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The suction pressure on an ogee spillway is caused when the head on the spillway is:

The suction pressure on an ogee spillway is caused when the head on the spillway is: Correct Answer > the design head

Concepts:

An ogee spillway is designed for maximum head up to RL of maximum reservoir level.

There are almost no chances of operating head exceeding the design head. However, when the operating head of the spillway is more than the design head, then the lower nappe of the falling jet may leave the ogee profile. This generates negative pressure at the point of separation which leads to the formation of cavities in the water resulting in Cavitation

 If the head of water over the spillway is less than the design head, the falling jet would adhere to the crest of the ogee spillway. This causes positive hydrostatic pressure and thereby reduces the discharge coefficient of the weir.

Related Questions

An ogee spillway of a concrete gravity dam having FRL of 328 m and MWL of 340 m is provided with vertical gates between piers erected on the spillway. The effective length of the spillway is 60 m. The discharge through the spillway when gates are opened up to the actual reservoir level of 331.0 m will be_____________
The discharge passing over an ogee spillway is given by (where, L is effective length of spillway crest and H is the total head over the spillway crest including velocity head.)
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.
Two centrifugal pumps P and R are available for use in a pipe flow system and their head-discharge characteristics are as follows. Pump P Pump R Discharge (cumecs) Head (m) Discharge (cumecs) Head (m) 0 40 0 45 0.12 35 0.14 40 0.2 28 0.24 35 0.3 18 0.3 28   The following statements (S1, S2, and S3) pertain to the head and discharge values during actual operation. Neglect losses in the system. Which of the following statements are correct? S1: When the two pumps P and R are connected in parallel, the discharge is 0.36 cumecs corresponding to a head of 35 m. S2: When the two pumps P and R are connected in parallel, the discharge is 0.36 cumecs corresponding to a head of 70 m. S3: When the two pumps P and R are connected in series, the discharge is 0.3 cumecs corresponding to a head of 46 m.
The W.E.S downstream profile of an ogee spillway can be represented by an equation: Xn = K Hd n-1. Y Where x and y are the coordinates of the points on the crest profile and K and n for a vertical upstream spillway would respectively be___________________
When the gated sluices are provided through the body of the dam spillway then the ogee spillway structure shall normally be of Corbel type.
If the operating head on an ogee spillway is more than the design head then ________________
N.P.S.H. the net positive suction head, is calc. by the absolute suction head _________ the vapor pressure of the liquid being pumped.
N.P.S.H.A. is the net positive suction head available and must be _________ than N.P.S.H.R. the net positive suction head required.
In the functioning of an ogee spillway, the operating head ______________________