William Fiedler, Wayne King, Neil Schwanz, Jonathan Garton and Lori McDaniel
February 07, 2012 | 3 Comments
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.
Delhi Dam was breached on July 24, 2010, after two days of heavy rain in the drainage basin above the dam. What follows are details about the dam’s history, the Iowa Department of Natural Resources’ role in forming and assisting the Independent Panel of Engineers to investigate the breach, the dam breach investigation process, the likely cause of the dam breach and lessons learned.
The dam breach initiated about 1 p.m. on July 24, 2010, with an estimated peak breach outflow of 69,000 feet3/s. The flood and the dam breach resulted in extensive property damage in the reservoir above the dam and in the communities downstream of the dam. No loss of life occurred as a result of the dam breach.
There were a number of factors taken into consideration in the investigation of the breach. These included:
Delhi Dam: History and background
Delhi Dam is on the Maquoketa River, about 1.4 miles south of the town of Delhi. The Maquoketa River in northeastern Iowa is a tributary of the Mississippi River. The dam was constructed between 1922 and 1929 by the Interstate Power Company for hydroelectric power generation.
Generation of power was terminated at the dam in 1968. The dam is currently owned and operated by the Lake Delhi Recreation Association. In 1991, the Lake Delhi Combined Recreation and Water Quality Tax District was formed pursuant to Iowa Code Chapter 357E to allow lake residents to tax their property an additional $4 per $1,000 of assessed value, to support the dam and lake. In 2005 the District issued tax-exempt bonds to finance dredging of the lake.
The flood gates and wicket gates of the dam were damaged in 2008 and dangerous scouring of the underwater rock armor of the dam was discovered. At the invitation of Federal Emergency Management Agency representatives, the Tax District applied for and was approved for repairs to the lake and dam. Several repair projects were completed and additional projects were underway at the time of the 2010 flood.
While the lake frontage was primarily privately owned, public access was allowed and the lake was patrolled by Iowa DNR enforcement officials. The dam was inspected every five years by the IDNR dam safety staff. The last full inspection was in 2009. At this inspection, the need for repairs to the spillway gates were noted, and IDNR required the repairs to be completed by the Lake Delhi Recreation Association by the end of 2009.
Follow-up contact made by the IDNR in January 2010 found that repairs were currently underway but were not completed. The repairs do not appear to have been completed prior to the July 2010 breach of the dam
Delhi Dam was designed as a concrete dam and earthen embankment. The 704-foot long structure consists of: A 60-foot long concrete reinforced earthfill section abutting the left limestone abutment; a 61-foot long conventional reinforced concrete powerhouse containing two S. Morgan Smith turbines with two Westinghouse generators (each rated at 750 kW); an 86-foot long gated concrete ogee spillway with three 25-foot by 17-foot vertical lift gates; and, a 495- foot long embankment section with a concrete core wall.
The embankment section was originally constructed with 1V:3H upstream slopes and 1V:2H downstream slopes, and extends to the south (right) abutment of the dam. The crest of the south embankment section of the dam is 25 foot wide and the dam crest is at elevation 904.8 feet NGVD29.
The maximum section of the concrete portion of the dam has a height of about 59 feet and the embankment section has an estimated maximum height of 43 feet. Lake Delhi, the reservoir behind Delhi Dam has an area of approximately 440 acres and a storage volume of 3,790 acre-feet at normal reservoir (elevation 896 ft) and a reservoir volume of about 9,920 acre-feet at the crest of the dam (elevation 904.8 feet). The spillway crest is at elevation 879.8 feet and the hollow inside of the spillway crest structure is filled with rock.
The concrete reinforced earthfill section of the dam at the left abutment was originally constructed with two parallel concrete retaining walls, founded on rock and spaced 20 ft apart. Rock fill was placed between the walls.
In 1967, a concrete crib wall and additional fill was placed upstream of the original walls.
The area downstream of this section serves as a parking and staging area for performing maintenance in the powerhouse.
Formation of the dam failure investigation team
In response to the dam failure, the Governor of the State of Iowa requested assistance from the National Dam Safety Review Board in providing an Independent Panel of Engineers to evaluate the cause of the overtopping and breach of Delhi Dam. This request was made to the Administrator of the Federal Emergency Management Agency (FEMA) dated August 6, 2010. The National Dam Safety Review Board includes representatives from federal and state agencies as well as a member from the private sector and operates under the direction of FEMA. The National Dam Safety Review Board is statutorily established under the Dam Safety Act of 2006 (Public Law 109-460) and provides the Director of FEMA with advice in setting national dam safety priorities and considers national policies affecting dam safety.
In an Aug. 6, 2010, letter, the State of Iowa identified the scope of the Independent Panel of Engineers review as follows:
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.
The lack of maintenance of the embankment section immediately south of the spillway and the 2H:1V downstream slope made inspection of the dam for seepage flows difficult.
Dam performance during July 22-24 flood
During the July 22-24 flood, Gate 3 could not be opened more than 4.25 feet. This was a significant reduction in the spillway capacity.
Nothing out of the ordinary was observed related to the dam performance during the July 22-24, 2010, flood until the reservoir water surface exceeded the top of the core wall at elevation 898.8 feet. Within about 8 hours of this occurring, vortices in the reservoir and sinkholes on the upper portion of the upstream face of the dam were observed. The first vortex was noticed about 40 to 50 feet south of the concrete structure at 3:30 a.m.; the second, noticed later, was estimated to be about 100 feet south of the concrete structure.
