CE 3205 Water and Environmental Engineering

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CE 3205 Water and Environmental Engineering

• Spillways

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SPILLWAY

• A spillway is a structure used to provide for the
  controlled release of flows from a dam or levee
  into a downstream area, typically being the river
  that was dammed.
• to prevent overtopping and possible failure of
  the dam.

Four Mile Dam, Australia Ogee Spillway
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Upper South Dam, Australia Ogee Spillway
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Sourcehttp//www.leanhtuan.com/
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Hoover Dam Spillway Crest
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Hoover Dam Spillway
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New Cronton Dam NY Stepped Chute Spillway
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Sippel Weir, Australia Drop Spillway
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Four Mile Dam, Australia Ogee Spillway
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Upper South Dam, Australia Ogee Spillway
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Itaipu Dam, Uruguay Chute Spillway
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Itaipu Dam Flip Bucket
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• Common type of spillways
• Free over fall/straight drop spillways
• Overflow or ogee spillways.
• Chute spillways
• Siphon saddle spillway

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• Free overfall or straight drop spillway
• In this type, water drops freely from the crest.
• Occasionally the crest is extended in the form of
  overhanging lip to direct small discharges away
  from the face of overfall section.

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• Ogee or overflow spillway
• The Ogee spillway is generally provided in rigid
  dams and forms a part of the main dam itself if
  sufficient length is available. 
• The overflow type spillway has a crest shaped in
  the form of an ogee or S-shape.
• The upper curve at the crest may be made either
  larger or sharper than the nappe.

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• Chute spillway
• chute spillways are used in flow ways where water
  is to be lowered from one level to another and
  where it is desirable to avoid a stilling basin.
• These are mostly used with earth dams and have
  the following merit.
• It can be provided on any type of foundations.
• Simplicity of design.
• However this type of spillway should not be
  provided where too many bends are to be given as
  per topography.

Baffle apron or chute spillway
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• Saddle spillways
• A siphon spillway is a closed conduit system
  formed in the shape of an inverted U.
• This type of siphon is also called a Saddle
  siphon spillway.
• Siphonic action takes place after the air in the
  bend over the crest has been exhausted.

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Required spillway capacity

• Spillway capacity should be equal to the max.
  outflow rate determined by flood routing. The
  following data are required for the flood
  routing.
• Inflow flood hydrograph-Indicates rate of inflow
  respect to time.
• Reservoir capacity curve-indicates the reservoir
  storage at different reservoir elevations.
• Outflow discharge curve-indicates the rate of
  outflow through spillways at different reservoir
  elevations.

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Overflow Spillway
Basic equation flow over weirs,
Where Qdischarge m3/s Cdcoefficient of
discharge Leeffective length Heactual effective
head
Hddesign head Hahead due to velocity of
approach (sometimes neglected)
Le effective width of crestL net width of
crest(clear waterway x no.of spans)N
number of piersKp pier contraction
coefficientKa abutment contraction coefficient
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Contraction Coefficients
Table 1 Pier Contraction Coefficient (Kp)
Table 2 Abutment Contraction Coefficient (Ka)
Pier contraction coefficient depends on several
factors such as shape and location of pier nose,
thickness of piers and velocity of
approach. Abutment contraction coefficient
depends on factors such as shape of abutment and
velocity of approach.
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• Design head, Hd

• Downstream profile
• d/s profile of spillway can be represented by

x,y coordinates of the point on the spillway
surface Hddesign head K,n constant, depend on
inclination of the upstream face of spillway
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Different inclination of upstream face of spillway
For overspillway/ogee, the upstream face is
vertical

• The slope of the d/s face of the overflow dam
  usually varies in the range of 0.71 to 0.81
• Z is total fall from the upstream water level to
  the floor level
• P is height of spillway crest above the bed.
• Y depth of flow at toe
• R is radius
• V is velocity of flow at toe

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slope of the d/s face of the overflow section
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C. vs. ?
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Cd. vs. (P/Hd)
(P/Hd)gt1.33, velocity is neglected
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Tailwater Effect on C
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Problem 01

• Problem 01 An overflow spillway with the
  upstream face vertical is to be designed for a
  flood peak of 3000 m3/s. The height of the
  spillway crest is kept at RL 130.50 m. The
  average river bed level at the site is 102.50 m.
  The number of spans is 6, clear waterway between
  piers is 12 m, thickness of the pier is 2 m, pier
  contraction coefficient, Kp 0.02 and abutment
  contraction coefficient, Ka 0.20 for the effect
  of end contraction. Assume the coefficient of
  discharge is 2.20 and the slope of the d/s face
  of the overflow section is 0.8 1.
• Determine the design head by neglecting the end
  contraction.
• What will happen if the design head is determined
  by taking the effect of end contraction of piers
  and spans?
• Determine the tangent point of x ordinate of
  the downstream
• profile from the origin of the crest.

R.L is reservoir level
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Solution

• Peak flow, Q3000 m3/s.
• The no. of spans is 6,
• clear waterway between piers is 12 m,
• thickness of the pier is 2 m,
• pier contraction coefficient, Kp 0.02
• abutment contraction coefficient, Ka 0.20
• Coefficient of discharge, C is 2.20
• Slope of the d/s face of the overflow section is
  0.8 1.

Determine the design head by neglecting the end
contraction.
Neglecting the end contraction, so we calculate
L L L clear waterway x no.of spans L12 x 6
72m
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a) Determine the design head
b) design head is determined by taking the effect
of end contraction of piers and spans
N6 Kp0.02 Ka0.2
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PHeight of spillway crest at R.L- average river
bed level at the site 130.5-102.5 28 m
Check, P/Hd 28/7.11 3.94 greater
than 1.33 So effect of velocity can be
neglected HeHdHa(due to velocity0) He Hd
c) Determine the tangent point of x ordinate of
the downstream profile from the origin of the
crest. .
For vertical upstream K2, n1.85
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Differentiate both sides with respect to x
Since slope of the d/s face of the overflow
section is 0.8 1, So...
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End