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

With self-cleansing velocity in sewers, scouring occurs at the bottom. Self-cleansing velocity refers to the minimum velocity required in a sewer to prevent sediment deposition and maintain a self-cleaning state. When the flow velocity in the sewer reaches or exceeds the self-cleansing velocity, it has enough energy to keep sediments suspended and prevent them from settling at the bottom of the sewer. Instead, the flow is capable of dislodging and carrying away any sediment that may have accumulated, leading to scouring of the bottom surface. Therefore, the correct statement is: scouring occurs at bottom.