Simulating head losses in hydraulic drop structures in SWMM, with a little help from CFD

Peter Klaver and Jason Rutyna, LimnoTech, Ann Arbor, MI, USA, Dave Collins, David J. Collins Engineering, Portland, OR, USA and Kurt Robinson, City of Portland, Portland, OR, USA


The City of Portland, Oregon has cause to model instances of street flooding in the vicinity of hydraulic drop structures that feed into the Willamette Combined Sewer Overflow tunnel system. Reproducing the events with a collection system model based on the U.S.EPA’s Storm Water Management Model (EPA SWMM) requires the representation of friction losses in the vortex generator structures. A modeling study was undertaken to better understand flows and losses in these structures, leading to enhancements to the City’s collection system model to benefit the assessment of mitigation alternatives. A conceptual approach to representing the head losses in vortex structure generators within a SWMM-based modeling framework was developed, and the general applicability of the approach was tested through a series of computational fluid dynamics (CFD) model simulations covering a range of flow conditions. The CFD modeling demonstrated that the conceptual approach could represent head losses under the conditions of primary interest, which were fully submerged conditions with the potential to cause surface flooding. When the approach was implemented in EPA SWMM, the observed flooding events were reproduced at a qualitative level.

The model study area focused on the Alder Conduit, which includes two vortex generator structures: one that drops wet weather flows from a higher-level sewer into the Alder Conduit, and a second that drops the full Alder Conduit flows into the tunnel system by way of the Alder Shaft. The interaction between the two structures was important in determining the flooding potential. The CFD modeling was performed using OpenFOAM, which is a freely available, open-source platform for a wide variety of CFD applications. Model runs included simulations of the observed events, and a wider range of elevation and flow boundary conditions for the development of stage-discharge curves. Relationships between flow and head loss were developed and then applied to an EPA-SWMM model of the Alder Conduit system, which was then able to reproduce the observed flooding at a qualitative level; that is, the location and extent of flooding was consistent with the observations, for which only anecdotal and photographic evidence was available.

This paper will provide an overview of the circumstances leading to the observed street flooding, details of the modeling approach, and a description of how the CFD results were translated into flow/head loss relationships for use in the SWMM-based model. The overall goal of the paper is to demonstrate how a focused modeling study can be used to enhance a larger-scale approach to operating and managing a collection system.