Flow Over Streamlined Weir

 

 Quaid-e-Awam University of Engineering, Science and Technology Department of Civil Engineering ydraulics ! "rrigation #a$oratory %&'ECT(

Characteristics of flow over streamlined weir

A))A*ATUS(

Self-contained glass sided Tilting Tilting flume, streamlined weir, Instrument

TE%*+(

carrier, Hook and point gauge, Q = 1!"# Cd $H%&'

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( structure, used to dam up a stream or river, over which the water flows, is called a weir ( weir is $asicall) an o$struction in an open channel flow path *e *eirs irs are commonl) used for measurement of open channel flow rate ( weir functions $) causing water to rise a$ove the o$struction in order to flow over it The height of water wa ter a$ove the o$struction correlates with the flow rate, so that measurement of the height of the flowing water a$ove the top of the weir can $e used to determine the flow rate through the use of an e+uation, graph or ta$le

A))E . C*EST(

The $ottom edge, over which the li+uid flows or top part of weir is k known nown as sill or crest of  the notch or weir and the sheet of li+uid flowing over a notch or a weir is known as nappe or vein/

T+)ES %0 E"*( A1 %n the &asis &asis of of the width width of of crest crest 21 &roa &road d Cres Creste ted d ei eir( r(

If the height of water a$ove the weir crest is not greater than two times of the width of the crest of weir, the weir is called a .road Crested weir /ost weirs in rivers are effectivel) $road crested, although with man) variations in shape/ ( $road-crested $road-crested weir is a flat-crested structure, with a long crest compared to the flow thickness *hen the crest is 0$road0, the streamlines $ecome parallel to the crest invert and the pressure distri$ution distri$ution a$ove the crest is h)drostatic h)drostatic .road crested weirs are ro$ust structures that are generall) constructed from reinforced concrete and which usuall) span

 

 Quaid-e-Awam University of Engineering, Science and Technology Department of Civil Engineering ydraulics ! "rrigation #a$oratory

the full width of the channel The) are used to measure the discharge of rivers, and are much more suited for this purpose than the relativel) flims) sharp crested weirs This t)pe of weir is commonl) used in the gauging of discharge in open channels, particularl) where accurac) and relia$ilit) are re+uired to $e com$ined with ease of construction and maintenance It is further divided into two t)pes i1 *ectangular Sharp Cornered eir ii1 *ectangular Streamlined eir 31 arr arrow ow Cre Creste sted d e eir(

If the height of water a$ove the weir crest is greater than two times of the width of the crest of weir, the weir is called a 2arrow 2 arrow Crested weir ( narrow crested weir is h)draulicall) similar to an ordinar) weir or to a rectangular weir

&1 %n the &asis &asis of of the natur naturee of crest crest 21 Sharp Sharp Cre Creste sted d ei eir( r(

It is a special t)pe of weir having a sharp-crest as shown in figure a The water flowing over the crest comes in contact with the crest line and then springs up from the crest and falls as a tra3ector) ( sharp-crested weir allows the water to fall cleanl) awa) from the weir In a sharp-crested weir, the thickness of weir is kept less than half of the height of water on the weir, ie,

31 %gee e eir( ir(

  It is another special t)pe of weir, generall) used as a spillwa) of a dam as shown in figure The crest of an ogee weir slightl) rises up from the point  A ie, crest of the sharpcrested weir and after reaching the ma4imum rise of where is the height of the water a$ove the point  A falls in a para$olic form as shown in figure d

 

 Quaid-e-Awam University of Engineering, Science and Technology Department of Civil Engineering ydraulics ! "rrigation #a$oratory C1 Types Types of weirs $ased $ased on Effect of of the sides on the emerging emerging nap nappe pe •

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*eir with end contraction contracted weir *eir *eir *e ir without end contraction suppressed weir

D1 %n the &asis &asis of 4eomet 4eometrica ricall Shape Shape 21 *ecta *ectang ngula ularr eir( eir( 31 56-n 56-not otch ch11 weir weirs( s( 71 Tra rape8 pe8oida oidall 5Cipol 5Cipollett letti1 i1 weirs( weirs(

 

 Quaid-e-Awam University of Engineering, Science and Technology Department of Civil Engineering ydraulics ! "rrigation #a$oratory )*ACT"CA# A))#"CAT"%S %0 E"*( •

5sed to alter the flow of rivers to prevent flooding

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5sed for the measurement of discharge of flow

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Help rivers naviga$le

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6roper distri$ution of water carried $) a main canal among the $ranch canals depending upon it

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7educing the h)draulic slope gradient in a canal if canal water slope is greater than the allowa$le water slope

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7educing head on e4isting structures

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Collecting sediments at u&s of structures sand strap

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Slowing of water velocities and reducing erosion $) directing water flow through weir into stilling $asin where the energ) of flow is dissipated

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8ischarge through $ridges openings especiall) afflu4 and $ack water can $e determined *eirs *e irs have traditionall) $een used to create mill ponds p onds

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Can $e used for fishing purpose

 

 Quaid-e-Awam University of Engineering, Science and Technology Department of Civil Engineering ydraulics ! "rrigation #a$oratory

)*%CEDU*E( 9nsure the flume is hori:ontal with the down stream weir at the $ottom of its travel 6osition the streamlined $road crested weir and introduce water into the flume until it discharges over the weir Stop the flow of water into the flume and when the water stops flowing over the weir, set the hook 

and point gauge some wa) upstream from the weir and o$tain the water datum reading (d3ust flow into the flume to o$tain heads, H, increasing in a$out 1"mm steps ;or each step measure Q, du and dc and ta$ulate the results 2ote the water profiles   *ESU#TS AD CA#CU#AT"%S(

du  

$=

dc

H

Q

H%&'

.readth of weir