Improvement of the EXTRAN Block in Storm Water Management Model (SWMM4.4h)

The EPA Storm Water Management Model (SWMM) is a dynamic rainfall-runoff model for single-event or long term simulation. The EXTRAN block of this model can simulate storm sewer systems. Under flood condition, if there is not a surface channel, the current EXTRAN block loses water to the streets and it does not return to the storm sewer system. In reality, flooded water may actually return to the storm sewer system again through nearby junctions or manholes. While it is possible to specify a system of surface channels to route surface flooding, EXTRAN limits inflows from the surface back into the system, based only on the flow capacity available in the storm sewer system. It does not take into account that the rate of return to the system may be restricted by the street inlet through which the water must pass. Therefore, the existing EXTRAN block has a problem with returning flow to the system, and it is difficult to characterize the actual behavior of the storm sewer system under this type of condition. The City of Sacramento addressed this problem and made extensive modifications to the SWMM version 2.2 so that inflow back into the sewer system at a junction is restricted to the smaller of (1) available flow capacity of sewer system or (2) inlet capacity at junction. This algorithm works well in the Sacramento SWMM (S-SWMM) model, but the EXTRAN block of this model is a version older than the current EXTRAN block of the SWMM 4.4h model. Thus, it was decided to modify the EXTRAN block of the SWMM 4.4h to incorporate the inflow modifications in the S-SWMM model. This study will mainly add the subroutine OVFLOW of the EXTRAN block of the S-SWMM model to the EXTRAN block of the SWMM4.4h model since it models street flow. The purpose of this study is to integrate the FORTRAN source code of the EXTRAN block from S-SWMM model to SWMM 4.4h model. The revised EXTRAN block is tested on an experimental sewer system and is compared to SWMM 4.4h and S-SWMM. In addition, we investigate how the street flow contributes to inflow and direct rainfall runoff in S-SWMM and SWMM 4.4h. Simulation results generated by the models are expected to show the inlet restriction effect and the available flow capacity of the sewer system. It is proved that the S-SWMM 4.4h can embody the real urban storm water phenomenon more closely than the S-SWMM and SWMM4.4h.