Finite Crest Length Weir Nappe Oscillation

Abstract

Low-head discharge over weirs and crest gates can produce a nappe instability in curvilinear weir nappe flow, referred to as nappe oscillation or vibration, that is characterized by horizontal waves (sinusoidal wave pattern in the longitudinal cross section) and significant low-frequency sound pressure waves. This topic has previously received minimal documentation in the literature. The current study documents weir-flow nappe oscillation research conducted using a prototype-scale quarter-round linear short-crested weir (4.67 m wide×3.63 m tall with a 152-mm radius) with confined and unconfined nappe conditions. The objective of this research was to develop a better understanding of the mechanisms behind nappe oscillation, evaluate oscillation frequencies (three different potential sources), and investigate potential mitigation techniques. Crest roughness modifications were evaluated as potential nappe vibration mitigation alternatives. The results suggest that the source of nappe oscillation is located at or near the weir crest downstream edge. Some modifications effectively suppressed nappe oscillation, whereas other factors functioned as oscillation amplifiers.