Q1: Self-cleansing velocity is

ANS:C - velocity at which no accumulation remains in the drains

Self-cleansing velocity refers to the velocity of water in drainage systems, particularly sewers, at which no accumulation of solids or debris remains in the drains. It is the minimum flow velocity required to prevent sedimentation and deposition of solids along the sewer walls. When the flow velocity in a sewer reaches the self-cleansing velocity, it is sufficient to keep the sewer pipes clear of debris by transporting solids along with the flowing water, preventing blockages and buildup. This velocity ensures that sediments and other materials are effectively flushed out of the sewer system, maintaining its hydraulic capacity and preventing operational issues. Therefore, the correct definition of self-cleansing velocity is: Velocity at which no accumulation remains in the drains.