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Type II 72-hr Rainfall

Type II 72-hr Rainfall

(OP)
I was able to download and add Type II 48-hr.hcr storm to the HydroCAD Rainfall Library. Is there a way to Use the 6-hr, 12-hr, 24-hr, and 48-hr to derive the 72-hr storm? We are getting into Dam projects that require looking at PMP/PMF events.

RE: Type II 72-hr Rainfall

Have you looked at the "spillway" distributions in HydroCAD?

Otherwise HydroCAD can be used to generate a synthetic rainfall distribution based local NOAA data. For details please see www.hydrocad.net/rainfall/generation.htm This procedure allows you yo define the overall rainfall duration and the time of peak.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Type II 72-hr Rainfall

(OP)
I am just getting started with HydroCAD, so I have not explored all the options; I was hoping it was as simple as grabbing the Type II 72-hr.hrc file from somewhere. I do not know all of the specific project requirements yet, but I have read in the TCEQ (Texas Commission on Environment Quality) Dam Safety Program that with respect to the PMP depths, "Then each possible duration up to 72 hours should be reviewed in order to determine the critical duration." Unfortunately Texas is one of the states where information is lacking, except for the larger metro areas. I have the PMP rainfall from the HMR51 charts and all the storms except the 72-hr. I'll look into your suggestions. Thanks for your response.

RE: Type II 72-hr Rainfall

There is no standard Type II 72-hour distribution, so you will have top scrutinize your stormwater regulations to determine exactly what is required.

Frankly, the text you quoted ("each possible duration up to 72 hours should be reviewed in order to determine the critical duration") tends to imply the use of the Rational method, and not the SCS/NRCS method. As such, the analysis would be based solely on IDF data and would not use a temporal rainfall distribution, 72-hour or otherwise. Again, a careful reading of the regulations is necessary.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Type II 72-hr Rainfall

(OP)
TCEQ: 4.3 Temporal Distribution of Design-Storm Precipitation "Distribute the total depth of the PMP, for both the entire basin as well as for each subbasin, as appropriate, temporally...". I assumed that meant using the a 72-hr distribution with the 72-hr rainfall. I should have remembered what I learned about ASSUME in college (so many years ago). However, subsequent charts and graphs imply no difference in 48 to 72-hr distribution; only the PMP rainfall depths. So I guess the end result is Type II 48-hr with 72-hr rainfall. I will dig deeper before posting any questions in this forum. I'll also study HydroCAD more with your very good YouTube tutorials before asking questions about HydroCAD.

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