Rainfall Intensity and Design Parameters
Storm drainage design begins with the design rainfall intensity for the geographic location and design recurrence interval. NOAA Atlas 14 (Precipitation Frequency Atlas of the United States, available at hdsc.nws.noaa.gov) provides precipitation frequency data at specified recurrence intervals (2-year, 10-year, 100-year) and durations (5 min, 15 min, 1 hour, 24 hour) for all U.S. locations. Roof drainage design typically uses the 100-year, 1-hour intensity — a conservative design event that represents the storm intensity expected to be equaled or exceeded once in 100 years on average.