Which of the following statements are true with regard to energy dissipation below a dam? (i) If the upper conjugate depth of a hydraulic jump is below the tailwater, little energy will be dissipated (ii) An upturned bucket will protect the dam from scour by moving material toward the dam. (iii) A secondary dam may increase tailwater height, thereby causing a hydraulic jump to form at the toe of the main dam. (v) A slopping apron above streambed level may be used to control the hydraulic jump so that it occurs on the apron. Correct Answer (I), (II), (III) and (IV) are correct Concept: Energy Dissipators: (i) These are the arrangements made to dissipate the kinetic energy of water on the downstream side near the toe of the dam and move away from it. (ii) Based on the relation between Tail water curve (T.W.C) and sequent depth or Jump height curve (J.H.C) (y2), there are different ways in which the energy dissipation may occur- 1. When TWC coincides with y2­ curve at all discharge: It is the most ideal condition of jump formation and the hydraulic jump will form at the toe of the spillway at all discharge A Simple concrete apron of length 5(y2 – y1) is sufficient. 2. When TWC is lying above y2 at all discharge: The hydraulic jump formed will be drowned by tail water and will dissipate less energy, since water is continuously flowing at high velocity along the channel bottom for a considerable distance. Expected solutions/devices to be used:- Constructing sloping apron above river bed level Roller Bucket type energy dissipator 3. ​When TWC lies below y2 at all discharges: (i) Expected solutions/Devices to be used:- If tail water is very low, the water may shoot up out of the above bucket and fall harmlessly into the river at some distance d/s of the bucket. This bucket is then known as ski bucket or upturned bucket and can be used for energy dissipation when tail water depth is insufficient or low at all discharges. Ski Bucket type dissipator requires sound and rocky river bed (i.e. stiff rock). Sloping apron can also be provided below the river bed and should be entirely in cutting. Subsidiary dam can also be provided. If tail water deficiency is small, a baffle wall or row of friction blocks may be provided. 4. When TWC lies above y2 at low discharges and lies below the y2 at high discharges: In this case, at low discharge, the jump will be drowned and at high discharges, tail water depth is insufficient. Thus the sloping apron partly above and partly below the river bed is to be provided so that it will control the hydraulic jump. 5. When TWC lies above y2 at low discharges and lies above the y2 at high discharges: Same as that of the above case.