Statement (I): The ogee spillway is a control weir having an S-shaped crest profile which provides a high discharge coefficient without causing cavitations. Statement (II): The crest profile of ogee spillway conforms to the lower nappe of flow over a ventilated sharp-crested weir and ensures a constant discharge coefficient for all heads.
Statement (I): The ogee spillway is a control weir having an S-shaped crest profile which provides a high discharge coefficient without causing cavitations. Statement (II): The crest profile of ogee spillway conforms to the lower nappe of flow over a ventilated sharp-crested weir and ensures a constant discharge coefficient for all heads. Correct Answer Statement (I) is true but Statement (II) is false
Concept:
Ogee spillway is an improvement upon the free overall spillway and is widely used with concrete, masonry, arch and buttress dams. Such a spillway can be easily used on valleys where the width of the river is sufficient to provide 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 the lower nappe of a free-falling jet over a ventilated sharp-crested weir. The ogee spillway having an S-shaped crest profile which provides a high discharge coefficient without causing cavitations.
∴ Statement (I) is true
The shape of the lower nappe of freely falling jet over a sharp-crested weir can be determined by the principle of the projectile. It generally rises slightly as it originates from the crest of a sharp-crested weir and then falls to make a parabolic form.
In practice, the actual head of water on spillway crest, called the operating head may be less or more than the designed head.
If the head of water over the spillway is more than the designed head, cavitation may occur.
If the discharge is higher than the designed discharge then negative pressure will develop on the underside of the lower nappe. Hence cavitation will occur.
When the ogee crest is formed to a shape differing from the ideal nappe shape or when the crest has been shaped for a head larger or smaller than the one under consideration, the coefficient of discharge will differ. A wider crest shape will reduce the coefficient of discharge while a narrower Crest Shape will reduce the coefficient.
The application of this concept is required to deduce the discharge flowing over a spillway when the flow is less or more than the design discharge.
∴ .Statement (II) is false as crest profile does not ensure a constant discharge coefficient for all heads.
