Using Dynamic Hydraulic Modeling to Understand Sewer Headspace Dynamics - A Case Study of Metro Vancouver's Highbury Interceptor Yuko Suda, P.Eng. Kerr Wood Leidal Associates Ltd. 200-4185A Still Creek Drive Burnaby, BC, V5C 6G9, Canada (604) 294-2088 ABSTRACT Metro Vancouver's Highbury Interceptor (HI) is a 6.1 km long 2,900 mm diameter combined sewer with significant odor and headspace pressurization issues identified along its length. During winter storms large amounts of air have been observed expelled from manholes and vents, resulting in howling noise. These events are significant enough that manhole covers have been lifted off and residents have reported observing heaving of the asphalt pavement around the interceptor manholes. Pressure monitoring found that two distinctly different mechanisms are influencing the air pressure within the sewer head space. A fully dynamic computer based hydraulic model in XP-SWMM revealed that the unique characteristics of the Highbury Interceptor profile resulted in the headspace in the sewer becoming completely isolated from upstream, downstream and tributary sewers under certain flow conditions. The results of the hydraulic model correlated well with the monitoring data, revealing that the extreme pressurization events occurred immediately following isolation of the headspace. An air displacement model was created, based on the hydraulic model, to develop the design parameters for an air extraction and odor control facility. KEYWORDS Sewer, pressurization, differential pressure monitoring, fully dynamic hydraulic modeling, air displacement modeling. INTRODUCTION The Highbury Interceptor(HI) is one of the principletrunk sewers in Metro Vancouver's (MV's) Vancouver Sewerage Area (VSA). It services the majority of the City of Vancouver and a portion of the City of Burnaby. The VSA is currently a combined sewerage network. In recent years the number of complaints related to significant odor and headspace pressurization issues along the length of the HI have increased. Large volumes of air have been observed expelled from manholes and vents during winter storms. These events are significant enough to result in loud howling sounds, manhole covers being lifted off, and residents having reported seeing heaving of the asphalt pavement around the interceptor manholes. SYSTEM DESCRIPTION The HI is 6.1 km long, starts at 1st Avenue in Vancouver, and travels south along Highbury Street. Figure 1 shows an aerial schematic of the HI. Three major interceptors enter the HI at the upstream end of the system; the English Bay Interceptor (EBI), the 8th Avenue Interceptor (8AI),
and the Spanish Bank Interceptor. The EBI is a 2,400 mm diameter pipe that runs along 1st Avenue. The 8AI is a 2,600 mm diameter pipe that enters the HI system at 8th Avenue and Highbury Street. Together EBI and 8AI service the majority of the north side of the City Vancouver and a portion of the City of Burnaby. The Spanish Banks Interceptor is a 1,200 mm pipe that services parts of the University of BritishColumbia Campus and the West Point Grey residential area. In addition, at the upstream end of the HI are two overflow siphons; the Alma- Discovery Street Overflow Siphons. From 4th Avenue, to approximately Marine Drive the HI is a tunnel, which consists of a combination of 2,950 mm dia. circular tunnel sections and 2,900 mm dia. Boston Horseshoe shaped (BHS) tunnel sections. The deepest portion of the tunnel is approximately 100 m below ground level. There are only two 300 mm diameter air vents (at 18thAvenue and 33rd Avenue) along the tunnel portionof the sewer. At Marine Drive, the HI flows southwest through the Musqueam Park and the Musqueam Indian Reserve. Inside the Musqueam Indian Reserve the HI crosses Musqueam Creek. At this point the HI becomes a partial siphonfor approximately 18 m. On either side of the creek crossing are 450 mm diameter vents to atmosphere. The HI continues through the Musqueam Indian Reserve to the North Arm of the FraserRiver, at which point it enters the Fraser River Siphon Chamber, which has three 300 mm diameter vents to atmosphere. The HI subsequently turns into a triple barrel siphon, crosses under the Fraser River, and enters the Iona Island Waste Water Treatment Plant (IIWWTP). The HI is a combined sewer system, and thus conveys both sanitary flows and storm flows. Therefore, the flows and air dynamics in the interceptorare affected by daily sanitary diurnal flow patterns and particularly by storm events. MONITORING In order to determine the headspace dynamics within the sewer a differential pressure monitoring program was carried out. The program consisted of two monitoring periods; first from June 25, 2010 to July 27, 2010 (summer program), and the second from September 29, 2010 to October 28,2010 (fall program). The differential pressure monitors record the difference in pressure between the sewer interior and exterior atmospheric pressure. The differential pressure of a sewer reflects the headspace dynamics of the system with positive pressure corresponding to the release of air and odours to the atmosphere and negative pressure corresponding to air drawing in. The pressure monitor is capable of detecting differential pressures between + 50 mm and 50mm water column (W.C.), with a resolution of 0.025 mm W.C. Monitors were placed at the following locations: 4lh Avenue Manhole; 33rd Avenue Vent; Marine Drive Manhole; Musqueam Creek Crossing North Side Vent; Musqueam Creek Crossing South Side Vent; and Fraser River Siphon Chamber Vent.
18th Avenue Vent 300 mm Dia. 33rd Avenue Vent 300 miiii ['..-.: Musquearr Creek Crossinq North Vent 450 mm Dia South Venl 450 rrm Dia Fraser River Siphon Chamber 3-300 mm Dia Vents Highbury nterceptor Other GVRD Trunk Figure 1 - Layout of the Highbury Interceptor
Differential pressure monitoring along the interceptorrevealed that the differential pressure in the sewer typically ranges from -2.5 to 5.0 mm of W.C; however, during some storm events pressure in the sewer increases rapidly, exceeding the differential pressure monitor's range of 50 mm of W.C. Pressures of this magnitude are considered significant and are rarely seen in sewer systems. The data revealed that the pressurization occurs abruptly, indicating a rapid change in displacement in the sewer. Conventional collection system air transport models did not explain this abrupt pressurization (KWL, 2011). Figure 2 shows the differential pressure monitor data for a storm that occurred from October 23-25, 2010, overlaid with the hourly rainfall data. -4th Avenue (0+348) Musqueam Creek South (5+311) -33rd Avenue (3+208) -Fraser River (5+779) Figure 2 - Winter Monitoring Differential Pressure Data -Musqueam Creek North (5+293) Rain Data The following observations are made for each of the time intervals labeled on Figure 2: Period A: This is during the dry weather period, before any rainfall. The graph shows that the 4" Avenue monitor has a distinctly different pattern than any of the other monitors, indicating that its head space is influenced by a different system than the rest of the HI. This makes sense as the 8AI and the EBI are both upstream of the monitor and are influenced by their ventilation dynamics, rather than that of the HI. The four remaining monitors appear to have a similar pattern during normal dry weather flows. Period B: This period occurs during the first portion of the storm event. Up to this point the four monitors, mentioned above, have a similar pattern, however the pressures at 33rd Avenue and Musqueam Creek north abruptly dip to below -50 mm of W.C. and the