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"The Evolution of the 100-year Flood"

Posted: Wed, Sep 12, 2018 10:45 AM

In 1930, a river gauge on the Upper Iowa River in Decorah measured water flow at 11,000 cubic feet per second.  By contrast, recent river flows have been 27,000 cubic feet per second, says Iowa Flood Center researcher Larry Weber.

Weber tells that there's been "an evolution of the 100-year-flood (level) and the 500-year flood (level)."  That's why the Iowa Flood Center has revisited its July study of the potential impact of a Menards store in the Freeport floodplain.

The updated study released this week still shows the Menards store would have a small impact on Upper Iowa River flooding, increasing the river levels by less than two inches during a flood. 

But the updated study also looked further into the question of what impact increased water flow levels would have on the Upper Iowa River.  Weber and other researchers took data from the 2008 flood in Decorah and added 2 percent, 5 percent, 10 percent and 25 percent to those figures.  Under such conditions, flooding increased, of course, with the Freeport Bridge acting as a barrier and water backing up.

Weber says 100-year floods and 500-year floods are definitely increasing in Decorah.  He says changing agricultural practices and changing land use have led to higher water flow levels on the Upper Iowa River, as have increased storm precipitation amounts.  The Iowa Flood Center returned to the July study and created the updated September study to give Decorah residents an idea of what the future might look like.

The updated study concluded, "Typically, any single proposed development project within the floodplain or floodway will not adversely affect water surface profiles or increase severity of flooding such that it will be rejected on those criteria alone. Concerns about continued floodplain development and their cumulative impacts might be addressed more efficiently by updating long term planning and zoning policies, rather than case-by-case analysis."