Priming first occurs at the smaller diameter branch sections that connect each roof drain to the main horizontal carrier pipe or manifold. At this point, each siphonic roof drain acts independently as a mini-siphonic system. As water accumulates in the manifold, air is purged out of the point of discharge until the manifold and stack is 95% to 100% full-of-water. The system is then completely siphonic and under predominantly negative pressure.

Chart 1 represents a design where the rainfall intensity (Id) is less than the statistical rainfall intensity (Is) of a storm of return rate (T) and duration (t). The excess water (Ir) is retained safety on the roof until the point of overflow.

Wavy flow (Pattern 1) is seen during rainfall events far below the piping system’s ability to prime. Light showers will typically produce this flow condition until rainfall intensity increases to a point where branch pipes can fully prime.

The so-called pulsating flow (Pattern 2) ordinarily happens at the junctions of the branch pipes with the main collection piping. This is due to the sudden decrease in pipe velocity as the water transitions from the smaller diameter branch pipes to the larger main collection pipe.

At this juncture, a hydraulic jump occurs as the fluid transitions from super-critical to sub-critical flow. At this stage, sudden increases in velocities take place accompanied by decreases in pressure. Eventually the peaks of these hydraulic jumps come in contact with the crown of the pipe and begin to propagate downstream and (if the dimensional rainfall intensity continues) the plug flow

pattern (Pattern 3) becomes prominent. As the rainfall event increases in intensity or the time of concentration is approaching, the pipe becomes more full of water and less full of air. The high flow velocity of the water captures and emulsifies the remaining air and a frothy

“bubble” flow forms (Pattern 4). This frothy flow condition becomes gradually clearer until all of the remaining air is purged out of the point of discharge and only water is present. Although a small percentage of air is always

induced by the siphonic drains, it is quickly carried downstream and a full-bore (Pattern 5) condition occurs.

It is rare that a rainfall event will occur at the exact design intensity (Id) for any sustained period. Therefore, a system will typically experience flow Pattern 3 to Pattern 5 during heavy rainfall. During light rainfall events, Pattern 1 and 2 may develop, but roof drainage is still accomplished and with a more efficient sized pipe system.