Question

1. Write at least two sentences in your own words to explain the difference of using flexible boundary channel and the rigid boundary channel (based on the design considerations presented here).

2. Answer True or False and justify your answer:

    a. The rigid boundary channel is recommended for channels with very high velocities because it can easily resist the shear stress caused by the water flow.

    b. The trapezoidal cross section (with a value of m = 4 or more) is always more appropriate to be used instead of the rectangular channel in areas in which the neighboring land use restricts the channel top width.   

Answer 1

Q.1 the difference between the flexible boundary channel and the rigid boundary channel

i) Rigid channels are those in which the boundary is not deformable. The shape and roughness magnitudes are not functions of flow parameters. For example, lined canals and nonerodible unlined canals.

the wherein flexible channel, not only depth of flow but also bed width, the longitudinal slope of the channel may undergo changes with space and time depending on the type of flow.

ii) A general mobile boundary channel can be considered to have four degree of freedom. In rigid channel we have one degrees of freedom.

Q. 2

a) True, In Rigid channels the flow velocity and shear stress distribution will be such that no major scouring, erosion or deposition will take place in the channel and the channel geometry and roughness are essentially constant with respect to time. Rigid linings are traditionally used on steep grades due to high velocities and shear stresses and may be used for areas where channel width is restricted and the higher flow capacity is needed. Rigid channels require channel protection or energy dissipation at the termination point to prevent scour due to the high outlet velocities.

b) True,  The trapezoidal cross-section appropriate choice for this condition because if value of m is greater or equal to 4, top width will not increase to much due to the inclination of sides. And also trapezoidal section is Best hydraulic section that is maximum discharge is obtained at the same velocity and Velocity shear is less due to more contact area.