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Mar 24, 2009 11:01 pm US/Central
Good Question: How Do You Know When A River Crests?
(WCCO)
The images out of Fargo and the Red River Valley are generating a lot of Good Questions from WCCO viewers -- everything from what happens to sandbags to how forecasters know when the worst of the flooding will hit.
How does the Weather Service measure the flood levels? -- Jim from New Brighton
"It's actually very localized to each community," said Diane Cooper, a National Weather Service Hydrologist based at the Chanhassen office.
"We have a sundry of different gauges. Some uses Doppler, which would be sound," she said.
All of the gauges send information via satellite back to the National Weather Service, so they can monitor changes. Some, she said, are triggered by certain water depths, and start reporting more frequently.
"But then we have the old type still out there still where you have staff gauge or basically a ruler painted on a bridge, and someone goes out and reads it for us," said Cooper.
How do forecasters predict when the river will crest? -- Judy in Minnetonka
"We use a fairly simple model that looks at how much rain has fallen, and then how much of it is going to directly runoff into your streams and your rivers," said Cooper.
She explained that National Weather Service hydrologists have two main models, but in the future they expect to have more tools at their disposal.
"We have temperature information that we're looking at, wind information that we're looking at, because those factors are a part of how fast our snow will melt," she added.
But there's another, more difficult-to-predict factor hidden deep beneath the surface of the water.
"The bottom of the river is a huge influence to how high the river level will get. Well that bottom of the river changes through an event," said Cooper.
Rivers don't just move water, they move sediment and sand. That sand builds up at the bottom of a river, depending on how fast the water is raging.
This is why it's not practical to dredge a river in order to make it deeper to hold more water. The expense would be massive, and over time, the river would simply fill back up with soil and sediment.
What happens to sandbags when the flood waters recede? -- Brenda in Rogers
According to the Army Corps of Engineers spokesman Mark Davidson, that's a local decision that varies all over the country. In Minnesota, in general, dry sandbags can be reused. Some are stored in a filled condition, but most are busted open and the sand is used as fill for new roads or sidewalks.
Wet sandbags are tricky. No one knows what kind of junk is in flood water. So, for environmental reasons, the wet ones typically go in landfills.
Isn't there a better way to do this than sandbags? -- Jerry in Anoka.
There may be, according to Davidson. Right now in Fargo, the Army Corps of Engineers is using HESCO Bastion Buckets. They dump sand, clay, dirt, whatever, inside them, and it's a temporary structure that looks like a wall. According to Davidson, it's far less labor-intensive than sandbags.
The Corps is testing several other methods,
outlined in their 2009 Flood Fighting presentation.
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