Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev

Water Resources Engineering

Civil Engineering (CE) : Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev

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Spillway profile with breast wall 

Spillways, generally the ogee-crested type, are sometimes provided with a breast wall from various considerations such as increasing the regulating storage of flood discharge, reducing the height of the gate, minimizing the cost of gate operating mechanism, etc.

For the spillways with breast wall, the following parameters are required to be determined:

  • Profile of the spillway crest including the upstream and downstream quadrants,
  • Profile of the bottom surface of the breast wall, and
  • Estimation of discharge efficiency of the spillway. 

The flow through a spillway with breast wall has been idealised as two-dimensional flow through a sharp edged orifice in a large tank. The following guidelines for determining the parameters mentioned above may be used for preparing preliminary designs and studies on hydraulic model may be conducted for confirming or improving on the preliminary design. Figure 36 shows pertinent details of various profiles of the spillway with a breast wall. 

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev  

Figure 36, Spillway with breast wall

Ogee Profile - Upstream Quadrant’ 

The upstream quadrant may conform to an ellipse with the equation: 

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev  (5) 

where 

A3 = 0.541 D (Hd/D)0.32   and 

B3 = 0.3693 D (Hd/D)0.04 

Ogee Profile - Downstream profile

The downstream profile may conform to equation: 

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev           (6) 

where 

Κ4 = 0.04 − 0.025 Hd/D                    (7) 

and 

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev       (8) 

Bottom Profile of the Breast Wall 

The bottom profile of the breast wall may conform to the equation: 

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev             (9) 

where 

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev  (10) 

n5 = 0.4D                 (11)

The upstream edge of the breast wall is in line with the upstream edge of the spillway and the downstream edge is in line with the spillway crest axis, as shown in Figure 36. 

The details of the upstream curve of the crest and bottom profile of breast wall are shown in Figure 37. 

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev

Figure 37. Upstream profile of ogee crest and bottom profile of breast wall details

Discharge Computation 

The discharge through the breast wall spillway may be estimated by the equation:

Spillways and Energy Dissipators (Part - 5) Civil Engineering (CE) Notes | EduRev       (12) 

The following equation relates Ch with the parameter (H/Hd) in the range of H/Hd = 0.8 to 1.33. 

         Cb = 0.148631 + 0.945305(H/Hd) – 0.326238(H/Hd)         (13) 

Typical values of Cb are: 

H/HdCb
0.80 0.696 
1.000.769 
1.15 0.797 
1.330.829 

 

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