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Design Rainfall Distributions Based on NRCC Data

This article explores the development and application of NRCC rainfall distributions and proposes the adoption of NRCC data in applicable states. It also discusses data smoothing procedures and future directions for software development.

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Design Rainfall Distributions Based on NRCC Data

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  1. Design Rainfall Distributions Based on NRCC Data

  2. General Considerations • Standard NRCS rainfall distributions were developed between 1960 and 1985. • NRCS is proposing adoption of NRCC data in the states where it is applicable. • Will a new set of rainfall distributions be required ? • If so, how to develop and apply them ? October 2010

  3. Standard Rainfall Distributions October 2010

  4. Geographic Application March 2009

  5. Ratio of shorter duration to 24 hour rainfall October 2010

  6. NOAA 14 Data for Bergen County, NJ October 2010

  7. NRCC Data for White Plains, NY October 2010

  8. Ratio of shorter duration to 24 hour rainfall, Bergen Co, NJ October 2010

  9. Ratio of shorter duration to 24 hour rainfall, White Plains, NY October 2010

  10. Steps in developing a distribution • Determine ratios of x hour / 24 hour rainfall. • Place the rainfall ratio for the shortest duration in the center of the distribution. • Symmetrically place each larger duration to include the shorter durations. October 2010

  11. 0.7 6 hr Example of a Nested Distribution 0.6 3 hr March 2009

  12. Data Smoothing Procedure • Why is data smoothing recommended? • How are data smoothed ? October 2010

  13. Example – original data October 2010

  14. Example – smoothed data October 2010

  15. Data Smoothing Procedure • Define incremental intensity: • Difference in rainfall divided by difference in duration, units inches/hour. • Data: 10-minute rain = 0.54 inches • 15-minute rain = 0.67 inches • Incremental Intensity = • 0.13 / (5 / 60) = 1.56 inches / hr October 2010

  16. Data Smoothing 1-year, NJ October 2010

  17. Data Smoothing Procedure • Incremental intensity is smoothed such that the 60-min and 24-hour rainfall values are not changed. 5-min through 30-min values may be changed. • 120-min through 12-hour values may be changed. • There are two line segments joined at the 60-min value. October 2010

  18. Data Smoothing 100-year, NJ October 2010

  19. Data Smoothing, Virgin Islands October 2010

  20. Data Smoothing, Virgin Islands October 2010

  21. Data Smoothing, Phoenix, AZ October 2010

  22. Data Smoothing, Phoenix, AZ October 2010

  23. Rainfall ratio maps based on NRCC data • 81 GIS maps were developed showing ratio of shorter duration to the 24-hour. • 9 durations from 5-minutes to 12-hours. • 9 return periods from 1-year to 500-year. October 2010

  24. 1-yr ratio, 15min/24hr October 2010

  25. 500-yr ratio, 15min/24hr October 2010

  26. Draft: 60 minute 25-yr ratio October 2010

  27. Draft: 3 distribution regions October 2010

  28. Steps to determine draft rainfall distribution • Determine average 25-year ratios for durations from 5-minutes to 12 hours in each of the three regions. • Develop 24-hour rainfall table at 0.1 hour increment for each region. October 2010

  29. Draft: average 25-year ratios October 2010

  30. Draft: RF Distributions October 2010

  31. Future Directions Develop software to easily use the data. • Use GIS data layers to derive site-specific rainfall data for hydrologic modeling. October 2010

  32. October 2010

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