Looped sewer networks can be formed by adding relief sewers to increase the conveyance capacity of an existing dendrical sewer system. It has been observed that looping creates oscillation and reversal of Bows in sewers under certain conditions which have adverse effects on systems' operation. Effective use of hydrodynamic modeling for detailed hydraulic evaluation is instrumental for investigating the potentials for such incidents and providing proper remedies. Flow variations over time at critical junctions and sewers in loops that are obtained from the dynamic modeling are reviewed so that relief sewers may be designed to prevent them in an iterative process. A case is studied to illustrate the occurrence of such incidents in a real sewer system and their impacts on the overall operation. In addition, the case study includes in-depth discussions of development and application of hydrodynamic models. Results of the study suggest that designing relief sewers to form loops be avoided in order to eliminate the potential of any such incidents. Otherwise, detailed hydraulic analysis for looped networks with dynamic flow routing models (such as EXTRAN block in the USEPA SWMM) is necessary with a wide range of drainage conditions which extend beyond design conditions and parameters that they may encounter during their lives of service. (C) 1998 Elsevier Science Ltd. All rights reserved.