WEIRS AND DROP STRUCTURESCIVE 401 FALL 2015

KACY WILLIAMS AND KARLA YOUNG

10/22/2015

WHAT IS A WEIR?

• AN OVERFLOW STRUCTURE DESIGNED TO MEASURE THE DISCHARGE OF WATER IN A RIVER OR OPEN

CHANNEL

• PLACED PERPENDICULAR TO THE FLOW OF THE WATER

• IS ALSO USED TO PREVENT FLOODING OR TO MAKE A RIVER MORE NAVIGABLE

• TWO MAIN TYPES: BROAD CRESTED & SHARP CRESTED

CALCULATING FLOW RATE

𝑄 = 𝐶 ∗ 𝑏 ∗ 𝐻𝑁

• Q = VOLUMETRIC FLOW RATE OF FLUID

• C = DISCHARGE COEFFICIENT, VARIES FOR

DIFFERENT WEIR STRUCTURES

• B = WIDTH OF THE CREST

• H = HEIGHT OF THE HEAD OF WATER OVER

CREST

• N = VARIES WITH DIFFERENT WEIR

STRUCTURES

ASTM Standards for calculating flow rate:

• ASTM D 5242 - Thin Plate Weirs

• ASTM D 5614 - Broad Crested Weirs

• ASTM D 5640 - Guide for selection of weirs/flumes

Common Weir Terms

• Crest = Area of weir where water flows over

• Nappe = Sheet of water flowing over weir

• Notch = Opening where water flows in

different types of weir structuresSource:

http://ocw.usu.edu/Biological_and_Irrigation_Engineering/Irrigation___Conveyance_Control_

Systems/6300__Weirs_for_Flow_Measurement_Lecture_Notes.pdf

BROAD CRESTED WEIR

• FLAT TOPPED, EXTENDS ENTIRE WIDTH OF

WATER CHANNEL

• USUALLY USED FOR LARGE CHANNELS OR

RIVERS

• USED ALMOST EXCLUSIVELY FOR MEASURING

WATER DISCHARGE

• ABLE TO WORK EFFECTIVELY WITH HIGHER

DOWNSTREAM WATER LEVELS COMPARED TO

OTHER WEIRS

• Q = C ∗ B ∗ H3/2

- WHERE C = 2/33/2 ∗ G1/2

- G = GRAVITY

Source: http://ponce.sdsu.edu/onlinechannel14.php

SHARP CRESTED WEIR

• HAS A SHARP UPSTREAM EDGE AT THE CREST, WHERE WATER WILL FALL AWAY FROM

THE WEIR

• USUALLY UTILIZED IN SMALLER RIVERS OR IN LABORATORY SESSIONS

• DESIGNED WITH SMOOTH THIN PLATES

• CAN BE VERY ACCURATE, +/- 2%

• 3 MAIN TYPES OF SHARP CRESTED WEIR:

• V-NOTCH OR TRIANGULAR

• RECTANGULAR

• TRAPEZOIDAL OR CIPOLLETTI

Source: http://content.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub20/pub20-h5.0.pdf

V-NOTCH (TRIANGULAR) WEIR

• MOST ACCURATE OF THE SHARP CRESTED WEIRS, BUT THE MOST DELICATE

• CAN ONLY BE USED IN CHANNELS WITH SMALL DISCHARGE

• DESIGNED FOR THE WATER TO NOT SPILL OVER THE CREST OF THE WEIR, BUT TO STAY WITHIN

TRIANGULAR PORTION OF WEIR

• FOR A 90 DEGREE V-NOTCH WEIR THE EQUATION FOR DISCHARGE IS: 𝑄 = 2.49 ∗ 𝐻2.48

Source: http://www.jfccivilengineer.com/sharp_crested_weir_2.htm

CIPOLLETTI (TRAPEZOIDAL) WEIR

• SIMILAR TO A RECTANGULAR WEIR, EXCEPT THE SIDES ARE ANGLED

• LESS ACCURATE THAN RECTANGULAR AND V-NOTCH WEIRS, BUT MORE STABLE

• CIPOLLETTI WEIR EQUATION FOR DISCHARGE: Q = 3.367 ∗ B ∗ H3/2 (B IS THE MEASURED

BOTTOM WIDTH)

http://web.deu.edu.tr/atiksu/ana52/2-2.gif

Source: http://www.lmnoeng.com/Weirs/cipoletti.php

DROP STRUCTURES CAN BE….

