Objectives
The principal objectives of this study were to:
1. Determine site-specific PMP values for durations from 1 hour to 10 days using a physically consistent, storm-based methodology.
2. Develop AEP rainfall frequency relationships that extend into the PMP range to support risk-based dam safety and flood hazard analyses.
3. Conduct hydrologic modeling of basin-scale runoff and reservoir inflows under extreme rainfall conditions to evaluate system response and capacity.
4. Provide recommendations for adaptive management, design resilience, and data integration for future updates.
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Methodology
The study employed a storm-based PMP approach, incorporating the most extreme observed rainfall events from across the Pacific Northwest and adjacent regions. Each storm was reconstructed using AWA’s Storm Precipitation Analysis System (SPAS). Each storm was maximized, adjusted for transposition potential, and accounts for orographic enhancement through the Geographic Transposition Factor (GTF).
PMP values were derived for durations ranging from 1 hour to 5 days, producing spatially distributed grids that reflect basin-specific topography and atmospheric dynamics.
Rainfall frequency analyses (AEPs) were developed using L-moment statistical methods applied to regional rain gauge and reanalysis datasets. These AEP relationships provide a rainfall recurrence intervals extending through to the most extreme PMP values, allowing for both deterministic and probabilistic applications in hydrologic design.
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Conclusions
The Snake River Dams PMP–AEP Analysis provides a scientifically sound, risk-informed foundation for flood hazard evaluation to support dam safety management.
The results confirm that:
· PMP-based design standards continue to represent the upper physical limits of precipitation potential,
· Adaptive management and periodic updates will ensure continued resilience.
By integrating storm-based PMP methods, probabilistic rainfall analyses, and hydrologic modeling this study offers a comprehensive, unified approach to understanding and managing extreme precipitation risk for this complex system.
