Frequently Asked Questions (FAQ)

Question:
Does the County publish maps of return frequency for a variety of durations? Have regression equations been developed to include different factors such as elevation and area? I know that Sacramento County has developed these maps and equations to supercede TP-40 and NOAA Atlas 2 and was hoping that something similar had been created for your county.

ANSWER:
The Los Angeles County Department of Public Works only has one map with the rainfall depths for the 50-year 24-hour storm. However, with these rainfall depths, the normalized intensity/duration relationship, and the conversion factors that convert the 50-year depth to other recurrence interval depths, the depth at other locations for other recurrence intervals can be found. Our Hydrology Manual has the 50-year, 24-hour rainfall depth map, the normalized intensity/duration relationship, and the conversion factors. The normalized intensity/duration relationship is It/I1440=(1440/t)^0.47. It is the intensity for a duration in minutes, t is the duration in minutes, I1440 is the 24-hour intensity. Our Hydrology Manual is available at: http://ladpw.org/wrd/publication/.

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