flow velocity.

Therefore the actual

invert at the lower manhole is the

velocity

can

be

determined

by

elevation of the invert of the upper

multiplying the velocity ratio by the

manhole less the product of the slope

full

velocity.

Continuing

the

multiplied by the length of the sewer

example, the full flow velocity was

between

manholes.

The

invert

found to be 3.2 feet per second and

elevations of the upper manhole will

the velocity ratio 1.045.

Therefore

be known for each section except the

the actual velocity will be (3.2 x

first.

The invert at the first

1.045) = 3.34 feet per second.

The

manhole will usually be made as close

actual velocity must be between 2 and

to the ground level as possible while

10 feet per second.

If lampholes are

still maintaining adequate cover.

to be used, then the velocity must be

at least 3 feet per second.

Figure

16

shows

a

plan

view

f. Invert

elevations.

Invert

and

profile

of

an

area

for

elevations can be determined once the

which

a

sewage

collection

slope is known.

The elevation of the