(2)
M.J.Deodhar 1 R.D.Mahadeokar 2 S.Y.Kute 3 K.S.Bandi 3
1 Principal (retd) K.K.Wagh College of Engineering , Nasik ,India.
2 Prof.&Head.,Civil Engg.Dept.Pune Vidyardhi Graih College.of Engg.Pune,India.
3 Asst Prof. of Civil Engg. K.K.Wagh College of Engineering , Nasik ,India.
1,Ampraphal Apartments. 34, Bharatkunj Society no.1.
Erandwana , PUNE (Mah) 411038
INDIA.
Ph.No. 091 020 5442030.
Abstract: Energy dissipation downstream of spillway is one of the most challenging jobs in the design of a dam.With the past experience it is now observed that the hydraulic jump type energy dissipator is the most efficient and reliable divice . However its performance depends on a number of factors . The important being the tail water rating curve .If the tail water level matches with the water level after jump for all the discharges then the energy dissipation in the stilling basin is quite satisfactory .
Efforts are made in this article to evaluate the range of tail water parameters so that the tail water level matches with the water level after jump for all the discharges .
Keywords: spillway, maximum flood discharge, discharge intensity, river water rating curve, stilling basin, energy dissipation arrangement, coefficient of rugosity, slope of river
A hydraulic jump type energy dissipator is found to be the most effecient energy dissipation arrangement for dams of heights ranging from 10 m. to 200 m. It may be slightly expensive than the other ones . However it is most effecient and reliable and water can be safely let into the tail channel without any risk of retrogression.
However the design of this type depends on a number of parameters as follows .
(1) Discharge intensity
(2) Rock available at the spillway site.
(3) Tail water rating curve
Out of these , very important is the tail water rating curve as it is expected that performance of spillway to be satisfactory for all the discharges upto the designed one .
If the tail water is less than the water level after jump , it will be necessary to raise it by artificial means. So also if the tail water level is more than the required one , the hydraulic jump in the basin will be submerged affecting its performance.
The tail water curve depends on its slope and its rugosity coefficient and these two parameters are interdependent and supplementary . The factor s1/2 /n is of consideration .
For the design of a spillway , the important aspects are the height of the dam and the maximum flood discharge .
For these two known things the range of rugosity coefficient and the bed slope can be evaluated so that the tail water level & the water level after jump match each other for all the discharges .
Figure1 shows the definition sketch .
The following range of basic data is considered .
(1) Height of dam : - 10 m to 200 m .
(2) Maximum flood discharge : - 250000 m3 / sec .
(3) Slope of tail channel : - 1 in 500 (0.002) to 1 in 10000 (0.0001)
(4) Rugosity coefficient of tail channel : - 0.015 to 0.040
(5) Length of spillway : - 100 m to 2000 m.
For simplicity the following assumptions are made.
(1) The river cross-section is rectangular
(2) Spillway length = river bed width
(3) Height of end sill of stilling basin is equal to 2.0 m
(4) Spillway pier width is neglected
(5) Spillway is located in the river gorge portion .
Since the height of the end sill is assumed of to be 2.0 m the depth in tail channel 'D' = Depth after jump minus two meters i.e d2-2.0
The following basic equations have been used with usual notations.
E = d1 + v12 2g/ = d1 + .051 q2 d1/3 (1)
(2)
D = d2 – 2.0 (3)
(4)
Therefore
There are seven parameters [ E , d1 , q , d2 , S ,n , D ] and four equations.
With the help of a computer, the parameters d1 & d2 are eliminated and the results are presented on a multiple co-ordinate system graph wide Fig. 2.
Consider a specific case of a dam as follows
(1) Height -------------------------50 m
(2) Length of spillway ------------1000 m
(3) Max .flood discharge ---------115000 m3/sec
From the multiple co-ordinats system graph in Fig. 2 the discharge intensity can be calculated from quadrant .I for various values of Q & L upto max values of 115000 m3/sec & 1000 m resp. and may be read on the y axis.
These range from 0.0 m3/sec to 150.0 m3/sec/m .
The values of S1/2/n corresponding to these discharge intensities can be evaluated for various values of H from quadrant II of the graph.
This factor S1/2/n ranges from 0.52 to 0.67
From quadrant III a band of graph (duly shaded ) may be considered for this range of S1/2 /n which will give the range of S & n in the practical range as follows.
n = 0.020 to 0.040
S = 1 in 1200 to 1 in 2200
If the available values of S & n lie between this range , then it may be concluded that a hydraulic jump type stilling basin can be designed for the available tail water.
Naturally if it does not then some alternative has to be adopted.
(1) For lower values of 'H' the range of S1/2 /n is wide & for higher values of 'H' the band width of S1/2 /n is comparatively narrow.
(2) With the help of the multiple co-ordinate system as presented in fig 2 , it can be varified whether the available tail water characteristics viz S & n are adequate to have a hydraulic jump type stilling basin without any modifications in the tail channel.
(3) For values above this band there is a possibility of submergence of jump so also for values below this band the jump may sweep away & needs raising of the water levels.
These are guidelines . However detailed design as well as hydraulic model studies are obligatory before finalysing the design .
Fig. 1
Fig.2