sewspillwaysurpluswaterdisgharger (1).pptx

Straight Drop Spillways
„
Overflow Spillways
„
Side Channel Spillways
 Chute Spillways
 Baffled Chute Spillways
„
Shaft Spillways
„
Siphon Spillways
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Choice of Spillway in the field
Ogee Spillways Used in concrete and Masonry dams
Chute Spillways Used in earthen and rock fill dams
Shaft (Tunnel) Spillways Used in earthen and rock fill dams
Side Channel and Shaft Spillway When gorge is very narrow
Siphon Spillway Almost constant head for design range
of discharge
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Shape and Hydraulics of Ogee-Crest
The ogee shape which approximates the profile of the lower nappe of a sheet of
water flowing over a sharp-crested weir provides the ideal form for obtaining
optimum discharges.
The shape of such a profile depends upon the
 head,
 inclination of the upstream face of the flow section, and
 height of the overflow section above the floor of the entrance channel (which
influences the velocity of approach to the crest)
Ogee crested control structures are also sensitive to the upstream shape and
hence, three types of ogee crests are commonly used and shown in Figure. These
are as follows:
1. Ogee crests having vertical upstream face
2. Ogee crests having inclined upstream face
3. Ogee crests having over hang on up stream face
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1. Ogee crests with vertical upstream face
The following equation as given by U.S. corps of engineers may be used for finding
coordinates (X , Y) for the D/S profile
x1.85 = 2 (Hd) 0.85 y
Where X & Y are coordinates as shown in the figure and Hd is the design head.
For U/S profile following coordinates with origin at crest are recommended
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2. Ogee crests with sloping up stream face
The D/S profile may conform to the following equation:
xn = K (Hd)n-1 y
Where: n., K are variable parameters which depends on the inclination of U/S face
of the dam. These are called standard “WES standard spillway shapes”
Values of constant K,n , R1,R2, a and b
Shape of U/S
face
K N R1/Hd R2/Hd a/Hd b/Hd
Vertical 2.000 1.850 0.5 0.20 0.175 0.282
3V: 1H 1.936 1.836 0.68 0.21 0.139 0.237
3V: 2 H 1.939 1.810 0.48 0.22 0.115 0.240
3V: 3H 1.873 1.776 0.45 0.00 0.119 0.000
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The curved profile of the crest section is continued till it meets tangentially the straight
sloping portion of the overflow dam section (spillway).The slope of the d/s face of the
overflow dam usually varies in the range of 0.7(H):1(V) to 0.8:1 and is basically decided
on the basis of stability requirements. The location of the point of tangent depends upon
the slope of the d/s face, where the value of dy/dx for the curved profile and the straight
segment must be equal at the end of the sloping surface of the spillway.
U/S profile of the Weir Crest Vertical U/S face: The u/s profile should be tangential to
the vertical face and should have zero slopes at the crest axis to ensure that there is no
discontinuity along the surface of the flow. The u/s profile should conform to the
following equation:
y = {0.724 (x+ 0.270 Hd)1.85/(Hd )0.85 }+ 0.126 Hd - 0.4315(Hd)0.375(x + 0.270 Hd)0.625
The details of the upstream profile are shown in Fig.1.7. It may be noted that the values of
x are negative according to the chosen axes of coordinates. The maximum absolute value
of x is 0.270 Hd, for which the value of y is equal to 0126 Hd when the u/s face is
vertical.
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The values of (y/Hd) for different values of (x/ Hd ) can be obtained from Table 1.2
(b) Sloping upstream face : The coordinates of the upstream profile in the case of
sloping upstream face can be determined from Table l.2 for slopes of 1:3, 2:3 and 3:3.
For intermediate slopes. the values may be interpolated.
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Table 1.2 Values of y/Hd for the u/s profile
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Discharge Computation for an Ogee Spillway
The discharge over an ogee spillway is computed from the basic equation of flow over
weirs, given below:
Q=CdLeHe
1.5
where Q is discharge (cumecs), Cd is the coefficient of discharge, Le is the effective length
and He is the actual effective head including the head due to the velocity of approach. i.e.
He= Hd+ Ha
(Note Sometimes, the coefficient Cd is also, written as C]
1.Coefficient of discharge (Cd) An ogee spillway has a relatively high value of the
coefficient of discharge (Cd) because of its shape. The maximum value of Cd is about
2.20, if no negative pressure occurs on the crest. However, the value of Cd is not constant.
It depends upon the shape of the ogee profile, and also upon the following factors.
i. Height of spillway crest above the stream bed
ii. Ratio of actual total head to the design total head
iii. Slope of the upstream face of spillway
iv. Extent of the downstream submergence of crest
v. Downstream apron
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Various text books give a plot of Cd versus Hd which is reproduced here in
the form of a table
P/Hd Cd P/Hd Cd P/Hd Cd
0.0 1.7 0.1 1.875 0.2 1.97
0.3 2.025 0.4 2.06 0.5 2.09
0.6 2.12 0.7 2.135 0.8 2.15
0.9 2.16 1.0 2.17 1.5 2.185
2.0 2.195 2.5 2.2 3.0
4.0
2.205
2.210
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2. Effective crest length
Where the crest piers and abutments are shaped to cause side contractions of the
overflow, the effective length, Le, will be the net length of the crest, L. The effect
of the end contractions may be taken into account by reducing the net length of
crest as given below:
Le= L-2(N. Kp + Ka).He
Where, Le = Effective length of the crest for calculating
discharge
L = Net length of the crest
N = number of piers
Kp = Pier contraction coefficient
Ka= Abutment contraction coefficient
He= Total head on the crest
The reason for the reduction of the net length may be appreciated from Figure 32.
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 The pier contraction coefficient Kp depends upon the following
factors:
1. Shape and location of the pier nose
2. Thickness of the pier
3. Head in relation to the design head
4. Approach velocity
For the condition of flow at the design head, the average values of pier
contraction coefficients may be assumed as shown in Figure 33 or
S.No Pier shape Coefficient (Kp)
1 Square-nosed piers, with corners rounded on a
radius equal to about 0.10 of pier thickness
0.02
2 Rounded-nose piers 0.01
3 Pointed -nose pier 0.00
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The abutment contraction coefficient is seen to depend upon the following factors:
1. Shape of abutment
2. Angle between upstream approach wall and the axis of flow
3. Head, in relation to the design head
4. Approach velocity
For the condition of flow at the design head, the average value of abutment contraction
coefficients may be assumed as shown in Figure 33 or
S.No Abutment shape Coefficient (Ka)
1 Square abutment, with head wall at 90º to the
direction of flow
0.20
2 Rounded abutment, with head wall at 90º to the
direction of flow, when 0.5 HD =radius =0.15 HD
0.10
3 Rounded abutment where radius =0.5 HD and head
wall is placed at not more than 45º to the direction
of flow
0.00
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Design ogee spillway for the following data:
 Height of the spillway crest = 100.0 m
 Number of spans = 10
 Length of each span (clear) = 12.5
 Thickness of each pier = 3 m
 D/s slope of spillway = 0.8(H): 1(V)
 Rock condition = good
 Design discharge = 8500 m3/s
Cut water (90o) round nosed piers are supposed to be used with rounded abutments
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Design an ogee spillway with the following data:
 Height of spillway crest above river bed = 100 m
 Design discharge= 12,000 cumecs
 Number of spans = 6
 Clear distance between piers= 15m
 Thickness of pier= 3 m
 Slope of d/s face of the overflow section = 0.8:1
 Slope of u/s face of the overflow section =3V:1H
Cut water (90o) square nosed piers are supposed to be used with square
abutments
Assume any other data if required.
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