Flow regimes in a vertical drop shaft with a sharp-edged intake

The results of an experimental analysis are provided concerning the hydraulic behaviour of a vertical drop shaft with a sharp-edged circular intake, also taking into account a peculiar venting system consisting of a coaxial vertical pipe, projecting within the drop shaft with different plunging lengths. Three different flow regimes are experienced: a “weir flow” for low head values, where an air–water mixture at the atmospheric pressure can be observed; a “pressurized flow” for high head values, where water flows in a pressurized regime along the shaft; a “transitional flow” for intermediate water head values, consisting of a continuous, spontaneous switch between weir flow and pressurized flow with consequent water head variation for a fixed discharge. Pressurized flow regime is analytically explained, so that a head–discharge relation can be provided both for the non-vented and for the vented configurations, with specific assumptions about intake losses.