Purely functional, dissipating energy and reducing

the velocity in the channel.

Aesthetically pleasing in addition to

functional, for public places.

Source: Urban Drainage & Flood Control District: Drainage Criteria V.2

SOURCE: COLORADO FLOODPLAIN AND STORMWATER CRITERIA MANUAL BY COLORADO WATER CONSERVATION BOARD

Backwater Control Structure

Primary Drop Structure

Typical Spacing:

0.3W to 0.6W

COLORADO GUIDELINES FOR DROP STRUCTURE DESIGN

• PRIMARY DROP STRUCTURE

• GENERAL “V” SHAPE POINTING UPSTREAM

• DOWNSTREAM ANGLE BETWEEN 120˚ AND 180˚

• UPSTREAM POINT LOWERED 4 TO 18 INCHES

• TO CONCENTRATE FLOW

• TO PROTECT FROM BANK EROSION

• BACK WATER CONTROL STRUCTURE

• OFTEN STRAIGHT ACROSS THE CHANNEL, BUT CAN BE

CONSTRUCTED BETWEEN 135˚ AND 180˚

• TO MAINTAIN A PLUNGE POOL BETWEEN STRUCTURES

• FURTHER DISSIPATE KINETIC ENERGY

• TO MINIMIZE SCOUR ON THE DOWNSTREAM SIDE OF THE

PRIMARY DROP STRUCTURE

• SPACING IS TYPICALLY BETWEEN 0.3 AND 0.6 TIMES THE

WIDTH OF THE CHANNEL

Source: Urban Drainage & Flood Control District: Drainage Criteria V.2

DROP STRUCTURES IN BOATABLE CHANNELS

• SPECIAL DESIGN CONSIDERATIONS SHOULD BE TAKEN, WITH REGARD

TO PUBLIC SAFETY, FOR BOATABLE CHANNELS

• “THE DESIGNER SHOULD NOT SET THE STAGE FOR HAZARDOUS HYDRAULICS

THAT WOULD TRAP A BOATER, SUCH AS AT A DROP STRUCTURE HAVING A

REVERSE ROLLER THAT MAY DEVELOP AS THE HYDRAULIC JUMP BECOMES

SUBMERGED. “

• “HYDRAULIC STRUCTURES ON BOATABLE CHANNELS SHOULD NOT CREATE

OBSTRUCTIONS THAT WOULD PIN A CANOE, RAFT OR KAYAK, AND SHARP

EDGES SHOULD BE AVOIDED.”

• “DROP STRUCTURES OR LOW-HEAD DAMS IN BOATABLE CHANNELS SHOULD

INCORPORATE A BOAT CHUTE DESIGNED IN ACCORDANCE WITH CAREFULLY

PLANNED COMPONENTS THAT ARE CONSISTENT WITH RECREATIONAL

REQUIREMENTS FOR BOATER SAFETY”

STRAIGHT DROP STRUCTURE DESIGN EXAMPLE

FIND THE DIMENSIONS FOR A STRAIGHT DROP STRUCTURE WITH A RECTANGULAR WEIR USED TO REDUCE

CHANNEL SLOPE.

GIVEN:

• Q = 250 FT3/S

• H = 6.0 FT.

• WO = 10.0 FT.

(UPSTREAM AND DOWNSTREAM CHANNEL -TRAPEZOIDAL)

• B = 10.0 FT.

• Z = 1V:3H

• SO = 0.002 FT./FT. (AFTER PROVIDING FOR DROP)

• N = 0.030

Source: U.S. Department of Transportation Federal Highway Administration: Hydraulic Engineering Circular No. 14, Third Edition

SOLUTIONStep 1. Estimate the required approach and tailwater channel elevation difference, h. This is estimated and given above as 6.0 ft. This drop

forces the slope of the upstream and downstream channel to 0.002 ft./ft., as given.

