Non-linearities at Circular Weirs under Environmental Flow Conditions

Abstract: At a control structure for free-surface flows, the shape of the passage controls the discharge capacity. The circular weir design has been known for its relatively large discharge capacity, its simplicity of design and construction, and the ease of passing floating debris. Recent studies reported a number of large experimental discrepancies in the absence of nappe ventilation, with hysteresis as well as some cyclic behaviour between attached and detached nappe, for a constant discharge. This study provides a detailed characterisation of the transient flow properties above an un-ventilated circular weir, including flow instabilities, hysteresis and non-linearities. The markedly different flow patterns were investigated physically, showing periods up to more than 10 minutes for some flow conditions. The occurrence of such instabilities implied direct adverse effects on physical and numerical modelling, despite the apparent simplicity of the geometry. Keywords: Circular weir, Environmental flows, Nappe instabilities, Hysteresis, Non-linearities, Enviromental Fluid Mechanics