Abstract

This chapter describes the capabilities and features of the recently developed Illinois transient model (ITM) for simulating the flow dynamics (transient and non-transient conditions) in combined storm sewer systems, ranging from dry bed flows, to gravity flows, to partly gravity partly surcharged flows (mixed flows), to fully pressurized flows (water hammer flows). ITM, which was originally developed at the University of Illinois at Urbana-Champaign, is a finite volume (FV) model that can handle complex boundary conditions such as drop shafts, reservoirs, closing and opening of gates as a function of time, and junctions with any number of connecting pipes and any types of horizontal and vertical alignment. ITM is open source code that is in constant development and its releases are made available on a regular basis. In the current version of ITM (v. 1.3, September 2010), the free surface region is modeled using the one dimensional (1-D) Saint-Venant equations. The pressurized region is modeled using the 1-D compressible water hammer equations. Open channel pressurized flow (mixed flow) interfaces are modeled by enforcing mass, momentum and energy relations across the interfaces together with Riemann solvers at the sides of mixed flow interfaces. This version of ITM is referred to as the two equation model. The current version of ITM is superior to other models of its kind because it is robust, can simulate mixed flows (simultaneous occurrence of free surface and pressurized flows) when using actual pressure wave celerities (~1 000 m/s), and because no Preissmann slot assumption is made to simulate pressurized flows (water hammer flows). ITM has been applied to several existing combined sewer systems in the U.S. and around the world, to improve the understanding of the flow dynamics of these systems.

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