• This spillway consists of a control weir whose shape is ogee or S-shape.
  • The shape of the ogee spillway conforms to the profile of lower nappe of water, falling from a sharp crested weir.
  • The profile of the spillway is so shape that the overflowing water remains in touch with the spillway surface throughout the fall, when the rate of overflow corresponds to the maximum designe capacity.
  • When head on the spillway crest is less than the maximum designe head, the falling nappe would remain adhering to the profile of the spillway.
  • This causes positive hydrostatic pressure and thus reduces the discharging capacity of the spillway.
  • On the contrary, when head on the crest exceeds the maximum design head during unexpected large floods, the lower nappe of falling water may not remain in touch with the spillway surface.
  • This phenomenon causes negative pressure and cavitation.
  • Cavitation has a damaging effect which is caused at points where lower surface of the falling nappe does not remain in touch with the profile.
  • At such points vacuum is develop which results in cavitation effect.
  • In addition to cavitation, vibrations are also set up in the dam due to alternate making and breaking of the contact between falling nappe of water and profile of the spillway.
  • The D/S curve of the ogee follows following equation

\[x^{1.85}= 2H^{0.85}\times y\]

  • where x and y are the co-ordinates of the crest profile measured from the apex of the crest. H is the design head. Other elements of the crest.overflow or ogee type spillway
  • It should be taken care of, that the upper curve at the crest should be neither too sharp nor too broad.Fig. 14.2. Crest details of ogee type spillway.
  • Discharge over an ogee spillway is found out by following equation

\[Q= CL_{e}H^{1.5}_{e}\]