figure of 2 gallons per minute per

right hand sides of the chart are flow

1,000 feet of sewer may be assumed for

rates. MGD stands for million gallons

infiltration.

Of

course

if

any

per day, gpm is gallons per minute,

information on infiltration in the

and cfs is cubic feet per second.

area is available from other sources

Along the top and bottom of the chart

(such as sewer systems already in

is slope in feet per hundred which is

operation nearby) then that value

also percent.

Inside the chart are

should be used.

lines moving up and to the right.

These lines are different pipe sizes

and range in size from 4-inch to 48-

inch

diameter.

The

lines

Since the easiest excavation is along

perpendicular to the pipe size lines

the natural ground slope, the sewer

are velocity lines and range from 1

system is most easily placed along the

foot per second (fps) to 12 feet per

slope of the ground.

This slope is

second.

The pipe chosen must handle

usually used as a first estimate for a

at least as great a flow at the chosen

pipe

slope.

If

this

slope

is

slope as the actual flow found in 13b.

unacceptable (usually because it is

To find a pipe size which will do

obviously too small or too great, or

this, enter the chart on the left or

because it does not give an acceptable

right side at the actual flow rate.

velocity) a new slope must be tried.

Draw a horizontal line until the

It is often at this point that drop

chosen slope is intersected. The pipe

manholes should be considered.

size below this point of intersection

is too small.

The pipe above this

point must be used.

As an example,

find a pipe size to handle an actual

Once a slope is chosen, the pipe size

flow of 300 gallons per minute at 1-

necessary can be determined by use of

percent slope. Enter the chart at 300

formulas or charts. The formulas that

gallons

per

minute

on

the

right

are most commonly used for sewer

and

move

horizontally

to

the

design are Kutter's formula and the

left

until

the

1-percent

slope

Manning formula.

The form of the

line

is

intersected.

The

point

Manning formula is somewhat simpler,

of intersection is above the 6-inch

but the results less accurate for pipe

diameter

pipe

and

below

the

sizes smaller than 12 inches.

Figure

8-inch pipe.

Therefore, 6-inch pipe

14 is a pipe-flow chart based on

cannot handle 300 gallons per minute

Kutter's

formula.

It

should

be

at

a

1-percent

slope

and

8-inch

understood that this chart is for

diameter

pipe

must

be

used.

pipes running full.

The left and

Integrated Publishing, Inc. |