1st International Conference on Rain Water Cistern Systems
Honolulu, Hawaii, USA - June 1982

Section 6: Related Topics

Page 360

Transient Mixed-Flow Models for Storm Sewer Systems

Charles C.S. Song
University of Minnesota, USA

Introduction

The traditional role of a sewer network has been primarily that of conveyance of waste water and storm water. A common design criterion was to ensure an adequate hydraulic capacity to convey a maximum design flow at a steady rate without surcharging the sewer. The design objective and criterion of a sewer network have undergone a fundamental change in recent years due to upgraded pollution control rules. To meet these goals, it has become increasingly clear that a substantial saving can be achieved by utilizing the in-line storage capacity of a sewer network. Most existing sewer networks have enough capacity to store the entire runoff due to a storm equal to or less than the one-year storm. The majority of storm runoffs can thus be stored for future treatment if there are provisions for a suitable control mechanism or some precautions against transient pressure.

A sewer network designed to store as well as convey storm water would undergo changes in flow regimes following a large storm event. Typically, the flow would initially be free surface flow and then begin to pressurize starting at the downstream end of the sewer system. At this time there would be a combined flow regime consisting of free surface flow and pressurized flow separated by one or more moving surge fronts. The magnitude of such a surge may build up to a significant amount as it moves upstream, and would generate severe water-hammer pressure upon collision with an upstream boundary. Clearly, a steady state or a kinematic wave type model, such as the SWMM model, is not suitable for a sewer network used as a storage-conveyance system. A complete dynamic model capable of simulating unsteady mixed-flow is necessary.

This paper describes some experiences with two dynamic models: (1) the Priessmann-Cunge-Wagner model and (2) the Song-Cardle-Leung model developed at the St. Anthony Falls Hydraulic Laboratory.

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