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Separate Sanitar& and stormwater sewers&stems


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Stormwater sewer s&stem


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)&pes of inet


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)&pes of inet


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)&pica Storm Sewer with *+L and "+L


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Design of Sewer Network

A sewer system is a network of pipes usedto convey storm runo and/or wastewaterin an area.

The design of sewer system involves thedetermination ofdiameters,

slopes, and

crown or invert elevations for each pipe in thesystem


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Design Criteria

Storm water uantities to !e transported aredetermined with the "ational method #othermethods are availa!le as well$.%ndicate what design freuency #return period$is usedDetermine the rainfall intensity & durationcurve for the reuired freuency%ndicate runo coe'cients to !e used in"ational method(enerally )anning*s formula is used for thedesign of storm sewers


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Constraints and assumptions

To prevent or reduce e+cessive deposition ofsolid material in the sewers, a minimumpermissi!le ow velocity at design dischargeor at !arely full&pipe gravity ow is speci-edTo prevent scour and other undesira!leeects of high&velocity ow, a ma+imumpermissi!le ow velocity is also speci-edAt any unction or manhole, the downstreamsewer cannot !e smaller than any of theupstream sewers at that unctionThe sewer system is a dendritic, or !ranching,network converging in the downstreamdirection without closed loops


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,ain Speci-cations

)inimum velocity #self&cleansing velocity$ 0.1 m/s

)a+imum velocity 2.3 m/s

)inimum pipe diameter 430 mm.

5nlike sanitary sewers, storm sewers are allowed toow full.

Storm sewers are even allowed to have surcharge#pressuri6ed ow$ as long as energy grade lineremains !elow a certain level #as per localspeci-cations$ with respect to ground surface.


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Design Steps

Step . / )opographica map7!tain or develop a map of the contri!uting areaAdd location and level of e+isting or proposed detailssuch as8

Contours

9hysical features #e.g. wadis$ "oad layout :uildings Sewers and other services 7utfall point #e.g. near lowest point, ne+t to

receiving water !ody$


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Design Steps

Step 0 / Preiminar& hori1ontaa&o!tSketch preliminary system layout #hori6ontalalignment$8

locate pipes so that all potential users canreadily connect into the system

try to locate pipes perpendicular to contours try to follow natural drainage patterns

locate manholes in readily&accessi!le positions

Step 2/ Preiminar& sewer si1ing;sta!lish preliminary pipe si6es and gradients


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Design Steps

Step 3 / Preiminar& vertica a&o!tDraw preliminary longitudinal pro-les #verticalalignment$8

;nsure pipes are deep enough so all users can

connect into the system try to locate pipes parallel to the ground surface ensure pipes arrive a!ove outfall level avoid pumping if possi!le

Step 5 / 4evise a&o!t"evise the hori6ontal and/or vertical alignment tominimi6e system cost !y reducing pipe8

Documents

Design Stormwater Network