Corbett asks, Corps answers on dredging, upstream reservoir, 5-in-1 dam

On Jan. 13, in Mayor Ron Corbett's second week in office, Corbett met with the U.S. Army Corps of Engineers and asked for answers to a handful of questions that have lingered in the community since the June 2008 flood -- some almost like urban legends.

In a letter to Corbett, Col. Shawn McGinley, commander and district engineer for the Corps' Rock Island, Ill., district office, now has answered those questions, though none in a way that would confirm suspicions some in the public might have had.

Take dredging. Some have thought much of the risk of flooding in Cedar Rapids would lessen if someone just dredged the river, making it deeper.

McGinley reports that dredging would provide “minor and localized reductions” in water levels, reductions that would vanish as the river bottom filled in and a new round of dredging would be required. He puts the initial cost to dredge the river, which would include rock excavation, at $26 million.

How about an upstream reservoir, not unlike ones above the cities of Iowa City and Des Moines?

McGinley says previous Corps studies in 1964 and 1982 considered a reservoir on the Cedar River just above Cedar Rapids. The studies concluded that the reservoir could not provide adequate water storage to reduce flooding without “significant and adverse social and environmental impacts.” Building a reservoir just above Cedar Rapids, he says, would have required the flooding of the Duane Arnold Nuclear Center and required the relocation of Palo, Vinton, Shellsburg and Toddville.

“There are no suitable, economically justifiable, larger or smaller reservoir sites in the Cedar River watershed that would have prevented the 2008 flood,” he says.

One large reservoir, he adds, would cost $600 million; smaller, multiple ones, $920 million.

As for the 5-in-1 dam in downtown Cedar Rapids, McGinley says the dam has a “slight impact” on the river's level. The dam caused the water surface level in June 2008 to increase .45 feet, he says.

McGinley also reports that the Corps did not make a calculation for what happened to the river as a result of the collapse of the railroad bridge below Eighth Avenue SE during the June 2008 flood. The new bridge, now in place, “is not an impediment” to the river's flow, he says.

Corbett also asked the Corps if the June 2008 flood supported a “claim” that Cedar Rapids is subjected to a 13-year flood cycle. McGinley says the city isn't. He said the city has had major floods in 1961, 1993 and 2008, “but there is no indication of a 13-year flood cycle.”

Corbett, asked, too, if the June 2008 flood was really a once-in-every-250-year event, rather than a 500-year event. Someone at the University of Iowa apparently has suggested the former, and the Corps apparently is using the latter in its flood-protection analysis. McGinley says the University of Iowa has not responded to date to the Corps' request for data, but in any event, such input would not change the “prescribed methodologies” which the Corps must follow in determining the Corps' important benefit-cost ratio, he says.

By federal law, the Corps can't recommend a flood-protection project for construction if the ratio is not at least at 1 -- the value of property protected by a flood-protection system must at least equal the cost to put the system in place.

To date, the Corps has said the city's proposed, “preferred” flood protection system, which would protect against a repeat of the historic June 2008 flood, would cost too much for what it protected.

The Corps has acknowledged that flood-protection systems can get funded even without the Corps' recommendation.