The role of local roughness in the hydraulic capacity of sewer pipes

摘要

The sewer conduits which, are usually made of concrete, economically should have the least of cost. This leads to the smallest diameter of pipe, which can pass the required flow rate of wastewater with an acceptable hydraulic performance. The capacity of passing the flow in pipes is affected by the roughness of the wall. On the invert of sewer pipe deposited material not only block the flow but also leads to the loss of hydraulic capacity by increasing the roughness. Corrosion of concrete in urban drainage system can be caused by the generation of hydrogen sulfide, which can be accumulated in condensation water on the soffit of the pipe. There, it is oxidized to form sulfuric acid, which can cause serious damage to pipe material. This will change the roughness of the flow and leads to a higher degree of hydraulic loss. Observations show that the most serious corrosions take place on the levels of frequently changing wastewater surface on the sides of the pipe cross section as well as the soffit. The effect of roughness particularly at the end of design period is crucial. This paper presents the result of calculation on the hydraulic capacity of sewer pipes with different local roughness along the wetted perimeter. An equivalent roughness for a certain level of wastewater based on Darcy-Weisbatch formula is computed. Next, a comparison is made between the capacity of smooth and roughened pipe. Finally, as a second approach, calculations are performed on the discharge capacity of roughened pipes by the use of equivalent roughness in Manning equation and compared with results of the first approach. It seems that Darcy-Weisbatch equation leads to a more reliable information. The results show that under moderate damage around 25% of the capacity of sewer conduits will be reduced due to this unwanted roughness.