Hatch Mott MacDonald (HMM) computer modeled 5.5 miles of the Overpeck Valley Trunk Sewer (OVTS) and approximately 7.8 miles of tributary interceptor sewers of the Bergen County Utilities Authority (BCUA). The modeling was performed to assess the hydraulic capacity of the sanitary sewer system, project rainfall related extraneous flow rates and determine how extraneous flows activate sanitary sewer overflows (SSOs) at a siphon crossing of the Overpeck Creek.
HMM created a dynamic model was created by using the XP-SWMM computer program, specifically the CaiCe Visual Hydro program version. Important capabilities of the model are its ability to model pressure flow, backwater influences, and reverse flow, and predict the duration and volume of SSOs from any manhole.
Project tasks included developing the model from as-built plans and inputting wastewater flow data from 26 BCUA meters and seven HMM meters. The modeled pipe diameters ranged from 8-60 in. The model was run for dry and wet weather conditions.
Field tasks included the installation and maintenance of seven flow meters and many manhole inspections at the Overpeck Creek siphon chambers and key manholes along the OVTS. The flow metering and manhole inspections revealed flow obstructions and poorly designed manholes which contribute to the SSO incidents.
American Sigma area-velocity meters were installed so that flow depth and velocity was continuously measured. Redundant flow sensors were used at the temporary metering sites along the OVTS.
The manhole inspections revealed a 43% reduction in the Overpeck Creek siphon design capacity due to the accumulation of grease, sediment, stones, and debris in the siphon barrels and the OVTS. Proactive sewer cleaning was immediately implemented to restore sewer conveyance capacity.
Very favorable rainfall events occurred during the metering period including a greater than 100-year frequency event. The model was calibrated by using the field investigation data to crosscheck model predictions. The modeling identified capacity bottlenecks along the OVTS, indicated a 1.5 MGD dry weather flow and predicted peak rainfall induced flows to the OVTS. Furthermore, the field investigations confirmed the 1.5 MGD dry weather infiltration is potable water main leakage entering the sanitary sewer system.