Wednesday, March 3, 2010
The Parting of the Sea: Low Tide in the Barra Lagoon
We've had interesting tides over the last few days in the lagoon where we are anchored, in Barra de Navidad (19 degrees 13.077 North, 104 degrees 42.762 West). Given that it was a full moon, the tidal ranges tend to be fairly wide to begin with, and the night we arrived it was no different: a six-foot-plus differential from high to low tide.
We came in to the lagoon as the tide was going down, but luckily it was still deep enough to maneuver without running aground. We watched another monohull come in after us - it had to back up and find a deeper route as it was not so lucky. We found one of the deeper spots to anchor in the anchorage with still only 3 feet under our keels and still another foot and a half to drop. This means that at low tide, there was only a foot and a half between our keels and the muddy lagoon bottom. Because we are a catamaran, our draft (the length from the water line to the bottom of the keels) is small (about 4 feet). Generally, monohulls have a much greater draft, and many sat on the bottom during this unusually low tide. Luckily, because the lagoon bottom is mud, sitting on the ground was not disastrous.
One boat, Moon Tide, even decided to purposely beach their catamaran to get some bottom work done. This was done by observing the lagoon over a couple of days to see where the shallowest point was, and knowing the tides. The boat was then driven over this shallow point at high tide, anchored, and awaited the drop in water level. The boat was then cleaned, and its crew then waited for the tide to rise again that night to be able to float off.
There are generally two high tides and two low tides daily on the west coast. While we have paid attention to tides at various intervals (we have them on our chart plotter), it's only been relevant pretty much in four situations. The first is when we've wanted to go up a river as you do for the jungle river dinghy trip in Tenacatita. This type of river has brackish water - part salt and part fresh water - that runs into a lagoon through mangroves. Here, you want to go into the river from the bay with a falling tide so that the current going in helps you motor upriver, and return downriver at a rising tide for the same reason. Of course our motor can take us against the current, but it's nice to have some help.
The second situation in which we follow the tides is going in or out of a marina or estuary leading to a marina or anchorage, as the tides affect the depth of the dredged channels as well as the currents which in turn affects your docking strategy.
The third situation is when you need to make a beach landing with your dinghy. When tides are down, the water is obviously shallower; the wave breaks at the shore are much bigger so greater caution must be exercised when you bring your dinghy ashore or try to set out back to your boat. Also, if you are coming ashore at low tide, you must be sure to pull your dinghy high up beyond the high water mark or you'll lose your dinghy as the water rises. With our hard bottom dinghy which weighs about 200 lbs., and another 100 lbs. for our outboard motor, we've become pretty strong. Another example of the family working together...
Finally, and perhaps the most important, we pay attention to the tide differential whenever we've dropped anchor somewhere: Your chain length-to-water depth ratio should be about 5 to 1 in relatively good conditions, and greater in large waves or heavy winds. The depth should be the high tide level, which we learned the hard way early on in our journey -- you may recall our blog post the night we dragged our anchor in Los Muertos as we hadn't considered that we anchored at low tide and didn't take into account that the tide would rise four feet - multiply that by 5 and you've got another 20 feet of chain that should have been let out. The more chain you let out, the lower the plane of its angle, which creates less pull on the anchor dug into the bottom. Make sense?
Here in the lagoon, the tides are much more relevant everyday for just dinghying to shore because there are areas in the lagoon that are so shallow they are above the water line during extreme low tides. As a result of such shallow water near the dinghy dock (perhaps only a foot deep), and for the first time since we began our journey, we got a fishing line caught in our dinghy motor (although quickly got it off without any damage). It's odd to see kids standing in only knee deep water in the middle of a lagoon, or fishermen doing the same. And it is quite a site to see most of this large lagoon dried up, giving the feeling that this is what it must have been like when the Red Sea parted - a good lesson as Passover approaches.
P.S. Some of you have asked why we put our GPS coordinates (otherwise known as our Lat-Long, or Latitude and Longitude coordinates). I believe you can cut and paste them into Google Earth (though you made need to convert them to decimal format) to see exactly where we are. It's pretty cool - check out this lagoon!