Storm Drain (Pipe) Sizing

Storm Drain (Pipe) Sizing

General Goals:

• List and describe drainage design steps

• Size storm drains in a drainage system

• Judge adequacy of drainage system design

Specific Objective:

• Determine (quantify) size of pipe needed to

accommodate peak discharge to a given drain

Drainage Elements: Storm Drain Pipes CVEEN 4410 - Engineering Hydrology

Constraints Hydraulic Design Procedure Objectives Pipe

Characteristics

1. Determine contributing area to upstream end of

drain

2. Determine tc and runoff coefficient for

contributing area

• Determine design rainfall intensity for tc

• 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)

CVEEN 4410 - Engineering Hydrology Drainage Elements: Storm Drain Pipes


Characteristics

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



Characteristics

Local ordinances often require the size of storm

drains to not decrease in the downstream direction

When making a

transition at a

junction, the

vertical

alignment of the

incoming and

outgoing pipes

should be such

that the crowns

line up (not the

inverts) Invert = elevation of the lowest level of the pipe at a specific point along the run of the pipe



Characteristics Contributing Area and tc

Side note:

Ensure that

you account

for tc and

contributing

area

properly!

Area 2

Area 3



Characteristics

After the peak discharge has been computed

you must select an appropriate pipe material,

shape, slope, and length

One approach:

Assume a pipe size, then determine the capacity

of that pipe using standard design charts

(charts 35 - 51 in file “hds3.pdf” - downloadable

from

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_num

ber=4&id=9)

If the capacity is inadequate alter the size of the

pipe and recheck the capacity



Characteristics

OR:

Given a design flow, the exact pipe dimension to

convey the flow can also be determined using

Manning’s equation, Darcy-Weisbach equation, or

numerous other equations, or software:

Manning:

Darcy-Weisbach:

where d = minimum design pipe diameter

n = Manning’s roughness coefficient

Kn = constant; 1.0 for SI, 1.49 for USCS

Qp = design peak discharge

So = local slope

f = friction factor

g = gravitational constant



Characteristics

The final step in designing a storm drain system is

to check the HGL

Computing the HGL will determine the elevation,

under design conditions, to which water will rise

in various inlets, manholes, junctions, etc.



Characteristics

1. When hydraulic calculations do not consider minor losses

(e.g., expansion, contraction, bend, junction), the elevation

of the HGL for the design flood should be at least 1.0 ft

below ground elevation

2. If all minor losses are accounted for, it is usually

acceptable for the HGL to reach the gutter elevation

3. Minor losses should be included when the velocity

exceeds 6 ft/s






Characteristics

In-Class Exercise…see you in class!

Documents

Storm Drain (Pipe) Sizing