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Dam outlet works lecture 3

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ENERGY DISSIPATORS
stilling basin
A stilling basin is defined as a structure in which a hydraulic jump used for energy dissipation is confined either partly or entirely.
Certain auxiliary devices such as chute blocks, sills, baffle walls, etc. are usually provided in the stilling basins to reduce the length of the jump and thus to reduce the length and the cost of the stilling basin.
Moreover, these devices also improve the dissipation action of the basin and stabilize the jump.
Chute Blocks :
These are triangular blocks with their top surface horizontal. These are installed at the toe of the spillway just at upstream end of the stilling basin.
They act as a serrated device at the entrance to the stilling basin. They furrow the incoming jet and lift a portion of it ab0ve the floor.
These blocks stabilise the jump and thus improve its performance, these also decrease the length of the hydraulic jump.
Basin Blocks or Baffle Blocks or Baffle Piers :
These are installed on the stilling basin floor between chute blocks and the end sill. These blocks also stabilise the formation of the jump.
Moreover, they increase the turbulence and assist in the dissipation of energy.
They help in breaking the flow and dissipate energy mostly by impact. These baffle blocks are sometimes called friction blocks.
Sills and Dentated Sills :
Sill or more preferably dentated sill is generally provided at the end of the stilling basin.
The dentated sill diffuses the residual portion of the high velocity jet reaching the end of the basin. They, therefore, help in dissipating residual energy and to reduce the length of the jump or the basin.
particular location of these blocks mainly depends upon the initial Froude number (F1) and the velocity of the incoming flow. The stilling basins are usually rectangular in plan. These are made up of concrete.
[A] U.S.B.R. Stilling basins :
[B] Indian Standards Basins :
1. Horizontal apron - Type-I
2. Horizontal apron - Type-II
3. Sloping apron - Type-Ill
4. Sloping apron - Type-IV
Type I basin (F1 between 2.5 to 4.5)
Provide chute blocks and end sill
Length of basin = 4.3 y2 to 6.0 y2
Width of chute block = y1
Spacing = 2.5 y1
Height of chute block = 2y1
Length of chutes = 2y1
U.S.B.R. Type-II basin for F1 greater than 4.5 and v1 less than 15 m/sec.:
U.S.B.R. Type-Ill basin for F, greater than 4.5 and V1 greater than 15 m/sec :
Chutes and dentated sills provided
Baffle is not provided because of –velocity is high and cavitation is possible.
[B] Indian Standards Basins :
1. Horizontal apron - Type-I
2. Horizontal apron - Type-II
3. Sloping apron - Type-Ill
4. Sloping apron - Type-IV
1. Horizontal apron - Type-I
2. Horizontal apron - Type-II
3. Sloping apron - Type-Ill
4. Sloping apron - Type-IV
1. Horizontal apron - Type-I
2. Horizontal apron - Type-II
3. Sloping apron - Type-Ill
4. Sloping apron - Type-IV
IS Type-Ill basin is usually provided with a sloping apron for the entire len

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Dam outlet works lecture 3

  1. 1. 17/2/2014 1 PREPARED BY V.H. KHOKHANI,ASSISTANT PROFESSOR, DIET.
  2. 2. Arrange ments Height Width Length Locatio n spacing s Chutes (1) 2Y1 Y1 2Y1 At toe 2.5 Y1 Baffel (3) NA NA NA NA NA End sills (4) 1.2 to 3 y1 2 in 1 slope Througho ut spillway section At the end of basin NA
  3. 3. Arrange ments Height Width Length Locatio n spacing s Chutes (1) Y1 Y1 2Y1 At toe y1 Baffel (3) 1.5 to 3.3 Y1 0.75 h3 2.3 TO 2.8 y1 0.8 y2 from toe 0.75 h3 End sills (4) 1.2 to 3 y1 2 in 1 slope Througho ut spillway section At the end of basin NA
  4. 4. Arrange ments Height Width Length Locatio n spacing s Chutes (1) Y1 Y1 2Y1 At toe y1 Baffel (3) NA NA NA NA NA End sills DENTAT ED END SILLS (4) O.2 Y2 0.02 Y2 0.15 Y2 At the end of basin NA0.15 Y2
  5. 5. USBR TYPES FR NO VELOCIT Y PROVISI ONS I 2.5 TO 4.5 NA CHUTES END SILL II MORE THAN 4.5 LESS THAN 15 CHUTES BAFFLE END SILL III MORE THAN 4.5 MORE THAN 15 CHUTES DENTATE D SILL
  6. 6. Arrange ments Height Width Length Locatio n spacing s Chutes (1) 2Y1 Y1 2Y1 At toe 2.5 y1 Baffel/ba sin blocks (3) 0.8 to 3.3 Y1 0.02 y1 Height of blocks 0.8 y2 from toe Height of blocks Dentated sills (4) 0.20 y2 0.02 y2 Height of sills At the end of basin Height of sills

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