Observations on Flow in Vertical Dropshafts in Urban Drainage Systems

Exploratory physical-model studies were made of the hydraulics of stormwater dropshafts in the city of Edmonton. Under certain conditions, it is necessary to provide a curved inlet at the top of the dropshaft to increase its carrying capacity. Except for very small discharges, the 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 periphery of the dropshaft. Such a flow 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 (Ds) equal to about twice the diameter (D) of the inflow sewer and a fall of 6.6Ds, and over a wide range of flow rates, the relative energy loss in the dropshaft was about 80–95%. Further, the air flow Qa required by the falling water, relative to the water flow rate Q, decreased from about 1.4 to 0.5 as Q* increased from 0.1 to about 1.0. Q*=Q/(gD5).Q= water discharge and g= acceleration due to gravity.