For much of '96, tide stayed high Anomaly: Some years have had one tide higher than any recorded last year on the Chesapeake Bay, but 1996 was unique for the sheer number of above-normal tides. Several factors could explain the phenomenon.

On the Bay

April 04, 1997|By Tom Horton | Tom Horton,SUN STAFF

THROUGHOUT Virginia and Maryland, people who observe the Chesapeake Bay know 1996 was the most unusual year for high tides that they can recall.

There have been several years with a single tide higher than any in 1996. (The highest recorded came in 1933.) But there has been nothing like the number of higher than normal tides last year.

There were weeks and months when, it seemed, the tide never really went out. Homeowners who get tide in their waterfront yards once every few years got it several times in 1996.

Their inconvenience was my delight.

My avocation is running aground while trying to go where no canoe has gone before, progging the tiniest, winding marsh creeks and guts.

But last year, I floated wherever I went. Just a few inches added onto the normal tide can open huge acreages to small craft along the gentle slopes of the bay's shore.

Is there an explanation for what elevated the tides in 1996, I asked Bill Boicourt?

Boicourt is an oceanographer with the University of Maryland's bay laboratory at Horn Point near Cambridge, and I've learned over the years that you always learn something fascinating from him, even if it is not the answer you asked for.

No one keeps long-term averages of the annual frequency and duration of high tides, so there is nothing official to compare with 1996, Boicourt said.

But it clearly was a remarkable year for high tides, and several factors could explain it.

We were at the peak of a long-term astronomical cycle, in which the tilt of the Earth and the tilt of the moon's orbit conspire every 18 1/2 years to exert more gravitational pull on the tides.

It was also a record-setting year for freshwater running into the bay from its rivers, and fresher water, being less dense than saltier water, "stands" higher.

The most noticeable high tide period of last year was during the warmer months. Thermal expansion alone -- in other words, warmer water takes up more space than cold water -- means the summer bay is about seven inches higher than the winter bay.

But all the above were likely no more than sideshows to the real action that drove the bay so high so often, Boicourt said.

In years both normal and abnormal, the major reason for most tidal caprice is the wind.

The Chesapeake Bay, Boicourt explains, is "unusually" disposed to the wind's influence. It is long, nearly 200 miles, and reasonably straight over that whole length, "so the wind can act, consistently, over a large area of it."

Blowing steadily from one direction, wind can, in effect, "tilt" the whole bay. This in turn can substantially add to, or subtract from, the tide as predicted on the basis of the sun's and moon's gravitational pull.

For example, think of the fierce northwest blow early this week as a giant hand depressing the bay around Baltimore and elevating it toward its mouth at the ocean.

The tides here still went up and down like lunar clockwork (two highs and two lows every 24.8 hours). But because of the nor'wester, they were a foot or more below normal.

Conversely, southwest winds can tilt the bay the other way, causing tides higher than normal.

And the initial tilt given by the winds is just the start. Once the wind relaxes, or changes direction, the bay does not just calmly return to equilibrium.

Rather, it begins to "seiche", or slosh, as water does in a dishpan if you tilt it sharply and then let it go. The dishpan will slosh several times in a few seconds. But the bay's sloshing frequency (north to south and back) is on the order of two days. That means that long after a wind has passed, a tidal surge, or slosh, can occur that is very much a product of that wind.

Now, take that sloshing a step further, Boicourt says.

Think of how, if you push someone on a swing and pump it with just the right timing, you can get it soaring way up and way back, even with minimal effort.

So it is that winds, blowing repeatedly with just the right timing, can greatly enhance the sloshing and tidal effects. They need not even be powerful winds, just timed to resonate with the bay's slosh cycle.

Finally, some winds, notably nor'easters, can pile up so much ocean water along the coast that it prevents the normal flow of water out of the bay on ebb tide, raising tides well above normal along the length of the Chesapeake.

And so it probably was during 1996: an unusual number of wind events whose power, direction and timing raised the tide. Throw in more freshwater inflow, along with the peak of the 18 1/2 -year Earth-moon cycle, and it was a remarkably soggy year.

One of Boicourt's colleagues at Horn Point, J. Court Stevenson, says he thinks yet another factor might have contributed.

Stevenson has been studying how long-term sea-level rise affects marshes in the Chesapeake. By analyzing how fast sediment was deposited historically in cores of mud taken from marshes, he knows the rate of sea-level rise jumped in the 1890s and again in the 1930s. (The reasons are not well understood.)

He says he thinks once his data for this year and last year are analyzed, "We'll see there was another little buzz [jump] in the rate of rise during 1996 and 1997."

Boicourt says the day is at hand when real-time monitoring buoys in the bay, coupled with improved computer models, will give us 24-hour advance forecasts of how high and low the tides will be.

The key will be better wind data, to add to the easily predictable gravitational effects of sun and moon.

Pub Date: 4/04/97

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