Many people arriving in our coastal waters are surprised by our tidal range, which is regularly between six and nine feet, vastly greater than other areas of the coast. Even if you see these tidal ranges every day, it’s easy to take them for granted. Let’s go back to school for a few minutes and discuss why and how our tides are so unique.
As you may recall, the moon’s gravity draws a bulge of water to the moonward side of the earth. Surprisingly, a second bulge (generally of lesser height) is created on the opposite side due to the centrifugal force of the spinning earth. These two bulges each day result in high tides. The two troughs on either side result in low tides. Due to the earth’s slant on its axis and other geographical forces, some locations experience semi-diurnal (two high and two low) tides like we do here on the east coast. Others have a diurnal (a single high and low) tide and other areas experience mixed tides (one major high and low, but with small secondary peaks).
With our semi-diurnal tides, the two highs and two lows are not equal in size and are distinguished as: Higher High Water (HHW), High Water (HW), Low Water (LW), and Lower Low Water (LLW).
These levels vary quite a bit over a 28-day cycle. Averages are calculated, not for just one month or one year, but over a 19-year period. These averages, or means, serve as vertical datum points (the agreed upon standard) for measuring depths of water and clearance under bridges. The datum now used for measuring charted depths in the U.S. is the Mean Lower Low Water (MLLW), the lowest of these four averages. Remember though, it is only an average, some low tides will be lower than average, resulting in less water than noted on the chart. Use extra caution at low tide when you see negative heights on tide tables.
Mean High Water (MHW) is used to chart bridge clearances. Always check bridge clearance gauges on approach. Do not rely on clearances marked on the chart. This is particularly true during spring tides (see explanation below).
So why are tide highs and lows different each day? As the earth rotates in its 24-hour cycle, the moon is also moving along its (roughly) 28-day cycle around the earth. The direction of rotation is the same for both bodies. In one day the moon has traveled 1/28 of its orbit, which also represents 1/28 of the spin of our earth. Since our earth takes 24 hours to make a complete spin, then the moon has traveled an additional 1/28th of 24 hours or about 52 minutes each day.
Therefore, tomorrow morning’s high tide will be on average, 24 hours and 52 minutes later than today’s. The time between the morning high (or low) and the afternoon high (or low) is half that much. However, those 52 calculated minutes can vary by plus or minus 10 minutes or more, depending on several factors – local rainfall, geography, river characteristics, moon state, wind, etc. For this reasons, it is always best to consult a tide chart for accurate times.
Not all high tides are the same height (ditto for low tides). As the moon orbits the earth, it alternates between a straight line with the earth and sun and right angles. During new moons and full moons, this straight alignment combines the gravitational pull of the sun and moon, producing higher highs and lower lows called spring tides. Neap tides are of lesser amplitude and occur during quartering moons. Each week we transition from one to the other. In addition, the moon’s orbit is elliptical. Ever wonder why the marshes get completely flooded some times? When a spring tide coincides with the moon being at its closest point of approach (perigee) we get those 10 and 11 foot tides. A strong easterly wind can have the same effect.
Georgia and South Carolina sit in the South Atlantic Bight. (A “bight” is a loop in a rope or an arc shape.) The implications of this geophysical fact are quite amazing. As the earth rotates east encountering the bulge we call high tide, the shape of our coast in the South Atlantic Bight, along with our shallow water, act as a funnel causing water to “bunch up” in our area, often several times higher than other parts of the coast. Georgia is pretty much in the apex of this funnel so we tend to get tides ranging from 6 to 9 feet; a big contrast to the one or two foot tides of South Florida.
Other parts of the world have even higher tides: The Bay of Fundy (40 feet), Anchorage Alaska (30 feet), and Seven River in Britain (20 feet) to name a few.
As you may have noticed, these large amounts of water funneling in and out of the area can have a huge impact on current, making docking difficult. In future articles we’ll address this, as well as some characteristics that affect handling on different boats. In the meantime, know your tide state. Be safe.