Soft Maple Development Comprehensive Assessment
As part of the updated FERC Engineering Guidelines, GEI performed a Comprehensive Assessment (CA) for a high-hazard dam located on the Beaver River near the town of Croghan, New York. This project was one of the first CAs performed in the Northeast under the updated Engineering Guidelines. The SoftMaple Development consists of two hydraulic fill embankments (Terminal Dam and Diversion Dam), a concrete gravity spillway with two vertical lift gates, a concrete intake buttressed with an earthen embankment, two penstocks, an unused penstock
stub, a powerhouse and five saddle dikes.
GEI developed a Part 12D Inspection Plan and a CA Pre-Inspection Preparation Report (CA-PIPR) and coordinated a field inspection and workshops involving the Independent Consultant (IC) Team, Licensee, and FERC. Prior to the workshops, GEI developed a Seismic Hazard Analysis (SHA), Hydrologic Hazard Analysis (HHA), and Consequence Analyses, and also prepared a detailed agenda, an introductory risk presentation, multiple technical presentations, and tools for brainstorming, screening, collaboration and anonymous voting.
GEI conducted the Potential Failure Modes Analysis (PFMA) and Level 2 Risk Analysis (L2RA) workshops over a period of approximately two and a half weeks, with one week in person and one and a half weeks virtual. The workshop started with a thorough review of background information, including project construction history, hydrologic hazard, seismic loading, geotechnical and structural stability, mechanical/operations, and consequences. In accordance with Chapter 17 of the FERC Engineering Guidelines, the workshop participants first developed the potential failure modes (PFMs) from scratch and did not use previous PFMA outcomes as reference. The PFM development process included brainstorming PFMs related to each structure and conveyance system and screening the PFMs to identify those that were considered Credible (including both life loss and financial/damage state PFMs) and carried forward to risk estimation.
For the PFMs that were carried forward, detailed tables (including step-by step descriptions/progressions, influence factors, and background information) were developed for each PFM. Following the PFM development, the likelihood of failure was estimated using either a quantitative or descriptive approach. Subject matter experts (SMEs) provided order-of-magnitude estimates for the overall annual probability of failure using a blind elicitation process. Led by the facilitator, the SMEs discussed their estimates and the group decided on a consensus order-of-magnitude estimate. Following the estimation of failure likelihood, the consequences associated with each PFM were also estimated and discussed. Confidence levels on both the likelihood of failure and the consequences were given.
GEI summarized the risk estimates and plotted each PFM on a risk matrix for either total incremental life safety or financial / damage state. The risk analysis process, the potential failure modes, and the estimated risks were documented in a Risk Analysis Report. The CA, including a review of the project design and construction, analyses of record, field inspection, emergency action plan and public safety plan, and the risk workshops were documented in the Comprehensive Assessment Report.