Velocity at Minimum Flow

In order to prevent settlement of sewage solids, the sewers are designed for minimum flow velocity. The minimum velocity at which no solid gets deposited at the bottom of sewer is called self cleansing velocity. The self cleansing velocity in sewer is determined by Shields formula:

V = 1/n × R^{2/ 3} × (K_s × (S_s - 1) d_p) ^{1 / 2}

In which

S_s = Specific gravity of particle,

d_p = Particle size,

Ks = A dimensionless constant with a value of about 0.04 to start motion of granular particles and about 0.8 for adequate self cleansing of sewers,

R = Hydraulic radius of the sewer, and

n = Manning's coefficient.

The Shields formula indicates that velocity required to transport material in sewers is only slightly dependent on conduit shape and depth of flow but mainly dependent on the particle size and specific weight. It is a practice to design sewer for self cleansing velocity at ultimate peak flow to be achieved at the end of design period. This is done on the assumption that although silting might occur at minimum flow, but the same would be flushed out during the peak flow.