Probable Maximum Precipitation (PMP) is used in the design of high hazard and critical infrastructure that must be able to withstand the largest physically possible flood (often referred to as the Probable Maximum Flood). PMP is most often estimated with a storm-based approach to calculate a deterministic (single maximum) value. However, in recent years, AWA has also developed a process to calculate Annual Exceedance Probabilities (AEPs) that extend to the PMP event and probability estimate of the PMP depths. This information is extremely valuable when evaluating critical infrastructure from a risk perspective and in combination with deterministically derived PMP depths. In addition, AWA analyzes climate change projections with a specific emphasis on extreme precipitation and other variables related to temperature and atmospheric moisture. These projections are evaluated in detail on a site-specific basis in context with our unique understanding of the site-specific meteorological conditions to determine how they will affect a given site, while quantifying uncertainty and providing unbiased recommendations. Precipitation trends, precipitation frequency, daily maximums, monthly and annual climatologies are provided with these studies.
OVERVIEW:
Applied Weather Associates (AWA) will complete a site-specific Probable Maximum Precipitation (PMP) and associated meteorological and climatological analyses for the Donlin Gold Mine proposed operations in southwest Alaska. This study will utilize the storm-based approach, the same approach AWA has used in hundreds of PMP studies world-wide, including several in the region and numerous studies completed for mining operations (Figure 1). AWA recently completed this same type of analyzes for large mining operations in the Yukon and British Columbia, which will benefit this study by reducing cost and time to complete this complex process. This study will utilize numerous storm analyses and the overall storm database that were completed during those studies that are directly applicable to this study.
As a result of this previous and ongoing work, AWA has a detailed and extensive understanding of the data and methods required for PMP development, storm types, and seasonality in the region. AWA’s extensive work in the mining industry results in an intimate understanding of the inputs required for mining operations, GISTM standards and the required inputs for water balance, water conveyance, and water storage for current and future conditions.
The storm-based approach utilized by AWA is the same process that was utilized by the National Weather Service (NWS) in the development of Hydrometeorological Reports (HMR), Technical Paper 47, and is recommended by the World Meteorological Organization Manual for PMP (2009). Previous and ongoing work in the region has shown that the Probable Maximum Flood (PMF) is likely to result from a tropical system, local intense thunderstorms, or stalled frontal system. The storm types can produce a combination of intense short duration rainfall and a large volume of rainfall over several days, along with snowmelt will all be analyzed using AWA’s gridded PMP processes. This will allow for hydrologic and engineering analyses to be completed for all potential combinations of runoff scenarios within the basin, from a point location to the overall basin. In addition to the storm-based, deterministically derived PMP estimates, AWA will develop annual exceedance probability estimates for several important durations, uncertainty analyses to quantify risk, and climate change assessments specifically related to the site-specific characteristics that can be utilized for future design and closure conditions.
