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 principle trunk 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 British Columbia 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 18th Avenue and 33rd Avenue)

along the tunnel portion of 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 siphon for 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 Fraser River, 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 interceptor are 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:

• 4th

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.

Figure 1 - Layout of the Highbury Interceptor

Differential pressure monitoring along the interceptor revealed 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.

-80.0

-70.0

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

10.0

20.0

30.0

40.0

50.0

60.0

Oct 23

00:00

Oct 23

06:00

Oct 23

12:00

Oct 23

18:00

Oct 24

00:00

Oct 24

06:00

Oct 24

12:00

Oct 24

18:00

Oct 25

00:00

Oct 25

06:00

Oct 25

12:00

Oct 25

18:00

Oct 26

00:00

Dif

fere

nti

al P

ress

ure

(m

m H

2O

)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

Ho

url

y R

ain

fall

(m

m/h

ou

r)

4th Avenue (0+348) 33rd Avenue (3+208) Musqueam Creek North (5+293)

Musqueam Creek South (5+311) Fraser River (5+779) Rain Data

A B C D

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 4th

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

pressures at Musqeuam Creek south and Fraser River increase. The pressures of the latter

two monitors do not increase beyond 3.5 mm of W.C. likely due to the monitor location

and vent configuration.

• Period C: As the initial storm subsides, the four above mentioned monitors converge

again, and start showing a similar pattern.

• Period D: During the final storm, the pressures at 33rd

Avenue and Musqueam Creek

north again drop, though not immediately, and the pressures at Musqeuam Creek south

and Fraser River increase, again staying around 3.5 mm of W.C. Part way through the

final storm the Fraser River monitor fluctuates significantly. This is due to the

surcharging of the sewer at the monitoring location, rendering the data unusable.

Period A reflects the type of pressure range and pattern that is expected due to air transport in a

conventional collection system, however the abrupt pressurization during Periods B and D are

not so easily explained.

MODELLING

In order to determine the cause of the rapid pressurization events a fully dynamic hydraulic

model in XP-SWMM was developed. XP-SWMM was selected over other models because it is a

dynamic, non-steady state model which solves the full St. Venant Dynamic equation, thus

providing a more accurate hydraulic grade line (HGL) than a steady state model. The data was

extracted and used to model the remaining volume above the wastewater (i.e. the sewer head

space storage within the HI). This was critical to identifying and quantifying the air displacement

in the sewer. Flow monitoring data from the same period as the differential pressure monitoring

period was used to load, calibrate, and verify the model.

The model revealed that the unique profile of the HI resulted in the headspace in the sewer

becoming completely isolated from upstream, downstream and tributary sewers under certain

flow conditions. This occurs during high sewage flow events, where surcharging cuts off the

headspace at the upstream end of the system and the siphon at the Musquem Reserve near the

downstream end prevents any entrapped air from escaping. Thus, as the sewage level increases

with the storm event, a large amount of the trapped air can only be displaced through the few

relatively small vents located along the sewer, resulting in high pressures and very high

velocities through the vents. The results of the hydraulic model correlated well with the

monitoring data, revealing that the extreme pressurization events occurred at the same time as

the headspace being isolated.

SEQUENCE OF EVENTS

During dry weather flows air is forced into the HI by the EBI and the 8AI, as the two upstream

pipes have a larger airflow capacity than the HI. The maximum air capacity of the HI is less than

the incoming air, and in addition the headspace on the downstream side is blocked due to the

Fraser River Siphon, resulting in generally positive pressures within the HI.

However during storm events, as the flow rate increases from the two upstream tributaries, the

sewage in the HI begins to backwater at the tie-in for the 8AI. The water level continues to rise

through the storm, decreasing the cross sectional area of the headspace in this area. This

restriction, decreases the amount of air that can be supplied into the HI from the 8AI and the

EBI, however the increase in the water velocities in the HI continue to pull air downstream. The

vents along the interceptor cannot supply the required air due to the flapper style vents. Thus the

HI quickly becomes starved for air, causing a sudden vacuum in the HI.

When the water level at the 8AI connection finally reaches the crown of the pipe, no more air

can be supplied resulting in the maximum negative differential pressure readings. At

approximately the same time, when the 8AI becomes isolated, the water level at the partial

siphon at Musqueam Creek reaches the crown of the siphon as well. The result of this is two air

pockets forming in the HI, one to the north of the creek and another to the south. When this

occurs, only the vents are connected to the headspace in the HI. Figure 3 shows the HGL profile

in the HI at this instant.

As the storm continues the water level continues to rise, and the headspace within the HI,

continues to decrease. However with the two upstream tributaries now isolated from the

headspace, and both the Musqueam Creek Partial Siphon and Fraser River Siphon, blocking the

headspace on the downstream end, the only places that the air can vent is through the 18th

Avenue, 33rd

Avenue, Musqueam North and South, and Fraser River vents. Given the size and

length of the HI, the amount of air that needs to be displaced is immense compared to the size of

the vents. Thus, as the water level continues to rise the pressure in the HI increases significantly

and a large amount of air is forced out of the vents at very high velocities.

As the water level continues to rise and reach the crown of the pipe, each of the vents in turn

becomes isolated from the headspace, forcing the air to be expelled out of fewer and fewer vents.

CONCLUSION

Based on the results from the hydraulic model, in conjunction with a collection system air

transport model, a conceptual design and estimate for an air management and odor control

facility was prepared. The facility included air pressure relief functionality to allow excess

pressure peaks developed during storm events to be dissipated without danger to the public or

property damage.

Without a fully dynamic hydraulic model, the cause and magnitude of these pressurization events

could not have been determined, and management of the peak air flows may not have been

adequately addressed in the conceptual design for the odor control facility.

REFERENCES

Kerr Wood Leidal Associates Ltd. (KWL). Odour Control Strategy Development – Highbury

Interceptor, Final Report. Vancouver, 2011.

.

Figure 3 - Screenshot of XP-SWMM.

33rd

Avenue

Vent

Musqueam Creek Partial

Siphon and Vents

Fraser River

Siphon Chamber

and Vents

8AI Tie-in

18th

Avenue

Vent

EBI Tie-in