Ogee Spillway

Ogee Spillway :-

• Ogee spillway is an improvement over straight drop spillway and it can be used for masonry, concrete, arch and buttress dam.
• Ogee spillway can be used easily on valley regions where the width of the river is sufficient to provide the required crest length and the river bed below can be easily protected against scour at a moderate cost.
• The profile of an ogee spillway is made in accordance with the lower nappe of  free falling jet over a ventilated sharp crested weir.
• The lower nappe of a falling jet over a sharp crested weir is determined using principle of projectiles.
• The space between the sharp crested weir and lower nappe is filled with a concrete or masonry and the weir so formed has the profile similar to that of an "ogee" and hence is called as ogee spillway.
• Normally the upstream face of an ogee spillway is vertical and the crest shape conforms to the lower nappe of a vertical sharp crested weir, but if the upstream face of an ogee spillway is sloped then the crest shape should also conform to the lower nappe that would be obtained from an inclined sharp crested weir.
• However the crest of an ogee spillway is made to conform one particular nappe that would be obtained at one particular head. This head is called designed head.
• But in actual practice, the head of water over the spillway ( operating head ) is more or less than the designed head.
• When the operating head is more than the designed head, then cavitation may occur.
• When the operating head is less than the designed head, then the falling jet would adhere to crest of ogee spillway, causing positive hydrostatic pressure, thereby reducing discharge coefficient of th weir.