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Design flows for sewers have evolved from steady state, constant flows calculated from the number of fixtures connected to complex dynamic calculation based on storm conditions and expected, although often illicit, interconnections with the storm drainage system. This paper will examine the historical evolution of the theory behind defined design flow conditions through the selection of specific design storm conditions. A recommended decision flow chart for selecting design storm(s) conditions for sanitary sewer systems subject to rainfall derived infiltration inflow is presented. This paper also includes discussion of storm(s) conditions appropriate for evaluation of long term control plans and design of subsequent combined sewer improvements.

A variety of design flow criteria used in the latter part of the 19th century and early part of the 20th century are discussed. The theories underlying these early criteria will be described to clarify the basis of design for the many legacy sewers that form the backbone of today's urban sewer networks. The evolution of the infiltration and inflow criteria impacted by the Clean Water Act construction grants regulations of the 1970s and 1980s is also presented including a synopsis of wet weather criteria that have evolved in the past two decades.

This paper concludes with a discussion of design storm selection processes aligned with the risk based wet weather approaches recommended in the WEF Guide to Managing Peak Wet Weather Flows (2006). Each of the previously discussed design flow methodologies is evaluated for applicability in the risk management framework. A decision flow chart is presented that will assist the reader in selecting design storm (or flow) conditions most applicable to the level of risk considered appropriate for specific communities.

Specific examples drawn from a number of municipal experiences across North America will be described throughout the paper.

Document Type: Research Article


Publication date: 2007-01-01

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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