Record rainfall, faulty gates and inherent design issues contributed to the July 2010 breach of Iowa's Delhi Dam, investigators found. The overtopping caused the earthen embankment to erode and eventually emptied Lake Delhi, causing millions in property damage.

In a letter from the Deputy Administrator of FEMA to the Director of the Iowa Department of Natural Resources dated Aug. 27, 2010, a commitment was made to convene a three-member Independent Panel of Engineers under the auspices of the National Dam Safety Review Board. The three members represent federal agencies with extensive experience in dam safety and include: William Fiedler, Bureau of Reclamation; Wayne King, Federal Energy Regulatory Commission; and Neil Schwanz, U.S. Army Corps of Engineers.

Each of the respective agencies absorbed the time and travel costs for this investigation. State funding was not used for this investigation.

The investigation process

In order to fulfill its mission, the independent panel initially collected and reviewed key information. The panel operated independently and access to individuals and to any requested information was freely granted.

Areas of focus included: the design and construction of Delhi Dam, subsequent modifications to the dam, the operational and performance history of the dam, past examinations and reviews of the dam, the timeline of events leading up to and including the breach of Delhi Dam and the emergency response to the dam breach.

A key activity for the panel was convening in Iowa during the week of Sept. 6, 2010. On Sept. 7, 2010, the panel reviewed records at the Iowa Department of Natural Resources Offices in Des Moines, Iowa, and conducted interviews with personnel from the IDNR, as well as the dam operators, owner’s representatives and local residents. On Sept. 8-9, 2010, the team inspected the dam site and the upstream and downstream areas and conducted additional interviews with personnel from local government agencies and from the Lake Delhi Recreation Association. The team spent Sept 10, 2010, in Des Moines at the Iowa Department of Natural Resources Office and reviewed additional records and conducted additional interviews.

After the information gathering was complete, the panel set out to draft their report. A thorough review of the geotechnical aspects of the original design and past modifications was completed. The original drawings and documentation were not very clear on the original foundation and geotechnical design, Therefore, a soil sample of the remaining embankment was tested and two shallow hand augers of the foundation were obtained.

A hydrology and hydraulic analysis was undertaken by the panel. The analysis utilized a HEC-RAS model developed by the IDNR shortly after the breach. There were many questions that arose after the failure in regards to one of the spillway gates that was not able to be fully opened. It was critical for the panel to complete some “what-if” scenarios to begin to answer those questions. Flow data were available from a U.S. Geological Survey-gage in the City of Manchester upstream of the lake. However, there were also significant local inflows to the lake that had to be estimated.

The panel also provided a review of the emergency management response and a timeline of events based on testimonials and photos.

Iowa’s role of information provider

In order for the investigation team to be completely impartial, the Iowa DNR facilitated the process to gather witnesses and provided records of the dam’s history. Iowa DNR did not have an active role in developing the investigation report.

This allowed the investigation team to not only review the failure itself, but to also review the State’s dam safety program and regulation of the dam up to the time of failure.

Findings of the panel

The following describes the panel’s key findings. Complete findings and recommendations are available in the full IPE report.

Dam design and construction

There was limited information on the dam materials in terms of gradations of the materials and density of the in place embankment. It appears that the dam embankment consisted of a homogeneous material, with a reinforced concrete core wall placed upstream of the centerline of the dam. A sample from the remnant of the embankment was tested and it was determined that the material was a sandy clay with low plasticity (plasticity index of 9).

The concrete core wall was placed on top of steel sheet piling that extended to rock for some distance near the right abutment wall of the gated spillway. The core wall was founded directly on bedrock from the spillway wall to a distance of about 20 feet from the wall. South of that point the sheet piling is shown extending to bedrock for a short distance where the depth to bedrock was known. The top of the cutoff wall extended to within about 6 feet of the crest of the dam.

The concrete cutoff wall on top of a sheet pile wall created a vertically rigid element in the dam that would not settle over time, as compared to the embankment on either side of the cutoff wall which could settle. This situation likely created differential settlement in the area of the cutoff wall that caused low stress that could have lead to cracks in the embankment fill emanating from the top of the cutoff wall. The potential seepage path created by the cracks from the cutoff wall and the low plasticity embankment material created a situation where internal erosion of the embankment could initiate and progress quickly.

The spillway was the primary waterway for passing flood flows at Delhi Dam. The wicket gates in the old power plant have a discharge capacity of about 500 feet3/s but this flow is relatively small compared to the spillway capacity. The spillway is regulated by three 25-foot wide by 17-foot-high vertical lift gates. With all three gates fully opened and the reservoir at elevation 904.8 NGVD29 (130 feet local datum), the estimated spillway capacity is about 32,000 feet3/s.

Dam performance prior to flood

No adverse performance of the dam was reported to the IPE. No significant seepage had been reported at the downstream toe or on the downstream face of the dam.

Although the embankment performed well up to the recent event, it is very possible that prior loadings did not achieve a water surface elevation that exceeded the top of the core wall (EL 898.8 ft) or have a sufficient duration to develop internal erosion.

The spillway gates have been difficult to operate in the past. The gate guides are tapered at the bottom and sometimes the gates would stick in the closed or nearly closed position. A crane had been used in previous floods to operate the spillway gates.