Abstract

The City of Portland, Oregon, Bureau of Environmental Services (BES) and a consultant team are currently evaluating alternatives to reduce the potential for basement flooding in the Holladay, Stark, and Sullivan combined sewer basins. To assist with hydraulic analysis for this project, BES developed a detailed SWMM model of the basin using a suite of geographic information system (GIS) and database tools to automate many of the steps for data extraction, variable assignments, and model building.

The detailed model contains essentially all of the pipes in the collection system and utilizes surface water and direct connection subcatchments to place runoff into the system in the correct location. The subcatchments are small, generally representing one city block, therefore much of the uncertainty associated with lumping RUNOFF parameters is eliminated. A virtual raingage program was used during model calibration to triangulate between real raingages and create a separate rainfall interface file for each quartersection in the model.

The detailed GIS approach and virtual raingage produced model results that correlated very well with flow monitoring data. A GIS tool was developed to analyze flooding of individual parcels by overlaying modeled hydraulic grade lines with estimated finished floor elevations obtained from a digital terrain model (DTM). The detailed SWMM model shows that much of the system capacity problems are in the local areas at the upstream ends of the system and that the local collection system tends to hold water back from the downstream trunklines in the system. This better understanding of where system capacity constraints occur allows BES a top down view of the basin during analysis. Local inflow control alternatives are considered first and pipe upsizing along with passing the problem downstream is the second alternative.

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