Predicting Potential Disasters

How Tidal Information May Save You From a Coastal Crisis


The memorable blizzard of February, 1978, caused coastal flooding and $500 million in damages to Massachusetts alone, much of the loss to coastal properties. The storm began a few hours after the moon was in perigee (closest to the earth) on February 6, and the day before a new moon. The property damage resulted not only from the severity of the storm and its accompanying storm surge, but also from the extreme high water caused by the nearly coincident new moon tide, or "spring tide," and a perigean tide.

Though meteorological conditions, such as those that produced the Blizzard of '78, are predictable only days or hours in advance, astronomical high tides are predictable centuries in advance. If you are a coastal property owner in Massachusetts, or a boat owner, you may want to note dates on your calendar when a perigean high tide is predicted, as it could combine with a storm to produce coastal flooding and property damage. You can find the dates for perigean tides -- and information about tides and tidal ranges in general -- in several reference materials that are listed in this bulletin.

Perigean Spring Tides

The term "spring tide" does not refer to the season, but rather to the higher high tides and lower low tides which occur at new and full moons. At new and full moons, the sun, earth and moon are aligned such that the pull of the sun on the oceans adds to the pull of the moon on the oceans. Spring tides alternate at one week intervals with "neap tides." Neap tides occur during the first or third quarter moons when the sun and moon are aligned at right angles with respect to earth, and the sun tides subtract from the moon tides.

A perigean tide refers to a tide that occurs when the moon is closest to the earth. The moon's orbit around the earth is elliptical rather than circular, which means that the distance between earth and moon is always changing. Perigee refers to the time when the moon and the earth are closest to one another. At perigee, the moon is about 30,000 miles closer to earth than at apogee, when the moon is farthest from earth. Perigee is reached about once a month, roughly the time it takes for the moon to revolve around the earth. When the moon is closest to earth, its effect on tides is greatest.

The largest astronomical tides, perigean spring tides, occur when spring tides and perigean tides coincide. Perigean spring tides occur at intervals that are slightly more than six months long, so each year they are later in the season than the preceding year. For this reason, we must refer to tables of predicted tides to know exactly when to expect these unusually high tides.

The term "storm surge" denotes the high sea levels -- those not related to astronomical tides -- that often accompany severe storms. There are two major causes of storm surges: strong onshore winds the low atmospheric pressure accompanying such storms.

As in the Blizzard of '78, major coastal flooding and storm damage is likely to occur when perigean spring tides coincide with a major storm and storm surge. The direction of the wind (onshore) and low atmospheric pressure (which effectively contributes to a higher sea level) will add to the severity of storm damage. Because Massachusetts has coastline facing the ocean at virtually every point of the compass rose, coastal flooding during storms is likely to occur somewhere regardless of the direction of the wind. The role of tides in increasing storm damage is more significant in areas having larger tidal ranges (i.e., the North and South Shores, Boston, Cape Cod Bay and the open ocean side of outer Cape Cod) than in areas with smaller tidal ranges (i.e., the south shore of Cape Cod, Buzzards Bay, Nantucket, Martha's Vineyard, Fairhaven, and New Bedford).

While damaging storms can occur under other sea and tidal conditions, high astronomical tides caused by the near coincidence of a spring tide and perigee can be especially dangerous. By becoming more informed of the timing of these predictable events, you will also be preparing yourself for the unpredictable events of nature.