Paleoflood Enhancements within HEC-SSP

The U.S. Army Corps of Engineers (USACE) owns and/or operates over 700 dams and over 15,000 miles of levees throughout the United States with authorized purposes including flood control, water supply, hydropower generation, and navigation. The performance of these structures are evaluated via risk assessments which incorporate hydrologic hazard curves, system response, and consequence information. A well-developed hydrologic hazard curve provides estimates of flow, duration, and stage across the entire range of possible loadings. Multiple inputs are required to fully develop a hydrologic hazard curve including flow- and stage-frequency. Paleoflood data often consists of geologic, geomorphic, and botanical evidence indicating evidence of large floods (paleostage indicators) and/or the lack thereof (non-exceedance bounds). Paleoflood data help improve flow-frequency curves especially when extrapolating to remote annual exceedance probabilities. This can reduce uncertainties and improve confidence in results for decision makers during risk analyses.

The Hydrologic Engineering Center's (HEC) Statistical Software Package (HEC-SSP) contains an array of computational abilities that aid users when performing hydrologic frequency analyses. Existing features range from computing flow-frequency curves using Bulletin 17C procedures to quickly assessing the relative goodness of fit of 19 analytical distributions when fit to the same data set. New tools are under development at HEC that will aid the incorporation of paleoflood data as well as support the interpretation of flow- and stage-frequency curves. First, an updated HEC Data Storage System library that allows for the storage and usage of millions of years' worth of streamflow data has been incorporated within HEC-SSP. Enhanced computational options include a new expected probability adjustment specific to EMA as well as an improved equivalent record length algorithm. Finally, user interface enhancements include updates to the Bulletin 17 analysis EMA Data tab as well as the ability to visualize and interpret non-exceedance bounds within flow-frequency plots.