Excel formulas for civil engineering9/1/2023 this information is typically available in each state from some state agency. This data must come from a rainfall intensity duration frequency graph or table for the design location. There is also provision in the spreadsheet for entering data points for storm intensity (in/hr) vs storm duration (min) for the specified design return period. In the spreadsheet that can be downloaded through the link below, there’s a table with ranges of values of the runoff coefficient, C, for several types of land use. The input information needed is: i) the drainage area, A in acres ii) the runoff coefficient of the drainage area, C iii) the design return period in years and iv) the design storm duration, d, in minutes. The Excel formulas also calculate i, and then use the Rational Method equation to calculate the design peak storm water runoff rate, Q. The Excel spreadsheet image shown at the left is set up with Excel formulas to derive an equation for calculating the design rainfall intensity, i. The next section presents and discusses an Excel spreadsheet for calculating the design rainfall intensity, i, based on rainfall intensity duration frequency data for the design location, and then calculating the design storm water runoff rate, Q.įor more background information about the Rational Method and the parameters, Q, C, i, and A, see the article, “ The Rational Method for Calculation of Peak Storm Water Runoff Rate.” For background information about calculating design rainfall intensity, i, see the article, “ Calculating Design Rainfall Intensity for Use in the Rational Method.” Excel Formulas for Design Rainfall Intensity and Rational Method Calculations Some information on typical ranges for C values is given in the table at the right. ![]() units given above for Q, i, and A, the Rational Method equation is Q = 0.0028 Cia.Ĭalculation of the peak storm water runoff rate, Q, is quite straightforward if values are known for C, i, and A. The unit conversion comes out to be 1.008 cfs/(acre-in/hr), so the equation, Q = 1.008CiA could be used, but in view of the accuracy with which the runoff coefficient can be determined, the conversion constant is typically taken to be simply 1. This equation actually gives Q in acre-in/hr.
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