GIS Model Builder – An Automated Process for Building High Resolution SWMM Models for Quantifying RDII from Various Sources

Increasing need to build high resolution SWMM models that are capable of evaluating collection systems runoff and rainfall-derived inflow/infiltration (RDII) process at the source requires complex and lengthy process. We simplified this complex process by building a Model Builder script within ArcGIS environment that integrates and process more than 12 ArcGIS datasets to produce detailed description of the contributing RDII sources and their physical parameters. Constructed SWMM model facilitated a detailed evaluation of the potential impact of the proposed RDII remediation to reduce SSO activations to meet different level of services.

To address its RDII problems, City of Columbus has conducted detailed RDII studies within 12 priority areas, to mitigate and reduce RDII contributions. The plan include disconnecting all direct downspouts connections to sanitary sewers, rerouting downspouts away from foundations to streets, and also rehab manholes and lining public sewer pipes and right of way and private lateral service connections. Developing a successful RDII removal program demands robust hydrological and hydraulic models that can predict the quantity of RDII and its sources with certainty. Previous and ongoing research⁴ has shown that understanding and defining different sources of these RDII is critical for quantitation. This can be achieved by building high resolution models and calibrating them to long-term directed flow monitoring data. Increasing the resolution of the model parameters requires detailed and complete GIS data.

Primary sources of Inflow and Infiltration can be any combination of the following: Direct Downspout Connection, Foundation and Floor Drains, Lateral Service Connections, Manholes, Sewer Mains, and other sources. Building a high resolution model would require processing a large amount of GIS data to compute the above areas for each RDII loading node in the model. The process involved delineating detailed catchment per each conduit in the hydraulic model and computing each RDII sources such as residential roofs, commercial roofs, perimeters, sewer lengths in right of way, private properties, number of manholes in pervious area and impervious area, area of garages, length of stormwater pipes per each area etc. for each loading area.

As the service area becomes large and too many parameters involved, the process can quickly become involved and labor intensive. This presentation discusses using GIS model builder for creating an automated process that can be applied for generating SWMM models efficiently that can otherwise be impossible at large scales.

⁴ Quantifying Components of RDII as a Foundation for Integrated Solution, Gheith and Herr, CHI 2013