What do clearance gauges at bridges indicate?

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🔍 Key Concepts

• Clearance gauges are installed on bridge piers or fenders to help mariners assess if their vessel can safely pass under the bridge at the current water level • Think about the difference between vertical clearance (air draft under a bridge) and water depth (draft under the keel) • Consider how tide or river level changes would affect what the gauge needs to show in real time

💭 Think About

• Ask yourself: When a captain looks at the numbers on a clearance gauge, what decision are they trying to make about their vessel? Passing under the bridge or water depth under the keel? • Do clearance gauges give fixed, charted information, or do they show something that changes with the actual water level at that moment? • Which answer choice best describes a measurement from the water surface to a part of the bridge structure that could be hit by the vessel?

✅ Before You Answer

• Make sure the option you pick is about air draft under the bridge, not depth of water under the keel • Check whether the wording suggests a real-time indication versus a charted, average, or tidal datum • Eliminate any choice that describes tide height alone or subsurface depth rather than clearance under the bridge

🔍 Key Concepts

💭 Think About

✅ Before You Answer