Dropshafts, siphons, inclined conduits, and weirs are some hydraulic structures which involve air entrainment. This paper investigates three mechanisms of air entrainment suggesting broad-brush equations to predict the aeration rate. Air bubble transport in closed conduits down-stream of the entrainment point is also considered. This section deals with threshold conditions to transport air bubbles in a closed conduit, and how much air is transported, particularly relating to the conduit length/conduit dimension ration.