Generally a stage versus discharge relation for the outfall channel is useful. Refer to the in Chapter 7 for considerations and a procedure leading to the development of a stage versus discharge relation in an outfall channel.

As a normal design practice, calculate the hydraulic grade line when the tailwater surface elevation at the outlet is greater than the soffit elevation of the outlet pipe or boxes. If you design the system as a non-pressure system, ignoring junction losses, the hydraulic grade line eventually will fall below the soffit of the pipe somewhere in the system, at which point the hydraulic grade line calculation is no longer necessary. Generally, check the hydraulic grade line. However, such calculations are not needed if the system has all of the following characteristics:

If the proposed system drains into another enclosed system, analyze the downstream system to determine the effect of the hydraulic grade line.