Transitional and Weir Flow in a Vented Drop Shaft with a Sharp-Edged Intake

An experimental analysis of a vertical drop shaft with a sharp-edged horizontal intake section and a coaxial venting pipe is undertaken to obtain design criteria given the limited attention provided by literature. Three different flow regimes are observed, namely weir flow and pressurized flow for extreme discharges and transitional flow for intermediate discharges. Particular attention is paid to transitional flow, which is a periodical regime consisting of an alternate switch from weir to pressurized flow, with water heads ranging between a maximum and a minimum. For transitional flow a detailed characterization of water surface oscillations is provided, and nondimensional equations are given describing mean water head, oscillation amplitude and frequency as functions of pipe Froude number and dimensionless plunged length of the coaxial pipe. Also, a region of existence is provided in terms of pipe Froude number and different wave types are identified. For weir flow, a nondimensional head-discharge relation is provided by analogy with overfalls.