Floodplain Mapping: Kalona, Iowa
On June 15, 2010, the city of Kalona, Iowa, was soaked with more than two inches of rain in about an hour. Flash flooding from a nearby drainage ditch forced the evacuation of a mobile home park, where residents found themselves suddenly knee-deep in water.
The floodwaters receded quickly, and, compared to other recent floods in Iowa, this event could be considered minor. But Kalona is facing flooding issues that are actually quite complex. For almost a year, the city has been working to understand and modify a proposed floodplain map of the community presented by FEMA, the Federal Emergency Management Agency. The FEMA maps, which delineate the 100-year-floodplain (areas with a 1 percent chance of flooding each year, independent of previous years), would cost Kalona property owners almost $1 million in annual flood insurance premiums.
After the FEMA maps were released in 2010, the Kalona City Council asked the Iowa Flood Center (IFC) to help evaluate the maps. Established after the 2008 floods, the IFC offers a central resource of flood information and research. IIHR Director Larry Weber and IFC Associate Director Nathan Young met with the Kalona City Council several times over the ensuing 18 months to explain the floodplain mapping methodology and discuss refinements to the Kalona map.
The IFC team studied the methods used to create the maps and reran the models. In September, Weber presented the results of IFC’s own 2D modeling of the area. The map looked quite a bit different than the FEMA original. Using LiDAR (laser radar) data to develop a digital elevation model, the IFC team modeled the geometry of the river and creek beds, as well as the surrounding area. Using the new map, Kalona’s flood insurance tab would be cut almost in half.
“This is a more advanced engineering approach,” Weber explains. “And it’s more appropriate.” The new maps will undergo another round of revisions when the IFC team applies a major precipitation event to the model. This added layer of complexity will better show where the true risk is, Weber explains. “We want to be able to
provide the best possible map.”
The services of the Iowa Flood Center have been crucial to Kalona as the town finds its way through a complicated and costly situation, says City Administrator Ryan Schlabaugh. “The goal is to have the most accurate map possible,” he says, “and the Iowa Flood Center is helping us do that.”
Weber says that by working through this process with Kalona, the IFC has learned important lessons that can be applied elsewhere. IFC researchers are in the midst of a four-year Iowa Floodplain Mapping Project.
To complement the Kalona mapping project, five undergraduate engineering students are also working on a floodwater management plan for Kalona, suggesting flood
mitigation projects, such as rainwater storage structures and buffer strips.
Although the process has by no means been a simple one for Kalona, the community is in effect leading the way through complicated waters.