Simply put, inflow/infiltration involves fluids flowing into the sewage system that should not normally do so – i.e. rain water. The gravity feed, which is severely undersized for the amount of inflow it receives, fills up very quickly during a rain storm.
Combine this with the fact that there is a volume restriction mandated by FTMSA as to how much sewage is permitted to be pumped through the force main from the Pump Station to FTMSA within a given period of time, and the result is a backup of sewage in the gravity feed.
With all of the sewage forcing its way into the line from higher elevations, the stacked manholes, located toward the end of the gravity side of the system in the lower elevations, cannot hold back the pressure and thus they begin to spew sewage through the manhole covers and the seams of the concrete stacks out onto the ground and into Beaver Run Creek and the above-mentioned tributary, literally for hours at a time.