The normal nature of tsunamis is such that they are preceded by a long-range trough. This causes an atypical drop in water level. This occurred during the recent Indian Ocean situation, and was responsible for much of the loss of life at Hilo, Hawaii, many years ago when it was hit (many of those drowned at Hilo were out on the reefs looking for shells in the unusually low tide).
If you are anchored in an exposed location, especially if the water is shallow, if the water level drops unexpectedly, head to sea immediately. It will help if the anchor chain is set up to be slipped in an emergency. If ashore, head for high ground.
We were surprised to find some boats were held in place by their ground tackle, even inside of the “surf line”. There are photos elsewhere on SetSail showing some boats being held through the first wave. Big anchors, strong chain, a chain stopper, and bow roller structure sufficient to handle the full breaking strength of the chain are part of this system.
Where to Anchor
Anchoring in exposed roadsteads is always a risk. Not so much from tsunamis, but from waves generated by far-off storm systems. Boats have been driven ashore in Cabo San Lucas, the West Indies, and even Fiji by wave systems unexpectedly arriving from storms occurring thousands of miles away. The key to avoiding this problem, and those associated with tsunamis, is sufficient water depth. Required water depth is related to bottom slope, and the shape of the anchorage. Shorelines that funnel wave energy towards the beach require more depth than open shore lines. Bottom shape also has an impact. Deeper is always better.
Although there was loss of life on the Indian Ocean atolls, the boats at anchor seemed to fare OK-at least that’s the word we have so far. The water level fell and rose in cycle, but aside from substantial currents, there does not seem to have been significant breaking waves inside of the atolls. We hope to get more information on this in the future.
Boats tied up to marinas throughout the affected areas seemed to have not done well. Docks were pulled apart by current, taking the secured boats with them, which were then swirled around in the harbors. Some were driven ashore, others suffered little or no damage. A similar situation occurred in Los Angeles Harbor in the early 1960s as it felt current flux from a tsunami generated by an Alaskan earthquake.
Multiple Wave Cycles
The three wave cycles reported by those on the scene seems to tie to other tsunami experiences. The lesson here is don’t think it is over after the first wave. Reports from cruisers in this event indicate it was the third wave that did the most damage.
Here are some additional comments from Al and Beth Liggett, who were on the scene:
Re: the tsunami stuff…Most of it tallies with what we saw or heard from others.
As far as the marina paragraph, we don’t feel that is accurate as the two marinas in Thailand were not affected at all: at Boat Lagoon the water just rose up and receeded. It flooded the hard stand and paint, buckets, shoes, lumber, spinnaker poles and the odd dinghy went whirling about, but the boats at the floating docks were all just fine. Since it is about a 45-minute meander in a narrow, shallow channel, (we can only get in/out on certain tides) leaving is probably not an option. Yacht Haven Marina is situated in a channel that is directly open to the west, and the wave surges did come in under the bridges and rush through the marina. All yachts held without damage. This is the one place that has docks long enough for mega yachts to berth, and we are told that they all had engines on to help hold boat and docks in the currents. This marina regularly has the spring tide rushes of currents coming through it. The management will advise new arrivals to hang off until slack water to make their entrance/exit, and has two “pusher” dinghies to help yachts maneuver into/out of their berths because of the current flow.
There are three marinas in Langkawi, but only two of them were the ones with all the damage; Rebak Marina and Telaga Harbor Marina. They are both very enclosed basins, and the water going in had no place to go except around in whirlpools, thus creating the havoc. The Royal Langkawi Yacht Club had again the rising/falling surge and rushing currents come streaming through. But again, it is open both ends and the waters were able to go in and pass through.
As far as we can gather, the few marinas (small) on the rest of the Malaysian coast had barely noticeable effects. – Al & Beth.