Utilities official: Recent Iowa River overflow in City Park not sign of another devastating flood to come

	Ken Lloyd, associate utilities director. Photo by Tom Jorgensen.

As spring 2009 made its slow approach to Iowa City and The University of Iowa, people traveling along Dubuque Street were greeted with flooding in parts of lower City Park. After the devastating flooding of June 2008, it's only natural to ask questions at the first sign of high water...

Does this mean that another 500-year flood is in store for summer 2009?

Is Mayflower Hall in danger again?

Have we forgotten what a typical spring looks like—in other words, are we overreacting?

fyi decided to get some answers. We asked Ken Lloyd, UI associate utilities director, to assess the current situation and talk about the forecast for flooding in summer 2009.

Is there reason to be concerned about the recent high levels of the Iowa River?  

Lloyd: At this time there is actually little reason to be concerned. The Coralville Lake level is hovering near the low level maintained by the Army Corps of Engineers for this time of year, 679 feet (above sea level), and is projected to stay there for at least the next week. For reference, the spillway crest elevation is 712 feet. The flow into the lake from the 3,000-plus square mile Iowa River Basin above the lake also is projected to drop over that period as well as the outflow from the lake. The outflow is projected to stay near or below 3,000 cubic feet per second for the next several days and that is well below the normal maximum of 10,000 cubic feet per second for this time of year.

So those pools in Lower City Park weren’t particularly troubling?  

Lloyd: The Lower City Park overflow that occurred a few weeks ago is a normal consequence of the normal maximum release rate from Coralville Lake for this time of year, per the Army Corps of Engineers Lake regulation schedule. The Corps was releasing at that maximum rate of 10,000 cubic feet per second for a time this spring to allow the lake to catch up with accumulated snowmelt and other precipitation runoff, and to reduce the lake level. That frequently occurs at this time of year.

Are there implications for Mayflower Hall at this time?  Is there an evacuation plan in place for Mayflower should water spill out of the banks?

Lloyd: At this time there are not flood concerns for Mayflower Hall. As part of the University's Flood Emergency Response Plan, actions have been planned to secure and protect Mayflower, including the orderly evacuation of the residents as necessary. That would only become necessary if Coralville Lake levels are forecast to reach elevations that would ultimately require the Army Corps of Engineers to release outflows at rates high enough to inundate Dubuque Street.

So the forecast looks OK for May and June?

Lloyd: The National Weather Service exceedance probability graphs show that for the period of May 1–June 30, the conditional probability of flooding in Iowa City is actually forecast to be less than the historical probability for flooding during this time period. 

The graph’s high point for the coming months corresponds to an Iowa River stage elevation of about 19.7 feet, or a flow of about 10,000 cubic feet per second. The graph forecasts a probability of achieving that level at being less than 2 percent. Historically that gage level—or higher—could be expected about 9 percent of the time.  

In summary, the forecast is very good and it appears quite unlikely that Iowa City will have to deal with serious flooding this spring.

by Christopher Clair