Hydraulics Engineering

1. Hydraulics Engineering Engr. Muhammad Hassan Hassan25.arif@gmail.com

2. Flow Measuring Structures • Weirs are overflow structures build across channels to measure the volumetric rate of flow of water. • It is an obstruction that causes the liquid to rise behind the weir and flow over it. • The crest of the measuring weir is normally perpendicular to the direction of flow.

3. Constructed from metal, masonry or concrete etc. • Ussally Weirs are identified by the shape of their opening or notch.

4. Types of Weirs There are many types of weirs depending upon their shape, nature of discharge, width of crest and nature of crest. But the following are important from the subject point of view: • According to the shape: • Rectangular weir • Cippoletti weir

5. According to the nature of discharge: • Ordinary weir • Submerged or drowned weir

6. According to the width of crest: • Narrow crested weir • Broad Crested weir • According to the nature of crest: • Sharp crested weir • Ogee weir

7. Broad Crested Weir • A weir having a horizontal or nearly horizontal crest sufficiently long in the direction of flow • These support the falling jet over their crest in the longitudinal direction. • Discharge over a rectangular weir can be calculated by using the formula. • Cd is the coefficient of discharge and L is the length of crest.

10. Consider a broad-crusted wire as shown above. • Let A and B are u/s and d/s ends of the weir. • Let L is length and Cd is coefficient of discharge. • H is the head at u/s and h is at d/s of weir. • Let V is velocity of water at B now applying energy eq. at A&B (considering the approach velocity to be zero)

11. The discharge over weir at B is given by Q= Cd*Area of Flow*Velocity Substituting the value of V

12. If Cd, L and g are constant, then discharge will be maximum when is maximum. • Differentiating with respect to h and equating to zero

13. Substituting the values

14. Sharp Crested Weir • For relatively small flows, the rectangular weir could be quite narrow. • Q In this case the value of H over the weir will be so small that the nappe will not stay clear rather it will cling to the plate. • Thus the use of triangular weir comes into practice as it function for relatively low flows.

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16. The most common types of sharp-crested weirs are rectangular, trapezoidal (Cipolletti), and V-notch weirs.

17. Rectangular Weir

18. Cipolletti Weir

19. The Cipolletti weir is trapezoidal in shape. The slope of the sides, inclined outwardly from the crest, should be one horizontal to four vertical.

20. V-Notch Weir

21. The fundamental equation for discharge over a triangular v-notch weir is given by

22. Velocity of Approach • Sometimes a weir is provided in a stream or a river (in storage reservoir value of approach is taken as zero) to measure the flow of water. • In such a case, the water approaching the u/s of weir has got some velocity of approach and is assumed to be uniform over the whole weir. • In deriving formula for discharge over broad crested weir, we have ignored velocity of approach but in actual practice velocity of approach of water is sure to affect the discharge over the weir.

23. Let a = x- sectional area of channel on u/s of weir • Additional height / head due to velocity of approach

24. Thus if velocity of approach is also considered then total height above the weir is • Discharge over the weir will then be given by

25. Flumed, Unflumed Structures • In every hydraulic structure (i. e. Bridge, regulator etc) which is constructed across an open channel, a few openings are left to allow the water to pass. • If the total width of all these openings is practically the same as that of the channel, such a structure is called full width or unflumed structure. • But generally the total width of such a structure is kept much less than the width of the channel to keep economy in its construction and to increase its utility. • Such a structure whose width is less than the width of channel is called flumed structure.

26. A flumed structure used for the measurement of the quantity of water is called venturiflume. • The difference between venturimeter and venturiflume is that in venturimeter the flow is under pressure whereas in a venturiflume the flow is under gravity and pressure is atmospheric. • Following two types of venturiflume are important from subject point of view. • Non-modular venturiflume • Modular venturiflume

27. A non -modular venturiflume consists of Convergent cone • It is short part of the channel which converges from b1 to b2. • It is also called inlet of the venturiflume. Throat • It is the small portion of the channel in It is in which width b2 is kept constant. Divergent cone • It is the small portion of the channel which diverges from width b2 to again width b1.

28. Modular Venturi Flume • It is the type of V. F. in which the width of throat is decreased to such an extent that the depth of water in throat is equal to critical depth. • The velocity of flow through the throat corresponding to critical depth is also critical.