This document provides information about a presentation on hydraulic design of sewers and storm water drains given by Group 2. It introduces storm drains and sewer systems, and discusses the importance of their design. It then covers topics like types of sewerage systems, hydraulic formulae used in design, minimum self-cleansing velocity, hydraulic characteristics of circular and egg-shaped sewers, and the design of storm water drains.

1. Presented By
Group 2 (Sec ‘B’)
Indranil Dutta
TOPIC: Hydraulic Design Of Sewers And Storm
Water Drains

2. 1.Arkadeep Mallick -13001320056
2.Avijit Dandapat-13001320048
3. Anshuman Mandal-13001321073
4.Abhishek Mukherjee-13001320054
5.Nikhil Shukla- 13001320041
6.Amrit Raj-13001320060
7.Anurag Kaushal-13001320059
8.Nilendu Saha-13001321060
9.Jasan Saha-13001321041
10.Dasmeet Singh Saini-13001320046
11.Sachin-13001320055
12.Indranil Dutta-13001321062
13.Sayan Ghosh -13001321077
14.Nibedita Acharjee-13001321075
16.Govinda Jha -13001320057
17.Ritika Nayak- 13001321065
18.Sovan Ghosh-13001321074
19.Arunabh-13001320051
20.Akanksha Suman-13001320053
21.Aashu Saurabh-13001320058
22.Swagata Banik-13001320062
23.Anuprova Sikdar-13001320052
24.Abismaran Dinda-13001321056

3. INTRODUCTION
 A storm drain, is infrastructure designed to drain excess rain and
ground water from impervious surfaces such as paved streets, car
parks, parking lots, footpaths, sidewalks, and roofs. Drains
receive water from street gutters on most motorways, freeways
and other busy roads, as well as towns in areas with heavy rainfall
that leads to flooding, and coastal towns with regular storms.
 Storm drains vary in design from small
residential dry wells to large municipal
systems.

4. Sewer Systems
Sewer systems are designed to drain wastewater that is coming
out from various sources like houses, factories, roads etc. to keep
the environment clean and prevent stagnation of water from
causing diseases and unhygienic conditions.
• Separate Sewer System: In this system, there are two separate
sewers one for domestic and industrial wastewater and the other for
stormwater. The storm water is separately drained out.
• This is preferable in cases where there
is an immediate requirement to collect
domestic and industrial wastewater and
not storm water and also when storm
water does not require any treatment,
but the sanitary sewage does.

5. 1. Combined System- Only one set of sewers carry sewage
and storm water both.
2. Separate System- One set of sewers carry sewage and
another separated set of sewers carry storm water.
3. Partially Separate System- A portion of storm water can
enter the sewage carrying sewer set and another portion
flows through another sewer set.
Types of Sewerage System

6. Water supply pipes Sewer pipes
It Carries pure water without any kind
of solid particles (organic or
inorganic)
It carries contaminated water
containing organic or inorganic solids
which may settle in the pipe.
It carries water under pressure.
Hence, the pipe can be laid up and
down the hills and the valleys within
certain limits.
It carries sewage under gravity.
Therefore it is required to be laid at a
continuous falling gradient in the
downward direction towards outfall
point
Velocity higher than self-cleansing is
not essential, because of solids are
not present in suspension.
To avoid deposition of solids in the
pipes self-cleansing velocity is
necessary at all possible discharge.
Difference between Water supply Pipes and Sewer Pipes

7. Importance of Sewer and SW Drains Design
The sewerage water contains solid particles in suspension;
and the heavier of these particles may settle down at the
bottom of the sewers, as and when the flow velocity reduces,
thus ultimately resulting in the clogging of sewers. In order
to avoid such clogging or silting of sewers, it is necessary
that the sewer pipes be of such a size and laid at such a great
gradient, as to generate the self cleansing velocities at
different possible discharges.
While, Stormwater management is
the effort to reduce runoff of
rainwater or melted snow into
streets.

