Comparing SWMM 5.1 Calculation Alternatives to Represent Unsteady Stormwater Sewer Flows

Abstract

The Storm Water Management Model 5.1 (SWMM) is a widely adopted dynamic hydrologic and hydraulic model often used to estimate runoff quantity and quality in urban drainage systems. SWMM’s unsteady flow algorithm, EXTRAN, is based on a link-node solution that enables it to represent typical stormwater inflows well. Yet, for rapid inflow conditions associated with more extreme inflows, predictions yielded by SWMM underestimate surges and sometimes underrepresent sudden changes sewer flow conditions. Recent research showed the benefits of introducing artificial spatial discretization (ASD) models in SWMM to represent rapid inflows in sewers. However, with the recent addition of the Preissmann slot algorithm in the SWMM formulation, a systematic evaluation of the performance of this pressurization algorithm in complex and highly dynamic inflow scenarios is still missing. The present study applied the conditions presented in the Storm Water Management Model Quality Assurance Report and compared the use of either link-node or ASD along the original EXTRAN and the new Preissmann slot algorithm in modeling results. The performance of each of the selected modeling alternatives was evaluated in terms of continuity errors and numerical stability. The findings obtained in this study indicate modeling result improvements with an adequate selection of temporal and spatial discretization.