07/30/2025
What Boaters Need to Know About Tsunamis 🌊
Tsunamis are one of the most powerful and destructive natural forces, and as a boater, understanding them and knowing how to react is crucial for your safety and the safety of your vessel.
What is a Tsunami?
A tsunami is a series of exceptionally long waves caused by a large-scale disturbance, most commonly an underwater earthquake, but also by landslides, volcanic activity, or even asteroid impacts. Unlike regular waves generated by wind, tsunamis involve the entire water column from the surface to the seafloor, carrying immense energy.
How Tsunamis Affect Boats: The Deceptive Nature of Wave Height
The impact of a tsunami on a boat depends heavily on whether the vessel is in open, deep water or in a harbor/shallow coastal area. It's critical to understand that a tsunami is fundamentally different from a regular wind wave or boat wake, even if they share the same apparent "height." Equating them is a dangerous misconception.
The Core Difference: Wavelength and Water Column Involvement:
• Wind Waves and Boat Wakes: These are surface phenomena. The energy is largely concentrated near the surface, and the water particles move in a circular motion that diminishes rapidly with depth. Their wavelength (the distance between two crests) is relatively short. A 1-foot wind wave means only the very top layer of water is significantly disturbed.
• Tsunamis: These are entire ocean column phenomena. From the surface all the way to the seafloor, the entire body of water is displaced and carries energy. Their wavelengths are incredibly long—often tens to hundreds of miles. Imagine pushing a tiny ripple across a kiddie pool versus pushing the entire volume of water in the pool from one end to the other. That's the difference in energy and water movement.
In Deep Ocean (generally greater than 100 fathoms or 600 feet):
• Tsunami waves in the deep ocean are often barely noticeable. They may only be a few feet high, but they travel at incredible speeds (like a jet plane, over 500 mph).
• Boats in deep water are generally safer from the direct destructive force of a tsunami than those in harbors. You might not even feel it pass by.
• However, even in deep water, strong and unpredictable currents can be generated, which can be hazardous.
In Harbors, Shallow Water, or Near Shore: The Real Danger:
• As tsunami waves approach shallow water and the coastline, their speed decreases, but their height significantly increases due to a process called shoaling. This compression of energy as the wave "piles up" makes them extremely dangerous.
• Even a "small" forecast height is deceptive. A 1-foot tsunami in the deep ocean, or even a 1-foot forecast for the coast, can easily become a 5-foot, 10-foot, or much larger wave upon arrival, especially in bays, harbors, and estuaries that can amplify the wave. The Santa Cruz tsunami you mentioned, which was observed at only 5 feet but generated 15-knot currents and caused over $20 million in damage inside the marina, is a perfect example of this destructive power. A 5-foot wind wave simply cannot generate sustained currents of that magnitude throughout a marina basin.
• Harbors and coastal areas are highly vulnerable. Tsunamis can cause:
• Rapid and extreme changes in water levels: Sudden drops can ground boats, and sudden surges can overtop docks and lift boats over pilings.
• Strong and unpredictable currents: Because the entire water column is moving, tsunamis generate incredibly powerful, rapidly changing, and unpredictable currents. These include eddies and whirlpools, which can break mooring lines, drag boats, and cause collisions. These currents are sustained, unlike the brief surge of a wind wave, and can persist for minutes or even hours as multiple waves in the tsunami series arrive.
• Waves, surges, and bores: These can flip over boats, cause them to break moorings, and collide with other vessels, docks, or debris.
• Debris: The tsunami will pick up and carry enormous amounts of debris (docks, cars, trees, houses, other vessels), which can severely damage or sink vessels acting like battering rams.
• Spills of harmful chemicals and waste: From damaged facilities.
• Damage to infrastructure: Docks, piers, and other port facilities can be destroyed, making it unsafe or impossible to return.
• Uncertainty Factor: It's important to remember that initial tsunami forecasts for height do not guarantee the exact localized impact. Local bathymetry, coastline shape, and harbor configurations can significantly amplify or diminish incoming waves in ways that are difficult to predict with perfect accuracy. Furthermore, a tsunami is not a single wave but a series of waves, and the largest in the series might not be the first, often arriving hours later.
Tsunami Warning Signs and Alerts:
There are two main types of tsunami warnings:
Natural Warning Signs (Local Tsunamis): For tsunamis generated nearby, you might not receive an official alert in time. Your first and best warning might be natural signs:
• Strong or long earthquake: If you are near the coast and feel an earthquake strong enough to make it difficult to stand, a tsunami could be coming within minutes.
• Loud roar from the ocean: Like a train or an airplane.
• Unusual ocean behavior: Such as a sudden rise or wall of water, or a sudden draining away of water showing the ocean floor (a very low, low tide).
