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 AlmaDiscovery 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)
-33rd Avenue (3+208)
• Musqueam Creek South (5+311)
-Fraser River (5+779)
-Musqueam Creek North (5+293)
Rain Data
Figure 2 - Winter Monitoring Differential Pressure 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