Is the lagoon half empty or half full?


To the editor: During the past three seasons in French Polynesia, we have observed the frustration many captains feel as they leave their emotion-laden Marquesas landfalls to navigate the sometimes tricky passes of the atolls of the Tuamotus. 

Predicting the current in the passes is critical to safe navigation. Strong, current-opposing large swells or wind waves can produce standing waves that could stop a vessel’s forward momentum and sweep it sideways onto a reef. For this reason, on those passes with strong current it is critical to predict the timing of slack water.

Some crews will visit several atolls, always correctly predicting the slack, while others will always get it wrong. When comparing their very different experiences on the SSB radio or in person, some of these crews will come to believe that they have (or do not have) an accurate method of estimating slack. However, the truth is that all of the main methods will work when the lagoon is at normal levels, but none of the traditional methods will work exactly when the lagoon has been overfilled.

First, a bit of background on atolls and passes: An atoll is lagoon surrounded by an encircling coral reef that may or may not have one or more navigable passes. All things being equal, with the normal tidal flood, water comes into the atoll through any passes, breaks in the reef or low points, and — with the normal ebb —water exits the lagoon through the same channels. 

At its most basic, the strength of the current through these passes depends on two factors: 

1. How easily can water get in and out of the atoll? Water can come in and out through passes — navigable or false — and any broken or submerged areas of the reef forming the atoll. All other things being equal, an atoll with more deeper and wider passes and more areas of broken or submerged reef will have less current.

2. How much water is inside of the lagoon? The amount of water in the lagoon is influenced by the range of the tide at that time. While the tidal range is not large in French Polynesia, the larger the range, the stronger the currents will be in the passes. 

The amount of water in the lagoon is influenced by the size of the atoll. Even if the tidal range is not incredibly large in the Tuamotus, in a lagoon 35 nm long and 20 nm wide (e.g., Rangiroa) those few feet of tide add up to a lot of water. All other things being equal, a larger atoll will have stronger currents. 

If these were the only two factors affecting the current in the pass, most of the basic tidal calculations for high water and low water slacks and currents would be accurate both in their timing and in their estimates of the strength of the current. Mariners could use simple calculations based on the moon, calculators and tide tables available as freeware or as part of chart/navigation packages and find them accurate.

Those estimates become more and more inaccurate, however, the rougher the conditions become at sea. This is because the amount of water in the lagoon is also affected by the amount of water being pushed out of the ocean, over the reef and into the lagoon by the wind, wind waves and ocean swells. As the conditions strengthen, larger swells and stronger winds will begin forcing more and more water into the lagoon. The lagoon will fill, sandbars will disappear, tidal high water marks will be broached, and the amount of water trying to exit the passes on the ebb tide will increase.

Lagoon depth can be affected by tide and wind.

Livia Gilstrap

At this stage, all traditional methods of estimating the slack (e.g., tide tables, having the moon overhead or underfoot) fail because they do not take into account the effect of a full lagoon on the currents in the pass. A full lagoon causes two major problems with estimating slacks. First, the strength of the ebbing current increases and the strength of the flooding current decreases. As lagoon fills, the force of the extra water trying to exit the atoll adds to the strength of the current on the normal ebb and dilutes the current on the flood. 

Second, and this is the factor that is the least understood by incoming voyagers, the timing of the slacks change. Let’s imagine a hypothetical atoll with one pass. In this hypothetical atoll, the low water slack (the end of the ebb) for the day was estimated to be at 8 a.m. local time. At 10 a.m. mid-flood, the current was estimated to reach 2 knots and a max flood current of 4 knots at 11 a.m. The flood would begin to slow down, lowering to 2 knots at noon, and the high water slack (the end of the flood) was estimated to be at 2 p.m. local time. 

Hypothetical effects of a full lagoon: Now imagine that the lagoon is full to a point that the water trying to get out of the lagoon is running at 2 knots. This is the same as saying that regardless of the state of the tides, there is a baseline 2-knot ebb all day long. At 8 a.m., when resources available to mariners estimate low water slack, there will still be a 2-knot outgoing current. Neutral current (slack as experienced in the pass) will actually occur at 10 a.m. when the normal incoming current neutralizes the 2-knot extra outflow. Similarly, when the tidal flood decreases back to 2 knots at noon, the extra 2-knot outflow will cause neutral current to be reached two hours prior to estimated high water slack. The hypothetical effects of a full lagoon on the ebb will be the opposite. The extra outflow will cause the experienced high water slack (i.e., the change to the ebb) to occur earlier than expected.

Putting these two factors together, with the lagoon full the low water slack will be later than expected and the incoming tide will be weaker or possibly non-existent, the high water slack will be earlier than expected and the ebbing tide will be stronger. This is despite the fact that high and low water will still occur as expected at the normal time on the beaches inside the lagoon.

Because of concerns about wind over tide, during a full lagoon the mariner should be particularly careful of any passes where the ebbing tide, which is stronger than normal, opposes the ocean swell or the current wind and wind wave conditions. 

How do you know if the lagoon is full? If inside the atoll, watch the high tide mark near your anchorage. If the conditions change, you will see the high tide mark change. You can also usually judge whether the high tide mark is lower or higher than normal by examining the shoreline. If high tide is lower than the driftwood and shell line, then the lagoon is probably not full. I assume whenever the swell is high and the wind is over 20 knots that the lagoon is filling, and the longer that goes on the more full the lagoon will be.

If the traditional methods fail, how do you predict the current when the lagoon is full? I asked this question of a dive master in the Tuamotus and he said that the only sure way is to go to the pass and watch the tide change. Of course, he is correct and if you are inside the lagoon and can access the pass, this is the best method. Tide changes in the pass are visually obvious and any delays or changes in today’s slack are a good way to estimate the delays for the next day. A second option is to attempt to call boats inside the atoll via VHF radio or to talk to boats via the SSB who have transited the pass recently. Ask them to estimate the current they experienced and note the time. Then, compare their experience with your predictions. 

Because we often are entering atolls without other boats to contact by radio, another method we employ is to arrive at the pass several hours before the target slack. While waiting outside the pass, we observe the waves caused by the current in the pass. When the waves are outside the pass, the current is ebbing. As slack approaches, the waves will reduce in size and will move closer to the pass. Eventually during the flood we will see waves start rising inside of the lagoon. We time our entrance as best we can for when the waves are low and in the center of the pass.

Although our learning curve included some white-knuckle moments, we have now had more than 40 safe pass transits in the South Pacific, with more ahead of us. We fell deeply in love with the Tuamotus; many of our favorite cruising moments, many of our fulfilled dreams and many of our fondest memories come from our time in isolated tropical atolls. For us, and many other cruisers, atolls are well worth the effort!

—Livia Gilstrap and her co-captain Carol Dupuis have been cruising on board their Wauquiez Pretorien Estrellita 5.10b since 2010. They are currently in the South Pacific. Read more on their blog, 

By Ocean Navigator