Comparison of Unsteady Open Channel Flow Calculation Programs

M.P.Cherian and Karen Ridgway

ABSTRACT

The intent of this study is to compare the performance of three popular programs used to make unsteady flow calculations in open channels for specific example problems, including a dam break. The three programs evaluated include: HEC-RAS, SWMM5 and the Transient Analysis Program (TAP). The example problems include gradually varying and rapidly varying flow situations. The examples are not drawn from a real-world project and therefore the conclusions drawn from this study may only be problem specific.

The primary reason for the study is to enable the comparison of three programs which are based on different solution methods for the same governing equations, namely, the fully dynamic St. Venant’s equations. TAP is based on an explicit finite volume approach, the unsteady HEC-RAS program uses an implicit finite difference method, while the SWMM5 program uses an explicit finite difference method. Each program has intrinsic strengths and weaknesses.

TAP is best suited for rapidly varying flow situations and can be used for both open channel flow and closed conduit flow calculations. Its principal weakness is that it is computationally intensive in terms of run time, therefore it is not practical to model large systems using TAP. The time step it uses for computations is back calculated from the Courant condition and is of the order of milliseconds. It has good stability properties because of the small time step.

Unsteady HEC-RAS is best for open channel flow simulations and not well suited for closed conduit analyses. It has many built-in routines to accommodate many structures usually associated with riverine and open channel flow unlike the other programs in this study. Since it is based on an implicit solution, its stability properties are good. The smallest time step possible is 1 second.

The SWMM5 program is applicable to both open channel and closed conduit flow calculations and is useful in modeling large systems. Its drawback is that it is often plagued by instability issues which have to be mitigated using artificial elements.

The example problems utilized in this study demonstrate that for gradually varied open channel flow situations, all three models give somewhat similar results. However, for rapidly varying flow situations the best results are obtained with TAP and HEC-RAS. For extremely rapidly varying flow situations, it appears that TAP produces more reliable results; however, this cannot be verified since no comparison is made with field-verified measurements.


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