Design of Sewer | Environmental Engineering - Civil Engineering (CE) PDF Download

Hydraulic formulas for Determining Flow Velocities in Sewers and Drains

(i) Chezy’s Formula
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where,
C = Chezy’s constant
S = Hydraulic gradient
R = Hydraulic mean depth or Hydraulic Radius
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
for circular sewer of dia D (in Running full)
Discharge,
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where,
A = Flow area of cross-section.
(a) Kutters Formula
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where,
n = Rugosity coefficient
S = Bed slope
(b) Bazin formula
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where, k = Bazin constant.
(ii) Manning’s Formula
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(iii) Crimp and Burge’s Formula

Design of Sewer | Environmental Engineering - Civil Engineering (CE)

This formula is close to Manning’s formula for all those sewer materials for which,
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(iv) William Hazen’s Formula

Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)

Shields Expression for Self Cleansing Velocity

Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)where,
f = Friction Factor
n = Porosity
sin θ ~ tan θ - Slope
⇒ θ = tan-1 (Slope)
G = Specific gravity of solid
d = Avg size of solid.
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
where, k = (1 - n) sin θ

Hydraulic Characteristics of Circular Sewer Free Board Provision
(i) Depth of partial flow
Design of Sewer | Environmental Engineering - Civil Engineering (CE)

Design of Sewer | Environmental Engineering - Civil Engineering (CE)

where,
α = Central angle in degree
d = Depth of partial flow
D  Dia of full section
(ii) Proportionate depth, (d/D)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(iii) Proportionate depth, (a/A)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where,
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(iv) Proportionate perimeter
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where,
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(v) Proportionate Hydraulic Mean Depth
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(vi) Proportionate Velocity
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(a) If
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(vii) Proportionate Discharge
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(viii) For two sewers whose degree of cleansing is same.

Design of Sewer | Environmental Engineering - Civil Engineering (CE)

 i.e.
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(a) Ss/S = R/r
(b) Vs/V = N/n · (r/R)1/6 
(c) qs/Q = N/n ·(a/A) (r/R)1/6 

  • Design of Sewer | Environmental Engineering - Civil Engineering (CE) design to run 1/2 full of Q design.
  • Design of Sewer | Environmental Engineering - Civil Engineering (CE) design to run 2/3 full of Q design.
  • Design of Sewer | Environmental Engineering - Civil Engineering (CE) design to run 3/4 full of design.

where, ϕ Dia of sewer.
Qdesign = Design discharge.
Sewer is designed for Maximum Hourly Discharge
(i) Maximum hourly discharge
= 1.5 (Maximum daily discharge)
(ii) Maximum daily discharge
= 2.0 (Avg daily discharge)
(iii) Maximum hourly discharge
= 3.0 (Avg daily discharge)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)

Estimation of Peak Drainage Discharge
(i)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where, k = Runoff/rainoff
Suffix 1,2, … n used for k is runoff of 1,2, ….n.
A = Area.
(ii)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)
(iii)

Design of Sewer | Environmental Engineering - Civil Engineering (CE)
Where,
Maximum rainfall intensity of a particular frequency having duration equal to time of concentration.
po = (Point rainfall intensity of same frequency as PC) × Area dispersion factor. (cm/hr)
Tc = Time of concentration in hr.
(iv) 

Design of Sewer | Environmental Engineering - Civil Engineering (CE)
where,
QP = Peak discharge in m3/sec.
A = Catchment area in (Hectare)
Critical rainfall in (cm/hour)
Design of Sewer | Environmental Engineering - Civil Engineering (CE)

The document Design of Sewer | Environmental Engineering - Civil Engineering (CE) is a part of the Civil Engineering (CE) Course Environmental Engineering.
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FAQs on Design of Sewer - Environmental Engineering - Civil Engineering (CE)

1. What is the purpose of sewer design in civil engineering?
Ans. The purpose of sewer design in civil engineering is to establish an efficient and safe system for collecting and transporting wastewater and stormwater away from urban areas. This helps to prevent flooding, protect public health, and maintain the cleanliness of water bodies.
2. How is the sewer system designed in civil engineering?
Ans. The sewer system is designed in civil engineering by considering factors such as population density, land topography, rainfall patterns, and future growth projections. Engineers use hydraulic calculations to determine the size and slope of sewer pipes, as well as the location of manholes and pumping stations, to ensure effective wastewater flow.
3. What are the different types of sewer systems used in civil engineering?
Ans. There are primarily two types of sewer systems used in civil engineering: combined sewers and separate sewers. Combined sewers collect both wastewater and stormwater in a single pipe, while separate sewers have separate pipes for wastewater and stormwater. The choice of sewer system depends on factors such as local regulations, infrastructure availability, and environmental considerations.
4. What are the challenges in sewer design for civil engineers?
Ans. Civil engineers face several challenges in sewer design, including determining the appropriate pipe size to handle the anticipated flow, accommodating varying topography and ground conditions, addressing potential infiltration and inflow issues, and ensuring the system's resilience to climate change and extreme weather events. Additionally, engineers must consider factors such as construction costs, maintenance requirements, and the impact on existing infrastructure.
5. How do civil engineers ensure the longevity of sewer systems?
Ans. Civil engineers ensure the longevity of sewer systems by incorporating design elements that minimize corrosion, erosion, and structural failures. This includes selecting durable materials, such as corrosion-resistant pipes, and implementing appropriate maintenance and inspection programs. Engineers also consider factors like proper slope, hydraulic efficiency, and capacity for future growth to ensure the longevity and effectiveness of the sewer system.
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