At a junction of two sanitary sewers of different sizes, how should the crown (top) levels be arranged to avoid backwater and turbulence?

Introduction / Context:
Proper geometric joining of sewers at manholes prevents backing up, separation, and undue turbulence. A key rule is how crown (top) levels meet when two pipes of different diameters intersect.

Given Data / Assumptions:

• Two circular sewers of different diameters join at a manhole.
• Gravity flow, separate sanitary system.
• Objective: avoid backwater in smaller sewer and preserve hydraulic grade continuity.

Concept / Approach:

Standard practice is to keep crown levels equal (crown-to-crown). This ensures the smaller sewer does not discharge against a higher crown on the larger pipe, which would obstruct flow and cause surcharge. Invert levels will therefore differ, with the larger pipe having a lower invert to maintain equal crowns.

Step-by-Step Solution:

Verification / Alternative check:

Design guides and municipal standards specify crown-to-crown junctions to prevent headloss and deposition at the junction.

Why Other Options Are Wrong:

(b) and (c) cause hydraulic mismatches and potential surcharging; (d) is unsafe; (e) equal inverts are incorrect when diameters differ.

Common Pitfalls:

Equalizing inverts instead of crowns; neglecting energy losses at bends or drop connections when levels vary greatly.

Final Answer:

Tops of both the sewers are at the same level (crown-to-crown join)