Sand is the stuff of art, hourglasses and beaches

Last week, I ran across four men who were making a sand castle out of a pile of sand.

Normally, this wouldn't be worth mentioning except this was a really big castle, about 20 feet tall, and the sand pile was enormous - 50 tons of it to be exact. Also, this sand-castle construction was not at the beach.

I saw it in the middle of Pearlridge Mall.

The men, two from the mainland and two from Canada, are professional artists who travel throughout North America making sand sculptures.

This time, they're creating what they call an Elves' Castle, a temporary edifice built especially for the holidays.

Believe me, this is no ordinary sand castle. It's an amazing work of art that will take weeks to complete.

As I watched the artists work, I wondered if they mixed something special into the sand to hold it all together.

"It's just plain sand and water," one man told me. "North Shore sand, I believe."

I asked what happens when it dries out, and the man gently patted a part of the structure already dry. "It holds together quite well."

I touched the wall too. Pretty solid. Then I picked up a handful of the fine, dry sand from the floor.

It flowed from my hand like a stream of water.

That's sand for you. It's remarkable stuff with unusual physical properties.

And although we usually take it for granted, the behavior of sand has profound effects on our daily lives.

Dry sand, for instance, pours like water through your fingers. Yet, unlike water, it supports your weight when you walk on it at the beach. Add a little water to dry sand and it becomes excellent construction material.

Why does sand stick together so well when wet? The answer is surface tension. When moistened just the right amount, films of water surround each grain of sand. This water forms chemical bonds, much like bridges, that link the grains and hold them together.

Add enough water, though, and the sand will again pour like it did when it was completely dry.

Another interesting characteristic of sand is the hourglass phenomenon.

An hourglass works as a precise measure of time because the pressure at the base of a pile of sand does not increase as the height of the sand increases (unlike liquids or solids).

This means that being buried under 30 feet of sand feels about the same as being buried under 3 feet (given air to breathe, of course). It also means that sand grains will trickle through the narrow opening of an hourglass at a constant rate regardless of how many are left above.

To us Hawaii residents, sand doesn't usually mean either art or hourglasses - it means the beach. Here are a few facts about sandy beaches:

Sand beaches are always in motion, changing with both wave size and wave direction.

Big waves move sand seaward; small waves move sand shoreward.

Artificial structures interrupt the natural movement of beach sand and thus often cause extensive changes along the shoreline.

Natural beach erosion can't be stopped by seawalls. A seawall may initially protect the land directly behind it but it also increases the backwash strength of waves. This accelerates erosion of the beach directly in front of, and at the sides of, the seawall.

If seawalls are extended, erosion continues until finally there is no beach at all - just seawall.

You can watch the Elves Castle at Pearlridge go up from now until its completion on Wednesday.

The castle will stand intact until January. Then it will go back to being single grains of sand somewhere on Oahu.

Marine biologist Susan Scott writes the newspaper column, "Ocean Watch",
for the Honolulu Star-Bulletin, www.starbulletin.com