When is the rational method valid?
The rational method is appropriate for small, mostly impervious drainage areas under about 200 acres with a time of concentration under 30 minutes — parking lots, small developments, small culverts. It assumes uniform rainfall over the entire watershed at constant intensity, which breaks down at larger scales where rainfall varies spatially and routing effects matter. For watersheds over 200 acres, mixed land use with significant pervious areas, or any system requiring volume (not just peak flow) — switch to NRCS TR-55 (small watershed), HEC-HMS, or SWMM.
What is time of concentration?
Time of concentration (Tc) is the time required for runoff to travel from the hydraulically most distant point in the watershed to the outlet. It matters because the rational method assumes the design storm intensity is sustained for at least Tc — that's when peak runoff occurs at the outlet. Short Tc (5–10 minutes for paved parking lots) means you use high-intensity short-duration rainfall from the IDF curve; long Tc (30+ minutes for grassy hillsides) means you use lower-intensity longer-duration rainfall, often producing a lower peak Q despite the same total rainfall depth.
Where do I find local rainfall intensity?
The authoritative source for US sites is NOAA Atlas 14 Precipitation-Frequency Atlas, available free at hdsc.nws.noaa.gov/pfds. Enter your latitude/longitude and you get rainfall depth and intensity for return periods from 1-year to 1000-year and durations from 5 minutes to 60 days. Most municipal stormwater ordinances reference specific return-period storms (10-year for minor systems, 25- or 100-year for major systems and detention sizing). California, Texas, and Florida also publish state-level IDF curves that may take precedence over NOAA Atlas 14 in their respective jurisdictions.
What is a 10-year vs. 100-year storm?
The "X-year storm" is the rainfall event with a 1-in-X probability of being equaled or exceeded in any given year. A 10-year storm has a 10 % annual chance; a 100-year storm has a 1 % annual chance. They are statistical recurrence intervals, not predictions of when the storm will arrive — you can get two 100-year storms in back-to-back years. Most municipal codes require minor storm conveyance (gutters, inlets, small pipes) sized for the 10-year storm; major systems (detention ponds, outfalls, bridges) for the 25-year or 100-year. FEMA floodplain mapping is based on the 100-year storm. Climate trends are pushing many jurisdictions to use 25-year design where 10-year was historic standard.