Draw a line from the downstream pressure / head back to the mile post of the upstream station maintaining the calculated slope.
If the line passes through the elevation line, move to the first upstream high point and continue drawing the line with the same slope back to the next elevation intercept or the station.
Slack line exists between any high point and the elevation intercept point.
A few added comments:
The fluid is at its vapor pressure in the slack region
The fluid in the slack region is flowing down the hill like a creek flowing down a hill (Open Channel Flow)
Hydraulic Gradient – Slack Line Conditions
Slackline
*Liquid is in free-fall* and is at approximately 0 psi (Open Channel Flow)
Pipe in slack line can be >70% empty of liquids
Liquid velocities are high
Prevalent at very steep elevation changes.
Length of the slack line can be reduced by increasing the back pressure, which raises the gradient.
Ground Profile
Gradient
180 psig
708 psig
How Slack Line Helps