Large Interceptor Capacity Analysis Using a Dynamic Hydraulic Model

High Level Sewershed (HLSS) is one of eight sewersheds in the City of Baltimore with a population of about 100,000 people in its 4,600 acres of drainage area (Figure 1). The High Level Interceptor (HLI) was built in 1912, and collects flow from the HLSS and surrounding drainage areas. This interceptor has a triple barrel siphon (42", 42" and 36") that crosses under a major highway. At the downstream end of the siphon, the low spot on the HLI surface profile where HLSS and another sewershed meet, sanitary sewer overflows (SSOs) have been observed in the past. To understand the system performance and develop recommendations to alleviate the overflow, flow monitoring was conducted at both ends of the siphon as part of the City-wide sewershed study and SSO elimination planning efforts. Overflow volume during a wet event and Hazen-William coefficients of the siphon tubes were calculated using the measured flow and water depth data. In addition to this localized siphon analysis, a HLSS hydraulic model was developed and calibrated based on city-wide flow monitoring conducted at 40 locations for one year. Sonar inspection conducted along the HLI revealed a significant sediment accumulation in the downstream reaches. This sediment data was used to accurately calibrate the siphon behavior as well as the downstream portion of HLI. This paper describes the field observations and dynamic hydraulic modeling efforts pursued to characterize the capacity issues and make appropriate recommendations to the City of Baltimore.