Criteria of Minimum Shear Stress vs. Minimum Velocity for Self-cleaning Sewer Pipes Design

Sedimentation in Urban Drainage Systems is a huge problem that allow changes in cross-sectional area, and therefore changes in pipe's roughness. The increase of suspended solids deposition and consolidation in pipes affect the velocity and shear stress distribution inside these elements [1]. Some authors have proposed methodologies to prevent sedimentation phenomena in sewer pipes. However, such methodologies requires variables estimation that result very difficult in most cases and their estimation could affect the resulting network pipes design. This paper aims to evaluate different methodologies and equations proposed to ensure self-cleaning in sewer and pluvial pipes systems design. Comparison was made by graphical analysis of values established in literature in order to assess restrictions of each design. Three methodologies were compared: Traditional criteria, Equation of Macke (1982), and Tractive Force Methodology (ASCE). A design methodology was proposed as an optimization problem considering a cost variability analysis using cost functions. Results shows that self-cleaning criteria during design process must be included in cases where the relation between pipes' flow and terrain slope exceeds the relation found by this study.