In civil Engineering (Drainage works), What is the mechanism of cavitation in pipes and drains?

No Cavitation in Storm Water Because Water Going by the Flow, Cavitation will appear at pressure pipe due to change in pipes slope

Cavitation in pipes is caused by air entrapment.

Cavitation in pipes are caused by air entrapment in the pipes mainly due to sudden high points after a steady lower slopes. Generally, a pipe runs 75% full and rest is air. These air pockets can clause bockage in sewage pipes. Appropriate venting should be provided to eliminate these air pockets.

Velocities in the main runs of drainage pipes should be designed without excessive slopes to control the velocity so reduce the risk of damage due to abrasive scour and in extreme cases pressure reduction causing cavitation. Cavitation is usually only a problem in the pumps. 

It is the formation of Air bubbles in the liquid when the liquid flows  from a low velocity state to a high velocity resulting in the formation of bubbles in the couplers joints and can cause damages to drain systems too.