toward the dam’s powerhouse and spillway (Figure 1). The multi-bay buttress-type concrete spillway structure had one ice/trash bay with a 10-foot wide lift gate, four 33.5-foot wide Tainter gates, and five 33.5-foot-wide needle-beam stoplog bays. Each needle beam stoplog bay contained five needle beams and six stacks of wood stoplogs. The needle beams were operated by the positioning of a pneumatic jack between a welded bracket on the side of the needle beam and the opera- tor walkway. A hydroelectric powerhouse was also located adjacent to and north of the spillway, containing two turbines with a total capacity In general, the Panel conducted the investigation following the meth- odology stipulated in the ASDSO Dam Failure Investigation Guideline and drew from the experience of the Independent Forensic Team for the Oroville Dam Spillway Incident of 2017. Additionally, the Panel reviewed a large amount of data provided by both NebDSP and NPPD, conducted a site visit, and gathered and reviewed data from local, state, and federal agencies. The Panel also conducted in-person and telephone interviews with many of these agencies and with individuals who had insights to contribute. of 3 megawatts. Methodology 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, the 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 as described in the following section. Failure Scenario Based on the operators’ accounts, the evidence left after the failure, and other observations and data, the Panel found that the most likely failure scenario is as follows: 1. A wet autumn and colder than normal winter produced frozen ground, substantial thicknesses of river ice cover and snowpack. A winter storm, characterized as a bomb cyclone, affecting the entire Great Plains began around March 12 and initially produced temperatures above freezing result- ing in rainfall or mixed rain and sleet on the snowpack 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 Figure 2: Ice deposits upstream from the Stuart-Naper Bridge. For scale, note the roof sticking out of the ice on the left side of the photo. Photo: NPPD video
Figure3: Post-failure damaged powerhouse west wall and ice slabs remaining. Photo: State DNR photo 3/19/20
dam’s Tainter gates to their maximum six-foot opening on the spillway crest. They later released stoplogs from some of the other bays to increase outflow but were not able to open most of the stoplog bays due to ice-encrustation. 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 located upstream from Spencer Dam (Figure 2). 4. One or more ice jams occurred upstream from the dam, backed up flood waters and then burst sending a great amount of ice rubble and flood water toward the dam. 5. Ice rubble and flood water rapidly filled the reservoir, causing the reservoir to rise to the dike crest. 6. Continued inflow of ice and water into the reservoir pushed some ice rubble over the crest and downstream slope of the dike. Ice pushed through the upstream brick wall of the powerhouse (Figure 3) and may have clogged the spillway gates. 7. Flow overtopped the dike, causing the downstream side of the dike to erode. The erosion led to headcuts, which grew in several locations along the dike’s downstream slope. The dam’s embankment dike breached in two locations, the first breach occurring around 5:15AM. The breaches 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 de- clared 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 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 following factors that led to this conclusion: the small size of the Spencer Dam res- ervoir, 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 flood water as it traveled downstream in the wide river floodplain. The flood of water and ice greatly exceeded the capacity of the dam and
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may 2020
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