Thursday, December 14, 2017

Adventure at Isle of Palms - Water Levels

Today is the big day, you’re headed through the famous shallows around McClellanville in the Isle of Palms, South Carolina. It’s a bright and sunny day, perhaps a little windy but that’s no big concern in the protected waters of the ICW. You’ve done your homework and read all the posts and even one from yesterday that was corrected to MLW. You’re going through with 2 ft of tide (dropping…) so you don’t foresee any problem with a 4ft keel. – it will be tight but ought to be okay.  As you progress, the water depth seems a little less than expected so you slow down a bit to be sure to stay in the channel. Unfortunately, that allows the tide to drop even further, you hit bottom and you’re stuck! What happened?

I’m sure the first chorus will be, “Dummy, never challenge McClellanville at low tide!”  But we’re interested in just the facts. Let’s take a look at Feb 7, the day of his adventure.

The first thing you notice is the increase in wind speed with gusts up to 39 mph (34 kts). Although the chart doesn’t show it, the direction is out of the northwest. It’s the ideal direction to push water out of Charleston Bay and associated bodies of water nearby. Now let’s look at the tide level vs predicted (per tide tables).

Now you can see the problem here. The actual water level was following the predicted level until the northwest wind started blowing. At low tide and approaching low tide, there was more than a foot less water depth than predicted on Feb 9, 2017 at noon due to that west wind.  Look at how quickly that northwest wind pushed the water out! Now it’s true that the tide station measuring actual vs predicted is located in Charleston but my experience has been that it’s a pretty good predictor of what’s happening in the Isle of Palms too.

But, you may ask, how about those times when there’s more water depth than predicted? What causes that?  Let’s look at a wind report from 11/11/2017.

The wind was blowing out of the northeast (and had been for several days) with gusts to 26 mph (23 kts) on 11/11/2017. Any wind with an easterly component will tend to push water against the coast with the result shown below at the Charleston tide station for the same day. If you had transited the Isle of Palms that day, you would have enjoyed an extra foot of water above the predicted tide level. I like to see charts like the one below when I transit McClellanville!

So the moral of the story is to pay attention to the wind in your plans and take advantage of those tide stations that give the actual tide vs predicted tide. See Links, Weather, Surveys, Buoys, and Tides for the list. Of course, just an abundance of rain can also raise the water level but the same tide stations will also give you an indication of that effect. By now you should appreciate how two captains can go through the same stretch of shallow water and see a 2 ft difference in depths even when corrected to MLW! There was more than 2 ft of difference in just the two charts above. So take advantage of the data available on the web in this modern age, it can be a lifesaver (or at least a keel saver).

By the way, our friend had a nice view of the countryside around McClellanville while waiting for the tide to rise.