8. Hydraulic Formulae for Determining Flow Velocities
Sewers of any shape are hydraulically designed as open
channels, except in the case of inverted siphons and discharge
lines of pumping stations. Following formulae can be used
for the design of sewers-

9. This is most commonly used for design of sewers. The velocity of flow through
sewers can be
determined using Manning’s formula as below:
V=1/n (r2/3s1/2)
Where,
v = velocity of flow in the sewer, m/sec
r = Hydraulic mean depth of flow, m = a/p
a = Cross section area of flow, m2
p = Wetted perimeter, m
s= bed slope of the sewer
n = Rugosity coefficient, depends upon the type of the channel surface i.e.,
material and lies between 0.011 to 0.015. For brick sewer it could be 0.017 and 0.03
for stone facing sewers.
s = Hydraulic gradient, equal to invert slope for uniform flows.
1. Manning’s Formula

10. 2. Chezy’s Formula:
V= C r1/2 s1/2
Where,
C= Chezy’s constant
3. Crimp and Burge’s Formula:
V= 83.5 (r2/3 s1/2)
4. Hazen-William’s Formula:
V= 0.85 (CH r0.63 s0.54)
The Hazen-Williams coefficient ‘CH’ varies with life of the pipe and it has
high value when the pipe is new and lower value for older pipes. For example
for RCC new pipe it is 150 and the value recommended for design is 120, as
the pipe interior may become rough with time. The design values of ‘C; for
AC pipes, Plastic pipes, CI pipes, and steel lined with cement are 120, 120,
100, and 120, respectively.

11. Minimum Self-Cleansing Velocity
The velocity that would not permit the sewage from getting stale and
decomposed by moving it faster, thereby preventing evolution of foul
gas is called as self-cleansing velocity.
This minimum velocity should at least develop once in a day so as not
to allow any deposition in the sewers.
The self-cleansing velocity depends on the size of the solid
particles present in the sewage and their specific gravity.
The drag force exerted by the flowing water on
the surface of the channel equals the frictional
resistance. And, we also know that the drag
force or the intensity of tractive force which is
exerted by the flowing water on a channel of
hydraulic mean depth r is given by;

12. τ = γwrs
Hence we got;
γw (G-1)(1-n)tsinθ = γwrs
(G-1)K’t=rs
s= k’/r{(G-1)t}
or, self cleansing inverts slope (s) is given as ;
s= k’/r{(G-1)d’} *As volume per unit area (t) becomes a function of the
diameter of the grain i.e. d’ for single grain.
V=cr1/2
s1/2
, From Chezy’s formula
Hence, self cleansing velocity (VS) is ;
Vs= cr1/2
[ k’/r{(G-1)d’} ]1/2
Or, Vs= c[ k’{(G-1)d’} ]1/2
Using
(1-n)tsinθ= constant (k’)

13. By comparing chezy’s formula and Darcy Weisbach formula we get ;
C= (8g/f’)1/2
Therefore, Vs=[(8g/f’) k’{(G-1)d’} ]1/2.
For removing the impurities mostly present in sewage it is necessary that a
minimum velocity of about 0.45 m/s and an avg velocity of about 0.9 m/s is
developed in sewers.
While designing the sewers, the flow velocity at full design depth is generally
kept at about 0.8 m/s.
*In above equations;
G= specific gravity of the sediment, γw= unit weight of
water
t = volume per unit area,τ = drag force, r= hydraulic
mean depth of the channel, s= bed slope of the
channel, k= dimensional constant, c= chezy’s constant.

14. Hydraulic characteristics of circular sewer running full or partially full
Section of a circular sewer running partially full
A) Depth at partial flow
d = [
𝐷
2
−
𝐷
2
∗ cos(
𝑎
2
)]

15. B) Therefore proportionate depth:
𝑑
𝐷
= 1/2[1 − cos
𝑎
2
]
C) Proportionate area :
𝑎
𝐴
=
𝑎
360
−
𝑠𝑖𝑛𝛼
2𝜋
D) proportionate perimeter:
𝑝
𝑃
=
𝛼
360
E) proportionate Hydraulic Mean Depth:
𝑟
𝑅
= 1 −
360𝑠𝑖𝑛𝛼
2𝜋𝛼
F) proportionate velocity =
𝑣
𝑉
=
𝑁𝑟
2
3
𝑛𝑅
2
3

16. n all above equations except ‘α’ everything is constant (Figure 7.1). Hence,
for different values of ‘α’, all the proportionate elements can be easily
calculated. These values of the hydraulic elements can be obtained from the
proportionate graph prepared for different values of d/D (Figure 7.2). The
value of Manning’s n can be considered constant for all depths. In reality, it
varies with the depth of flow and it may be considered variable with depth
and accordingly the hydraulic elements values can be read from the graph for
different depth ratio of flow.
From the plot it is evident that the velocities
in partially filled circular sewer sections can
exceed those in full section and it is
maximum at d/D of 0.8. Similarly, the
discharge obtained is not maximum at flow
full condition, but it is maximum when the
depth is about 0.95 times the full depth.

