The correct answer is: D. Neither silting nor scouring occurs at the bottom.
Self-cleansing velocity is the minimum velocity required to prevent the deposition of solids in a sewer. If the velocity is too low, solids will settle out and build up on the bottom of the sewer. This can lead to blockages and other problems.
Sewers are designed with self-cleansing velocities to prevent this from happening. The velocity is calculated based on the size of the sewer, the type of solids that are likely to be present, and the slope of the sewer.
If the velocity is correct, then neither silting nor scouring will occur at the bottom of the sewer. Silting is the deposition of solids, while scouring is the removal of solids. Both of these can cause problems, so it is important to design sewers with the correct self-cleansing velocity.
Here is a more detailed explanation of each option:
- Option A: The bottom is silted. This is not correct because self-cleansing velocity is designed to prevent silting.
- Option B: The bottom is scoured. This is not correct because self-cleansing velocity is designed to prevent scouring.
- Option C: Both silting and scouring occur at the bottom. This is not correct because self-cleansing velocity is designed to prevent both silting and scouring.
- Option D: Neither silting nor scouring occurs at the bottom. This is the correct answer because self-cleansing velocity is designed to prevent both silting and scouring.