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Evaluation of 1-Day–1%AEP Rainfall Depths in Mississippi
Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Kronkosky B.C.

In Mississippi, the Federal Emergency Management Agency (FEMA), National Flood Insurance Program (NFIP), insures ~64,000 policies that total ~$16M dollars (9/30/2017 FEMA statistics)—in aggregate (since 1978), Mississippi has contributed to ~60,000 insurance claims that have exceeded $3B dollars. In almost all instances NFIP "base flood" flood plains are delineated using 1-day–1% rainfall depths (100-year floodplain). In 2013, the National Oceanic and Atmospheric Admiration (NOAA) released "NOAA Atlas 14—Precipitation-Frequency Atlas of the United States, Volume 9, Version 2,"—the most current source for establishing 1-day–1% rainfall depths for Mississippi (and other neighboring states). In addition to Atlas 14, there are seven other studies, dating as far back as 1917, which define 1-day–1% rainfall depths for Mississippi.

In this investigation, we present a detailed review of these rainfall depths (5 of the 7 studies) using Mississippi county centroids (88 counties). Homogeneous statistical tests are utilized to show differences amongst these estimates, which indicate most estimates are within 10%. It is also shown these studies are bound by each other's standard error, indicating these estimates are statistically indistinguishable (within margin of error). These results suggest 1-day–1% rainfall depths (for Mississippi) have not significantly changed in over 100 year of research, and that methods used 100 years ago produce near identical results as modern-day studies (e.g. Atlas 14).

Attendees will be presented with a breadth review of these historical studies and detailed comparison of their estimates. The objective is to demonstrate that newer research should not necessarily supplement historical practice unless there are significant differences. This calls to question…is newer better, or are we over complicating how 1-day–1% rainfall depths are prepared.

2017 MWRRI Annual Report
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