Seepage from the downstream slope was first observed around 6 a.m. July 24, 40 to 50 feet south of the spillway training wall. At 6 a.m., settlement of the dam crest was observed in the areas where the vortices and sinkholes were first observed. All of this evidence is consistent with internal erosion occurring in the portion of the embankment above the top of and downstream of the concrete core wall.
The dam breach began to accelerate around 12:30 p.m. on Saturday, July 24th. The dam breach was caused by internal erosion of the embankment, flows over the embankment and structural failure of the thin concrete core wall.
A full breach of the embankment dam occurred at about 1 p.m. on July 24th. The concrete core wall appeared to have failed due to differential loading caused by the flood and erosion of downstream embankment soils. As erosion of embankment soils continued, sections of the core wall also toppled, eventually ceasing at a maximum breach width of 235 feet. It is likely that the concrete core wall slowed down the rate at which the embankment dam breached.
The breach of Delhi Dam did not cause any loss of life. This is attributed to several factors: the concrete core wall likely slowed down the rate of the dam breach; warning of dam failure was issued several hours before the breach; the flood wave was dissipated in farm fields, which reduced the level of flooding in the downstream communities of Hopkinton and Monticello; and door to door warnings were issued in Hopkinton and Monticello resulting in evacuation of residents whose homes were subsequently inundated.
Alternative scenarios for reservoir operations during flood
A number of scenarios were evaluated to help determine if different spillway operations would have made a difference in reservoir levels and the breach of the dam.
One of the items explored was the effect of the partially closed spillway gate. A routing was performed in which all three gates were opened to 18 feet, which was the maximum opening achieved by Gate 1 during the July 22- 24, 2010 flood.
The flood routing results indicated that Delhi Dam would not have overtopped if all three gates had been fully opened.
However, the reservoir would have exceeded the top of the core wall by up to 2.4 feet for about a day, and it is likely that internal erosion would have initiated in the embankment.
Based on the duration of seepage that likely would have occurred through the embankment, it is judged that the dam would have suffered damage and possibly a total breach, even with all three gates fully open.
One of the criticisms from downstream residents is that the dam operators should have lowered the reservoir in anticipation of the peak flood inflows.
Routings were performed that evaluated the effect of lowering the reservoir by 2-, 4-, 8- and 10-feet at the beginning of the flood.
The results of these flood routings indicated that the dam would likely have still been overtopped and the reservoir would have exceeded the top of the core wall elevation for an extended period, even with the hypothetical drawdown levels. This reflects the fact that the reservoir volume is relatively small in comparison to the flood volume and any space that was created would have been filled prior to experiencing the peak flood flows.
Cause of dam breach
From the eyewitness descriptions, photographic and video evidence and limited excavation investigation, the cause of the dam breach was internal erosion in the embankment coupled with overtopping flow.
The internal erosion was most likely caused by a seepage path initiated along differential settlement of the embankment material adjacent to the core wall.
The failure mode was triggered by reservoir levels that exceeded the top elevation of the concrete core wall, which was exacerbated by the inability to open the third gate beyond the 4.25 feet measured in the post breach investigation.
The location and design of the concrete core wall and the fact that it did not extend to the crest of the dam created more favorable conditions for internal erosion of embankment materials once the reservoir reached the elevation corresponding to the top of the core wall.
The IPE believes that any flood of sufficient magnitude to raise the reservoir above the top of the concrete core wall for a more than several hours would have resulted in the embankment experiencing piping/internal erosion.
If internal erosion did not occur, the duration of 16 hours and maximum depth of 1.4 feet of overtopping predicted by the flood model (with one gate malfunctioning) would have likely caused a breach via overtopping and headcutting erosion.
Overtopping erosion to the point of breach was predicted with WINDAM, a NRCS erosion program. A summary of the WINDAM analysis is included in the full IPE Report.
Several other factors that would add to the likelihood of overtopping erosion are the downstream slope of 1V:2H, erosion features located at the toe as described in recent inspection reports, the rock toe and inclusion of the 1920’s roadbed, unknown (but likely low) insitu soil densities and the trees and vegetation on the downstream slope.
Conversely, if the dam had not experienced overtopping flows above the original dam crest elevation (this would have required that Gate 3 was fully functional July 22-24, 2010 flood event), it is possible that the internal erosion mechanism by itself would have lead to the breach of the dam.
Recommendations and lessons learned
The scope of the panel’s investigation was limited. Several recommendations were made that will add to a better understanding of the breach at Delhi Dam:
The IPE also made recommendations that address issues related to managing dam safety issues and a dam safety program:
The Lake Delhi Recreation Association is currently planning to reconstruct the dam and potentially restore hydroelectric power generation.
The funding source for this rebuilding effort is still unknown. The LDRA is beginning the process of a full engineering study to determine alternatives for dam reconstruction.
The Iowa DNR dam safety program continues to improve its program with dam owner outreach and additional staff for breach evaluation and mapping.
ASDSO has recently formed a committee to evaluate failure investigations of Delhi Dam and other dams which have failed with the goal of improving the current practice of investigating dam failures.
This article appeared in the Journal of Dam Safety Issue 9.2 and is reprinted with permission from the Association of State Dam Safety Officials (www.damsafety.org).
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