Chesapeake tides are full of surprises


December 04, 1993|By TOM HORTON RTC

SMITH ISLAND, Nov. 28 -- It's 7 a.m., and something both ominous and lovely, as regular as clockwork yet quite unpredictable, is beginning to happen here.

The tide is coming up.

Normally, that's not a big deal.

Twice every 24.8 hours the pull of moon and sun, and Earth's rotation, knead and mound the oceans into great, elongated waves that break on the coasts, not as surf but as tide.

So lengthy is the travel time of such waves up the Chesapeake Bay that by the time one crests at Havre de Grace, another wave -- the next high tide -- is entering the estuary's mouth. In between, at mid-bay, the tide is low.

The timing of each high and low is so predictable, for Smith Island or anyplace else on Earth, that it can be published to the minute, years in advance, or calculated by computer. So it is that we have known for a while, with wonderful certainty, that the tide this day at Smith Island will be at its fullest around noon.

Knowing when the tide will be high (or low) is easy. But what we can't predict, couldn't even suspect until yesterday, Nov. 27, and won't know for certain until noon today, is how high. For example, last night's tide was a tad higher than normal, though nothing remarkable. But by 7 a.m. today it was already well above a normal high and rising fast -- and it had another five hours until the crest.

By 10 a.m., the bay was covering our yard, where it had not reached for more than two years. In the street, a tabby cat crouched on a tuft of high ground and fished for minnows.

And still two more hours to rise.

It is the extreme shallowness of the bay, combined with the caprice of the winds, that make the extent of Chesapeake tides among the most difficult in the world to forecast.

Easterly winds, like the ones this Thanksgiving weekend, blow up from the bay's mouth, forcing extra ocean water into the estuary and blocking the outflow before another high tide arrives. Northwest winds have the opposite effect, causing extreme low tides. In April 1975, after nearly four days of nor'westers, the low tides got so low as to stop fully loaded ships from leaving the port of Baltimore.

The wind blowing on the bay's surface also routinely sets up what oceanographers call "seiching" (French for sloshing), a depression in one place causing a rise in another.

And because the bay is so "thin" (average depth about 21 feet), not much wind energy is needed to slosh the water around pretty good. Envision the phenomenon by extending your arm and fingers parallel to the ground. A small movement (wind pressing down) at the shoulder translates into quite a large movement at your fingertips. And these seiches/sloshes, depending on when the wind blows, can act either to amplify or dampen the normal tidal action.

High tide holiday

Eleven o'clock and the island kids are out in force, commandeering canoes and big chunks of plastic foam and paddling down streets and through the flooded yards. For youngsters, high tides are the equivalent of the mainland's occasional big snowstorms, which Smith seldom gets because of the surrounding bay's moderating effect on temperatures.

Sunday school and church have been canceled -- too wet for the preacher to get to the boat that normally takes him every Sabbath to the island's three towns.

And another hour still to rise.

From numbers alone, tidal fluctuations along the Chesapeake might seem unremarkable compared with the 20-foot and greater tidal ranges one can find in Alaska and Canada.

The difference between mean low water and mean high water on the bay is seldom more than a couple of feet -- tame even by comparison to Long Island Sound and New Jersey's coast.

Even the highest tide at Baltimore in nearly 150 years of record-keeping -- on Aug. 23, 1933 -- was 8 feet above normal.

But a few feet of vertical rise can translate into hundreds, even thousands of feet of horizontal flooding around the Chesapeake's flat edges.

And Smith Island is the flattest of the flat among all Maryland's inhabited lands. In the whole county (Somerset), the highest elevation is 46 feet above sea level. Even marshy Dorchester has higher ground.

And on the island itself, nine miles out in the bay off Crisfield, the average elevation is just 2 feet, with no point in any of the three towns of Tylerton, Ewell and Rhodes Point more than about 5 feet.

Many of the neatly mowed yards here, on close inspection, contain enough tidal-wetlands plants that cutting them probably bruises, if not breaks, federal environmental laws.

And the phone exchange, HA-5, is named for the high waters and winds from Hurricane Hazel that slammed the island in the 1950s. A repeat here today of the August 1933 tide would be exciting indeed.

Tylerton under water

It is nearly noon now. Perhaps 80 percent of our town, Tylerton, is under water, but not far under -- inches to a couple of feet is all.

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