Developing an Accessible Database of Extreme Storms for Hydrologic Modeling and Research

The development of Probable Maximum Flood (PMF) estimates for critical infrastructure such as dams and levees requires the quantification of several variables, including Probable Maximum Precipitation (PMP) for the watershed or basin of interest. These PMP depths have been calculated in both generalized formats, such as the series of NOAA Hydrometeorological Reports (HMRs), and site specific studies. These products are produced through the analysis of observed extreme storm events that are subsequently maximized and transposed across a climatological region. The present lack of a readily accessible and comprehensive database with gridded precipitation data of these extreme storms increases the time and level of effort for hydrologic modelers to determine the appropriate estimate of a PMF for risk assessment purposes. An ongoing project within the U.S. Army Corps of Engineers (USACE), however, provides a path forward to create such a database and ensure compatibility of storm data with hydrologic modeling and water control requirements.

Analysis of extreme storm events has historical been relegated to the review of previously created isohyetal maps or a limited number of observational reports. To further complicate the detailed exploration of a specific event, various portions of data was often contained under the auspices of multiple agencies at the federal (NOAA, USACE, USBR, USGS, etc.), state, and local levels. Modern GIS capabilities enable the digitization of historical maps as well as the recreation of the original storm analysis. The latter can be accomplished by consolidating data into a point shapefile and then performing different interpolation techniques to compare with the previous efforts. To compensate for large areas of missing data, which is particularly relevant in storms prior to 1950 and in sparsely populated areas of the country, or for regions of extreme topography, base maps that utilize precipitation frequency or climatological averages can also be used. Some of these techniques have been incorporated into the development of daily PRISM data and can be captured for specific storm events. To further aid the eventual usage of the data in hydrologic modeling, storm typing procedures can be added to the analysis of specific storms. While such standards have been used in the development of various HMRs, such as HMR 55A for the Continental Divide eastward to the 103^rd Meridian, a more focused degree of storm typing based on regional climatology has recently been explored and holds the potential to further aid hydrologic engineers in the PMF analysis.

The overall effort of the USACE Extreme Storm Database is predicated on the availability of gridded precipitation data for hydrologic modeling. This requires the aforementioned storm analysis, gridded data development, and database management. The development of gridded data, preferably in time steps of an hour or less, can be accomplished with the USACE program HEC-MetVue (Meteorological Visualization Utility Engine). With HEC-MetVue, spatially distributed precipitation patterns developed either through digitized historical maps or storm total point data can be given a temporal pattern based upon hourly gauge data. The result can be in a variety of file formats including DSS or ASCII for follow-on usage in hydrologic models such as HEC-HMS. Future improvements will create an even greater interoperability between the USACE Extreme Storm Database and HEC-MetVue to ensure a seamless transfer of data. Such features will include, but not be limited to, modification of historical storms and the development of watershed specific PMP estimates or Probable Maximum Storms.

Hydrologic Engineers frequently require observed storm data to not only develop an Inflow Design Flood for projects but also to calibrate certain hydrologic models. The limited availability of gridded precipitation data in a format for use in such computer programs has created unnecessary challenges that have resulted in additional time and money to complete these studies. As these needs within the hydrometeorological community have been identified, USACE has taken the lead to create and improve a database geared toward the preservation of extreme storm data as well as their availability to members of the hydrologic, water control, and emergency management communities. These efforts are anticipated to take multiple years for full completion but will provide a vital benefit to stakeholders at all levels.