The Hazen-Williams Nomograph (the upper chart) was set-up for an 8-inch force main, to show the various hydraulic conditions for a fixed coefficient of 150. The Manning's Nomograph (the lower chart) was set-up for an 8-inch gravity sewer, to show the various hydraulic conditions corresponding to a fixed pipe slope of 0.004. The purpose here was to show the effect of discharging an 8-inch diameter force main into an 8-inch gravity sewer. The common denominator is flow. This is an actual project, where the Land Developer downstream from an existing pumping station,
is being required to build a new pumping station. The existing station has no flow meter, and historical data based on hourmeter readings, only provided average flow, not peak flow. But the existing station was known to have a severe Infiltration/Inflow problem. The operator had not been on the job long, but knew exactly what was going on at the station. Responding to the engineer's question, "what happens at the station during a hard rain" he replied using such words as:

"responds almost immediately to rain"
"pumps 24-hours a day, you know, they just run"
"clear water"
"pumping rain"

In this actual case history, the design consultant worked for the Land Developer, who wanted to insure that the new station does not overflow. If the entire collection system was new, the new station could simply be oversized to handle the unknown flow, but tying into any existing system can be problematic. In this actual case history, It was determined that an 8-inch force main was definitely needed, but the only available tie-in was to an 8-inch gravity sewer. This is how the question of pipe coefficients came into play.