• If you experience any of these, act immediately!
Official Tsunami Alerts (Distant and Local Tsunamis):
These are issued by warning centers and broadcast through various channels:
• Tsunami Warning: This is the highest level of alert. A dangerous tsunami is expected or occurring, with widespread flooding and powerful currents. Immediate evacuation is recommended.
• Tsunami Advisory: A tsunami with potential for strong currents or waves dangerous to those in or very near the water is expected or occurring. Flooding of beach and harbor areas is possible. Stay out of the water and away from beaches and waterways.
• Tsunami Watch: A distant earthquake has occurred, and a tsunami is possible. Stay tuned for more information and be prepared to take action.
• Tsunami Information Statement: An earthquake has occurred, but there is no tsunami threat, or the threat is very far away and has not been determined.
How to receive official alerts:
Sign up here https://mil.wa.gov/alerts
• Marine VHF Radio: The U.S. Coast Guard issues urgent marine information broadcasts on Channel 16.
• NOAA Weather Radio: Broadcasts warning and post-event information for all types of hazards, including tsunamis.
• Wireless Emergency Alerts (WEA): These are automatic alerts sent to mobile phones in affected areas.
• Local Radio and Television: Tune into local news.
• Outdoor Sirens: Some coastal communities have tsunami sirens. Local Emergency Management Offices: Sign up for their email and text message alerts.
• Tsunami.gov: The official U.S. Tsunami Warning System website.
•Get a NOAA Weather Radio. Emergency NOAA Weather Radio: You get NOAA emergency weather broadcasts along with AM/FM stations. Https://amzn.to/442JGES It has 3 power sources: a hand crank, a solar panel, and AAA battery compatibility, so you'll always have juice for the radio, lights, and SOS This little powerhouse has a 7400mWh (2000mAh) power bank to charge your phone or small tablet.
What to Do if a Tsunami Warning is Issued:
Your actions depend on your location:
If you are on land or tied up at the dock (in a harbor/marina):
• DO NOT try to take your boat offshore. You will likely not have enough time, and attempting to do so could put your life at extreme risk due to rapidly changing water levels and powerful currents.
• Immediately leave your boat and evacuate on foot to high ground as soon as possible. Follow designated evacuation routes if available. Aim for at least 100 feet above sea level or at least one mile inland.
• Even seemingly small tsunamis can cause dangerous currents and flooding in harbors.
If you are in deep water or can get to deep water quickly (generally 100 fathoms/600 feet or deeper):
• Head for deeper water. If you are already at this depth, maintain your position.
• Do not return to port if a tsunami warning has been issued. Port facilities may be damaged and hazardous with debris and powerful, lingering currents.
• Sail directly into the waves, but be aware that tsunami currents can amplify wind waves.
• Maintain as much separation as possible from other boats to avoid collisions.
• Be prepared to stay at sea for a day or more, as coastal areas and harbors will be unsafe.
• Important: This option is primarily for those already offshore or very close to deep water with enough time to reach it safely. For a local tsunami (generated nearby), there will be no time for this maneuver.
If you are in shallow water, a harbor, or a lagoon and cannot reach deep water in time:
• Return to shore, secure your vessel as best you can, and move quickly to higher ground or inland. Your personal safety is paramount. Do not risk your life for your boat.
Tsunami Preparedness for Boaters:
Being prepared is key to minimizing risk:
Know Your Risk:
• Understand if your boating areas are in tsunami hazard zones. Check maps that show potential flooding and current speeds.
• Learn about local tsunami warning signs and alerts.
Stay Informed:
• Sign up for tsunami alerts from your local emergency management office.
• Have a battery-operated NOAA Weather Radio on board.
• Regularly monitor marine radio channels (e.g., US Coast Guard on VHF channels 13, 14, and 16).
Make a Plan:
• Develop a plan with your crew for what to do if a tsunami happens, both on the water and ashore.
• Identify quick ways to release anchors or fishing gear so your boat isn't dragged by currents.
• If you live on your boat, have at least two weeks of supplies stored onshore, and a "go bag" ready.
• Practice and update your plans regularly.
• Check with your local port and emergency management about their tsunami procedures.
Prepare Your Boat:
• Securely tie your boat when docked and replace aging mooring lines.
• Have enough food, fuel, and supplies (at least 3 days) on board in case you need to remain at sea.
• Be aware that shore facilities may be damaged, so plan for self-sufficiency if you're at sea.
Heed Natural Warnings:
• Never wait for an official alert if you experience natural tsunami warning signs. Act immediately.
By understanding these points and taking proactive steps, you can significantly increase your safety as a boater in the face of a tsunami threat. Remember: any tsunami alert should be taken seriously, regardless of predicted heights. Your life is more valuable than your boat.
