the incipient failure conditions of large angular riprap. A total of 32 tests were performed on angular riprap dumped on steep bed slopes of 0.333 to 0.5, and against a steep side-bank slope of 0.4 in a wide trapezoidal channel. The critical movability number value defining the critical incipient failure conditions of angular riprap on steep bed slopes and steep side-bank slopes was determined to be 0.12 and 0.227, respectively. Based on the HEC-RAS 1-dimensional (1-D) steady-state flow analysis, it was identified that HEC-RAS overestimates the critical incipient failure movability number of the steep bed and steep side-bank riprap by a critical factor of 1.91 and 1.35, respectively. The applicability of the study’s findings is limited to prototype riprap D₅₀ sizes of 0.57 m to 1.125 m, and a trapezoidal canal bottom-width to D₅₀ ratio of 16:31 (W_base: D₅₀).