When an inch means a ton. Or 267 tons, to be precise

July 5, 2011

Kate Davidson

Who knew an inch could make such a difference?

In our piece this week on the Great Lakes Dredging Backlog, we introduced you to Mark Barker, president of The Interlake Steamship Company.  I called him “a man who measures revenue with a ruler.”

To see what that really means, check out this nifty chart from the Great Lakes Maritime Task Force. It shows how much cargo a ship can hold for every inch of water it occupies. For the biggest vessels – the “thousand- footers” – one inch of draft corresponds to 267 tons of cargo. That’s why every bit of clearance matters to shippers trying to get the most bang from every trip. 

These are industry-provided numbers. But they’re figures officials with the Army Corps of Engineersquoted as well.

There was one math question that neither industry nor government experts could help me with, though. An estimated 15 to 18 million cubic yards of sediment have built up in federal navigation channels.  So how much is that dredging backlog, in human terms?

“You mean like how many football fields?” they both asked. No idea.

So I decided to do a little math of my own. 

A cubic yard is a little bigger than a standard oven. So think 18 million ovens.

Or, think gardening. Let’s say the typical bag of garden mulch is between two and three cubic feet.  You would need to throw about 200 million bags of it into the Great Lakes to build up 18 million cubic yards.

Or take the largest pyramid in the world, by volume. The Quetzalcóatl Pyramid at Cholula de Rivadavia has a volume of 3.3 million cubic meters, according to Guinness World Records.

That means the amount of excess sediment in Great Lakes shipping channels could conceivably fill four ancient pyramids.

Now, if you don’t trust my DIY math, no worries. I ran these numbers by one economist and one math professor and they check out.

The economist made a great closing point, too. William Holahan is chair of the Department of Economics at the University of Wisconsin-Milwaukee and he does a lot of work with water.  He said pyramids give a sense of monumental size, but the problem is sediment spreads more evenly through the Great Lakes.

“In 'human terms' it is more like plaque build-up in a person's arteries!! At key points in the body there are blockages,” he wrote in an email.

Check out our radio piece on the dredging backlog.  And feel free to leave your own comparisons below.