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June 12th , 2012

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Interactive maps show Boise River flooding scenariosIdaho Statesman

Published: June 12, 2012

If the Boise River were flowing at 34,800 cfs — five times flood stage — Bronco Stadium would look like an island. But you’d be OK if you work in the Capitol or live in the North End. This image from the National Weather Service shows the range of floodwater depth, with deeper water in shades of blue and shallow water in white.

When would your street be under water? Thanks to NOAA, which made Boise the first city in the West to get the maps, it’s easy to get a good idea. At 15,000 cubic feet per second, Julia Davis Park and Zoo Boise are flooded.

“You’ll probably want to come up with a plan to move the animals…I suppose the giraffes would be OK,” said Jay Breidenbach, warning coordination meteorologist for the National Weather Service in Boise.

“Boise State University would be safe for flows up to 20,000 cfs. Beyond that, it’s probably going to spill over that embankment and flood the university.”

Last month, the Boise River rose to its highest flow in 14 years — about 8,100 cfs — and it could have gone higher if weather conditions had played out differently. Now Boise residents can go online and see maps of where water is expected to go when the river is running above flood stage, which is 7,000 cfs at Glenwood Bridge.

The maps show more than a dozen river flow rates — as high as 34,800 cfs — which is much more practical information than FEMA’s 100-year and 500-year flood maps. You can zoom in and look at your street or neighborhood by double-clicking on the floodwaters, which will give you information on how deep the water might be.

The National Oceanic Atmospheric Administration has posted flood-inundation maps for 68 cities in the country — but Boise is the first city west of Texas. Breidenbach said there was a move by NOAA to create flood maps after Hurricane Floyd caused massive flooding in North Carolina in 1999. Maps also were made for Gulf Coast states affected by catastrophic flooding after Hurricane Katrina in 2005. Since then, so-called 500-year floods have occurred in communities that are protected by big dams nearby, like Boise’s Lucky Peak. In 2010, dams upstream from Nashville were pushed beyond capacity.

“The dams sort of make us feel safe. They make us feel like there’s going to be plenty of space to capture a big flood — but it can happen below dams,” Breidenbach said.

Boise River reservoir inflows peaked at 26,000 cfs this past April — with capacity in the Boise River dams to absorb most of that.


The Boise maps were the product of more than five years of work and collaboration between local and federal agencies, including Ada City-County Emergency Management, U.S. Army Corps of Engineers, U.S. Bureau of Reclamation, Idaho Department of Water Resources, Flood Control Districts 10 and 11, and local cities.

The maps are extremely precise because much of the data was gathered by using green lidar — laser beams shot from a plane flying at 1,000 feet. The term lidar is an acronym for “light detection and ranging.” NASA signed off on the data-gathering flight in 2007, and a NASA scientist flew the plane. The lidar data collection cost about $140,000, a bill paid by local and federal agencies and a few private groups, including Trout Unlimited and Harris Ranch. Another $70,000 was contributed by the Ada City-County Emergency Management and Corps of Engineers to show more flow and flood options.The National Weather Service worked with the Corps to polish up the maps for public use, making them easy to use online. A user’s guide and FAQ are included on the site.


The main goal of the flood maps is emergency planning and communicating flood risk more clearly to the public, Ada City-County Emergency Management Director Doug Hardman said.

“It’s an excellent planning tool for the individual homeowner, as well as emergency responders,” he said.

But he offered one caveat: Users should not assume that there won’t be flooding outside the areas shown on the maps.

“The maps don’t talk about debris problems at bridges, or bank problems,” he said. “They are lines on a map, an educated guess. If a bank fails, or a tree topples and goes against a bridge, it could change all that.”

The flood maps show the area from Diversion Dam in East Boise to the head of Eagle Island. There are plans to extend the maps at least to the Canyon County line.

“It’s critical that we do that piece as well,” Hardman said. “In many respects, it’s the most at-risk from high flows in the Boise River.”

Katy Moeller: 377-6413

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