Tides
All coastal boaters need a basic understanding of tides because they pose significant impacts on boating. With regard to marina life, tides are the primary process governing daily water level changes and associated currents. While tides are totally predictable at a given location, their characteristics vary greatly with location around the coastal U.S. as discussed below.
Tidal Effects on Water Level
Semi-diurnal tides are common in U.S. coastal waters and characterized by two highs and two lows daily. Driven by the gravitational pull from the moon and sun, they result in two simultaneous bulges on the ocean surface: one in the direction of the extra-planetary pull and the other on the opposite side of the earth. The gravitational pull from the moon is strongest due to its close proximity to the earth. Despite the massive size of the sun, its great distance from the earth causes its gravitational pull on our ocean to be only 46 percent of that from the moon.
The semi-diurnal (S-D) tide is a combination of the lunar tide with period 12.42 hours and the solar tide with period of 12 hours. This can result in varying heights of the two high-water (and two low-water) events during a single day; such variability are called inequalities to tidal enthusiasts.
Along the U.S. East Coast, tides are purely S-D, with no significant diurnal components. In Maine near the Canadian border, the S-D tide has a range of roughly 16 feet, partly due to its proximity to the Bay of Fundy in Nova Scotia, which sometimes experiences ranges approaching 30 feet on account of the coastline geometry that causes natural resonant amplification (increased vertical height) of the tide. With further distance to the south along the New England coastline, S-D tidal ranges decrease to 10 feet at Boston. South of Cape Cod in Massachusetts, S-D amplitudes are much less, ranging from 3 to 4 feet along the entire Atlantic coast between Nantucket and the Outer Banks of North Carolina. S-D amplitudes rise to near 5 feet along South Carolina and northeast Florida but drop again to 2 feet in southeast Florida and throughout the Florida Keys.
Eastport, Maine water level showing Semi-Diurnal tide over seven days. 20-ft tidal range with two highs each day.
Note that every two weeks, the moon and sun are in alignment, causing intensification of the gravitational pull and greater ranges (amplitudes) of the S-D tide. These events are called Spring tides (moon tides in some regions), with no connection to our climatological seasons. Relatively weaker S-D tides, called Neap tides, occur when the moon and sun are at 90 degrees to each other in relation to the earth’s surface and the gravitational pulls are consequently misaligned.
Diurnal tides have a period of 24 hours, equivalent to our solar day and resulting in one high water and one low water event per day. Generally, the diurnal tide is substantially weaker than S-D tidal amplitudes because it results from two relatively minor factors: 1) the inclination of the Earth’s rotational axis from the horizontal, orbital plane of the moon and 2) effects associated with the earth’s revolution around the sun.
Biloxi, MS water level showing Diurnal tide over seven days. 2-ft tidal range with one high each day.
Along the East Coast of the U.S., diurnal tidal amplitudes are negligible so tides appear purely S-D. In direct contrast, S-D tides are negligible in the Gulf of Mexico such that diurnal tides dominate there, although they are weak. Along the entire coast of the Gulf of Mexico, tidal ranges are only 1 to 2 feet. Outflow from major rivers (e.g., Mobile, Mississippi and Rio Grande) certainly have major impacts on local water levels but their effects on hourly water level fluctuations are small except during storm events.
Mixed tides occur when both S-D and diurnal tides are significant at a specific location. Two highs may occur in a single day but they typically have different amplitudes; the same for amplitudes of low water. On some days there is only one high tide depending upon how the S-D and diurnal tidal amplitudes conflict and cancel each other. A single low-water event can occur on other days. Overall, water level records in regions of mixed tides are very irregular compared to the nearly sinusoidal shape of purely S-D records.
San Diego, CA water level showing Mixed tide over seven days. 5-ft tidal range with different daily patterns.
The entire West Coast of the U.S. is characterized by mixed tides, from the Mexican border to Canada and along the southern Alaska coast to the Aleutian Islands. Ranges of mixed tides increase from about 6 feet in San Diego and Los Angeles to 10 feet along the Olympic Peninsula in Washington. Ten- to 12-foot ranges are normal from Juneau to Kodiak but decrease westward along the Aleutian Island chain.
In the Bering Sea of Alaska, Mixed tides have only 2-foot range and along the North Slope, ranges are even smaller. Tides in Hawaii also are Mixed with 2-foot range. The same for Puerto Rico in the Caribbean—less than 2-foot mixed tides.
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AUTHOR OBSERVATION |
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These widely varying geographic variations in tidal characteristics should be in the forefront of captains’ minds if they cruise throughout the coastal waters of the U.S., especially in harbors where the entrance channel is prohibitively shallow near low water. |
Tidal Currents
Horizontal currents are driven by the vertical movement of water associated with tidal forcing from the moon and sun. In simplest terms, flood tidal currents occur during the incoming, flood tide with strongest flow around midway between the times of high and low water; ebb tidal currents occur similarly during the opposite stage of the tide.
Tidal currents are typically not a major concern in marinas, especially those situated near the heads of harbors where currents are weak or non-existent. If however, a marina is located alongside a major harbor channel or is situated on a riverbank, then tidal flow can impact vessels and marina living in different ways, as listed below and discussed in other sections of this guidebook:
- Docking a vessel can be challenging if tidal currents are perpendicular to the direction of approach, especially for side-tie slips.
- Flushing by tidal currents can be beneficial if wastewater and nuisance algae are swept out of the marina by the horizontal flow.
The best way to prepare for tidal currents in your marina is to become familiar with the times of high and low water, and understand the variations in daily tidal ranges which affect the strength of tidal currents. NOAA websites provide excellent tidal current predictions for numerous locations around the coastal U.S. One of the main NOAA portals for such information is: http://tidesandcurrents.noaa.gov/currents14.