Terminology for depressed sewer crossings: The arrangement used to carry a sewer below an obstruction and beneath the hydraulic grade line is commonly called what?

Introduction / Context:
Sewer alignments sometimes must pass beneath roads, streams, or utilities. When the crown of the sewer must dip below the hydraulic grade line to cross such obstacles, special depressed arrangements are used and referred to by several equivalent terms in practice.

Given Data / Assumptions:

• The sewer crosses below an obstruction.
• The pipe invert is below the hydraulic gradient.
• Gravity flow conditions with flowing full at the depressed reach are possible.

Concept / Approach:

Such depressed sections are widely called inverted siphons, sag pipes, or depressed sewers. Although “siphon” suggests suction, in sewerage it simply denotes a depressed pressure-flow reach conveying flow under a head differential between upstream and downstream manholes.

Step-by-Step Solution:

Verification / Alternative check:

Hydraulic checks confirm that energy grade line allows passage without surcharge at design flow; CFD or steady-state models may be used.

Why Other Options Are Wrong:

Options (a), (b), and (c) are not wrong—they are synonymous; therefore the comprehensive correct choice is “All of these”.

Common Pitfalls:

Underestimating headloss; omitting flushing arrangements; allowing deposition in low-velocity conditions.

Final Answer:

All of these