Introduction:
Stone pitching or riprap is a method used in civil engineering to prevent erosion of the upstream (U/S) and downstream (D/S) faces of structures like embankments, dams, and bridges. It involves placing large stones or rocks on the slopes to protect them from the erosive forces of water.

Explanation:
Erosion of U/S and D/S face:
One of the main purposes of stone pitching or riprap is to prevent erosion of the U/S and D/S faces of structures. When water flows over these slopes, it exerts a force that can remove soil particles and cause erosion. This erosion weakens the slope, leading to instability and potential failure of the structure. By placing stones or rocks on the slope, the erosive forces of the water are dissipated, protecting the U/S and D/S faces from erosion.

Over-topping:
While stone pitching or riprap can help reduce over-topping to some extent, it is not the primary purpose of this method. Over-topping refers to the situation where water flows over the top of a structure, such as a dam or embankment. This can occur when the water level exceeds the capacity of the structure or when the structure is not designed to handle the anticipated flow. While stone pitching can provide some resistance to over-topping, its main function is to protect the slopes from erosion.

Cracking due to frost action:
Frost action occurs when water freezes and expands within the pores and cracks of rocks and soil. This expansion can cause cracking and damage to the slope. While stone pitching can provide some protection against frost action by covering the surface and reducing direct exposure to water, it is not the primary purpose of this method. Other measures such as proper drainage and insulation may be required to minimize the effects of frost action.

Gully formation:
Gully formation refers to the creation of deep channels or ravines due to the erosive action of water. When water flows over unprotected slopes, it can concentrate its energy on specific areas, leading to the formation of gullies. These gullies can cause further erosion and instability of the slope. Stone pitching or riprap helps to distribute the energy of flowing water, preventing the concentration of erosive forces and minimizing the chances of gully formation.

Conclusion:
In conclusion, stone pitching or riprap is primarily provided to prevent erosion of the upstream and downstream faces of structures. It helps to dissipate the erosive forces of water, protecting the slopes from erosion and maintaining the stability of the structure. While it can provide some resistance to over-topping and minimize the effects of frost action, its main function is to prevent erosion and gully formation on the slopes.