Experimental studies were undertaken to examine the flow in long vertical conduits with particular reference to the design of storm water drop shafts. A distinguishing characteristic of such flow is the potential cavitation regime. Its existence depends upon the design of the structure. The cavitation regime will develop when the conduit is sufficiently long and the head sufficiently large. It can also be generated at a lower head if a control valve is installed in the supply line so that the net head can be negative. If a small amount of air is introduced into the system, the cavitation region is eliminated, the pressure gradient is more uniform, and the flow consists of a uniform mixture of air and water. (Author)