Continuous Simulation Modeling for Sewer Systems

by Eric C. M. Bergstrom, R. W. Beck and Associates, Seattle, United States,

Abstract: The peaking-factor method of sanitary sewer design was developed to simplify the calculations in determining design flow rates. This method of analysis is conservative to the point that a large margin of safety is often provided. For new sewers, the peaking factor method can allow for the uncertainties of eventual development. However, in evaluating constructed systems or designing improvements to serve older development, the peaking-factor method of analysis may overestimate the actual flows and subsequently lead the engineer to overlook capacity available in existing sewers. This can result in the construction of unnecessary facilities. As an alternative, wastewater flows in a sewer collection system may be modeled using continuous simulation methods. This type of analysis routes diurnal curves or hydrographs through a sewer collection system. Continuous simulation computer software models available today can be used to reflect significant features of a system, such as long, minimally sloped interceptors. This type of modeling is more precise than the peaking-factor method in the depiction of flow rates. For an existing system, when a continuous simulation model is used in conjunction with flow monitoring, hourly flow rates into a treatment facility can also be predicted fairly accurately. In the design of new systems, an appropriate safety factor may be applied to the peak estimated flow rate determined from the model. The computer software models and techniques available today make it unnecessary to simplify sewer design calculations. This paper addresses the application of continuous simulation modeling for sewer analysis.

Subject Headings: Simulation models | Flow simulation | Computer models | Flow rates | Hydraulic models | Computer software | Sanitary sewers

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