LWI Region 3 Modeling Overview

4 Precipitation and Frequency Analysis

4.1 PRECIPITATION AND FREQUENCY ANALYSIS: Atlas 14 gridded precipitation conversion into NRCS hyetographs. 4.1.1 Atlas 14 Precipitation

NOAA’s National Weather Service Hydrometerological Design Studies Center Precipitation Frequency Data Server (PFDS)

The LWI modeling and methodology document recommends using NOAA Atlas 14 precipitation frequency estimates for design storms, obtained from the Precipitation Frequency Data Server maintained by the Hydrometeorological Design Studies Center/ Office of Water Prediction.

{ Atlas 14 provides precipitation data in tabular, graphical, and gridded depth-duration-frequency (DDF) formats. The "Atlas 14 Everywhere" methodology was applied consistently across all basins. { Gridded rainfall depths as hyetograph (approximately 3,000 x 3,000 ft per cell) were assigned to each precipitation grid cell, providing a spatially variable and more accurate representation of rainfall across the basin compared to uniform depth assumptions (see Figure 4-1). { Precipitation grids were downloaded for 5-, 10-, 25-, 50-, 100-, 200-, and 500-year recurrence intervals across 6-, 12-, 24-, 48-, and 72-hour durations.

{ The NRCS nested distribution methodology was applied to each grid cell to embed shorter duration rainfall within longer-duration storms, producing distributed DDF values (see Figure 4-2 for distribution and Figure 4-3 for accumulated precipitation). { Resulting gridded hyetographs, generated through the Spatial Hydrologic Grid (SHG), were used as meteorological input for the ROM-RAS model. { The precipitation input was applied to the HEC-RAS model using a HEC-DSS file, with data provided in 6-minute intervals.

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LWI REGION 3 TRAINING

LOUISIANA WATERSHED INITIATIVE