Crest Shape

• The ogee shaped crest is commonly used as a control weir for many types of spillways- Overflow (Figure 5), Chute (Figure 8), Side Channel (Figure 12) etc. 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, the inclination of the upstream face of the flow section, and the height of the overflow section above the floor of the entrance channel (which influences the velocity of approach to the crest).

• The ogee profile to be acceptable should provide maximum possible hydraulic efficiency, structural stability and economy and also avoid the formation of sub atmospheric pressures at the surface (which may induce cavitations).

• 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 21. These are as follows:

  1. Ogee crests having vertical upstream face

  2. Ogee crests having inclined upstream face

  3. Ogee crests having overhang on up stream face

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• However, the same general equations for the up stream and down stream quadrants  are applicable to all the three cases, as recommended by the Bureau of Indian Standards code IS:

• 6934-1998 “Hydraulic design of high ogee over flow spillways- recommendations” and are outlined in the following paragraphs.

Ogee Crest With Vertical Upstream Face

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Ogee Crest With Sloping Upstream Face

• In this case, the desired inclination of the upstream face is made tangential to the same elliptical profile as provided for a crest with a vertical face. The down stream face equation remains unchanged.

Ogee Crest With Overhang

• Whenever structural requirements permit, the upstream vertical face of an ogee crested spillway (Figure 22) may be offset inside, (Figure 24). It is recommended that the ratio of the rises M to the design head Hd, should be at least 0.6 or greater for flow conditions to be stable. The crest shapes on the upstream and downstream may be provided the same as for an ogee crest with vertical up stream wall if the condition M/ Hd >0.6 is satisfied.