CNN is reporting today that a "boil" has caused an evacuation of about 600 people from Hamburg, Iowa. Hmmmm...I hadn't a clue about boiling levees! Turns out that it's a well-known and well-studied phenomenon because it can lead to rapid levee failure. The boil noticed today appears to be a hole less than 1.5" diameter that created a small geyser of water erupting onto the dry side of the levee. A-ha--if they had said "geyser," I'd have gotten the idea much sooner! Black Hawk helicopters are being used to drop sand bags onto the levee because it is deemed to dangerous to send in a ground crew.
Here's a primer from the Army Corps of Engineers on levee failure and repair. A "sand boil" from a levee is a small hole created by water seepage through the levee. Water flows through (sometimes being so turbulent that it appears to boil), creating the "geysers" noticed today. Sand boils occur in earthen dams, but the phenomenon is usually referred to as "piping," not boiling. If the seepage water is dirty, it indicates that material is being eroded from the levee and that the integrity of the levee is threatened. If you ever need to sandbag your premises, check out the instructions on the primer sheet! Even shows how to reduce fatigue while filling sandbags!