OBSERVATIONS ON FLOW IN VERTICAL DROPSHAFTS IN URBAN DRAINAGE SYSTEMS

Exploratory physical-model studies were made of the hydraulics of stor mwater dropshafts in the city of Edmonton. Under certain conditions, i t is necessary to provide a curved inlet at the top of the dropshaft t o increase its carrying capacity. Except for very small discharges, th e falling water in the dropshaft does not form a coherent central jet but takes the form of a complex jet with a spreading flow around the p eriphery of the dropshaft. Such a how also forms a pool at the bottom of the shaft. The energy loss in the dropshaft is significant. For the case studied, with the diameter of the shaft (D-s) equal to about twi ce the diameter (D) of the inflow sewer and a fall of 6.6D(s), and ove r a wide range of how rates, the relative energy loss in the dropshaft was about 80-95%. Further, the air flow Q(a) required by the falling water, relative to the water how rate Q, decreased from about 1.4 to 0 .5 as Q increased from 0.1 to about 1.0. Q* = Q/root(gD(5)). Q = wate r discharge and g = acceleration due to gravity.