17. The sewers flowing with depths between 50% and 80% full need not to be placed
on steeper gradients to be as self cleansing as sewers flowing full. The reason is
that velocity and discharge are function of tractive force intensity which depends
upon friction coefficient as well as flow velocity generated by gradient of the
sewer. Using subscript ‘s’ denoting self cleansing equivalent to that obtained in full
section, the required ratios vs/V, qs/Q and ss/S can be computed as stated below:

18. Hydraulic elements for circular sewer

19. Consider a layer of sediment of unit length, unit width and
thickness ‘t’, is deposited at the invert of the sewer (Figure 7.3).
Let the slope of the sewer is ș degree with horizontal. The drag
force or the intensity of tractive force (Ț) exerted by the flowing
water on a channel is given by:
𝝉 = 𝜸𝒘𝑹𝑺
Where,
𝛾𝑤 = 𝑢𝑛𝑖𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
R = Hydraulic mean depth
S = Slope of the invert of the sewer
per unit length

20. With the assumption that the quantity of tractive force intensity at full flow and
partial flow implies equality of cleansing, i.e., for sewers to be same self-cleansing
at partial depth as full depth:
𝝉 = 𝑻
Therefore , 𝛾𝑤𝑟𝑠𝑠 = 𝛾𝑤𝑅𝑆
Hence, 𝑠𝑠 =
𝑅
𝑟
𝑆
Or,
𝑠𝑠
𝑆
=
𝑅
𝑟
Therefore,
𝑣𝑠
𝑉
=
𝑁
𝑛
∗ (𝑟/𝑅)
2
3 ∗ (𝑠𝑠/𝑆)
1
2

21. Or, by substituting r/R = S/𝑠𝑠
𝑣𝑠
𝑉
=
𝑁
𝑛
∗ (𝑟/𝑅)
1
6
And
𝑞𝑠
𝑄
=
𝑁
𝑛
𝑎
𝐴
(
𝑟
𝑅
)
1
6

22. The Advantages of Circular Sewers
 The perimeter of circular sewer is the least with respect to
the sewer of other shape.
 The inner surface is smooth hence the flow of sewage is
uniform and there is no chance of deposition of suspended
particles.
 The circular sewers are easy to construct.

23. Hydraulic Characteristics of Egg Shaped Sewer
For computing the egg shaped sewer of an equivalent section, the dia
of the circular section (D) is multiplied by a constant factor so as to get
the top horizontal dia width (D') of the egg shaped section.
Thus D’ = 0.84 D
where D' = width of egg shape section
D = dia of circular sewer of the same cross-sectional area, obtained for
passing the requisite discharge.
Knowing D, D' can be easily worked out,
and the dimensions of the egg shaped sewer
are thus established.

24. The hydraulic mean depth of egg shape sewers of equivalent section is
the same as that of the circular sewers when running full, but it is higher
for smaller depths of flow ; and hence the velocity generated in them at
smaller depths of equivalent sewers.

25. Design of storm water drains:
• The design of surface drainage consists of two stages one is a
hydrologic design and then the hydraulic design.
• Hydrologic designs involve the quantification of excess water to
be drained and the rate at which it is to be drained, these two
things we need to determine. Whereas, hydraulic design, involves
calculating the drainage channel geometry and the drainage
network layout.
• The rate of discharge at any point in the
storm drain is not the sum of the design
inlet flow rates of all inlets above that
storm drain section. It is generally less
than this total

26. • For ordinary conditions, storm drains should be sized on the
assumption that they will flow full or practically full under the design
discharge but will not flow under pressure head. The Manning's
Formula is recommended for capacity calculations.
• The exceptions are depressed sections and underpasses where ponded
water can be removed only through the storm drain system. In these
situations, a 50 year frequency design should be used to design the
storm drain which drains the sag point.
• The main storm drain which drains the
depressed section should be designed by
computing the hydraulic grade line and
keeping the water surface elevations
below the grates and/or established
critical elevations.

