CU06997 Fluid dynamics

Hydraulic structures 13.1 Hydraulic structures (page 430)

13.2 Thin plate weirs (page 430-440)

13.3 Long based weirs (page 440-445)

13.4 Flumes (page 445-452) Just read

13.5 Spillways (page 452-465) Just read

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Weir?? What?? Why??

http://en.wikipedia.org/wiki/Weir

• Regulation

• Overflow

• Flow measuring

• Sometimes it is just a obstacle

• Thin plate (sharp crested) weirs [korte overlaat]

• Long based (broad crested) weirs [lange overlaat]

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Free or submerged flow

Volkomen of onvolkomen overlaat

• Free flow (volkomen)

If the water level downstream changes,

the water level upstream does not change

• Submerged flow (onvolkomen / verdronken)

If the water level downstream changes,

the water level upstream also changes

• http://vimeo.com/16847389

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Thin plate (sharp-crested) weirs

Free flow

Water level upstream >> Water level downstream

Use specific energy=total head measured from the crest 2

Formula’s Thin plate weirs

There are a lot of different formula’s which try to be

as accurate as possible.

Most important to remember is that in the formula’s

water level and total head (specific energy) is

measured from the crest and the formula’s can only be

used with free flow (volkomen)

Next slides some examples, you will find more in the

book.

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Basic formula 𝑄 = 𝑚 ∙ 𝐵 ∙ 𝐻32

Q = discharge overflow [m3/s]

m = runoff coefficient [m1/2/s]

B = Width crest overflow [m]

H = Head at overflow [m]

measured from top crest!!

H

Total head

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Thin-plate Rehbock weir

2324,0

78,1 HBd

hQ

Free weir flow

Crest weir

Rehbock weir

2

Discharge measurement weirs

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86,1 HbQ rHqv 28,2 21tan46,1 2

5

Hqv

mhHaHdHb 40,0,2,3,3

rH 25,0h

2

Romijn weir

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7,1 HBqv

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Long based (broad crested) weirs

[Lange overlaat]

Most of the time these weirs are not meant for

measuring discharge.

More for regulating purposes or just because there is

a object in the river which act as long based weir

Free flow (volkomen) and submerged (onvolkomen)

flow can occur

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Submerged flow [onvolkomen]

Free flow [volkomen]

Total Head or Energy line H

Total Head or Energy line H

Super critical flow

Hydraulic Jump

Bottom eddy

Long-based (broad-crested) weirs

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Remarks

• When the flow downstream is supercritical it is

always a free flow at the weir

• If the water level downstream is lower than 2/3 H ,

it is a free flow.

• If the water level downstream is higher than 2/3 H,

it is a submerged flow.

• With free flow, the water level at the end of the

crest is 2/3 H.

• H and water level are measured from the top of the

crest. H is measured upstream of the weir

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Submerged broad crested weir flow

)(2 33 hHghBcq olv

col=discharge coefficient submerged broad crested weir [1]

B = width weir

h3 = water level downstream

Total Head or Energy line H

Submerged broad crested weir [onvolkomen lange overlaat]

Bottom eddy

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Free flow broad crested weir

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HBcq vv

Cv=discharge coefficient free flow broad crested weir [m1/2/s]

B = width weir

h3 = water level downstream

Total Head or Energy line H

Free flow broad crested weir [volkomen lange overlaat]

Super critical flow

Hydraulic Jump

3

Partially filled Culvert

acts as a broad crested weir

4

DownstreamUpstream

Cross-section

Length Culvert

Flow velocity

Partially filled Culvert

acts as a broad crested weir

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• The bed level of the culvert is the crest level

• The width of the culvert is the width of the crest (B)

• The water level (h3) and total head (H) are measured

from the bed level of the culvert.

• If the water level (h3) downstream is lower than 2/3 H ,

it is a free flow.

• If the water level (h3) downstream is higher than 2/3 H,

it is a submerged flow.

• When submerged flow :

tot

olc

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