Control StructuresControl Structures

Control StructuresControl Structures

Control structures are used to:Control structures are used to:

• Control discharge and/orControl discharge and/or

• Control water elevationControl water elevation

Sharp Crested WeirSharp Crested Weir

• Sharp-crested weirs are those Sharp-crested weirs are those overflow structures whose crest overflow structures whose crest width in the flow direction is small (< width in the flow direction is small (< 25 cm).25 cm).

• There are many types of such There are many types of such structures, the following two types structures, the following two types are the most common:are the most common:– Rectangular sharp-crested weir.Rectangular sharp-crested weir.– V-notch sharp-crested weir.V-notch sharp-crested weir.

• Rectangular Sharp-Crested weirRectangular Sharp-Crested weir

• The weir has a rectangular section.The weir has a rectangular section.

• There are two geometrical factors There are two geometrical factors affects the discharge:affects the discharge:

– bbcc/B/B

– hh11/p/p1 1

The following equation can be used either The following equation can be used either to design the weir or to evaluate the to design the weir or to evaluate the discharge:discharge:

5.112

3

2hbgCQ ee

Where Where Q is the discharge in m3/sec,Q is the discharge in m3/sec,

CCee is a discharge coefficient, is a discharge coefficient,g is the gravitational acceleration,g is the gravitational acceleration,

bbee is the effective weir’s width, is the effective weir’s width,

hh11 is the water head above the is the water head above the crest.crest.

For practical reasons, the effective width For practical reasons, the effective width

is taken to be the weir’s width, bis taken to be the weir’s width, bcc..

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

The following steps show how to design The following steps show how to design the weir:the weir:

• Assume a reasonable value for hAssume a reasonable value for h11 such such as half the water depth of the as half the water depth of the waterway.waterway.

• PP11 is then equal to ( d – h is then equal to ( d – h11 ). ).

• Assume the ratio ( bAssume the ratio ( bcc/B ) equal 0.6. /B ) equal 0.6.

• Obtain CObtain Cee from the graph. from the graph.

• Apply in the equation to solve for bApply in the equation to solve for bcc..

• This is the first trail.This is the first trail.

• Recalculate the ratio ( bRecalculate the ratio ( bcc/B ),/B ),

• Obtain a new value for CObtain a new value for Cee,,

• Apply again in the equation to obtain Apply again in the equation to obtain a new value for ba new value for bcc,,

• Repeat until convergence.Repeat until convergence.

If the characteristics of the weir are If the characteristics of the weir are known, the discharge can be known, the discharge can be evaluated from the equation.evaluated from the equation.

• Example:Example:Design a sharp crested weir according Design a sharp crested weir according to the following data;to the following data;

Q = 0.4 mQ = 0.4 m33/sec, B = 1.0 m, d = 0.8 /sec, B = 1.0 m, d = 0.8 mm

• Sol.Sol.

Assume pAssume p11= 0.4 m … h= 0.4 m … h11 = 0.4 m = 0.4 m

hh11/p/p11 = 1 … assume b/B = 0.6 = 1 … assume b/B = 0.6

from graph … get Cfrom graph … get Cdd= 0.61= 0.61Apply the equation …Apply the equation …get b = 0.88 mget b = 0.88 m

b/B = 0.88 … get Cb/B = 0.88 … get Cdd=0.65=0.65

Apply the equation … get b = 0.82 mApply the equation … get b = 0.82 m

b/B = 0.82 … get Cb/B = 0.82 … get Cdd=0.645=0.645

Apply the equation … get b = 0.83 mApply the equation … get b = 0.83 m

Close … then b = 0.83 mClose … then b = 0.83 m

weir height = 0.4 mweir height = 0.4 m