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Saturday, December 20, 2014

3-4 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, time-consuming 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 self-cleansing, 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 formula1 for self-cleansing velocity is as follows:

Equation 3-5

Where:

V

=

minimum self-cleansing velocity (fps)

B

=

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

sg

=

specific gravity of the soil particle (unitless)

Dg

=

particle diameter (in.)

Or, in metric units:
Equation 3-5(a)

Where:

V

=

minimum self-cleansing velocity (m/s)

B

=

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

sg

=

specific gravity of the soil particle (unitless)

Dg

=

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 self-cleansing velocity for storm sewers2 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 3-5 or 3-5(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 self-cleansing velocity. The design velocity for full-flowing pipes can be approximated with Equation 3-6:

Equation 3-6

The design velocity for storm sewer applications should be a minimum of 3 fps (0.9m/s) or the value calculated through Equation 3-5 or 3-5(a).

 

MINIMUM VELOCITY CONSIDERATIO