Sewer Design

SEWER DESIGN

 

Hydraulic Elements of Circular Sewers

Advantages of Circular Sewer Sections:

(i) Easy to manufacture.

(ii) Efficient Section: A circular section gives the highest H.M.D. (Hydraulic Mean Depth) when running full (or) half full. When depth is more, velocity and discharge will be high.

(iii) It is the most economical section since it utilises minimum quantities of material.

(iv) Circular section have uniform curvature all around and it offers less opportunities for deposits.

Sewers should run atleast half full.

 

a) Circular section running full:

b) Circular Sewer running partially full:

Let d be the depth at partial flow and 0 be the central angle subtended.

Let a = Area of cross section.

p = Wetted perimeter.

r = Hydraulic mean depth.

V = Velocity of flow.

 

(i) Depth at partial flow:

(ii) Area of cross-section running partially full:

(iii) Wetted Perimeter:

(iv) Hydraulic Mean Depth (HMD):

(v) Velocity of Flow:

(vi) Discharge running full:

❖ When N/n does not vary with depth of flow and when sewers flow more than half full, velocity in partially filled circular sewer exceeds those in full section.

❖ Maximum velocity achieved when depth of flow is 0.81 times full depth. (12.5% greater than when running full).

❖ Maximum discharge obtained NOT when sewer is running full but when depth is 0.95 times full depth (7% greater than when running full).

(vii) Partial Depth Self Cleansing Flow:

Sewers flowing 0.5 to 0.8 times full need not be placed on steeper gradients to achieve self-cleansing velocity,

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q, discharge when sewer are partially full.

Q = discharge when sewers are running full.

 

Ventilation of Sewers:

Sewers are not designed to run full. Sewers are always designed to flow half or three- fourth for the purpose of ventilation of foul gases.

❖ Small sewers (< 0.4 m ) - designed for flow at ½ depth.

❖ Sewers (0.4 m to 0.9 m) - designed for flow at 2/3 depth.

❖ Large sewers (ultimate peak discharge) - designed for flow at 3/4 depth.

 

Egg-Shaped Sewers (Ovoid Sewers)

Advantages:

 (i) Used in combined sewerage system where discharge (flow) highly varies. There will be heavy flows during rainy season and only 5 to 10% flow (dry weather flow) during summer. Egg-shaped sewers provide greater depth (than circular) during low flow conditions.

(ii) Gives higher velocity for low flows than circular sewers of equal capacity.

Disadvantages:

(i) Unstable section. Small end (down) has to support broader upper sections weight.

(ii) More difficult to construct, expensive, material required is more.

(iii) Self cleansing velocity requires suitable gradient.

 

Two Forms:

i) Standard (or) Metropolitan section.

ii) New shaped section. Sveidos

 

Hydraulic Elements of Standard Form:

a) off Sewer running full:

Area of crown portion = 1.57 b²

Area of central portion = 2.80 b²

Area of invert portion = 0.23 b²

Total Area = = 4.6 b²

Similarly, Total Perimeter = 7.82b

R=H.M.D. = 0.58b

 

b) Sewer running 2/3rd full:

Total area = 3.03b²

Area of central portion = 2.80b²

Area of invert portion = 0.23b²

Total perimeter = 4.68b.

H.M.D= R = 0.64b.

 

c) Sewer running half-full and one-third full:

At running half full, R = 0.54b.

At running one-third full, R = 0.41b.

 

Hydraulic elements of the New Form:

a) Sewer running full:

Area of crown portion = 1.57 b²

Area of central portion = 2.86 b²

Area of invert portion = 0.03 b²

Total area                       = 4.46 b²

Perimeter = 7.82 b

H.M.D, R = 0.57 b.

 

b) Sewer running 2/3 rd full:

c) Section running one-third full:

R=0.38b

 

Hydraulically equivalent Circular Sewers:

• The design of egg-shaped sewers is complicated.

• The computations of various hydraulic elements (such as area, wetted perimeter, hydraulic mean depth, etc.) of egg-shaped sewers involves complicated mathematical calculations.

• Therefore while designing egg-shaped sewers, approximate diameter of hydraulically equivalent circular sewer is calculated first, with the same discharge (running full) and laid at same gradient, and then converted into dimensions of egg-shaped section.

Let D = Diameter of hydraulically equivalent circular section.

D₀ = Top horizontal diameter of egg shaped section.

 

Other Sewer Sections

In soft soils with difficulty of providing foundations for circular/ovoid sections,following  shapes are used.

• Semi-elliptical

• Rectangular

• Horse-shoe

• U-shaped.

• Parabolic

 

Shapes of sewer Pipes

Sewers are generally circular pipes laid below ground level, slopping continuously towards the outfall. These are designed to flow under gravity. Shapes other than circular are also used.

Other shapes used for sewers are:

a. Standard Egg-shaped sewer

b. New egg-shaped sewer

c. Horse shoe shaped sewer

d. Parabolic shaped sewer

e. Semi-elliptical section

f. Rectangular shape section

g. U-shaped section

h. Semi-circular shaped sewer

i. Basket handled shape sewer

Standard egg-shaped sewers, also called as ovoid shaped sewer, and new or modified egg-shaped sewers are used in combined sewers. These sewers can generate self cleansing velocity during dry weather flow. Horse shoe shaped sewers and semi-circular sections are used for large sewers with heavy discharge such as trunk and outfall sewers. Rectangular or trapezoidal section is used for conveying storm water. U-shaped section is used for larger sewers and especially in open cuts. Other sections of the sewers have become absolute due to difficulty in construction on site and non availability of these shapes readily in market.

Shape of sewer depends on:

(i) Hydraulic (flow) conditions.

(ii) Construction Ease.

(iii) Foundation conditions.

(iv) Availability of space.