Frequently Asked Questions (FAQ)

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.

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:

WRD hydrology sedimentation hydraulic hydrological wells groundwater rainfall conservation depth precipitation alert gauges