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### Storm Drain Pipe Size Requirement Formula and Calculator

Storm Drain Pipe Sizing Formula and Calculator

The following formulas can be used to quantify the size of pipe required to accommodate the peak discharge to a given yard or industrial drain system.

1) Determine contributing area to upstream end of drain,

2) Determine runoff for contributing area

• Determine design rainfall intensity for the area to be drained,
• Determine peak discharge (typically using Rational Method)
• Size pipe for peak discharge and given constraints (typically using Manning’s equation), assume pipes will flow full under design discharge but not be placed under pressure head
• Repeat process for downstream drains (account for increasing contributing area),

Design considerations:

• Velocities should be > 2.0 ft/s to avoid siltation and < 10-15 ft/s to avoid damage to system components,
• Minimum slope of 0.5% is often required,
• Head losses should be minimized,
• Minimum drain size is also usually specified in local drainage ordinances; 12-15 inches is a common minimum for street collection culverts,
• Local ordinances often require the size of storm drains to not decrease in the downstream direction.

Figure 1.0

Formula 1.0, Manning Equation:
d = [ 3.208 · ( n / Kn ) · ( Qp / ( So0.5 ) ] 3/8

Formula 2.0, Darcy-Weisbach Equation:
d = [ 0.811 · ( f / ( g · So ) Qp2 ]1/5

Where:

d = minimum design pipe diameter, ft (m)
n = Manning’s roughness coefficient,
Kn = constant; 1.0 for SI, 1.49 for USCS,
Qp = design peak discharge, ft3/s (m3/s)
So = local slope, %
f = friction factor,
g = gravitational constant ( 32.17405 ft/s2, 9.80665 m/s2 )

To determine your peak discharge requirements see:
Rainfall Runoff Rate Formulas and Calculator

Typical Manning's Roughness Coefficients

Concrete Box Culverts n = 0.012,
Concrete Pipes n = 0.012

Metal Pipes: Pipe and Pipe Arch - Helical Fabrication Re-corrugated Ends - All Flow Conditions
12” to 24” n = 0.020
30” to 54” n = 0.022
60” and larger n = 0.024

Pipe and Pipe Arch - Spiral Rib Fabrication Re-corrugated Ends - All Flow Conditions* All Sizes n = 0.012

Plastic Pipes:
Polyvinyl Chloride-PVC (external rib/smooth interior), All Sizes n = 0.012
Polyethylene (All Sizes) Single Wall n = 0.024, Double Wall (Smooth) n = 0.012
Polypropylene (All Sizes) Single Wall n = 0.024
Double & Triple Wall (Smooth) n = 0.012

Related:

References:

• Design Charts for Open - Channel Flow, Federal Highway Administraton Hydraulic Design Series NO. 3
• Hydraulic Engineering Circular No. 22, Third Edition Urban Drainage Design Manual, National Highway Institute