What factor significantly affects the slope of a gravity wastewater line?

The factor that significantly affects the slope of a gravity wastewater line is the pipe bedding. Proper bedding ensures that the pipe is well supported along its entire length, which directly influences how the pipe settles and its alignment. If the bedding is inadequate, it can lead to improper slope and alignment, causing issues such as insufficient flow or blockages due to pooling or uneven settling of the pipe.

While other factors like pipe material, pipe diameter, and flow rate are important in designing and assessing the performance of wastewater systems, they don't affect the slope as fundamentally as the bedding does. For instance, while the material can influence durability and resistance to corrosion, and diameter can affect flow capacity, the slope itself is mainly determined by how well the pipe is situated within its bed.