Hydraulics of Flow in Sewers

HYDRAULICS OF FLOW IN SEWERS

Factors that influence flow of sewage in sewers.

• Slope of sewer.

• Geometry of sewer.

• Roughness of interior surface of sewer.

• Bends, transitions, obstructions etc.

• Flow conditions.

• Characteristics of sewage.

Hydraulic Formulae:

(i) Chezy's Formula:

V=C √R.S

V = Velocity of flow (m/s)

S = Hydraulic gradient of sewer

R = Hydraulic mean radius (m) = A/P

C = Chezy's constant

A = Area of cross-section (m²)

P = Wetted perimeter (m)

Chezy's constant C is found using Bazin's formula:

Q=AV

Q = Discharge in m^3/s.

(ii) Kutter's Formula:

R = Hydraulic mean radius

S = Slope

N = Rugosity co-efficient (depends on nature of inner surface of sewer)

When N decreases, diameter increases.

Example: For Cement concrete sewer of 600 mm, N = 0.013.

(iii) Bazin's Formula:

K - Bazin's constant (Depends on nature of inner surface of sewers).

(iv) Manning's Formula:

Velocity of flow,

where N, R, S have same meaning as given by Kutter's formula.

(v) Crimp and Burge's Formula:

V=83.5 R2^/3 S^1/2

(Similar to Manning's formula where 1/N =83.5 or N = 0.012).

Example: For a circular pipe,

(vi) William-Hazen's Formula: For flow under pressure for designing water pipes.

V=0.85 C R^0.63 S^0.54

C value depends on the type of pipe material.

Note: Apart from the above formulae; Nomograms, Tables, Charts are also available for designing sewers.