In light of recent advances in both hardware and software, there is an increasing trend toward more comprehensive analyses.

What if, for example, you wanted to include a carpark culvert with a 2-minute catchment response time in addition to a proposed detention basin with a 1-hour response time – and perhaps also a larger river basin with a 24-hour response time – all in the same model? The frequency storm would max them all out. A hydrological analysis is generally based on a set of conditions that represents the critical event at a single node a typical rainfall-runoff model that uses a more realistic, critical storm pattern would need to be run separately for each of these different scales. What are the chances that a 7-day, 100-year storm event would also include the 100-year 1-minute intensity, the 100-year 1-hour intensity, and the 100-year 24-hour intensity, all in the same event? I would venture to say that it has never happened in the entire history of this planet! That said, the frequency storm can be a very useful bookend, particularly for a rain-on-grid hydraulic analysis. Some rainfall patterns are based on actual, observed events, with patterns that you might expect to see and capture with gauge data the frequency storm, however, is purely hypothetical and its occurrence would be very improbably to say the least. For example, the 25-year 5-minute, 25-year 10-minute, 25-year 15-minute, and up the 25-year 24-hour rainfall occurs within the same design storm and are centrally nested within each greater storm duration listed.” (.27) The primary assumption made in the development of the rainfall distribution is that the rainfall values for all durations for a single return period occur within one 24-hour period.

“An important characteristic of NRCS synthetic rainfall distributions is that the maximum rainfalls for all durations from 5 minutes to 24 hours are represented accurately. So what is the frequency storm all about?Įssentially the frequency storm builds the intensity associated with every duration into a single, hypothetical storm event.Ĭhapter 4 of the NRCS National Engineering Handbook (Part 630) outlines Storm Rainfall Depth and Distribution techniques, including the NRCS synthetic design storm, which is an example of a frequency storm: The screen shots above show a hypothetical set of rainfall depths that I have applied to the Tenk model (example file that ships with the software) as a centrally-loaded frequency storm. The frequency storm asks for rainfall depths corresponding to a range of durations: If you have used HEC-HMS to develop rainfall-runoff relationships for your catchment/watershed, you may be familiar with the option of selecting the “frequency storm” as the precipitation input for your meteorological model.

Run a flt file on hec ras