Half-Round Circular Crested Weir: On Hysteresis, Instabilities, and Head–Discharge Relationship

Waters flowing over rounded weirs experience a rapidly accelerated flow region near the crest. The head-discharge relationship of a half-round crested weir was tested physically under carefully controlled flow conditions for a weir design with a small radius of curvature. The experiments were undertaken for a wide range of discharges, over two orders of magnitude, and the results were compared to the literature on circular weirs. The nappe was not aerated. With increasing discharges, the nappe was initially attached to the weir’s downstream wall, until some nappe detachment occurred. With a further increase in flow rate, the detached nappe reattached at large flow rates. The transitions, i.e., both nappe detachment and reattachment, were characterized by large instabilities, change in flow properties, and sometimes loud noise, and the processes were subject to some hysteresis. The finding demonstrated the complicated features of an unaerated round-crested weir overflow. The importance of careful experimental procedure is discussed.