Ogee Spillways
The ogee spillway is a modified form of drop spillway. Here, the downstream profile of the spillway is made to coincide with the shape of the lower nappe of the free falling waterjet from a sharp crested weir. In this case, the shape of the lower nappe is similar to a projectile and hence downstream surface of the ogee spillway will follow the parabolic path where “0” is the origin of the parabola. The downstream face of the spillway forms a concave curve from a point “T” and meets with the downstream floor. This point “T” is known as point of tangency. Thus the spillway takes the shape of the letter “S” (i.e. elongated form). Hence, this spillway is termed as ogee spillway.
The shape of the lower nappe is not same for all the head of water above the crest of the weir. It differs with the head of water. But for the design of the ogee spillway the maximum head is considered. If the spillway runs with the maximum head, then the overflowing water just follows the curved profile of the spillway and there is no gap between the water and the spillway surface and the discharge is maximum.
When the actual head becomes more than the designed head, the lower nappe does not follow the ogee profile and gets separated from the spillway surface. Thus a negative pressure develops at the point of separation. Due to the negative pressure, air bubbles are formed within the flowing water. These air bubbles air responsible for the frictional force (i.e. abrasion) which causes much damage to the spillway surface.
Again, if the head of water is less than the designed head, the waterjet adheres to the body of the spillway and creases positive pressure which reduces the discharge through the spillway.
The shape of the ogee spillway has been developed by U.S Army Corps Engineers which is known as “Water-way experimental station spillway shape”. The equation given by them is, X^{n} = K×H^{n–1}×Y, where, x and y are the coordinates of a point P on the ogee profile taking O as origin. K and n are the constants according to the slope of the upstream face of the spillway (figure below).
The value of K and n are given as follow:
Shape of u/s face of spillway |
K |
n |
Vertical |
2.0 |
1.85 |
1:3 (H:V) |
1.936 |
1.836 |
1:1½ (H:V) |
1.939 |
1.810 |
1:1 (H:V) |
1.873 |
1.776 |
Thus for different values of y, the values of x are determined considering the slope of u/s face. The value of r_{1}, r_{2}, a and b are also calculated. The results are tabulated for the constructional guidance.
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