Lec 2(Design of Sewer System).pptx

8/9/2019 Lec 2(Design of Sewer System).pptx

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Step of Design of Sewer

System

Lecture 2


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Steps

1. Preliminary Investigations

2. Design Criteria Considerations

3. Actual Design. Preparation of Drawing and !"#

$. Su%se&uent 'odi(cation


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1.Preliminary Investigation

• "%tain maps and drawings t)at furnis)information of t)e area

1. Population Density

2. *ater consumption3. Soil c)aracteristics + ,atural slope

•. 'aps s)ould also )ig)lig)t t)e location ofstreets- pars - %uildings etc.

Note: Sewerage systems are operatedunder gravity where as water supplysystems are operated under pressure


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• If a map or needed information is notavaila%le conduct a detailed survey of t)e

area to do t)e needful• /sta%lis) !enc) 'ars t)roug)out t)e area

and mae contours pro(les and crosssections

• 0se a dierent scales e.g 11444 to13444 for maing map5


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• Soil conditions s)ould %e investigated for

t)e type of stratum

• Location of water ta%le

• Presence of any underground roc

• Collection of rainfall and runo data.

• Study of natural slopes of t)e area and

selection of a suita%le disposal point.


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2.Design CriteriaConsideration

15Design 6low 7 Calculation of avg. sewage 8ow ont)e %asis of water consumption and t)e population

at t)e end of design period

a5Sanitary Sewer

# design9 Pea sewage 8ow : In(ltration

%5 Partially Com%ined Sewer

# design9 Pea sewage 8ow :Storm 8ow:

In(ltration

*ASA Criteria (Peak sewage fow = Storm fow)

# design9 2 ; Pea sewage 8ow : In(ltration


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25 Design /&uation


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35'inimum Self cleansing velocity

• Sewage s)ould 8ow at all times wit)

suBcient velocity to preventsettlement of solid matter in t)esewer

• Sel cleansing velocity is minimumvelocity t)at ensures non


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5'a;imum >elocity

S)ould not %e greater t)an 2!m"s


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Fa%le Commercially availa%le diameters of sewer in mm


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5 'inimum cover.

< 'inimum 1 m eart) cover on sewer crown toavoid damage from live loads

E5'an)oles


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6ig 'an)ole


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5 Direction of Sewer lines


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J5 # d@# f ?atio

• # d9 Design 6low

• # f 9 6low w)en sewer is 8owing fullIn order to provide air space in t)e upper portionof sewers for ventilation purposes *ASA

recommends to maintain t)e following ratios orsanitary sewer

Sewer SiGe

22$


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3. Actual Design of Sewer

• SiGe of sewer 0sing #9A> for thecalculation of diameter

• Slope of sewer 0sing manningformula

> 9 1@n ?2@3 S1@2

used for either calculation of slope or checkingvelocity


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. Preparation of drawings and

!"#s

• +ypical drawing includes


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6ig Sewer Pro(le


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-ill o.uantity


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F)an Mou

Lec 2(Design of Sewer System).pptx

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