Introduction
This provides an overview of the Probable Maximum Precipitation (PMP) analysis completed for the Lyulyakovitsa Tailings Dam in Bulgaria. The study was undertaken to quantify the magnitude and spatial characteristics of extreme precipitation that could influence the TSF dam and its contributing watershed. PMP represents the theoretical upper limit of precipitation that is meteorologically possible for a given location and duration. As such, it is a cornerstone for evaluating the safety of high-hazard facilities, including tailings dams, under the most severe and credible hydrometeorological conditions.
Applied Weather Associates (AWA) conducted the study using physically based methods that reflect current scientific understanding of storm behavior, moisture availability, and topographic influences on extreme rainfall. The resulting PMP values provide a scientifically defensible and site-specific basis for assessing hydrologic risk and ensuring the long-term safety and regulatory compliance of the Lyulyakovitsa Tailings Dam.
The PMP analysis forms part of a broader effort to integrate updated hydrologic and climatic data into tailings management planning, ensuring that design standards reflect both current and projected future conditions.
Objectives
The objectives of this study were to:
1. Develop site-specific PMP estimates for the Lyulyakovitsa Tailings Dam, covering durations from 1 hour to 10 days, using a storm-based analytical approach.
2. Identify representative extreme rainfall events from regional and global historical records for reconstruction, transposition, and maximization.
3. Quantify the spatial and temporal distribution of PMP rainfall across the dam catchment to support hydrologic modeling and dam safety analysis.
4. Evaluate consistency and reliability of PMP results with established international methodologies for high-hazard infrastructure.
5. Provide technical guidance on the application of PMP results for risk assessment, regulatory compliance, and long-term dam safety planning.
Methodology
The PMP analysis followed AWA’s storm-based methodology, which emphasizes physically realistic storm reconstruction and moisture maximization rather than statistical extrapolation. This approach is widely accepted as the most accurate method for deriving site-specific PMP estimates, particularly in regions with complex topography such as the Lyulyakovitsa basin.
The analysis included the following key components:
· Storm Selection and Reconstruction: Historical extreme rainfall events were selected from the Balkan Peninsula and comparable synoptic regions. These storms were chosen based on spatial extent, moisture availability, meteorological intensity, and data completeness. Each storm was reconstructed using multi-source datasets, including rain gauge observations, radar, satellite precipitation products, and upper-air soundings.
· Moisture Maximization and Transposition: Each storm was analyzed to determine the maximum potential atmospheric moisture content, adjusted to represent theoretical maximum conditions at the Lyulyakovitsa site. The transposition process assessed whether storms occurring in nearby basins could physically occur at the study site, accounting for topographic and synoptic comparability.
· Orographic Adjustment and Spatial Analysis: The Storm Precipitation Analysis System (SPAS), developed by AWA, was used to model storm rainfall fields at high spatial resolution. This tool integrates terrain data and moisture indices to create detailed PMP grids for multiple durations. The spatial distribution was validated against regional climatology to ensure physical realism and consistency.
· Quality Assurance and Consistency Checks: The final PMP estimates were compared with published values from other regional studies to confirm continuity across basin boundaries. Statistical and meteorological consistency was evaluated for all durations.
· This rigorous, physics-based process ensures that the resulting PMP values represent realistic upper limits of rainfall potential for the Lyulyakovitsa Dam area under both current and foreseeable climate conditions.
Key Findings
· The PMP results demonstrate that the Lyulyakovitsa catchment is influenced by a combination of convective and synoptic storm systems, each contributing differently to rainfall extremes depending on duration.
· Short-Duration Storms (1–6 hours): These events are dominated by intense convective activity, often associated with local instability and short-lived but high-intensity rainfall bursts. Such storms typically produce the highest rainfall rates over limited areas.
· Long-Duration Storms (1–10 days): Multi-day rainfall totals are governed by slow-moving or stationary frontal systems that bring sustained moisture inflow from the Mediterranean or Black Sea regions. These synoptic-scale systems produce broad and continuous rainfall, leading to significant cumulative totals across the basin.
· The maximum PMP depths for 24-hour and multi-day durations are comparable to, or slightly exceed, those from earlier regional assessments, reflecting improved resolution and more comprehensive storm data.
· These findings confirm that the current PMP-based design approach remains appropriate for dam safety evaluations, providing robust protection against the most severe hydrometeorological events conceivable at the site.
Conclusions
· The storm-based PMP approach yields results that are physically realistic, regionally consistent, and defensible for engineering applications.
· The derived PMP values should serve as the primary design inputs for hydrologic and hydraulic analyses used in spillway sizing, freeboard determination, and flood routing assessments.
· The current PMP magnitudes adequately encompass the range of future climatic variability projected for the region. PMP therefore remains a valid and conservative design criterion.
· Continued monitoring and periodic reassessment of PMP (every 10–15 years) are recommended to ensure alignment with evolving observational datasets, climate models, and regional hydrometeorological understanding.
· Integration of PMP findings with ongoing risk assessment and dam safety management programs will further strengthen infrastructure resilience and regulatory compliance.
