A sloping apron is provided partly above the river bed and partly below the river bed in case of when TWC lies above the JHC at low discharges and below the JHC at high discharges. This is the correct option among the given choices (D).

Explanation:

Sloping apron:
A sloping apron is a structure that is provided in rivers or streams to protect the riverbed from erosion. It is constructed using suitable materials such as concrete, stone, or riprap. The apron is designed in such a way that it slopes gradually from the bank towards the riverbed.

TWC (Threshold Water Current) and JHC (Just Hauling Current):
TWC is the minimum velocity of water required to start the movement of sediment particles on the riverbed. It is the critical velocity at which the sediment particles just begin to move. On the other hand, JHC is the velocity of water at which the sediment particles are continuously transported without any deposition. It is the critical velocity at which the sediment particles are in a state of equilibrium and are being transported downstream.

Situation when TWC lies above the JHC at low discharges and below the JHC at high discharges:
In rivers or streams, the velocity of water varies with the discharge. At low discharges, the velocity of water is relatively low, and sediment particles tend to settle and deposit on the riverbed. In this case, the TWC is higher than the JHC. However, at high discharges, the velocity of water increases, and the sediment particles are continuously transported downstream without any deposition. In this case, the TWC is lower than the JHC.

Purpose of a sloping apron in this situation:
A sloping apron is provided partly above the riverbed and partly below the riverbed to prevent erosion and protect the riverbed from scouring. When the discharge is low, and the TWC is higher than the JHC, the sloping apron above the riverbed helps to dissipate the energy of the water flow and reduce the erosive forces. It acts as a transition zone and prevents the water from directly impacting the riverbed, reducing the chances of erosion.

On the other hand, when the discharge is high, and the TWC is lower than the JHC, the sloping apron below the riverbed helps to control the flow and prevent the formation of scour holes. It provides a smooth transition from the riverbed to the apron, reducing the velocity and energy of the water flow. This helps to minimize the erosive forces and protect the riverbed from scouring.

Overall, the sloping apron, provided partly above and partly below the riverbed, is designed to cater to different flow conditions and ensure the stability and protection of the riverbed in situations when the TWC lies above the JHC at low discharges and below the JHC at high discharges.