Author(s): H. Chanson

Linked Author(s): Hubert Chanson

Keywords: Circular crested weir; Discharge relationship; Hysteresis; Instabilities; Nappe detachment

Abstract: Water flowing over weirs experiences a rapidly accelerated flow region near the crest. A common type of weir crest is the circular-crested weir. Advantages of the round-crested weir shape are typically listed as a stable overflow pattern compared to a sharp-crested weir, the ease to pass floating debris, the simplicity of design, with an associated lower cost compared to the ogee crest, and a relatively large discharge capacity, compared to broad-crested and sharp-crested weirs. As part of the data sets presented by Tullis et al. (2019), the headdischarge relationship of a half-round crested weir was tested physically under carefully controlled flow conditions by the author himself. The relationship was obtained for a wide range of discharges, over two orders of magnitudes, and the results were compared to the literature on half-round crested weirs, i. e. circular weirs. The nappe was not ventilated. 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 re-attached at large flow rates. The transitions, i. e. both nappe detachment and re-attachment, were characterised by large instabilities, change in flow properties, sometimes loud noise, and the entire processes were subjected to some hysteresis. The finding demonstrates the complicated features of an un-ventilated roundcrested weir overflow, while the importance of careful experimental procedure is discussed.

DOI: https://doi.org/10.14264/uql.2020.578

Year: 2020