ogee weir, shape of an ogee,
The ogee spillway has a control weir which is ogee or S-shaped in profile. The upper curve of the ogee ordinarily is made to conform closely to the profile of the lower nappe of the ventilated sheet falling from a sharp-crested weir. Flow over the crest is made to adhere to the face of profile by preventing access of air to the underside of sheet. For discharges at designed head, the flow glides over the crest with no interference from the boundary surface and attains near-maximum discharge efficiency.
The upper curve at the crest may be made either broader or sharper then the nappe profile. A broader shape will support the sheet and positive hydrostatic pressure will occur along the contact surface. The supported sheet thus creates a backwater effect and reduces the efficiency of discharge. For a sharper shape, the sheet tends to pull away from the crest and to produce sub atmospheric pressure along the contact surface. This negative pressure effect increases the effective head and thereby increases the discharge.
This type of spillway can be easily used on valleys where the width of the river is sufficient to provide the required crest length and the river bed below can be protected from scour at moderate costs.
The profile of this spillway is made in accordance with the shape of lower nappe of a free falling jet, over the ventilated sharp crested weir. The shape of lower nappe of freely falling jet over a sharp crested weir can be determined by the principle of projectile. It generally rises slightly as it originates from the crest of sharp crested weir or and then falls to make the parabolic form. Now if the space between the weir and the lower nappe is filled with concrete or masonry, the weir so formed will have a profile similar to an "ogee" and hence called an "ogee weir" or "ogee spillway".
This lower nappe will then become the crest of spillway. Since the lower nappe of the free falling jet will be different for different heads over the crest of the sharp crested weir, the profile of the ogee weir is generally confined to the lower nappe that would be obtained for maximum head over the spillway (i-e up to the maximum reservoir level).
Normally, the upstream face of the spillway is kept vertical and the crest shape confirms to the lower nappe of the vertical sharp crested weir under maximum head.