Inverted siphons used to ...

Inverted siphons used to convey canal water by gravity under roads, railroads, other structures, various types of drainage channels, and depressions. A siphon is a closed conduit designed to run full and under pressure. The structure should operate without excess head when flowing at design capacity.

Inverted siphons (also called depressed sewers) allow storm water or wastewater sewers to pass under obstructions such as rivers. Our inverted siphon calculation allows up to five parallel siphons to go under the river. Unlike the main sewer pipe, the siphon pipes flow under pressure and must have flow velocities greater than 3 ft/s (0.9 m/s) to keep material suspended. Therefore, several siphons having smaller diameters than the main sewer may be required. The calculation computes siphon diameters (or siphon flows), velocities, inlet chamber wall heights, and siphon invert elevations.

Storm water and wastewater sewers often encounter obstructions such as rivers, other pipes, subways, tunnels, or valleys. To pass these obstructions, a common method is for the sewer pipe to drop sharply, then run horizontal under the obstruction, and finally rise to the desired elevation. The piping going under the obstruction is traditionally called an "inverted siphon", but since the pipe is not actually acting as a siphon, a better term is "depressed sewer"

Unlike the main sewer pipe, the siphon pipe(s) flow under pressure. Special care must be taken in inverted siphon design since losses are greater for pressurized flow, and the velocity in each siphon pipe must be at least 3 ft/s (0.9 m/s) for sewage or 4 ft/s (1.2 m/s) for storm water (Metcalf and Eddy, 1981). Therefore, even if there is only one main sewer pipe, several siphons may be required. If minor losses due to bends or elbows in the siphon are significant compared to the siphon length, include the equivalent length of the elbows. Increase the siphon length (Ls) so that Ls is the physical length of a siphon plus the equivalent length of minor losses due to elbows in siphon.