For preliminary harbour planning, the width of a harbour entrance is often restricted to a practical upper limit to control waves and currents while allowing safe navigation. Which standard nominal limit is commonly cited?

Introduction / Context:
Harbour entrances must balance safety and environmental control. Wider entrances admit more wave energy and can induce stronger currents during tides or storms, while too-narrow entrances compromise navigational safety. Many classical references provide nominal limits for preliminary checks.

Given Data / Assumptions:

• Question targets a typical cited nominal restriction, not a universal rule.
• Entrance must accommodate expected traffic while limiting wave penetration.
• Breakwater layout and local metocean conditions influence the final value.

Concept / Approach:
A nominal upper limit around 150 m is often taught in classical harbour-engineering problem sets to prevent excessive wave ingress and to maintain manageable entrance currents, pending a more rigorous site-specific design that considers diffraction, ship size, and metocean statistics.

Step-by-Step Solution:
Identify the purpose of restricting width: manage wave penetration and currents.Select the commonly cited nominal limit from classical sources: about 150 m.Recognize that actual designs may adjust this value based on local conditions.

Verification / Alternative check:
Historic MCQs and study notes frequently use 150 m as the indicative value, with caveats for larger modern ships and complex wave climates.

Why Other Options Are Wrong:

• 100 m or 125 m may be overly restrictive for many harbours.
• 180 m admits more wave energy, which classical guidance sought to limit.
• 90 m is too narrow for many vessel classes.

Common Pitfalls:
Treating the nominal limit as a universal code value; ignoring diffraction analysis and navigation simulation in final design.

Final Answer:
150 m