Transforming the Future of Dam Safety
Proud sponsors of the 2022 USSD Conference, our team is delighted to see our industry colleagues and friends in person once again! We have an enormous amount of timely information to share and invite you to consider choosing from among Gannett Fleming’s seven engaging presentations.
Be sure to stop by Booth 203 to say “Hi,” and meet with our experts eager to discuss your unique infrastructure and operational needs. Plus, you can recharge your mobile device using one of our charging ports.
See you in San Diego!
Join Our Presentations
Application and Performance of NGA-Subduction Ground Motion Models for Several High Hazard Dams in Cascadia
Tuesday, April 12 • 10:30 a.m.
The Next Generation Attenuation (NGA) program released three subduction ground motion models (GMM) as part of the NGA-Subduction (NGA-Sub) project in 2020. These GMMs are applied in probabilistic seismic hazard analyses (PSHA) and deterministic seismic hazard analyses (DSHA) for four dams in Cascadia, which are classified as “High Hazard” under Federal Energy Regulatory Commission (FERC) guidelines.
As the models were only recently released, their performance was evaluated via sensitivity studies and understanding of the Cascadia Subduction Zone (CSZ) for proper application at the dams. In cooperation between the consulting team completing the PSHA and DSHA, Board of Consultants (BOC) overseeing the study and seismic retrofit efforts for the largest of the four dams, and FERC and its independent reviewer, a ground motion characterization was developed. This paper will compare the models against those previously adopted by practice and inform owners, regulators, and practitioners how updates to existing seismic hazard studies elsewhere in Cascadia may change when considering the NGA-Sub GMMs. Lessons learned from these four studies may inform the application of these NGA-Sub GMMs for dam projects elsewhere.
Seismic Hazard Assessment for High Hazard Dams in Hawaii
Tuesday, April 12 • 11:50 a.m.
The Hawaiian Islands are subjected to a high level of earthquake activity due to the volcanic activity that created them. The highest activity is on the Island of Hawaii and decreases westward along the island chain. The unique seismic hazard in Hawaii consists of shallow and deep earthquakes and is primarily characterized by areal source zones and gridded seismicity, within the framework of a probabilistic assessment. Working with the USGS as they updated the National Seismic Hazard Maps in Hawaii, a statewide PSHA was performed to better understand the seismic hazard on high hazard dams located on the islands of Hawaii, Maui, Molokai, Oahu, and Kauai.
Seismic hazard assessment of the dams classified by the Hawaii Department of Land and Natural Resources as high hazard was designed to increase dam safety and advance the field of seismic hazard in Hawaii. Safety evaluation earthquakes were developed and can be used for future stability analyses of these high hazard dams.
Semi-Quantitative Risk Analysis of the Yelm Canal, City of Centralia
Tuesday, April 12 • 4:45 p.m.
At the City of Centralia’s Yelm Canal, which is part of the Yelm Hydroelectric Project, the risk driver Probable Failure Modes (PFMs) are related to internal erosion and seepage. Because of the identified risks, FERC restricted canal operations to no more than 300 cubic feet per second (cfs). For the canal to generate sufficient power to be economically feasible, at least 600 cfs is required. Seven PFMs require risk reduction measures to enable the canal to operate at or above 600 cfs.
Hydrologic loading PFMs have a reduced influence on project risks compared to normal and seismic load PFMs. This is mainly because the canal and most of the project features are not subject to flood loadings. There are no direct drainages into the canal, and project features, such as the diversion dam, that are subject to flood loadings, do not result in uncontrolled releases into populated areas in the event of failure.
The following recommendations were provided to enable safe and continued canal operations into the future.
- Continued training, focused inspections, and documentation of conditions at the canal. Specific recommendations included:
- Stockpiling of materials at critical locations along the canal to facilitate successful intervention in the event of an incident.
- Updating and revising the city’s Dam Safety Surveillance and Monitoring Plan (DSSMP) to reflect current conditions.
- Developing training plans for personnel to ensure that the corporate expertise remains intact as changes in personnel occur.
- Developing a risk reduction plan that addresses each of the seven risk driver PFMs.
With the implementation of these recommendations, the Yelm Hydroelectric Project should be considered safe to operate at or above 600 cfs. The risk reduction plan will likely be a phased approach and as the most serious risks are mitigated, increased canal flows should be allowed. Collaboration with FERC will be required to determine allowable canal flows corresponding to reduced risks. It is suggested that the allowable canal flows be established early to help the city in its planning and reduction efforts.
Concrete Assessment and Service Life Extension for Morris Sheppard Dam
Wednesday, April 13 • 10:30 a.m.
