Obstructions caused by roots, sediment, grease and other materials increase head loss and reduce carrying capacity in gravity sewers. While hydraulic models such as SWMM or SewerGEMS can calculate the head loss from distributed roughness with Manning’s equation or the point head loss for a variety of manholes, it is difficult to determine the impacts of obstructions within the pipes. Representing the head loss due to obstructions is not automatically considered in conduit elements. Instead, pipes that have a known obstruction are split into two conduit elements with a bolted manhole element in between. Then, the manhole element uses the standard head loss method with a set minor head loss coefficient (K). Ideally, these losses should be modeled using the minor head loss equation in the energy equation for the pipe itself. There are also limitations to determinate analytically the minor head loss coefficient because it depends on type of the obstruction, its geometry, and hydraulic properties of the flow in the pipe. Laboratory experiments at Wilkes University were conducted to determine the relationship between K and other properties over a wide range of conditions in a 7 m long, 100 mm diameter clear PVC pipe with an adjustable slope, flow, and several depth measurement locations along the pipe. Different sizes, shapes and types of obstructions were inserted into the pipe to measure the impacts. The results for solid obstructions shows K values were function of the type of obstruction, percent blockage, and Reynolds Number.
