Broad-crested Weir

This is the simplest device for flow measurement. It is more suitable for large discharges. The width of the weir is taken as the width of the waterway. The following equations is used:

The discharge coefficient Cd equals 0.89. To design the weir, h1 is the only unknown and can be calculated from the equation. If the characteristics of the weir are known, the discharge can be evaluated from the equation.

Example: Design a broad crested weir for a maximum discharge of 3 m3/sec, where the channel width is 2.5 m and the depth is 1.4 m. Sol. B = 2.5 m Apply the equation get h1 = 0.86 m

Ogee type Weir

Ogee Type weir Ogee type weir is used as spillway for dams. The shape of the weir is selected such that the water flow is always in contact with the face of the weir. The shape of the weir face follows this equation:

Where x and y are the coordinates and h1 is the head of water above the crest level. The discharge can be evaluated using the following equation: Where Ce is the discharge coefficient (1.3) and bc is the crest width.

Example: Design an Ogee type weir to pass a discharge of 100 m3/sec. The crest width is 30 m and the weir height is 10m. Sol. Apply the equation … get h1 = 1.3 m Calculate the coordinates of the weir’s shape

Y 0 2 4 6 8 10 X 0 2.3 3.1 3.8 4.4 4.8