Step 2. Calculate normal flow conditions approaching the drop to verify subcritical conditions. By trial and error,

yo = 3.36 ft., vo = 3.71 ft/s, Fro = 0.36; therefore, flow is subcritical. Proceed to step three.

Step 3. Calculate the critical depth over the weir into the drop structure. Calculate the vertical dimensions of the stilling basin. Start by finding

the critical depth over the weir based on the unit discharge, q = Q/B = 250/10 = 25ft.2/s

yc=

q2

g

1 3

=252

32.2

1 3

= 2.69 ft.

Next calculate the required tailwater depth above the floor of the stilling basin:

y3 = 2.15yc = 2.15 2.69 = 5.77ft.

Now the distance from the crest down to the tailwater needs to be calculated:

h2 = -(h-yo) = -(6.0-3.36) = -2.64 ft. (negative indicates elevation below the crest)

Finally, calculate the total drop from the crest to the stilling basin floor:

ho = h2 − y3 = −2.64 − 5.77 = −8.41 ft. (round to − 8.4 ft. )

Since the nominal drop, h, is 6.0 ft., the floor must be depressed by 2.4 ft.

Source: U.S. Department of Transportation Federal Highway Administration: Hydraulic Engineering Circular No. 14, Third Edition

Source: U.S. Department of Transportation Federal Highway Administration: Hydraulic Engineering Circular No. 14, Third Edition

SOLUTION (CONT.)Step 4. Estimate the basin length.

Lf = −0.406 + 3.195 − 4.368hoyc

yc= −0.406 + 3.195 − 4.368

−8.41

2.692.69 = 9.94 ft.

Lt = −0.406 + 3.195 − 4.368h2yc

yc = −0.406 + 3.195 − 4.368−2.64

2.692.69 = 6.26 ft.

Ls =

0.691 + 0.228𝐿𝑡𝑦𝑐

2

−hoyc

yc

0.185 + 0.456𝐿𝑡𝑦𝑐

=0.691 + 0.228

6.262.69

2

−−8.412.69

2.69

0.185 + 0.4566.262.69

= 10.89

L1 =Lf + Ls

2=

9.94 + 10.89

2= 10.4 ft.

L2 = 0.8yc = 0.8 2.69 = 2.2 ft.

L3 > 1.75yc = 1.75 2.69 = 4.7ft.

LB = L1 + L2 + L3 = 10.4 + 2.2 + 4.7 = 17.3ft.

The total basin length required is 17.3 feet

SOLUTION (CONT.)

Step 5. Design the basin floor blocks and end sill.

Block height = 0.8yc = 0.8(2.69) = 2.1ft.

Block width = Block spacing = 0.4yc = 0.4(2.69) = 1.1ft.

End sill height = 0.4yc = 0.4(2.69) = 1.1ft.

Step 6. Design the basin exit and entrance transitions.

Sidewall height above tailwater elevation = 0.85yc = 0.85(2.69) =2.3 ft.

Armour approach channel above headwall length = 3yc = 3(2.69) = 8.1ft.

Source: U.S. Department of Transportation Federal Highway Administration: Hydraulic Engineering Circular No. 14, Third Edition

CONCLUSIONS

• WEIRS AND DROP STRUCTURES ARE BOTH IMPORTANT TO RIVER MECHANICS

• WEIRS ARE USED TO CALCULATE THE DISCHARGE IN A RIVER, AND SUBSEQUENTLY VELOCITY

• DROP STRUCTURES ARE PUT IN PLACE WHEN THE VELOCITY IS TOO HIGH TO PREVENT EXCESS

EROSION AND SCOUR

• WEIRS ARE USED IN LABORATORY ENVIRONMENTS AS WELL AS REAL WORLD SITUATIONS,

SUCH AS RIVERS

• THERE ARE A MULTITUDE OF DESIGNS FOR BOTH WEIRS AND DROP STRUCTURES. THE DESIGN

OF EACH IS SPECIFIC TO THE FLOW CHANNEL IN WHICH IT WILL FUNCTION.