9/20/13 Storm Sewer Hydraulic Calculations with the Manning Equation | Engineering Excel Spreadsheets

file:///Z:/1110/40 Calculations/40.4 Mechanical/Infrastructure Works/Reference/Storm Sewer Hydraulic Calculations with the Manning Equation.htm 1/3

Storm Sewer Hydraulic Calculations with the Manning EquationPosted on March 5, 2011 by Harlan Bengtson

Introduction

If you want to obtain an Excel spreadsheet to use for storm sewer hydraulic calculations, click here to visit our

spreadsheet store or visit our companion site, www.engineeringexceltemplates.com. Read on for information

about the use of storm sewer hydraulic calculations with the Manning Equation.

One part of storm sewer hydraulic design is determination of the design diameter and slope for the storm

sewer pipe between subsequent manholes. Although storm sewers are circular pipes, the storm water typically

flows under gravity, rather than as pressure flow, so the Manning equation for open channel flow can be used

for the calculations. Storm sewer hydraulic calculations are typically made for full pipe flow. Under these

conditions the hydraulic radius becomes: R = A/P = (πD /4)/(πD) = D/4.

The Manning Equation Applied to Full Pipe Flow

The general form of the Manning equation in terms of velocity is: V = (1.49/n)(R )(S ) for U.S. units and V =

(1.0/n)(R )(S ) for S.I. units. As noted above, R = D/4 for full pipe flow, so the Manning equation in U.S.

units becomes V = (1.49/n)[(D/4) ](S ) -U.S. units or V = (1.0/n)[(D/4) ](S ) – S.I units, for full pipe,

gravity flow in a storm sewer pipe. The parameters in the equations are as follows:

V is the flow velocity in the pipe (ft/sec – U.S. and m/s – S.I.).

n is the Manning roughness coefficient, an empirical, dimensionless constant.

D is the pipe diameter (ft -U.S. and m – S.I.).

S is the pipe slope, which is dimensionless.

The volumetric flow rate is related to the other parameters through the equation Q = VA or, for a circular pipe

flowing full: Q = (πD /4)V, where Q will be in cfs for U.S. units or m /s for S.I. units.

Calculation of Diameter and Slope for a Storm Sewer Pipe

The required diameter and slope for the length of storm sewer between

two manholes can be calculated using the equations presented in the last

section (Mannings equation and Q = VA) together with the typical

design criteria that 1) the full pipe flow rate that the pipe can carry must

be at least equal to the design peak storm water runoff rate to the inlet

for that section of storm sewer and 2) the full pipe velocity must be

equal to or greater than a specified minimum velocity. The diagram at

the right shows a sectional view of a storm sewer pipe between two

Engineering Excel SpreadsheetsEasy to Use, Low Cost Excel

Spreadsheets for Engineering

Calculations

2

2/3 1/2

2/3 1/2

2/3 1/2 2/3 1/2

2 3

9/20/13 Storm Sewer Hydraulic Calculations with the Manning Equation | Engineering Excel Spreadsheets

file:///Z:/1110/40 Calculations/40.4 Mechanical/Infrastructure Works/Reference/Storm Sewer Hydraulic Calculations with the Manning Equation.htm 2/3

manholes and the parameters being discussed here. The calculation procedure is illustrated by the example in

the next section.

Example Calculation of Pipe Diameter and Slope

Problem Statement: For a section of storm sewer between two manholes, the design flowrate is: Q = 6.4

cfs. The required minimum full pipe storm water velocity is: V = 3 ft/sec. The pipe roughness coefficient

(concrete pipe) is: n = 0.011. Find a standard pipe diameter and pipe slope that will meet the two criteria: Q

> Q and V > V for this section of storm sewer pipe.

Problem Solution: First the pipe diameter needed for a full pipe velocity of 3 ft/sec at design flow rate will

be calculated using the equation: Q = VA. Then the Manning equation will be used to calculate the pipe slope

needed to give full pipe velocity equal to 3 ft/sec with the next larger standard pipe size.

Step 1: The equation, Q = VA becomes: Q = V (πD /4). Substituting known values for Q and V , the

equation becomes: 6.4 = 3(πD /4). Solving for D gives: D = 1.65 ft = 19.8 in. From the list of standard storm

sewer pipe sizes in the next section it can be seen that the next standard size larger than 19.8 inches is 21 “, so

that will be used for the diameter.

The Manning equation will then be used to calculate the slope for D = 21 in. = 1.75 ft, and V = 3 ft/sec. The

Manning equation is: V = (1.49/n)[(D/4) ](S ). Substituting values for V, D, and n gives: 3 = (1.49/0.011)

[(1.75/4) ](S ). Solving this equation for S gives: S = 0.00148.

Thus, the solution is: D = 21″, S = 0.00148. These values of D and S will give Q > 6.4 cfs, because Q = 6.4

cfs for V = 3 ft/sec and D = 19.8″. With D = 21 ” and V = 3 ft/sec, Q must be greater than 6.4 cfs. The

equation Q = (πD /4)V can be used to check this.

Standard Pipe Sizes

Standard U.S. pipe sizes in inches for most types of pipe used as storm sewers: 4, 6, 8, 10, 12, 14,

16, 18, 21, 24, 27, 30, 33, 36, 39, 42, 48, 54, 60

Standard S.I. pipe sizes in mm for most types of pipe used as storm sewers: 100, 150, 200, 250,

300, 350, 400, 450, 500, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050

Use of Excel Spreadsheets for Storm Sewer Calculations

For information on making storm sewer calculations with Excel spreadsheets, see the related article: “Excel

Spreadsheets for Storm Sewer Hydraulic Design.” For low cost, easy to use spreadsheets for several types of

storm water calculations, including storm sewer hydraulic design, click here to visit our spreadsheet store or

visit our companion site, www.engineeringexceltemplates.com.

References

1. Bengtson, Harlan H., Hydraulic Design of Storm Sewers, Including the Use of Excel, an online, continuing

education course for PDH credit.

des

min

fu l l

des fu l l min

fu l l fu l l2

fu l l fu l l

2

2/3 1/2

2/3 1/2

fu l l fu l l

fu l l fu l l

2

9/20/13 Storm Sewer Hydraulic Calculations with the Manning Equation | Engineering Excel Spreadsheets

file:///Z:/1110/40 Calculations/40.4 Mechanical/Infrastructure Works/Reference/Storm Sewer Hydraulic Calculations with the Manning Equation.htm 3/3

About Harlan Bengtson

Dr. Bengtson has B.S. and M.S. degrees from Iowa State University and a PhD from the University of Colorado, all in Chemical

Engineering. He is now retired after 30 years in engineering education, in teaching and administrative positions. His areas of

expertise are environmental engineering, hydrology, engineering science and renewable energy. He is a licensed Professional

Engineer in Missouri. He did consulting work while in academia and had prior industrial experience. Dr. Bengtson has authored

numerous publications, presentations and technical reports. He is currently active as a technical writer.

View all posts by Harlan Bengtson →

2. McCuen, Richard H., Hydrologic Analysis and Design, 2nd Ed, Upper Saddle River, NJ, Prentice Hall, 1998.

3. Steele, E.W. and McGhee, T.J., Water Supply and Sewerage, New York, NY, McGraw-Hill Book Co, 1979.

This entry was posted in Storm Water Calculations. Bookmark the permalink.

One Response to Storm Sewer Hydraulic Calculations with the Manning Equation

Engineering Excel Spreadsheets

seo hosting says:

April 10, 2011 at 3:51 am

Does anybody know where I can discover free online grant applications?seo hosting

Reply

Proudly powered by WordPress. Install WordPress