Predicting river crests about more than rainfall

By Erin Jordan, The Gazette

Predicting floods is a complicated business involving not just rainfall, but also geometry of river channels, character of hill slopes and mathematical formulas.

Boiled down, the data can help Iowans know when to start sandbagging or evacuate before high water.

For the general public, what really matters is just how high the river can get, said Witold Krajewski, director of the Iowa Flood Center at the University of Iowa.

The Army Corps of Engineers predicted Wednesday morning the Coralville Lake would top the spillway next week, but later in the day scaled back the numbers slightly. Small revisions like this are constant because rain upstream can affect the whole system.

The predictions start with knowing the landscape in and around waterways.

Digital topographic maps from the U.S. Geological Survey show elevation points for every 100 yards across the country, Krajewski said. Scientists in Iowa also have access to LiDAR, or Light Detection and Ranging technology, that involves laser readings from planes that fly over the landscape.

Elevation maps allow scientists to figure out the locations of river channels and basin boundaries.

Also important to flood predictions is the character of the hill slopes that run into waterways, Krajewski said. A grassy field will absorb more water than a paved parking lot.

These base numbers go into computer models that help scientists understand how different rainfall amounts will affect the region.

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Predicting river crests about more than rainfall