The U.S. is facing a crisis: addressing its aging infrastructure. Morris Sheppard Dam in Graford, Texas, is an example of how one owner is dealing with an aging concrete dam—an 80-year-old, 150-foot-high 1,600-foot-long, Ambursen structure, which retains 750,000 acre-feet of water.
Earlier in the dam’s life, a 0.4-foot downstream movement of select buttresses driven by uplift pressures within the foundation’s horizontal rock strata lead to concrete cracking. While mass concrete ballasts and relief well modifications performed in 1991 stabilized the structure, concrete cracking, spalling, delamination, and corrosion of the reinforcement and spillway gates continue to advance. Corrosion-inducing water contaminants (i.e., chloride, sulfate ions, suspected hydrogen sulfide producers) are contributing to accelerated concrete and steel deterioration, which are also influenced by freeze-thaw, seasonal conditions, and drought periods.
Phases I and II of a multi-phase Concrete Assessment and Service Life Extension project are complete which included a structural analysis, risk analysis, nondestructive and destructive investigation of the reinforced concrete structure, and water quality testing. For the Phase II field investigation, rope access was used to accommodate challenging site conditions including accessing inverted portions of the structure. A coring program was also completed within Phase II to extract concrete and steel samples throughout the structure, included using divers to collect underwater cores at the base of the dam. Phase III of the project is underway and consists of additional investigation and testing to examine previously unidentified alkali-silica reactions (ASR) discovered in the Phase II investigation; a failure mode progression structural analysis; and a long-term structural concrete testing and repair program.
Instrumented Pressure Monitoring Evaluation of the Wanapum Development Future Unit Intake
Wednesday, April 13 • 1:30 p.m.
Concrete structure instrumentation and monitoring programs (IMP) have grown extensively to match the growing number of PFMs. To do this, the Wanapum Hydroelectric Development conducts visual inspections, while also monitoring several types of instrumentation, such as piezometers (standpipe, pneumatic, and vibrating wire), drain holes, weirs, settlement monuments, alignment monuments, two and three-dimensional crack gauges, borehole extensometers, inclinometers, plumblines, and lift joint drains.
Under federal regulations, instrumentation data requires set threshold values, which indicate a significant departure from the normal range of readings and prompt action. From time to time, IMPs should be reevaluated based on a variety of approaches, such as the historical performance of the instruments, design guidelines, and structural analyses of the concrete structures. This presentation summarizes the main approaches used in the determination of threshold values for the concrete structure instrumentation data gathered at the Wanapum Hydroelectric Development.
This session will be co-presented with Structural Project Engineer Jeremy Begley, PE.
Case Study: Priest Rapids Dam Spillway Post-Tensioned Anchor Design
Wednesday, April 13 • 2:30 p.m.
Portions of a concrete lift joint within the Priest Rapids Dam spillway had dis-bonded requiring stabilization to ensure the long-term integrity of the structure. Therefore, a two-dimensional structural stability analysis was performed for both static and seismic loading conditions in order to estimate a post-tensioned anchor (PTA) load required to ensure adequate rotational and sliding stability of the spillway. This presentation will provide a brief overview of these simplified two-dimensional analyses and associated parametric sensitivity studies to obtain the necessary PTA loading.
This session will be co-presented with Structural Designer Nicholas Contreras.
Dam Resiliency is More Than Just Dam Safety
Wednesday, April 13 • 4:05 p.m.
The most recent trend in critical infrastructure management is an increased focus on resiliency. There are multiple definitions of resiliency, but most of them refer to the ability to adapt to changing conditions and prepare for, withstand, and rapidly recover from disruptions. This concept encompasses a broad domain of hazards, threats, or incidents and includes natural disasters, pandemics, industrial accidents, cyber incidents, acts of terrorism, and criminal activity.
The increasing severity of climate-related natural disasters and the number of dam failures over recent years demonstrate that resilient dams not only require a sound dam safety program but also complementary programs addressing crisis management, security, and public safety through an integrated framework. The various aspects and principles associated with crisis management, public safety, and security, along with dam safety, can help achieve a level of prevention, preparedness, response, and recovery capabilities to achieve the desired resiliency objectives.
This paper takes a look at traditional dam safety programs and expands it to a more holistic process that incorporates crisis management, public safety, and security as part of a dam safety framework. Awareness of what resiliency entails provides a clear understanding of how each of the four areas (dam safety, security, public safety, and crisis management), contribute to this holistic approach. Resiliency of a dam safety program can be achieved through prevention, mitigation, preparedness, response, and recovery.