Control Structures

Control Structures Control structures are used to: Control discharge and/or Control water elevation

Sharp Crested Weir

Sharp-crested weirs are those overflow structures whose crest width in the flow direction is small (< 25 cm). There are many types of such structures, the following two types are the most common: Rectangular sharp-crested weir. V-notch sharp-crested weir.

Rectangular Sharp-Crested weir

The weir has a rectangular section. There are two geometrical factors affects the discharge: bc/B h1/p1 The following equation can be used either to design the weir or to evaluate the discharge:

Where Q is the discharge in m3/sec, Ce is a discharge coefficient, g is the gravitational acceleration, be is the effective weir’s width, h1 is the water head above the crest. For practical reasons, the effective width is taken to be the weir’s width, bc.

The value of the discharge coefficient can be obtained from the following graph.

The following steps show how to design the weir: Assume a reasonable value for h1 such as half the water depth of the waterway. P1 is then equal to ( d – h1 ). Assume the ratio ( bc/B ) equal 0.6. Obtain Ce from the graph. Apply in the equation to solve for bc. This is the first trail.

Recalculate the ratio ( bc/B ), Obtain a new value for Ce, Apply again in the equation to obtain a new value for bc, Repeat until convergence. If the characteristics of the weir are known, the discharge can be evaluated from the equation.

Example: Design a sharp crested weir according to the following data; Q = 0.4 m3/sec, B = 1.0 m, d = 0.8 m Sol. Assume p1= 0.4 m … h1 = 0.4 m h1/p1 = 1 … assume b/B = 0.6 from graph … get Cd= 0.61 Apply the equation … get b = 0.88 m

b/B = 0.88 … get Cd=0.65 Apply the equation … get b = 0.82 m b/B = 0.82 … get Cd=0.645 Apply the equation … get b = 0.83 m Close … then b = 0.83 m weir height = 0.4 m