DESIGN PROCEDURE:
- 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.
DESIGN CRITERIA:
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 half to
two 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)
Hence O.K.