Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
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