Critical duration under TC
Critical duration under TC
(OP)
Might be an easy one (yes or no) :
When seeking a volume from the Modified Rational Method,the calculation is performed at multiple durations with corresponding intensities to produce a series of trapezoidal hydrographs.
Once the allowable discharge is subtracted the hydrograph with the greatest volume is then your critical storm duration for that analysis.
But do you check for duration < time of concentration (e.g. duration of 5 min while the tc is 15 minutes) ?
When seeking a volume from the Modified Rational Method,the calculation is performed at multiple durations with corresponding intensities to produce a series of trapezoidal hydrographs.
Once the allowable discharge is subtracted the hydrograph with the greatest volume is then your critical storm duration for that analysis.
But do you check for duration < time of concentration (e.g. duration of 5 min while the tc is 15 minutes) ?





RE: Critical duration under TC
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Critical duration under TC
In my case, it's under tc : critical duration (max volume) occurs at 10 min while tc = 15 min.
RE: Critical duration under TC
RE: Critical duration under TC
It really depends on the IDF and the available outflow.
RE: Critical duration under TC
IDF parameters A and b = 64.96 and -0.44 (in I = A*Tc^b)
Area = 3,6 ha
C = 0,5
Qmax release = 0.5 m3/s (constant for the example).
I get 172 m3 at 10 min.
RE: Critical duration under TC
RE: Critical duration under TC
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Critical duration under TC
My point is the critical duration is 10 min. while someone could pretend that tc = 15.
We could pretend that tc = 30 minutes too... what would it do on the calculations!?
RE: Critical duration under TC
I'm still doubtful that the critical duration could be less than the Tc. This duration would have a smaller runoff peak and volume, so I don't see how it could yield more storage in the pond. I would re-check your assumptions about the discharge you are subtracting.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Critical duration under TC
The discharge is constant... this is not true but... reducing it wouldn't change the fact that the critical duration is 10 min (it could be worse e.g. at 5 min).
RE: Critical duration under TC
The critical duration depends on how you do the pond routing, which in turn depends on how you estimate the discharge. I never said the discharge was constant...
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Critical duration under TC
My question was relatively easy and I think you're trying to say that critical duration can't be before tc.
But I'm trying to get a formal answer about this.
RE: Critical duration under TC
Think back to the rectangular hyetograph that produces the trapezoidal hydrograph. The area of the hyetograph represents a volume of rainfall. Even though a longer Tc will produce a lower intensity rainfall, the area of the hyetograph is always higher for longer Tcs.
Right?
I don't use modified rational for hydrology down here, because review agencies quit accepting it because it was so heavily abused.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Critical duration under TC
RE: Critical duration under TC
Here's one excel sheet founded on Internet :
https://www.kalcounty.com/drain/new_pdfs/RationalM...
RE: Critical duration under TC
RE: Critical duration under TC
I don't get why this wouldn't be "Modified Rational".
There is no need to use Tc execept to adjust the runoff coefficient used in the Rational Method.
RE: Critical duration under TC
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Critical duration under TC
http://www.deq.virginia.gov/Portals/0/DEQ/Water/St...
starting on pg 134 of the PDF
RE: Critical duration under TC
This is solved by :
The shortest period selected should be the time of concentration, tc .