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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.