OMAHA, Neb. — The U.S. Army Corps of Engineers, Omaha District, will conduct damage assessments on Wednesday, May 9, of the spillway slabs at Gavins Point Dam.
The investigation is scheduled to begin at 8 a.m. and will include the use of ground penetrating radar and other investigative methods. To conduct this type of testing, releases from Gavins Point Dam will need to be reduced to zero to dewater the uppermost portion of the spillway. Flows from Gavins Point will remain at zero for no more than 8 hours.
Beginning at 6 a.m., releases will be reduced from the current rate of 25,000 cubic feet per second (cfs) in increments of 9,000 cfs per hour until a release of zero is reached. Following the completion of the testing, releases will be increased to pre-assessment levels over several hours. Releases have not been reduced to zero since the early years of Gavins Point. The lowest in recent memory was during the 1993 flood when releases were ranged from 6,000 to 9,000 cfs for about a month.
"Preliminary assessments of damages sustained at the Gavins Point Dam project in the aftermath of the Flood of 2011 indicate minor damage to a number of areas on the spillway slab," said John Remus, Chief of the Omaha District Hydrologic Engineering Branch. "In order to properly assess the damage and develop corrective measures, the upper portion of the spillway slab must be dewatered."
The assessments are being conducted now in order to develop and implement corrective measures by the spring of 2013.
"While the slab area has sustained damaged, interim repairs have been made to allow for the full use of the spillway if needed." said John Bertino, Chief of the Omaha District’s Engineering Division. "However, it is prudent from both a cost and dam safety point of view to fully restore the condition of the spillway to minimize the risk of developing more severe damage under future operation."
The flow reduction will result in the following approximate stage reductions:
- 8.0 feet at Yankton, S.D., during the afternoon of May 9
- 5.0 feet at Sioux City, Iowa, on begin late in the day on May 10
- 2.8 feet at Omaha, Neb., on May 12 and 13
Stage reductions at Yankton, Sioux City and Omaha are expected to last for approximately 12 hours, 24 hours and 48 hours, respectively. Missouri River stages in locations below Omaha will be impacted but to a lesser degree. Stage reductions are expected to be less if tributary inflows increase due to rain events below Gavins Point Dam.
The Corps has done extensive coordination with municipal and industrial water intake operators, marinas, boat operators and others that may be impacted by the reduction in stage along the river.
Releases from upstream reservoirs will be adjusted to maintain the pool levels at Gavins Point Dam. These adjustments will be coordinated with Western Area Power Administration.
The spillway slabs must be dry to successfully conduct the assessments. If there is inclement weather on May 9, the test will be moved to May 10 or 11. The Northwestern Division Missouri River Basin Water Management Office and engineers in the Omaha District will monitor the situation closely and make any necessary adjustments to releases.
During the investigation, the roadway over Gavins Point Dam will remain open. However, the northern parking lot adjacent to the spillway will be closed to enable the staging of equipment associated with the test.
This assessment is being paid for through funds provided by the Disaster Relief Appropriations Act signed Dec. 23, 2011, by President Barack Obama.
In response to the Flood of 2011, the District Engineer for the Omaha District identified a need for a special execution cell or team to engage in time-sensitive rehabilitation of flood control structures – including dams and levees – in the Missouri River Basin. The Omaha District Systems Restoration Team was established to focus the vast skills and abilities of the district to execute an estimated $280 million in repairs on 18 levee repair projects and $234 million on some 100 projects at the mainstem dams and related flood control structures along the Missouri River.
For regular updates on the repair efforts to flood control structures in the Missouri River Basin, visit the Omaha District’s Flood 2011 Repairs Systems Restoration Team web page.