Classification of Ogee Spillway

 

A Presentation on

Design Of Ogee Spillway

By

Sandeep Choudhary Civil Engineering, VI-Semester CE- 6001 Design of Hydraulic Hydraulic Structures Structures

D epa partm rtme ent of C iv ivil il E ng inee ineeri ring ng R ewa E ng ine inee ering C olleg e, R ewa (M (M.P .P.) .)

 

#Design of Ogee Spillway# Problem Prob lem sta statem tement ent::

D ionhafo lonwstp tion a ecsoingcnreatesugitraabvlietysedcatm vrintghethoevedrofw reoarm facoef  sloping at a slope of 0.7H:1V . The design discharge for the spillway is 8000 cumecs. The height of the spillway crest is kept at RL 204.0m. The average river bed level at site is 100m. The spillway length consists of 6 spans having a clear width of 10m each. Thickness of each pier may be taken to be 2.5m.

 

Elementary section of Ogee spillway

 

Step -1 Determine the Eff Effective ective length of spill spillway way



Le = L = 6 x 10 = 60 m.

 

Step -2 Determine the (He) design head For the given spillway the coefficient of discharge may be  Now , Q= C.L.He3/2 Where , Q = Discharge from the spillway le=Effective length of the spillway cre resst C=Coefficient of discharge which depends upon d v a r i o u s f a c t o r s s u c h a s r e l a t i v e d e p t h o f a p p r o a c h ( i . e d / H ratio , relation of actual crest shape to the ideal nappe shape ,slope of upstream face downstream apron interference, and submergence , etc )

He = total head over the crest including the velocity head SD

 

8,000=2.2 * 60 (He)3/2 He3/2 =8000/(2.2*60) He = 60.62/3 He = 1 5 .5 m The height of the spillway above the river bed = h= 204100= 104m

 

Step -3 Determ Determine ine the spillway type (high or low) ℎ 

> 1.33

ncee h/hd ,i. ,i.e. e. 104 104/15 /15.5> .5>1.3 1.33 3 Sinc It is a high spillway, the effect of velocity head can, therefore ,be neglected.

The discharge coefficient is not affected by tail water  conditions, and the spillway remains a high spillway

 

Step -4 Design criteria criteria for for effective effective length length

Where;

[N.K  .K P + K a ] He Le = L - 2[N

Where Le = effective length of the crest L= net length of the crest  N= number of piers K  p= pier contraction coefficient coefficient K a= abutments contraction coefficient He= Total design head on the crest cr est including velocity head

 

Step Ste p -5 Assum Assume e the Value of Kp and Ka S.No.

Pier shape

Contraction coefficient

1.

Square no nosed piers without any rounding

0.1

2.

Square nosed piers with corners rounded on radius equal to 0.1 pier thickness

0.02

3

Rounded nose piers and 900 cut water nosed piers

0.01

4

Pointed nose piers

0.0

 

# Value of K a for different cases:

S.No S. No.. Sha hape pe of ab abu utm tmen entt

Contraction coefficient

1.

0.2

Squa Sq uare re ab abut utme ment nt with with head wall at 900 to the direction of flow

2.

Rounded abutment with head hea d wall wall at 900 900 to the direction of flow

0.1

 

Effective length of spillway (Le ) : Le can be worked out as follows: Le = L-2[N.K P + K a ] He ⁰

Assuming that 90 cut water nose piers and rounded abutments shall be provided , we have K a = 0.01 and KP  = 0.1  No. of piers = N = 5 Also assuming that the actual value of He is slightly more than the approximate value worked out (i.e. 15.5 m ),eref sefor ayore , leet, itLbe=166.03 -m ,5w e0.h0a1v+ e 0.1]×16.3 =55.1 m. Ther Th 2 [ × e

 

Step -6 Determine the (He)design head, by (Le) effective length Hence Q = 2.2 × 55.1 × He3/2 or 8000 = 2.2 × 55.1 × He3/2 He3/2= 8000 2.2 × 55.1 He3/2 = 66.0

He = 16.4 m ≈ 16.3 m(assumed) Hence, the assumed He for calculating Le is all right. The crest profile will be designed for Hd = 16.4 m (neglecting velocity head).

 

Step- 7 Additional Step Step for for Justify The The Velocity Velocity Head Note : The velocity head (Ha) can also be calculated as follows : Velocity of approach a pproach = Va = 8000 (60 + 5× 2.5)(104 + 16.4) Va = 0.917 m/s Velocity head = Ha = Va2 /2g Ha = (0.917)2 = 0.043 m.

2×9.81 This is very small and was, therefore, neglected.

 

StepSte p- 8 Design criteria criteria Crest Crest profiles profiles for inclined inclined u/s u/s face WES-standard spillway crest and IS 6934-1973 has adopted and recommended shapes. These shapes can be represented by the fo foll llow owiing gen ener eral al eq equ uat atiion on::

xn= K Hdn-1 .y Where, (x,y) are co-ordinates of the point on the crest profile with the origin at the highest point Hd is the design head including the velocity head K and n are constants depending upon the slope of upstream upstr eam face.

 

Different values of K and n for Crest u/s profiles : Slope of u/s face of spillway

K

N

Vertical

2.0

1.84

1:3 (1H:3V)

1.936

1.836

1:1 ½ (1H: 1 ½ V)

1.939

1.810

 

Design criteria Crest Crest profiles for downstr downstream eam Profile water experimental station Indian standard , According to water for a vertical u/s face the d/s profile is given by: x1.85 = 2. H

0.85

y (K and n from table)

d

So, y=

x1.85 0.85 = x1.85 0.85 2 (Hd ) 2 (1.64)

y=

x1.85 21.6

 

Before we determine the various co-ordinates of the d/s profile, we shall first determine the tangent point. The d/s slope of the dam is given to be 0.7H:1V Hance,

dy = 1 dx 0.7 Differentiating the equation of the d/s profile pr ofile with respect to x, we get dy =

1.85x1.85-1

dx

21.6

 

x0.85 =

21.6 1.85 X 0.7

= 16.7

x= 22.4m

y = (22.4)1.85 21.6

= 14.6 m

The profile of the spillway has been determined and values in the following tables.

 

The co-ordinates from x=0,x=22.4m are worked out in table X(meters)

Y=x1.85 /21.6(meters)

1

0.046

2

0.166

3 4

0.354 0.60

5

0.905

6 7

1.274 1.710

8

2.162

9

2.684

 

X(meters)

Y=x1.85 /21.6(meters)

10 12

3.240 4.575

14

6.020

16

7.88

18

9.74

20

11.85

22

14.35

22.4

14.60