Spencer Dam

SPENCER DAM FAILURE was investigated early last summer by a board of experts. The experts released their findings in a report this week.

The Spencer Dam Independent Investigation Panel has prepared their final report on the failure of the Spencer Dam that occurred in northern Nebraska on March 14, 2019.

The report focuses on the physical causes of the failure, the human and organizational causes and the lessons to be learned from the failure.

The investigation panel submitted the Spencer Dam Failure Investigation Report to the Nebraska Department of Water Resources, the regulating authority over Spencer Dam, and to the Nebraska Public Power District, the dam owner. It is now publicly available at DamSafety.org/SpencerDamReport.

Thursday, April 23 at 11a.m., the Association of State Dam Safety Officials (ASDSO) will moderate a question and answer session with Mark Baker, P.E., the panel team leader, to answer questions regarding the report.

The investigation panel was comprised of dam safety engineering experts with decades of specific experience in dam safety failure investigations, hydrology (including cold-weather hydrology), hydraulics (including cold-weather hydraulics), ice and debris flow and hydraulic structures.

All work submitted by the investigation panel, as well as any conclusions and opinions presented, are the sole work of the team and completed without input or influence from Nebraska Department of Water Resources or the Nebraska Public Power District.

ASDSO, a national non-profit organization serving state dam safety programs and the broader dam safety community, selected the investigation panel members and established the Spencer Dam Failure Investigation Oversight Group to oversee the investigation. ASDSO appreciates the in-depth work of the team and supports the release of this report.

The panel found the most likely scenario for the failure left by the accounts of the operators and the evidence of the collapse was as follows:

The Panel's effort to reconstruct the events of March 13 and 14 were hampered by a lack of first-hand accounts due to the remoteness of the site, the evening and early morning timing of the failure and severe weather conditions during the failure. The dam's operators were able to provide descriptions of what they saw during the event but only at specific locations and times, and they were limited by visibility. Given the lack of first-hand accounts, this report describes the range of what might have happened and details the panel's opinion of the most likely scenario for the dam's failure.

Based on the operators' accounts, the evidence left after the failure and other observations and data, the panel found that most likely failure scenarios are as follows:

1. A wet autumn and colder than normal winter produced frozen ground, substantial thickness of ice cover and snow pack. A winter storm, characterized as a bomb cyclone, affecting the entire Great Plains began around March 12, and initially produced temperatures above freezing, resulting in rainfall or mixed rain and sleet on the snow-packed and frozen ground. This storm produced flooding and dynamic breakup of the river's ice cover. As time progressed, weather conditions became colder and windier.

2. During the evening of March 13, dam operators opened all four of the dam's Tainter (radial) gates to their maximum six-foot opening on the spillway crest. They later released stop logs from some of the other bays to increase outflow but were not able to open most of the stop logs bays due to ice.

3. Around midnight on March 13, a major ice run came down the Niobrara River, failing the Stuart-Naper Bridge and damaging the Highway 11 (Butte) Bridge. Both bridges are upstream from the Spencer Dam.

4. One or more ice jams occurred upstream from the dam, backed up floodwaters and burst sending a great amount of ice rubble and floodwaters toward the dam.

5. Ice rubble likely clogged the opened gates and stop logs of the dam's spillway and the reservoir close to the dike crest.

6. Continued flow of ice and water into the reservoir pushed some rubble onto and over the crest and downstream slope of the dike. Ice pushed through the upstream brick wall of the powerhouse.

7. Flow overtopped the dike, causing the downstream side of the dike to erode. The erosion led to head cuts, which grew in several locations along the dike's downstream slope. The dam's embankment dike breached in two locations, the first breach occurred around 5:15 am. The bridges widened and discharged water and ice rubble downstream.

8. The flow of water and ice failed the dam and swept through a house and other buildings located immediately downstream from the dam, causing their destruction and the disappearance of the lone resident (who was later declared dead by drowning). The flow spread over the channel and its floodplain downstream of the dam and was impeded by the approach embankment of Highway 281, located a short distance downstream of the dam. When the flow breached the highway embankment, it formed a major new channel through the breach.

9. The ice run carrying the ice and debris continued downstream, where several other bridges were damaged or destroyed. The panel completed hydraulic modeling of the river downstream. The panel concluded that the failure of the dam did not exacerbate flooding more than a few miles downstream and certainly not in the village of Niobrara 39 miles downstream. The factors that led to this conclusion were: the small size of the Spencer Dam reservoir, the several bridges and other restrictions that potentially caused ice jams, the massive size of the flood and ice run and the decrease in peak flow (attenuation) of floodwater as it traveled downstream in the wide river floodplain.

The flood of water and ice greatly exceeded the capacity of the dam and its spillways. In the panels opinion, there was nothing the operators at the dam could have done the morning of the flood that would have kept the dam from failing given the magnitude of the flood and ice run. If the dam had not been present, the panel believes that the structures immediately downstream would not have been safe during this flood of water and ice, and the highway bridge and the local structures including the house would likely have been washed downstream by the initial surge of water and ice. If the dam had been modified before the event to pass the flood and ice run, the downstream highway embankment would likely still have blocked up water and ice, flooding and damaging the house, before failing the highway embankment.

The panel identified two key human factors contributing to the dam failure and consequences

1. There is a notable lack of knowledge about ice-run-related potential failure modes generally in the dam safety industry. Specifically NebDSP did not know that Spencer Dam had previously failed and was damaged in ice run events. NPPD had limited knowledge of past ice run events at the dam.

ASDSO maintains a database of 380 dam failures. Although the database is weighted toward more recent failures (post-2010), no dam in that database was reported to have failed during an ice run. The National Performance of Dams Program lists one dam failure due to ice flow in 1976. The Dam Safety Industry generally lacks knowledge of how ice runs can impact the safety of dams in cold-weather regions. Current dam safety best practices do not include evaluating run-of-the river dams for stability during ice runs.

Ice was involved with the 1935 failure of Spencer Dam. In 1960 and again in 1966 and the dam's gates and powerhouse were damaged by ice. These incidents do not appear in ASDSO's database as ice-related failures. There was no consolidated history of the dam, and important records were lost, unorganized or unavailable. While the dam appeared to be well-maintained, no provisions were made to pass or prepare for ice run events. Furthermore NebDSP predominantly relied on its dam inspection program to bring dam safety issues to the attention of the dam owner; latent vulnerabilities such as performance during ice run floods are not addressed in the state's inspection reports.

2. The Panel believes that NebDSP and NPPD underestimated the potential of the dam to cause life-threatening flooding at the downstream house and property in the event of dam failure.

There was a lack of recognition that the house, Strawbale Saloon and RV campground situated just downstream from the dam would be at risk if the dam failed. One reason is that the Downstream Hazard Potential Classification (DHC) for the dam was "significant"when, in the Panel's opinion, it should have been "high." Its Significant DHC rating resulted in less dam safety regulation including no requirement for an Emergency Action Plan (EAP). If the dam were designated a "High" hazard potential dam, there would have been a requirement for an EAP and there might have been a requirement to modify the dam to increase flood handling capacity.

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