An inverted siphon must allow for a minimum low-flow velocity of 3 ft./sec.

• A. 3 ft./sec
• B. 1 ft/sec
• C. 2 ft/sec
• D. 4 ft/sec

Answer: (A)

Maintaining enough flow energy at low discharge is essential to keep solids from settling and clogging the inverted siphon. The number three feet per second is chosen as the minimum to ensure the flow remains capable of keeping materials in suspension and providing a self-cleansing effect even during the lowest anticipated flows. If the velocity drops below this, grit and organic matter can settle on the invert and gradually block the conduit. A velocity of one or two feet per second is simply too slow to prevent deposition, while four feet per second would still meet the requirement but represents a higher-than-necessary flow for the stated minimum.