Design of Sewers
- Data regarding slope, coefficient of roughness, discharge in the sewer, depth of flow in the sewer etc;
- Using the following Manning’s formula (v=1/n m2/3 i1/2), the diameter of the sewer is calculated.
(Q/4d2) = 1/n (d/4) 2/3 i1/2 (for half full condition)
3. After determining value of “D” the velocity is calculated by susing the following Manning’s formula, V = 1/n (d/4)2/3 i1/2
4. If the calculated velocity is within the available range. Then the design is O.K. if not the design should be revised.
1. Design period = 20 years
2. For this capacity of sewer may be increased by 25%
3. In separate system, carrying capacity = 3 to 6 times. Dry weather flow.
4. In case of combined system, carrying capacity = 2 times total flow.
5. Sewers are normally designed to flow third running full when discharging the maximum flow.
6. The minimum velocity for self-cleaning is 0.6 m/s in case of separate system; 0.75 m/s in case of combined system.
7. The maximum velocity is 3.0 m/s for both the systems.
8. The separate system is best suited for Indian conditions.
Consider a circular sewer flowing half full and carrying a discharge of 0.5 cumecs. Assume slope = 1 in 450, coefficient of roughness = n =0.012.
Velocity of flow V = Q/A
From Manning’s formula V = 1/n m2/3 i1/2
\V = [Q/1/2´ (ÕD2)/4] = 1/n (D/4)2/3 i1/2
[m = A/P = D/4; for circular flowing half full]
Substituting the given values.
[0.5/(1/2)´(Õ/4) D2] = (1/0.012) ´(D/4) 2/3 ´(1/450) 1/2
Þ D = 0.92 m.
CHECK FOR VELOCITY:
V = (1/0.012) ´ (0.92/4) 2/3 ´ (1/450) 1/2
= 0.74 m/s > 0.6 m/s (minimum velocity for self cleaning)
< 3.0 m/s (maximum velocity)