Using Sanitary Sewer I/I Field Data to Calibrate a Storm Sewer Model
Since 1992, the City of Columbus Division of Sewerage and Drainage (DOSD) has conducted detailed investigations of its infrastructure to identify, mitigate and eliminate sanitary sewer infiltration and inflow (I/I). In 2009, DOSD selected Camp Dresser McKee to perform a detailed evaluation of the wastewater and stormwater infrastructure for the approximately 6 mi2 (16 km2) capital improvement project (CIP) No. 650405.7, Northwest Alum Creek area sanitary sewer system inflow and infiltration remediation project (NWAC) as shown in Figure 15.1.
The goals of the project are to:
1. Conduct detailed studies of the sanitary sewer collection system in order to identify the locations and causes of sewage overflows from manholes, sanitary reliefs, sewage system surcharging and sewage backup into basements; and
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 or eliminate sanitary sewer surcharges and consequent overflows at manholes, structures and water in basements for a selected performance target.
Although the focus of NWAC is the sanitary collection system, a new model of the stormwater collection system was developed to evaluate the hydraulic connections between the sanitary and storm sewer systems and the stormwater impacts of inflow and infiltration remediation. For consistency with the NWAC sanitary sewer system model, the new stormwater model was constructed in the PCSWMM environment with the USEPA SWMM 5.21 engine.
The existing storm sewer system was analysed 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, fifteen flow meters were installed in the storm sewer system to allow for calibration of the SWMM5 model. Input parameters for the SWMM5 model were calculated from existing geographic information system (GIS) data of the study area, as well as leveraging detailed drainage reconnaissance (DDR) information that was originally gathered to identify potential sources of I/I to the sanitary sewer system.
The field data gathered to identify I/I sources was also used to refine the SWMM5 parameter that represents the percentage of impervious surfaces that are tributary to pervious surfaces, called disconnected impervious surfaces (DIS). The initial DIS values input to the storm sewer model, calculated from the DDR data, were sufficient to support adequate calibration of the model.
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