ogee weir, shape of an ogee,
OGEE SPILLWAY
OGEE SPILLWAY
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.
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