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### Storm Drain Pipe Size Requirement Formula and Calculator

Civil Engineering and Design

Fluid Flow Hydraulic and Pneumatic Engineering and Design Menu

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.

Preview: Storm Land Drain Pipe Sizing Calculator

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 / K_{n} ) · ( Q_{p} / ( S_{o}^{0.5} ) ] ^{3/8}

Formula 2.0, Darcy-Weisbach Equation:

*d = [ 0.811 · ( f / ( g · S _{o} ) Q_{p}^{2} ]^{1/5}
*

Where:

d = minimum design pipe diameter, ft (m)

n = Manning’s roughness coefficient,

*K _{n}* = constant; 1.0 for SI, 1.49 for USCS,

*Q*= design peak discharge, ft

_{p}^{3}/s (m

^{3}/s)

*S*= local slope, %

_{o}f = friction factor,

g = gravitational constant ( 32.17405 ft/s

^{2}, 9.80665 m/s

^{2})

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:

- Rainfall Runoff Rate Formulas and Calculator
- Pipe Roughness Coefficients Table Charts | Hazen-Williams Coefficient | Manning Factor
- Darci's Equation Fluids Flow Equation
- Head Loss Darcy - Weisback Equation
- Head Loss Darcy Calculator - Weisback Equation
- Fluid Flow Regimes Review
- Orifice Plate Flow Calculations and Design
- Pressure Drop Along Pipe Length - Fluid Flow Hydraulic and Pneumatic
- Function for open channel flow rate with input variable for inflow

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