designed storm event

If you're being required to calculate the 100 year depth in proposed stormwater infrastructure, any civil site engineer should be qualified to handle that. If you're looking for the stage in creeks or rivers, and you're on a studied reach, get your elevations here:


If you're not on a studied reach, you'll need to hire a hydrologist to figure it out for you. Lots of those frequent these forums.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

The 100-year design flood event is a pretty nebulous term. Typically the 100-YR (technically the 1% annual-chance) storm event rainfall depth is obtain from a source such as TP-40 , NOAA Atlas 14, or some other local source. These rainfall frequency depths are based on a statistical analysis of many years of rainfall data.

The design flood is a function of local design criteria and land use for the watershed in question.

FEMA floodplain maps (linked in the post above) are generally based on existing land use conditions at the time the study was done. If you are working in a watershed that has been fully developed for some time they are a good data source. If your watershed is undeveloped or has seen significant land use changes since the FEMA study was done you will need to do further study if the design criteria dictates you study full developed conditions. If that is the case you will need to perform a hydrologic analysis of the watershed to determine peak discharges for your design storm and a hydraulic analysis of the ditches to determine their capacity.

Storm return periods must have a duration. You would not design for a 100-year storm, you would design for a 100-year 24-hour storm or a 100-year 1-hour storm as appropriate. Normally 100-year storms are associated with longer rain events such as 24 hour storms.

Storm return periods must have a duration. You would not design for a 100-year storm, you would design for a 100-year 24-hour storm or a 100-year 1-hour storm as appropriate. Normally 100-year storms are associated with longer rain events such as 24 hour storms.

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..well.. ..yes and no. When someone says the "100 year flood," they may be referring to the worst flood condition that might happen in a 100 year period irrespective of storm duration. You determine this by modeling a bunch of different duration 100 year storm distributions, and picking the worst one. It's not often done that way, but sometimes it is, particularly down in Florida.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

Good point, beej. I work in Florida too and run different storm durations regardless of whether they are required. I still always thought of them as different storms.

Yes, the different durations for a specific frequency are known as different design storms in the industry.

What to choose? Depends on the typical storm makeup for your region and how large of a watershed area your are trying to accomodate.

An intense convective 2hr storm may quickly overwhelm the ditches, culverts and roads and cause most of the local flooding problems, but not fill up the large regional detention pond, while a larger volume, less intense, 24hr may not cause as many local problems as the 2hr storm, but will cause flood levels to rise over a larger area, and fill the regional detention pond.