Therefore, if this were an inflow point into the detention basin, the storm drain and the Conduit Outlet Protection would need to be sized for this QPEAK. Remember, the actual QPEAK will be based upon the Standard Rational Method Equation, not the MRM Equation. QPEAK = 38.65 cfs TPEAK = 0.10 Hours Volume = 41,743 cfĬomparison between TR-55 and Rational Method Hydrographs (Post-Dev.) QPEAK = 38.46 cfs TPEAK = 12.11 Hours Volume = 144,474 cfĬomparison between TR-55 and Rational Method Hydrographs Rational Method (Post-Dev.): Volume = 38,020 cfĬomparison between TR-55 and Rational Method Hydrographs (Pre-Dev.)ĭifferenceNegligible Significant SignificantĬompare Modified Rational Method (MRM) Hydrograph to a TR-55 Hydrograph (Post-Dev.) Drainage Area = 10.0 Acres TC = 0.20 hours Land Use = Industrial, HSG C Runoff Curve Number = 91 (TR-55) Runoff Coefficient = 0.88 (MRM) 10 Year Storm Pre-Development Q PEAK 10 YEAR = 14.03 cfs What is the difference between Runoff Volumes, Peak Flow Rates and Hydrograph Shape?Ĭomparison between TR-55 and Rational Method Hydrographs TR-55 (Post-Dev.): QPEAK = 14.03 cfs TPEAK = 12.38 Hours Volume = 76,775 cfĬomparison between TR-55 and Rational Method Hydrographs Rational Method (Pre-Dev.): The greatest difference in volume between the pre and post hydrographs becomes your critical hydrograph with respective critical storm duration.Ĭompare Rational Method Hydrograph to a TR-55 Hydrograph (Pre-Dev.)ĭrainage Area = 10.0 Acres TC = 0.50 hours Land Use = Meadow in Good Condition, HSG C Runoff Curve Number = 71 (TR-55) Runoff Coefficient = 0.44 (MRM) 10 Year Storm What is the difference between Runoff Volumes, Peak Flow Rates and Hydrograph Shape?Ĭomparison between TR-55 and Rational Method Hydrographs TR-55 (Pre-Dev.): The runoff volume from the pre-development hydrograph is subtracted from each of the runoff volumes (areas under the Trapezoid), for each storm duration. The I in the Rational equation is based upon the duration and not the Time of Concentration, However the hydrographs initially peak at the original Time of Concentration.
Conversion mgd to cfs series#
As the TC goes up, the I goes down.īut what is the Modified Rational Method (MRM) ?Ī series of Trapezoidal shaped hydrographs are created for different Storm Durations. Rainfall Intensities Based upon I-D-F Curves Rainfall Intensities are obtained by entering the Log- Log nomograph with the Time of Concentration along the abscissa, intersecting the Storm Return Period Curve, and proceeding horizontally to the Rainfall Intensity.
If the difference adversely effects the analysis, request a segmental analysis. Lets convert In-Ac/Hr to CFS: / 3600 Sec / Hr = 1.008 CFS / In-Ac / Hr The degree of accuracy does not warrant a units conversion factor (constant) of 1.008!!Īcre Single Family Dwelling with 38% Impervious Surface Coverage in HSG D would have a Runoff Coefficient of 0.80 from the above table, and a maximum of 0.50 from the table below.ĭepending upon the source of the Coefficients, there can be a significant amount of variability!Ĭompare the results from nomographs with the segmental TC method contained within TR-55.
How do you get Q in CFS, when you are multiplying In / Hr x Ac? It would seem that the math would equal In-Ac / Hr. Q = C x I x A x APFWhere: Q = The Quantity Flow Rate in CFS C = Dimensionless Runoff Coefficient I = The Rainfall Intensity in In / Hr A = The Drainage Area in Acres APF = Antecedent Precipitation Factor
Rational Method Equation:Based upon the Rational Method Equation: MRMBy: Paul Schiariti, P.E., CPESC Mercer County Soil Conservation District
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