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Sustainable stormwater removal systems (also called inflow controls or green solutions) are one of many tools being used by Portland's Bureau of Environmental Services (BES) to manage stormwater in its combined sewer system. In urban settings such as Portland, these systems are small scale and thus have relatively localized effects. Judiciously placed, inflow controls can help to relieve localized surcharging conditions in the sewer system and reduce sewer backups into homes. They can also help prevent more costly solutions from having to be implemented. Widespread use of inflow controls may substantially reduce peak flows and volumes to major components of the collector systems.

The difficulty lies in gaging, verifying, and predicting the effectiveness of these controls due to their small size relative to the overall systems they are serving. One particular challenge is to model these systems efficiently, even with the advent of newer sewer modeling engines and data management tools, such as Geographic Information Systems (GIS). The level of modeling resolution and requisite amount of information required can be beyond the capabilities of more traditional lumped modeling approaches and requires creative data management and model generation methods.

The City of Portland's Bureau of Environmental Services has, over the last six years, created a set of methods and tools for combined and sanitary sewer modeling called the Explicit Model Generator, Analysis, and Alternative Tool Set (EMGAATS). EMGAATS, using XP-SWMM as the core modeling engine, allows for the simulation of extremely local rainfall, runoff, impervious & pervious conditions, tree canopy, on-site flow routing, and sewer flooding conditions. This method and framework, presented at several past WEF conferences and in various WEF publications is explicit in that it models all facilities and impervious surfaces at the parcel level. BES has applied this technique to several of its combined sewer basin projects and has found that it more clearly defines the source of basement flooding problems, calibrates to flow monitoring with very little effort, and allows for the evaluation of low-impact inflow controls. The model provides Portland with an extremely flexible modeling framework, capable of scaling from service laterals all the way up to interceptor tunnels. This tool set is the basis for much of the planning and implementation of the City's inflow control and CIP programs. It has been shown to be a system capable of simulating site-specific inflow controls at an adequate level to determine their local and basin-wide effects as well as the resulting changes in the capital infrastructure that take advantage of their benefits.

This paper shall provide an brief overview of the primary components of the explicit model as they pertain to the modeling of green solutions (inflow controls), the approaches used within the explicit model for green solutions, examples of model results, and the model's applications relative to both planning and implementation of Portland green solutions overall.
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Document Type: Research Article

Publication date: 01 January 2007

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