NEW JERSEY BULLETIN NO. NJ210-9-2

October 14, 2008

SUBJECT: ENG – Engineering Field Handbook Supplement, Rainfall Frequency and Distribution Data

Purpose: To distribute a supplement to Chapter 2 of the Engineering Field Handbook revising rainfall frequency data and to provide information on new storm distributions for New Jersey.

Effective Date: Effective upon receipt.

In September 2004, county rainfall frequency data for New Jersey was revised by NRCS following a study conducted by the National Oceanic and Atmospheric Administration (NOAA) for the Ohio River Basin and Surrounding States. The revised data replaced TP-40 rainfall frequency values for New Jersey. Also undertaken by NRCS was a review and update of storm distribution data. Out of this effort, county based storm distributions have now been developed for the one year through 500 year, 24 hour storms. This data, along with a final update of the county average rainfall frequency table is now available at http://www.nj.nrcs.usda.gov/technical/engineering/

Comparisons of peak discharges generated using the current Type III distribution and the new county based distributions tend to indicate little difference for the more frequent storm events, up to the 10 year, 24 hour storm. For less frequent events, the new distributions tend to give lower peak discharges. For the 100 year, 24 storm event, the reduction has varied from about 4 to 9 % in the northwestern counties, 12 to 18% in the northeastern counties, to about 12 to 16 % in the central and southern counties.

The revised rainfall frequency data can be incorporated into current hydrologic models for immediate use. WinTR55 software will be updated to include the new data in the near future. An updated version of WinTR20 that facilitates the use of the new distribution data is currently available. Until the county based storm distributions are more readily accessible, it is acceptable to continue use of the Type III storm distribution, especially for analysis of the more frequent storm events up to the 10 year, 24 hour storm which is typically used for design of erosion control measures. Depending on the amount of acceptable error, the Type III storm distribution may still be used for less frequent storms, however, where more precision is necessary, the new distributions should be used. Also, it remains acceptable to use the more site specific data accessible through the Hydrometeorological Design Studies Center (HDSC) Precipitation Frequency Data Server at http://hdsc.nws.noaa.gov .

Filing Instructions: Remove Exhibit 2 NJ 2 dated September 2004 from Chapter 2 of the Engineering Field Handbook and replace with the attached revision.

/s/

DAVID LAMM, PE
State Conservation Engineer
Enclosure, Dist: O

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Last Modified: November 18, 2008