ANS:D - neither silting nor scouring occurs at the bottom.

1. The bottom is silted:
   • When sewers are designed with self-cleansing velocity, it means that the flow velocity within the sewer is sufficient to prevent sediment deposition and maintain the sewer in a self-cleaning state.
   • If the flow velocity is not adequate to prevent sediment deposition, silt and other debris may accumulate at the bottom of the sewer over time. This buildup of sediment can lead to reduced flow capacity, increased frictional resistance, and potential blockages or backups in the sewer system.
   • Siltation of the sewer bottom can occur if the flow velocity is too low to entrain and transport sediments, or if there are irregularities in the sewer profile that create areas of low flow velocity where sedimentation can occur. In such cases, periodic cleaning or maintenance may be required to remove the accumulated sediment and restore the hydraulic capacity of the sewer.
2. The bottom is scoured:
   • When sewers are designed with self-cleansing velocity, the flow velocity is typically high enough to prevent sediment deposition and promote scouring of the bottom surface.
   • Scouring refers to the process by which flowing water dislodges and transports sediments from the bottom of the sewer, maintaining it in a clean and self-cleaning state. The turbulent flow in the sewer helps to lift and suspend particles from the bottom surface, preventing sediment buildup.
   • Scouring is important for keeping the sewer clear of debris and sediment, ensuring unobstructed flow and minimizing the risk of blockages or backups. It helps to maintain the hydraulic capacity and efficiency of the sewer system, reducing the need for frequent cleaning and maintenance.
3. Both silting and scouring occur at the bottom:
   • In some cases, sewers may experience both silting and scouring at the bottom, depending on factors such as flow velocity, sediment load, and sewer design.
   • If the flow velocity is insufficient to prevent sediment deposition, siltation may occur, leading to the accumulation of sediment at the bottom of the sewer.
   • However, in areas of higher flow velocity or turbulence, scouring may still occur, especially during peak flow conditions or when there are changes in sewer gradient or alignment that promote scouring action.
   • The balance between silting and scouring depends on various factors, including the design parameters of the sewer, the characteristics of the wastewater, and the maintenance practices implemented by sewer authorities.
4. Neither silting nor scouring occurs at the bottom:
   • It is unlikely that neither silting nor scouring would occur at the bottom of a sewer designed with self-cleansing velocity.
   • The purpose of designing sewers with self-cleansing velocity is to ensure that flow velocity is sufficient to prevent sediment deposition and promote scouring action, thereby maintaining the sewer in a clean and self-cleaning state.
   • If neither silting nor scouring occurs, it suggests that the flow velocity may not be adequate to achieve self-cleansing conditions, or there may be other factors inhibiting sediment transport and cleaning action within the sewer.

In summary, while sewers designed with self-cleansing velocity aim to promote scouring and prevent silting at the bottom, the actual outcome depends on various factors including flow velocity, sediment load, sewer design, and maintenance practices. The ideal scenario is to achieve effective scouring while minimizing silting to ensure the hydraulic efficiency and longevity of the sewer system.