RDII contribution from new developments – design standard versus flow monitoring and continuous modeling approach

Shyam Prasad and Hazem Gheith, ARCADIS, Columbus, OH, USA, Rob Herr, Greg Barden and Fang Cheng, City of Columbus, Columbus, OH, USA


Estimating RDII (Rainfall Dependent Inflow and Infiltration) contribution for future developments is critical for designing sanitary sewer systems.  Municipalities and sewer utilities usually develop design standards based on: tributary areas contributing to the future development, population design densities, local zoning laws and an assumed RDII contribution.  While the hydraulic design standards such as sewer minimum grade, design depth from the ground surface elevation and cleaning velocities can be set easily based on literature values, hydrology and capacity assumptions can be challenging for design purposes.

The City of Columbus sanitary sewer design manual calls for a design flow defined as follows.  Design capacity shall not exceed the 50, 75 and 92 percentage of pipe flowing full capacities for 8 to 15, 18 to 27, and 30 inches and larger diameter pipes. Where, design flow is computed as sum of peak sanitary flow and the total RDII contribution up to the point in question.  The peak sanitary flow component is average sanitary flow (0.0002 CFS per capita based on 130 GPCD) times a peaking factor of 3.5.  The design standard for infiltration is 0.003 CFS per acre of total area tributary to the portion of the collection system under analysis. 

The design standards are essential in absence of flow monitoring data and hydrological and hydraulic models.  However, the City of Columbus maintains a system-wide SWMM model representing its complex collection system.  Under the Sewer System Capacity Model 2012 (SSCM2012) – capital improvement update project, the current system-wide SWMM model has been updated using physically based approach that utilizes SWMM groundwater modules for modeling RDII components to support its integrated planning.  The SWMM model has been built at house level detail to understand the RDII components from different sources entering the collection system. 

A new approach has been developed for estimating RDII contribution from future developments using calibrated high resolution SWMM model, understanding of I&I sources in the system along with historical flow monitoring data.  This approach includes analysis of sewer and house ages and their relative RDII contributions to the collections system.  This paper presents results from a comparative study that documents the merits and limitations of both methods with an example 

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