Boise Assesses Flood Risks Before Federal Maps Get Finalized

Apr 25, 2017

Boise officials have expressed concerns with federal maps that propose adding hundreds of acres to a flood plain in the city, which could increase the cost and difficulty of buying or developing nearby property.

The city disagrees with the Federal Emergency Management Agency over maps predicting the contours of a 100-year flood between Lucky Peak Dam and the Snake River. A 100-year flood is an event FEMA predicts has a 1 percent chance of being equaled or exceeded in a single year, The Idaho Statesman reported.

The city has hired its own outside experts to assess the flood risks. City spokesman Mike Journee said the findings will be presented to FEMA.

"Our residents wouldn't be able to do this on their own unless they went out and hired experts," Journee said. "And so we wanted to make sure that we had the right people putting it together, taking a look at it and giving FEMA feedback that would be useful to them."

FEMA released draft versions of the redrawn 100-year flood boundaries last year.

The maps are designed to inform communities about the risks of building near waterways. Once finalized, the maps determine construction and flood insurance requirements for buildings inside the flood plain.

Officials in Boise are concerned about the maps including the additional acres in the flood plain because it could affect economic development and lead to undesirable building designs.

Karl Gebhardt, one of the city's experts, said FEMA is relying heavily on topographical measurements taken from the air in 2007, while he and the other local experts are using data from 2015. The Boise research team is surveying the river's channels on the ground instead of from the air in an effort to attract more data, he said.

On Friday, the water flowing out of the dam and into the Boise River reached almost 9,500 cubic feet per second (269 cubic meters per second). Between early March and early April, the flows ran between 8,200 and 8,500 cubic feet per second (232 and 241 cubic meters per second). That is far below the indicator of a 100-year flood, which is 16,600 cubic feet per second (470 cubic meters per second).

The FEMA maps, which have not yet been finalized, are expected to go through a public comment process starting in May. During that period, the city of Boise will submit feedback from its experts' findings to the federal agency.

Gebhart said errors are likely during the mapping process, but the goal is to try "to get it as close as you can get to make a reasonable estimate for flood risk."