April 6, 2023

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Geosyntec Practitioners Contribute to Dam Safety at USSD 2023

Geosyntec will present at and attend the United States Society on Dams (USSD) 2023 Annual Conference and Exhibition at the North Charleston Convention Center in Charleston, South Carolina, on April 17 through 21, 2023. Presenters include Glenn Rix, PhD, PE (Georgia), Christie Hale, PhD, PE (Texas), and Jay Griffin, PE (California). Jamie Rosen, P.Geo., (Ontario) will participate in the associated International Symposium on Dam Safety on April 20. Other Geosyntec staff at the conference are Stephanie Owen, Rick Poeppleman, Lucas deMelo, Derek Morely, Lelio Mejia, Joe Kula, Kate Darby, Chris Hunt, Scott Deitche, Victor Damasceno, Joe Goldstein, Terry Sullivan, and Brian Martinez. We will be available at Booth 507 to meet you and talk about how we support dams and levees.

Geosyntec is a sponsor of the 2023 event.

The theme of USSD 2023 is “Infrastructure Resilience Through Risk Management,” and it will include over 100 technical sessions, two plenary sessions, a Legacy Lecture, and five workshops. USSD advances the role of dam and levee systems and builds the community of practice. As the United States member of the International Commission on Large Dams (ICOLD), USSD provides a forum for the exchange of knowledge and is charged with leading the profession of dam and levee engineering.


Glenn is a Senior Principal Geotechnical Engineer based in Georgia with more than 30 years of experience focused on geotechnical earthquake engineering and engineering seismology, including seismic risk assessment for civil infrastructure.

Christie is a seismic hazard analyst involved in seismic safety assessment projects. Her technical experience includes deterministic and probabilistic seismic hazard analysis, deaggregation analysis, target spectrum development, and time-history selection and modification.

Jay is a Senior Geotechnical and Geoenvironmental Engineer focused on mining, dam safety, and waste containment.

Jamey is a Senior Principal Geoscientist with more than 20 years of experience focused on Geospatial Information Management System design and development. Jamey has built systems to validate, manage, analyze, and visualize construction, geotechnical, and environmental data for projects and facilities in the United States and Canada.


Presenters: Glenn Rix
Coauthors: Martin McCann, Husein Hasan, Pete Zimmerman, Matthew Huebner
Time: Tuesday, April 18 at 3:45 p.m. EST

The Tennessee Valley Authority conducted a quantitative risk assessment (QRA) for the Blue Ridge Hydroelectric Project to evaluate risks associated with intrinsic (i.e., internal erosion), seismic, and hydrologic (i.e., spillway) events. The purposes of the QRA were to: (i) provide a complete representation of risk for the potential failure modes that were considered, (ii) be pragmatic and technically defensible, (iii) provide a basis to develop and evaluate the effectiveness of risk-reduction measures, and (iv) support the development of the dam safety case and inform dam safety decision-making.

Specific tasks involved (i) developing probabilistic descriptions of seismic and hydrologic loading conditions, (ii) performing fragility analyses for seismic, hydrologic, and internal erosion failure modes, (iii) developing breach parameters to estimate outflow hydrographs from an uncontrolled release of the reservoir (URR), (iv) developing event and logic trees for each failure mode, (v) performing downstream consequence analyses to estimate life loss associated with URR, (vi) quantifying the risk of individual failure modes and the total risk, and (vii) considering potential benefits from identified risk-reduction measures.

It is important for an assessment of risks to address sources of uncertainty in a systematic, quantitative manner to provide a measure of the uncertainty in the results in the context of available information. In the QRA this was achieved by identifying, modeling and quantitatively evaluating sources of uncertainty and calculating both mean and percentile estimates of the risk metrics. Through a systematic, quantitative evaluation of uncertainties in a manner that captures the range of technically defensible inputs to the risk analysis, the resulting percentile estimates of the risk metrics provide insight into whether conclusions drawn from those risk metrics are likely to change if new information becomes available.

Presenters: Christie Hale
Coauthors: Linda Al Atik, Chris Hunt (Geosyntec), Chris Conkle (Geosyntec), Matthew Muto
Time: Wednesday, April 19 at 10:30 a.m. EST

Seismic hazard assessments were previously conducted for six dam sites in the Sierra Mountain region of California utilizing a reference rock condition with a VS30 equal to 760 m/s. Site adjustment factors are needed to adjust the response spectra from the reference rock condition to the site-specific foundation conditions for each site.

For sites in California, a common approach for developing site adjustment factors is to use the site terms built into the NGA-West2 ground motion models; however, these models are poorly constrained for sites with VS30 values of 1000 m/s or higher, leaving a gap in the practice for the development of site adjustment factors for hard rock sites.

Recent work by Al Atik et al. (2022) addresses this gap by providing site adjustment factors that can be used to adjust NGA-West2 response spectra from a reference site condition of VS30 = 760 m/s to a site-specific condition with VS30 between 680 – 2200 m/s.

This paper describes the development of site adjustment factors for six dam sites, and includes use of the NGA-West2 site adjustment factors for a dam site with VS30 less than 1000 m/s, comparison of the NGA-West2 and Al Atik et al. (2022) site adjustment factors for a dam site with VS30 near 1000 m/s, and use of the Al Atik et al. (2022) site adjustment factors for dam sites with VS30 greater than 1000 m/s. The application includes an approach that differentiates between sites with measured and estimated VS30 values, as well as a discussion on the considerations for evaluating the applicability of available site adjustment models.

Presenters: Jay Griffin
Coauthors: Chris Hunt (Geosyntec), Robert Ellis
Time: Wednesday, April 19 at 1:30 p.m. EST

The Dixie Fire started on 13 July 2021 in the Feather River Canyon in southwestern Plumas County, California. The fire ultimately burned 963,309 acres (approximately 1,505 square miles) in Butte, Plumas, Shasta, Tehama, and Lassen counties. Considering the significant potential for increased post-fire debris loading within the local reservoirs, a series of steps were taken to support post-fire dam safety planning. These steps included: (a) field visits to several dams within the burn area, including observations along the reservoir rims via helicopter, (b) a focused review of relevant dam safety documentation, (c) review of post-fire documentation on burn severity and debris flow hazard to qualitatively assess the potential for debris loading in the reservoirs, and (d) identification of potential mitigation options related to debris management.

This presentation will describe each of the steps above with particular focus on field observations, assessment of the post-fire burn severity and debris flow hazard documentation, and types of mitigation options. A brief discussion of other relevant post-fire considerations, such as maintaining access to critical facilities, will also be presented.

More Information

About the Event: USSD Annual Conference
About USSD: USSD | United States Society on Dams
For consultation regarding dam safety, contact Glenn Rix at This email address is being protected from spambots. You need JavaScript enabled to view it.

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