After waters receded from the catastrophic the flood of 2008, an extensive inspection of the Coralville dam was conducted by the US Army Corps of Engineers to ensure that the integrity of the structure was not compromised.
Divers soon discovered that the bottom of the lake’s stilling basin, two walls that enclose the area where water initially leaves the dam, had suffered erosion and concrete damage. Beginning on Tuesday, September 6, repairs to those areas will begin to be made.
“Similar repairs were made to the stilling basin in 1988 and inspections of the structure are made every five years,” Coralville Lake Manager John Castle said. “After the flood of 2008 we saw that there was some missing concrete and erosion on the basin’s walls and floors.”
According to Castle, the repairs are important to make because there would be potential for the dam’s integrity to suffer if the concrete floor eroded so severely that dam’s water could permeate the limestone bedrock underneath.
The timing for the repairs to the stilling basin could not be more perfect according to Castle, who says that the unusually dry period that the area has been experiencing will allow the work to be done without causing issues with water levels at the dam.
“Right now the flow of water out of the dam is at approximately 450 cubic feet per second, and we will need to reduce it to around 300 cubic feet per second” Castle said. “At that rate, the lake [water levels] really won’t rise that much.”
Carolyn Taylor of Solon (left) and Maria Kaschmeder form the Iowa County Conservation Board try to identify the types of mussels they found during a search for endangered Higginsii mussels just upstream from the Hills access on the Iowa River Monday, August 29, 2005. The search will continue today on the Iowa River and then move to the Cedar River Wednesday and Thursday. File photo: 8/29/05. (Brian Ray / The Gazette)
A temporary dam will be placed across the Iowa River and metal pieces will be used to divert water out of the area while crews work on repairs which will include reinforcing worn areas of cement with steel plates.
Several organizations including the Iowa DNR will be taking advantage of the Iowa River’s temporarily loss of approximately 3-5 inches of depth by conducting mussel surveys at several sites down river.
“What we’re going to be doing is wading into the shallow waters to look for mussels that might be exposed into the air, counting and then moving them to deeper water,” Rock Island District Corps of Engineers biologist Joe Jordan said.
Gill breathers outfitted with a small “foot” to move, mussels travel very slowly and burrow themselves into the bottom of the river, filtering the water for their food and oxygen, Jordan said.
Although Mussels can adapt to droughts, the quick drop of water levels in the Iowa River over the next couple of weeks may leave some of the area’s mussels unable to move to deeper waters quickly enough, thus increasing the risk that they would be unable to survive.
“Mussels are important because they’re an indicator of the water’s overall ecosystem,” Jordan said.”If the population of mussels is declining it may be contributed to poor water quality.”
Iowa DNR fisheries biologist Paul Sleeper says that the section of the Iowa River directly down stream from the Coralville Dam all the way south to Hills is a very important area for mussels to be surveyed in the State of Iowa.
“There are 25 different species found in the Iowa River, and some of them are only found here,” Sleeper said. “We have the highest concentration of Pistolgrip mussels in the state and might even find a couple of different species — we’ve never been able to survey when the water has been this low.”
Approximately 25 to 30 individuals are expected to participate in the mussel survey, which will take up to a week to complete. They will work in crews of four to five and concentration on intervals between the area’s smaller dams, Sleeper said.
Repairs on the Coralville Dam spill basin are expected to be completely finished within two weeks of the start date. During that time, public access to the area around the spill basin will be restricted.
“The fishermen will be disappointed because we won’t be able to let them in near the area to fish during the construction,” Castle said. “But for the most part, people probably won’t really notice that anything [to do with the lake or river] has changed.”
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