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MODELING MODIFIED RATIONAL METHOD

thread789-332022

MODELING MODIFIED RATIONAL METHOD

MODELING MODIFIED RATIONAL METHOD

(OP)
When using the rational method, I want to create a modified rational hydrograph where the storm duration is 3 times the TC. The rising limb is 1 times Tc and the falling limb of the trapezoidal hydrograph is 8 times the Tc so the total length of storm event is 11 times the Tc. If I run the Hydrocad program it seems to compute the critical duration and shorten the storm to 9 times the Tc with no extended storm duration (the flat top of the hydrograph curve, making it a trapezoid). Whats the best way to model the modified rational hydrograph?
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RE: MODELING MODIFIED RATIONAL METHOD

Just enter a rainfall duration that is 3 times the Tc, and set the rise and fall factors to 1 and 8. This will produce a flat-topped runoff hydrograph with a total duration that is 11 times the Tc (3+8) If you have defined an IDF curve, HydroCAD will automatically determine the intensity for the specified duration. Or you can enter the intensity manually.

Do not use the critical duration analysis for this application, since it will automatically adjust the rainfall duration to attain the highest peak flow.

Peter Smart
HydroCAD Software
www.hydrocad.net

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