Excel Spreadsheet Templates for Pipe Outlet Control
The image at the left illustrates the Excel spreadsheet templates that can be downloaded to calculate the pipe diameter needed to control the outlet flow to no more than the pre-development peak storm water runoff rate for the design storm if values are provided for all of the parameters in the Inputs column. Those input parameters are shown in the diagram at the right and are summarized below:
Manning roughness coefficient of pipe, n, dimensionless
- pipe length, L, ft
water elevation in pond at design storage volume, Es, ft
pipe centerline elevation, Ec, ft
Pre-development peak runoff rate, qpb, cfs
- trial value for pipe diameter, D, inches
The equations used to calculate the required pipe diameter using the above parameters are:
Kp = 5087 n2 D-4/3; C = 0.456 + 0.047(L Kp) - 0.0024(L Kp)2 + 0.00006(L Kp)3; D = 12C qpb0.5(Es - Ec)-0.25
These calculations require an iterative (trial and error) solution, because a value is needed for D in order to calculate Kp, which is needed to calculate D. A trial value of D is chosen to start the process, then with values entered for all the inputs, the Excel formulas will calculate Kp, C and the pipe diameter, D. If the calculated value for D is different from the trial value, then repeat with the trial value of D set at the next standard pipe diameter less than the calculated value. Repeat if necessary.
Standard U.S. diameters for storm water drainage pipe (all in inches) are:
4, 6, 8, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 48, 54, 60, 6" increments up to 180"
Excel spreadsheet templates can be downloaded through the links below to make these calculations in either U.S. or S.I. units. The last section of this article (on page 3) gives information about the parameters and equations in S.I. units.
Click here to download this Excel spreadsheet template in U.S. units.
Click here to download this Excel spreadsheet template in S.I units.