34 MINIMUM VELOCITY CONSIDERATIONS
Sediment can reduce the capacity of a storm water pipe over time. In some
installations, it may render the pipe useless until the system can be
cleaned. This is an expensive, timeconsuming undertaking so preventative
measures should be taken during design. Sedimentation is of great concern
in storm sewer applications since large, heavy grit may be present. To
minimize potential problems, flow should be maintained at a minimum, or
selfcleansing, velocity.
Flow velocity can be increased by either increasing the slope of the
pipe or by using a smaller diameter. Modifying either slope or pipe size
requires careful consideration of site factors and flow needs. However,
by using a pipe with a lower Manning’s “n” a smaller diameter
pipe may be selected without adversely affecting capacities and without
modifying the slope of the line.
The potential for settling is determined by the specific gravity and
diameter of particle, its cohesion, flow velocity, and the roughness of
the pipe interior. The formula^{1} for selfcleansing velocity
is as follows:
Equation 35
Where:
V

=

minimum selfcleansing velocity (fps)

B

=

constant equal to 0.04 for clean granular particles or 0.8 for
cohesive material (unitless)

s_{g}

=

specific gravity of the soil particle (unitless)

D_{g}

=

particle diameter (in.)

Or, in metric units:
Equation 35(a)
Where:
V

=

minimum selfcleansing velocity (m/s)

B

=

constant equal to 0.04 for clean granular particles or 0.8 for
cohesive material (unitless)

s_{g}

=

specific gravity of the soil particle (unitless)

D_{g}

=

particle diameter (m)

Soil types vary widely across the nation as well as within states and
counties. Separate calculations in each specific installation may prove
impractical, so an optimum selfcleansing velocity for storm sewers^{2}
is usually accepted to be 3 fps (0.9m/s).
In some specialized installations where sediment is a known problem it
may be wise to perform a soil analysis prior to final drainage design
to determine the parameters necessary for Equation 35 or 35(a). By doing
so, much of the guesswork is eliminated and sedimentation is kept to a
minimum. In each design, a final check should be performed to compare
the expected velocity with the selfcleansing velocity. The design velocity
for fullflowing pipes can be approximated with Equation 36:
Equation 36
The design velocity for storm sewer applications should be a minimum
of 3 fps (0.9m/s) or the value calculated through Equation 35 or 35(a).
MINIMUM VELOCITY CONSIDERATIO