Sewer Design Practice – Matching Crown (Soffit) Levels at Junctions Assertion [A]: The crown (soffit) of the outgoing larger-diameter sewer is matched with the crown of the incoming smaller-diameter sewer at a junction. Reason [R]: Matching crowns prevents backing up of sewage in the incoming smaller sewer by avoiding a hydraulic step at the connection.

Difficulty: Easy

Both [A] and [R] are true and [R] is the correct reason for [A]
Both [A] and [R] are true and [R] is not the correct reason for [A]
Both [A] and [R] are false
[A] is true but [R] is false

Correct Answer: Both [A] and [R] are true and [R] is the correct reason for [A]

Explanation:

Introduction / Context:
At sewer junctions where a smaller-diameter line discharges into a larger one, detailing the relative soffit (crown) and invert levels is essential for smooth flow. A common rule is to match the crown of the larger outgoing sewer with the crown of the smaller incoming sewer. This practice influences hydraulic grade line behavior and maintenance performance.

Given Data / Assumptions:

Incoming sewer: smaller diameter.
Outgoing sewer: larger diameter.
Design objective: avoid hydraulic discontinuities that could cause surcharge or sediment deposition.

Concept / Approach:

Matching crowns ensures that the incoming stream does not encounter an upward step at the junction. If inverts were matched instead, the larger sewer’s crown would be lower, creating a hydraulic drop and a pocket where flow from the smaller sewer could back up or pond, increasing the chance of surcharge and grit deposition. Aligning crowns provides a continuous soffit, reducing turbulence zones and improving flow transition into the larger conduit.

Step-by-Step Solution:

State [A]: Crown of larger outgoing sewer aligned with crown of smaller incoming sewer ⇒ accepted good practice.State [R]: Doing so prevents backing up due to eliminating an adverse hydraulic step at the junction.Link: The reason directly explains the asserted practice.

Verification / Alternative check:

Standard sewer design manuals illustrate soffit alignment as a measure to prevent ponding at manholes and to maintain self-cleansing velocities, especially at low flows.

Why Other Options Are Wrong:

[B]: True/true but unrelated is incorrect; the reason is causally correct here.
[C]: Both are not false; they reflect standard practice.
[D]: The reason is not false; it is the technical basis for crown matching.

Common Pitfalls:

Matching inverts instead of crowns; ignoring energy losses at junctions; failing to consider drop connections when depth differences are large.

Final Answer:

Both [A] and [R] are true and [R] is the correct reason for [A]

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