The Missouri River flooding has undermined the south bridge approaches of dual Interstate 29 bridges at mile post 1.4 in the southwestern corner of Iowa, and is threatening the bridges. The seriousness of the damage is expected to delay the eventual reopening of the interstate when the waters recede.

I-29 has been closed for months since the Missouri River flooded.

The dual set of bridges in Fremont County were built in 1972, are 40 feet wide and 179 feet long, and located in the northbound and southbound lanes of I-29. The bridges take I-29 traffic over Drainage Ditch Number 6.

The full extent of the damage to the bridges, roadway approaches and embankment are not known at this time due to the high-velocity flood waters that are still rushing under the bridge. But there is damage readily apparent at the site, including collapsing of the pavement, loss of material and scouring of the ground beneath the Interstate at least 11 feet deep.

Efforts are under way to attempt to reduce further loss of the soil under the bridge approach pavement. But the efforts have had little success as the rushing flood waters continue to wash away the large rock placed at the site.

The piling that support the bridge are 35 feet long and have been partially exposed (no longer embedded in the river bottom) due to the scouring action of the flood water.

More bridge problems

In other flood-related news from Iowa, officials with the Iowa Department of Transportation, Nebraska Department of Roads, Federal Highway Administration, and Burt County (Nebraska) Bridge Commission met this week to discuss the eventual reopening of the toll bridge carrying Iowa 175/Nebraska 51 over the Missouri River at Decatur, Neb.

The bridge has been closed since June due the Missouri River flooding, which caused severe embankment erosion and material loss on the Iowa side of the bridge, as well as deep scouring around the east bridge abutment and pier.

Because the flood waters have not yet started to recede, there remain many unknowns regarding the condition of the bridge and surrounding area. Several problems are readily apparent at the bridge site that will require fixing when the waters recede. Those problems include slope instability, substructure concerns and major scour around the critical structural elements of the bridge and roadway approach.

The U.S. Army Corps of Engineers' current schedule is to reduce the output of water from Gavins Point Dam by the end of August. Barring additional rain that would change this schedule, it is possible that the engineers may able to perform a more detailed analysis of the problems at that time.

Once the analysis is complete, the next step will be to quickly develop a corrective action plan and issue emergency contracts to perform the work. The amount of time that will be required to complete the work hinges on the extent of the damage.