Optimization Technique for Design of Gravity Sewers and Related Aspects

Hydraulic design of piped gravity sewer systems needs to be optimized for stipulated design guidelines. Though separate systems for transporting storm run-off and sanitary flows are preferred, many cities are forced to use combined sewer systems to jointly transport both flows. Faulty and ad hoc design concepts can undermine sewer hydraulics and lead to flooding. At times, maximum depth of excavation for new sewers is stipulated at the project outset based on site conditions like congestion, soil characteristics, adjoining existing infrastructure, constructability, space constraints, and socioeconomic and environmental aspects. This paper summarizes a set of evaluation parameters used to assess the efficiency of a system's hydraulic design, including equivalent diameter (D_e), pipe utilization factor (f_PU), equivalent slope (S_e), equivalent invert depth (I_e), and equivalent velocity (Ve). The approach delineated can be a tool to ensure consistency in design, eliminate surcharge, and reconcile competing system operational constraints.