Utilizing Sanitary Sewer Inflow/Infiltration Field Data to Calibrate a Storm Sewer Model.

Josh Reinicke and Marc Lehmann

ABSTRACT

In support of the Northwest Alum Creek I/I remediation project’s sanitary sewer modeling effort in Columbus, Ohio, stormwater modeling was performed to determine whether the removal of inflow and infiltration (I/I) from the sanitary sewer would negatively impact the storm system. This presentation will describe how and why data from a detailed drainage reconnaissance (DDR) effort were gathered using a handheld global positioning system (GPS) unit for nearly 10,000 structures. These data were initially gathered to support the study objectives of the sanitary sewer system, but were found useful to support the storm sewer modeling. Conclusions about the use of the DDR data to project future conditions within the study will be discussed, as well as the value of gathering this type of information for future studies.

Since 1992, the City of Columbus Division of Sewerage and Drainage (DOSD) has conducted detailed investigations of its infrastructure to identify, mitigate, and eliminate I/I. In 2009, DOSD selected CDM to perform a detailed evaluation of the wastewater and stormwater infrastructure for the approximately 6-square-mile (16 km2) Northwest Alum Creek study area. The goals of the project are to:

  1. Conduct detailed studies of the sanitary sewer collection system in order to identify locations and causes of sewage overflows from manholes, sanitary reliefs, sewage system surcharging, and sewage backup into basements.
  2. Develop plans for specific sanitary and storm sewer system improvements and I/I remediation that successfully address the identified problems. The remediation plans will define cost-effective improvements to the storm and sanitary collection systems to mitigate and/or eliminate sanitary sewer surcharges and consequent overflows at manholes, structures, and water-in-basements (WIBs) for a selected performance target.

An analysis was performed of the existing storm sewer system to determine a baseline condition that could be used in the development of different I/I removal alternatives. As part of the storm sewer analysis, 15 flow meters were installed in the storm sewer system to allow for calibration of the SWMM 5 model that was developed of the storm sewer within the study area. Input parameters for the SWMM model were calculated from existing geographic information system (GIS) data of the study area, as well as leveraging DDR information that was originally gathered to identify potential sources of I/I to the sanitary sewer system.

The potential for disconnected impervious surfaces (DIS) being tributary to a pervious surface was calculated using downspout discharge information collected as part of the DDR. These field data gathered to identify I/I sources were also used to refine the SWMM5 parameter that describes the percent of impervious surfaces that are tributary to pervious surfaces. The initial DIS values input to the storm sewer model that were calculated from the DDR data were sufficient to support adequate calibration at the 15 flow meter sites.


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