New flood maps in Minnetonka show growing flood risks

New flood maps under review by Minnetonka officials this summer reflect the growing possibility of higher floodwaters.

Data from the National Oceanic and Atmospheric Administration (NOAA) show an increase in the amount of rain expected in major storms in Minnetonka, where flood maps were last revised in 1999.

"We just want to be better prepared as a community if we were to get these events," said Public Works Director Will Manchester.

Even on a sunny day, Minnetonka is wet. The city counts three creeks, 160 storm water ponds, some 800 wetlands and 13 lakes, Manchester said — including Lake Minnetonka lapping at the city's western edge.

Heavy rain can mean hundreds of pockets of flooding all across Minnetonka. And according to NOAA's new data, Minnetonka will see more severe flooding from major storms than was expected 25 years ago.

The amount of rain expected for a 100-year rainfall event in Minnetonka has increased from 5.9 inches to 7.4 inches. There's about a 1% chance of a storm bringing that much rain in any given year.

Rainfall predictions for more common storms are up slightly, too. A 10-year storm — that is, a storm that has about a 10% chance of happening in any given year — is now expected to bring almost 4.3 inches of rain, up from 4.2 inches in the old model.

The updated flood maps do not show new large flood-prone areas, but slightly expand the boundaries of areas that are likely to be flooded, Manchester said.

"The properties that had flood plain on them before will still have flood plain on them," he said.

Manchester said the new flood maps will not force anyone to buy flood insurance, though every Minnetonka homeowner is eligible for FEMA's National Flood Insurance Program. Flood insurance premiums have more than tripled in Minnesota since 1998, according to the Minnesota Pollution Control Agency (MPCA).

However, the maps are useful for developers and residents who want to avoid the city's riskiest areas when building. City officials also will use the maps as they plan infrastructure, which could be at risk in an extreme flood.

In the event of a 100-year flood, the new maps show that parts of Interstate 394 and Hwy. 7 would be underwater if Minnetonka were to get just over 7 inches of rain in one storm.