A drop manhole is provided if

ANS:B - a branch sewer joins the main sewer at higher level

A drop manhole is provided if a branch sewer joins the main sewer at a higher level. Drop manholes, also known as drop shafts or drop chambers, are constructed at points where a branch sewer connects to the main sewer at a higher elevation. These manholes facilitate the smooth transition of wastewater from the higher branch sewer to the lower main sewer while maintaining the required gradient for proper flow. When a branch sewer joins the main sewer at a higher level, a drop manhole is necessary to allow the wastewater to drop down from the branch sewer to the main sewer level. Without a drop manhole, the sudden change in elevation could cause hydraulic issues such as turbulence, air entrapment, and potential blockages. Therefore, drop manholes are specifically provided to accommodate situations where branch sewers join the main sewer at higher elevations, ensuring efficient and uninterrupted flow of wastewater within the sewer network.