27. • What does hydraulic design mean? It refers to the calculation of
geometric elements of the drainage such as width, depth, and side
slopes. All these design parameters are estimated by knowing the
maximum carrying capacity of the channel. The geometric
elements of the channel could be rectangular, trapezoidal,
parabolic channels, or semi-circular channels.
• Manning’s roughness coefficient is
important to avoid channels covering and
have enough channel carrying capacity.

28. The Following Procedure to be Followed for
Designing of Drainage:
Step 1: Determine inlet location and spacing
Step 2: Prepare plan layout of the storm
drainage system establishing the following
design data: location of storm drains, direction
of flow, location of manholes, location of
existing utilities such as water, gas, or
underground cables.

29. Step 3: Determine drainage areas and runoff coefficients, and a time of
concentration to the first inlet. Using an Intensity-Duration-Frequency (IDF)
curve, determine the rainfall intensity. Calculate the discharge by multiplying
AxC x I.
Step 4: Size the pipe to convey the discharge by varying the slope and pipe
size as necessary. The storm drain systems are normally designed for full
gravity flow conditions using the design frequency discharges.
Step 5: Calculate travel time in the pipe to
the next inlet or manhole by dividing pipe
length by the velocity. This travel time is
added to the time of concentration for a new
time of concentration and a new rainfall
intensity at the next entry point.

30. Step 6: Calculate the new area (A) and multiply by the runoff coefficient
(C), add to the previous (CA), multiply by the new rainfall intensity to
determine the new discharge. Determine the size of pipe and slope necessary
to convey the discharge.
Step 7: Continue this process to the storm drain outlet. Utilize the equations
and/or nomographs to accomplish the design effort.
Step 8: Check the design by calculating the
hydraulic grade line (HGL).

31. Q) Design an unlined trapezoidal section for the outfall reach of an open
urban storm water drain, drainage a catchment area of 120 hectares. given
the following additional data:
1. Inlet time = 18 minutes
2. Flow time in the upper reaches of the drain = 30 minutes
3. Coefficient of run-off for the area = 0.6
4. Design water surface slope = 1 in 3000
5. The drain has to be designed for a 5 years
rain frequency, and is situated near a place for
which depth duration curves are available, from
which the rainfall for 48 min duration is read out
to be 52 mm for 5 year frequency
6. The drain is to be constructed in cutting with
a maximum permissible flow velocity as 0.9
m/sec.

32. Solution:
As per given data, time of concentration
Tc= Inlet time + flow time
Tc= (18 +30) minutes= 48 minutes
The critical rainfall corresponding to a duration of 48 minutes for 5 years
frequency curve is given to be 52 mm
Therefore, pc=52 mm/ 48 min=6.5 cm/hr
From rational formula the peak storm run off,
Qp=1/36(KpcA)
1/36(0.6)(6.5)(120) m3
= 13 m3

33. Let us now assume the side slopes of the drain as 1H : IV and longitudinal slope
as 1/3000
Using N= 0.025 in Manning's formula, the discharge through this drain is given
as
Q=1/N(AR2/3√S)
Where A=(B+y)y
=(9.0+1.5)1.5
=15.75 sq.m.
P= B+2√2y
=9+2√2×1.5
=9+4.24
=13.24m

34. R= A/P
= 15.75/13.24 = 1.19m
Therefore, Q= 1/0.025 × (15.75) (1.19) % (1/√3000)
= 11.5 × 1.123
=1.293 m3 which is slightly less than the required value of 13 m
Hence, increase the bed width slightly say use B= 9.1m
Therefore, A= (9.1+1.5)1.5 = 10.6×1.5 = 15.9 sq.m.
P= 9.1+2√2×1.5 = 9.1+4.24 = 13.34 m
R= 15.9/13.34 = 1.192 m
Therefore, Q= 1/0.025 × (15.9)(1.192) %
(1/√3000)
= 13.05 m3 which is ok

35. Check for maximum velocity
Velocity generated in the drain at the peak discharge
=Q /A = 13/15.9 = 0.81 m/sec
Which is less than the maximum permissible velocity of 0.9 m/sec
Hence, use a trapezoidal drain section with bed width of
9.1m and water depth of 1.5 m with 1:1 side slopes, laid at
a longitudinal slope of 1 in 3000.

36. Conclusion
Sewerage and SW drain systems and their maintenance, if
neglected, could pose a threat in both community and healthcare
causing infections as well as emergence of multi-resistant bacteria
that could cause unpredictable clinical manifestations. That’s why
we’ve to design the systems very carefully.