This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Sediment Load”.
1. What are the types of sediment load?
a) Bedload and Suspended load
b) Bedload and Dissolved load
c) Bedload, Dissolved load and Suspended load
d) Suspended load and Dissolved load
Explanation: Bedload is that portion of sediment load that is transported along the bed by sliding, rolling or hopping. The dissolved load is the one that is chemically carried by water in the form of ions. When the material is maintained in suspension due to the turbulence of flowing water, it is called a suspended load.
2. Increased dissolved load gives the flooded stream its muddy color.
Explanation: Due to unusual high velocity in the flooded streams, there is heavy sediment transportation and deposition downstream. The increased suspended load may be visible giving the stream a muddy color.
3. At low velocity, the bed does not move at all but as velocity increases, it goes on assuming different shapes.
Explanation: Depending upon the discharge or the velocity of the water, the channel bed may be distorted into various shapes by the moving water. Ripples are superimposed on the bed when they first appear. At high velocities, ripples disappear and only dunes are left.
4. On increasing velocity beyond the threshold stage of motion, the bed develops ____________ shape.
a) dunes with ripples
b) saw-tooth ripples
d) flat surface
Explanation: A channel bed made of fine sand less than 2 mm diameter develops the shape of ripples of saw-tooth type when the velocity is increased beyond the threshold stage of motion. If the velocity is increased further, large periodic irregularities appear that are called dunes.
5. Which of the following statement is wrong?
a) Ripples do not occur if the size of the bed particles is coarser than 0.6 mm
b) Dunes are much larger in length and height than ripples
c) Crests of both ripples and dunes extend across the entire width of the stream
d) Ripples are less rounded than Dunes
Explanation: Both formations of ripples and dunes tend to occur in the form of short-crested waves. Sub-critical flow conditions exist in both these regimes at this stage i.e. Froude’s number is less than unity. Crests of both do not extend across the entire width of the stream.
6. When the velocity is further increased beyond the stage where sand waves are formed in association with the surface waves, the waves are then called as ____________
c) flat surface
Explanation: As the velocity is further increased, the surface waves become so steep that they move upstream and break intermittently and the sediment particles keep on moving downstream only. The flow becomes super-critical and sand waves are then called antidunes.
7. In the case of canals and natural streams, anti-dunes rarely occur.
Explanation: As compared to the ripples and dune regime, the resistance to flow is very small. The sediment transport rate is exceptionally high in this regime. Hence, it rarely occurs in natural streams.
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