Tide driven: Exhibit explores Kachemak waters

Posted: Monday, August 09, 2010

Visitors to Kachemak Bay often get confused -- and amazed -- by the bay's extreme tides. People who don't live on the ocean have a hard time understanding a bay where the difference between the highest high tide and the lowest low tide is more than 28 feet.

This summer, the Kachemak Bay Research Reserve and the Alaska Maritime National Wildlife Refuge collaborated on a new exhibit at the Alaska Islands and Ocean Visitor Center that illustrates local tidal ranges. As it happens, the height of one of the steel pillars supporting the roof of the Islands and Ocean main lobby is just over 31 feet. The tidal column exhibit wraps around one of those pillars, a sculpture of a post encrusted with sea creatures and a lighted, numbered gauge showing the tide at any moment from a minus 5.5 foot low tide at the floor to a 24.5 high tide.

"It's nice to see it physically. Oh, that's what 28 vertical feet means," said Kris Holdereid, a physical oceanographer and National Oceanic and Atmospheric Administration director of the Kasitsna Bay Laboratory.

Another exhibit that opened this summer on Kachemak Bay estuaries shows how tides influence a key ecosystem here, where rivers and streams meet the bay. To help people better understand the tides, Kachemak Bay Research Reserve held two Discovery Lab sessions, Driven by the Tides, last weekend. Driven by the Tides also supports an art and science collaboration, Living by the Tides, an exhibit at the Pratt Museum showing Nov. 12-Dec. 30. Artists are invited to attend the lab to get ideas.

Why do tides matter and why should people care? People who live by the sea set their schedules by the tides. For beach walkers, it's like having your hiking trail go underwater at erratic times. For cabin owners, the tides can mean the difference between an easy loading dock or a 25-foot staircase.

For Kachemak Bay marine life living near estuaries, tides are like a big soup spoon, stirring up the sea and bringing rich sea life at the cold ocean bottom closer to the surface where fish and other animals feed, said Catie Bursch, a marine educator with Kachemak Bay Research Reserve who will teach the Driven by the Tides lab.

"Tides are fascinating for people who aren't around them all the time," Bursch said.

People have a lot of misconceptions about the tides. One Bursch hears a lot is that Kachemak Bay has the second most extreme range of tides in the world. That's not true -- at least by how NOAA defines extreme range of tides. While Seldovia, Homer's nearest official tidal station, has an extreme range of record high and low tides of 32.68 feet, NOAA looks at the mean range between tides. That's calculated by taking all the high tides for a 13-year period and averaging them, taking all the low tides for that period and averaging them, and then calculating the difference.

The Bay of Fundy, Nova Scotia, holds the record, for a mean range of 38.4 feet. Ungava Bay, Quebec, is second with a range of 32 feet and Bristol, England, is third with a range of 31.5 feet. Turnagain Arm is number four at 30.3 feet. Kachemak Bay has a mean range of 15.83 feet, not even in the top 10.

Another misconception is that distance from the equator influences tidal extremes, Bursch said. Numerous factors affect tides, including the shape of a bay's bottom, the time and distance a tide has to move and an ocean shore's location. Distance does matter -- but for the distance from neutral points in the ocean called amphidromic points where a place doesn't have a tide.

Think of amphidromic points like the center of a whirlpool, Bursch said. The water isn't moving in the center.

"The farther away you are from an amphidromic point, the higher your tide," she said.

The biggest effect on tides -- and what causes them -- is gravity, the effect of the moon and the sun. Although the sun has more mass than the moon, it's farther away and its effect is dampened by distance so the sun has about half the effect as the moon.

"It basically really is all about gravity," Holdereid said.

Oceans covering the earth get pulled toward the moon and sun, creating a tidal bulge. On the other side of the earth, inertia -- what causes coffee to slosh out of a cup when a car makes a sudden stop -- pulls the ocean the opposite direction. The combination of inertia and gravity create two tidal bulges, or the tides.

"It's a tricky thing to understand," Holdereid said.

Every month relative to the earth, the moons line up in a straight line, perpendicular to the earth an angle in between. However, the lunar day -- how long it takes for the moon to orbit the earth -- is 24 hours and 50 minutes, which is why two daily high tides and two daily low tides are 12 hours and 25 minutes apart.

The earth is closest to the sun on Jan. 2 and farthest on July 2. The northern hemisphere is tipped closest to the sun on June 21. All of these factors affect tides at any time in the year and at any location, too.

"The tides are a great integrated measure of the gravitational effect of all these different cycles," Holdereid said. "We know we're closest at this point because of what the tides are telling you."

That's why when NOAA makes tide predictions it has to look at a 19-year epic -- a period that measures the cycles of all these rotations, orbits and revolutions. By analyzing the effects of these cycles at various times and places, scientists can forecast the tides based on the effects of those cycles.

In the Driven by the Tides lab, Bursch will look at some of these astronomical effects and other factors that influence the tides.

"The Discovery Lab will have more than you want to know about tides," she said.

Included in the exhibit are films that show tides in action. A popular film is a time lapse video of various locations around the bay as the tides wax and wane, like the harbor ramps, the Nick Dudiak Fishing Lagoon and the Homer Spit.

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