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This paper presents the methodology and results of engineering evaluations conducted to develop the Sanitary Sewer Overflow (SSO) Long Term Control Plan (LTCP) for the City of Norfolk, Virginia. This sanitary sewer system (System) is one of the oldest in the nation with some active sewers installed in 1913; it comprises 143 sewersheds, 867 miles of pipes and 152 pumping stations within 52.4 square miles. The LTCP needs assessment is based on the results of the field investigation program, the conditions of the sewers system assets and hydraulic modeling.

The existing conditions are an indication of potential asset deficiencies (and their causes) that need to be corrected. Parameters investigated for each sewershed include: pipe age and material, defect history, estimated I/I rates, record of SSOs, manhole conditions, structural conditions, surcharging, presence of inflow sources, geographic distribution of service calls and recurrent problems, and estimated wastewater flow produced in gallons per capita per day (GPCD) for various design storms. Field information used to determine sewershed characteristics included results of: manhole inspections, flow; rainfall; and groundwater monitoring, digital imaging of sewers and smoke testing. The sewershed conditions were entered into a project GIS, which was used as a tool for condition assessment.

Twenty-six temporary sewer flow meters throughout the City gathered flow data from September 2001 through March 2002. October and November 2001 were the driest months with accumulated monthly rainfall of 0.8 and 0.15 inches, respectively and were used for determining the dry weather Average Daily Flow (ADF). Dry day flows recorded at 15-minute intervals were averaged to determine the shape of the diurnal curve for each area and to estimate infiltration. The RDI/I factor, expressed as a percentage of the total accumulated rainfall that entered the System, ranged from 0.1% to 5.3% with an average of 1.3% for all of the monitoring sites. This RDI/I factor was applied to other storms such as the 2, 10, and 25-year return frequency rainfall events.

Water consumption data for the period of February 2001 through March 2002 was used to determine the portion of metered water consumption returned to the sewer system as sewage flow. Sewage Return Factors (SRFs) were developed for each sewershed based on land use. The SRF for Primarily Residential use is 80% and for Mixed Use is 87%. Dry weather infiltration in each sewershed was estimated by multiplying the sewershed land area by a unit infiltration rate. Infiltration rates for individual sewersheds range from a low of 14% to a high of 70% of ADF. The estimated total dry weather infiltration flow is 7.9 MGD, which is about 40% of the estimated dry weather ADF of 19.7 MGD.

The rating of the asset conditions determines the relative need for corrective actions in each sewershed and was based on the individual ratings for parameters such as: pipe age, pipe material, defect history, DWF infiltration, SSOs on record, manhole conditions1, manholes surcharged, % manholes subject to inflow, % houselines CCTV inspected2, records of service calls and wastewater flow expressed in GPCD for the 1-year storm.

Hydraulic Modeling results indicated that flows closely matched the actual metered flows during Wet Weather. To estimate the performance of the System under wet weather flows the storms with frequencies of recurrence of 1, 2, 10 and 25 years were superimposed on the ADF. Results of these simulations were used to identify the System capacity and rehabilitation needs for SSO Control.

Each of the sewersheds was evaluated based on application of the field program results, hydraulic modeling results and a detailed assessment of the sewer system asset conditions. Project priority ratings were used to establish priorities for gravity sewers, pumping stations and force mains. The resulting projects were organized into the City's SSO LTCP.

The Virginia Department of Environmental Quality has characterized the City's SSES and associated SSO Control LTCP as a “Success Story”.
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Document Type: Research Article

Publication date: 2004-01-01

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