Basic Hydraulics Alleviates a Thirty-Year-Old Overflow

A 36” forcemain experienced overflows since construction almost thirty years ago. Since prior attempts to alleviate the overflows proved unsuccessful, an understanding of the cause was deemed critical to developing a solution. Four hypotheses of possible causes of overflow were developed and subsequently tested with modeling software. The hypothesized causes included 1) Obstruction in the pipe downstream of the air release vault (high point), 2) Rapid pump ramp up, 3) Sudden flow meeting “standing” water, and 4) Air binding.Modeling indicated no problems with the forcemain or gravity sewer. A return to basic hydraulics and review of the actual conditions provided great insight. Pipe slope governs open channel hydraulics and review of the inspector’s marked up plans indicated that the pipe slope was not adequate to convey the design flow under open channel conditions. In addition, flow transitions from supercritical to subcritical were noted which would result in a hydraulic jump further exacerbating the situation. Design included replacing open pipe air vents along the pressure/gravity sewer with air release/vacuum valves. Construction was completed July 2017 eliminating overflows and achieving original pump station design flow. The subject project is an excellent example of the need to review basic theory even when the problem appears simplistic.