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Weirs Weirs

Weirs - PowerPoint Presentation

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Uploaded On 2016-03-11

Weirs - PPT Presentation

Generally any flow obstruction that causes water to rise to flow over it but used exclusively for intentional obstructions Uses include flow measurement sharpcrested weirs and control of water surface profile eg by inducing supercritical flow broadcrested weirs ID: 251957

weirs weir crested water weir weirs water crested notch flow surface typically channel sharp crest rectangular air contracted analysis

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Slide1

Weirs

Generally, any flow obstruction that causes water to rise to flow over it, but used exclusively for intentional obstructions

Uses include flow measurement (sharp-crested weirs) and control of water surface profile, e.g., by inducing super-critical flow (broad-crested weirs)Slide2

Definition Diagram and Analysis of Sharp-Crested Weirs

Drawdown at crest is typically ~0.15

HSlide3

Nappe

entrains air underneath it and can collapse onto the downstream side of the weir; following analysis assumes that this does not occur (can ventilate this area to assure an air space)Slide4

Consider thin layer

dh

in plane of weir in a rectangular channel. Assuming

V

2

>>

V

o

2 and negligible frictional headloss, V=(2gh)0.5, so (replacing L by b):

where

H

C

is the distance from the crest of th

e weir to the

water

surface at that location.Slide5

Measurements of the water surface elevation above the weir crest are typically made upstream of the weir, where the water surface has not been significantly affected, and this value of

H

is used instead of

H

C

in the calculation. To account for this and other approximations, an empirical coefficient is added to the equation:

C

w

can be approximated by 0.611+0.075(

H

/

P

w

) and is typically in the range 0.64-0.70.Slide6

Other Common Shapes

for

Sharp-Crested

WeirsV-notch weir useful if low flows are of interest, since they could cling to the plate of a rectangular weir. For V-notch weirs:

Contracted weir

V-notch or Triangular weir

Weir can totally block part of the channel width, in which case the channel is

contracted

.

C

w,V

-notch

typically ~0.6Slide7

Streamlined or Broad-Crested WeirsSlide8

Water Profiles (CEE 477)