its spillways. In the panel’s 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 have not been safe during this flood of water and ice; and the highway bridge and the local structures includ- ing the house would likely have been washed downstream by the initial surge of water and ice. If the dam had been modified prior to the event to pass the flood and ice run, the downstream highway embankment would likely still have backed up water and ice, flooding and damaging the house, before failing the highway embankment. Human Factors 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 an 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, 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 runs floods are not addressed in the state’s inspec- tion reports. 2. 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 regu- lation 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. Lessons Learned The failure of Spencer Dam was a tragedy. One of the most important
results of the investigation is the lessons that engineers and dam pro- fessionals can learn from the failure: • Engineers working on dams, bridges and other infrastructure facilities at rivers in cold-weather regions need to assess whether the rivers are sus- ceptible to periodic severe ice runs. If this susceptibility exists, it should be addressed in design. Dam facilities should be designed to be operated safely during these extreme weather events. Warning systems are one potential measure to reduce risk where ice runs form. • More research needs to be done on the dynamic nature of rivers in cold weather regions, including weather systems in such regions, ice run forma- tion, frequency, movement, damage, and how infrastructure like dams should be designed, maintained and operated to withstand ice run loading. • Dam inspections, while valuable, are not adequate dam safety evaluations in themselves. Evaluations must include review of critical documentation and records. Potential Failure Modes Analysis at an appropriate level should be conducted as part of a dam safety review. Once the potential failure modes (PFM) are understood, inspection checklists should be modified to identify signs these PFMs are developing and/or the dam is vulnerable to them. • Dam owners should maintain a complete and organized set of electronic records for their dam(s). A concise history of the dam with reference to key records and past incidents is invaluable. • One of the most important responsibilities dam safety regulators have is to periodically assess the areas downstream of low and significant hazard dams to evaluate whether the hazard classification is appropriate. Docu- mented formal procedures (including reviewing data such as aerial or satel- lite photography and verifying during the site inspection) should be adopted. • For dams with people at risk downstream, Emergency Action Plans should be developed and exercised. • Dams should have operation plans that include operations during extreme events. The Panel’s hope is that these lessons resulting from the investigation of the failure, including natural events and human factors, will be taken to heart by the community of practice resulting in safer and more sus- tainable infrastructure for the public. The Panel gratefully acknowledges the contributions of ASDSO and its Spencer Dam oversight group (Roger Adams, PE, Lori Spragens, Mark Ogden, PE, Dusty Myers, PE, Greg Paxson, PE and John France, PE), Irfan Alvi, P.E. (human factors), James Pawloski, P.E. (state dam safety regulatory issues). Full text of the report may be found at https://damsafety.org/Spencer- DamReport. MARTIN TEAL, PE, PH, D.WRE. is Senior Vice President with WEST Consultants (hydrology and hydraulics). MARK E. BAKER, PE was the panel leader and is Principal, DamCrest Consulting (dam safety programs and human factors) ROBERT ETTEMA, PE, PHD is a Professor in the Department of Civil & Environmen- tal Engineering, College of Engineering, Colorado State University (ice and hydraulic structures) JOHN TROJANOWSKI, PE is President, Trojanowski Dam Engineering (hydraulic structures, concrete dams)
30
csengineermag.com
may 2020
Made with FlippingBook Annual report