...
So, how is the depth of the pipe at the downstream end so much less than the depth of the pipe at the upstream end? This is because the ground is not level, and demonstrates the point that you can never use the pipe depths to establish if the pipe is flowing the correct way. The only way to prove this is by considering and calculating the altitudes of the ends of the pipe above sea level.
...
Understanding altitudes and depths.
This would normally be done by measuring the altitude of the cover level above seas level using GPS equipment, then measuring the depth of the pipe at each end with a tape measure, entering these values into WinCan VX, and then using the ‘Calculate Invert Levels’ tool in the ‘Tools’ ribbon to do the subtraction without user error.
...