ANS:A - 1 in 60

The gradient required for generating self-cleansing velocity in a sewer depends on various factors such as the diameter of the sewer, flow velocity, and the characteristics of the wastewater. Self-cleansing velocity refers to the minimum velocity required to prevent sediment deposition and maintain the sewer in a self-cleaning state. While the specific gradient required can vary depending on local standards, guidelines, and design considerations, a commonly used rule of thumb is that a gradient of approximately 1 in 100 (1%) is sufficient to achieve self-cleansing velocity for most sewer diameters. This means that for every 100 units of horizontal distance, there is a 1-unit change in elevation. Therefore, the closest option listed is:

• 1 in 100