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Critical Duration Analysis

Critical Duration Analysis

Critical Duration Analysis

I'm using the rational method within Hydrocad to model a site with 14 drainage areas.  9 of the drainage areas are routed through detention basins.  All of the runoff from the drainage areas and detention basins flows to a central stream, the bottom of which is the location for the site study point.  

Of course, each of the DA's has a different Tc, which under normal circumstances is a transgression of the rational method.  (Different Tc's for each drainage area mean that 14 different storms will be occouring over the site simultaneously.)

Hydrocad has a Critical Duration Analysis tool that can be used when using the rational method for multiple DA's.  When this analysis is activated for the node at the study point, does it work back upstream and change all of the Tc's to the critical duration.  I've read the help menu text and still don't get exactly what this funtion does.

Please, no comments on how I should be using SCS for this.  Rational is mandated by ordinance.

RE: Critical Duration Analysis

HydroCAD always applies the same rainfall duration to all subcatchments in the project, including those upstream of the study point.  This is true for a manual duration setting, or for the critical duration analysis.  However, the Tc is a physical characteristic of each subcatchment, so it remains constant.  For details please see www.hydrocad.net/rational.htm

RE: Critical Duration Analysis

I see, so Hydrocad unlocks the Tc from the storm duration.  The duration (and resultant intensity) are consistent over the site, but the sub drainage areas can still have their separate Tc times for travel down the various conveyances to the study point.  Thanks.

RE: Critical Duration Analysis

Exactly.  For a single subcatchment, the duration and Tc are generally considered to be synonymous.  But with multiple subcatchments they must be treated separately.

RE: Critical Duration Analysis

The minimum duration used to collect IDF rain gage data is five minutes. A 5-minute duration is associated with a raincloud approximately one square mile (625 acres) in size (assuming a raincloud velocity of 12 mph)passing over the rain gage.
If the drainage area is less than 100 acres using a 5-minute duation for the entire site seems reasonable.
What are the sizes of the rainclouds being generated by HydroCad?

RE: Critical Duration Analysis

When using the Rational method, the rainfall input data consists of a user-supplied IDF curve.  When using the SCS/NRCS method, a standard or custom rainfall distribution is used.  In each case, the selection of the rainfall data is the responsibility of the designer.  Any analysis of the raincloud dynamics would be performed by the designer in dertermining the rainfall parameters to use with HydroCAD.

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