APPENDIX F: DRAINAGE AND STORMWATER MANAGEMENT
Downtown Ottawa Transit Tunnel:
Tunney’s Pasture to Blair Station via a Downtown LRT Tunnel
Prepared for:
City of Ottawa
110 Laurier Avenue West
Ottawa, ON K1P 1J1
Prepared by:
Delcan Corporation
1223 Michael Street
Suite 100
Ottawa, ON K1J 7T2
January 2010
Table of Contents
List of Tables
1.0 DRAINAGE IMPACTS ................................................. 1 1.1 Water Quantity..................................................... 1 1.2 Water Quality ....................................................... 1
Table 1: Conceptual Tunnel Drainage Plan Summary .............. 2 Table 2: Maintenance and Storage Site SWM Facility.............. 3 Table 3: Major Pipe Relocation Requirements ......................... 3 Table 4: Potential Infrastructure Risk and Issue Locations ....... 4 2.0 DRAINAGE DESIGN CRITERIA ................................. 1 2.1 Level of Service ...................................................... 1 2.2 Water Quality ....................................................... 1
3.0 REQUIREMENTS OF THE NETWORK...................... 2 3.1 Ballasted Track ..................................................... 2 3.2 Tunnel Drainage ................................................... 2 3.3 East Portal to Rideau River .................................... 2 3.4 St. Laurent Station to Blair Station ........................ 3 3.5 Maintenance and Storage Facility .......................... 3 3.6 Maintenance and Storage Facility Connecting
Tracks ................................................................... 3
4.0 INFRASTRUCTURE RISKS AND RELOCATION
REQUIREMENTS ......................................................... 3 4.1 Major Pipe Relocations .......................................... 3 4.2 Potential Infrastructure Risks and Issues ................ 4 4.3 Tunnel Stations Shafts ........................................... 4
5.0 INFRASTRUCTURE DATA GAPS ............................... 4
6.0 IMPACT MITIGATION PLAN .................................... 4 1.0
DRAINAGE IMPACTS
1.1 Water Quantity
The LRT track will be supported by ballast and an impervious
sub-grade involving the existing paved surface of the
Transitway. Typically, the track structure will be built up from
the existing Transitway surface, using the existing curbs to
retain the ballast. While the ballast will provide flow
attenuation and storage, the impervious sub-grade will ensure
that runoff volumes will be essentially the same as the existing
Transitway.
conditions. Thus the redevelopment of the site is expected to
have a beneficial impact from a water quantity perspective.
Notwithstanding the limited impacts identified above, major
system drainage will need be given careful consideration for each
element of the system to ensure that the level of service criteria
are met under future conditions, the potential impacts on
adjacent properties are properly evaluated, and appropriate
mitigation measures identified. Particular issues related to
major system drainage include:
•
The existing Transitway infrastructure will be removed where
the preferred LRT alignment departs from the Transitway, and
will be typically reinstated with a pervious surface.
Generally speaking, in terms of water quantity, no negative
drainage impacts are anticipated as a result of the project. In
fact, the attenuation provided by the ballast is likely to result in
a positive impact due to the reduction in existing peak flows.1
In the vicinity of the tunnel portals there will be a significant
increase in grade, and the LRT surface will transition from
ballast to concrete slab. In these sections, an increase in peak
flow rates over existing conditions is anticipated as a result of the
project.
At some transit stations, new building structures will be
required, but most will be integrated with existing Transitway
stations or be constructed below ground for the tunnel stations.
However, for the sections addressed by this report, the change in
imperviousness at transit stations will be small, and can be
ignored for the purposes of functional design.
For the proposed maintenance and storage facility site, the
future imperviousness is expected to be lower than existing
1
Available storage in the order of 1,500 m3/ha of flat rail corridor, whereas the storage needed to manage
excess flow resulting from a 100 year storm is only about 200 m3/ha assuming just a 5 year minor system.
•
Vertical curb extensions to contain rail ballast will impact
major system drainage
Redevelopment of Transit Stations and the development
of the Maintenance and Storage Facility
1.2 Water Quality
The conversion from diesel buses to electric LRT is expected to
have positive impact in terms of the quality of the runoff from
the transit surface. Furthermore, given the lack of any
significant increase in impervious area, no significant water
quality impacts area anticipated. Based on preliminary
discussions with the Ministry of the Environment (MOE), there
will, in general, be no need for water quality control facilities.
Nonetheless, some water quality control has been proposed for
specific locations to improve on existing conditions, address
specific issues, and contribute to the overall mitigation plan for
the DOTT project. The proposed approach is discussed in the
following section on drainage criteria.
2.0
DRAINAGE DESIGN CRITERIA
2.1 Level of Service
For planning purposes, the level of service to be provided is
based on the Transitway Drainage Guidelines, as documented in
the City’s Sewer Design Guidelines. Current practice would
require a minimum ten (10) year level of service for the transit
corridor and the Transit stations. However, the project
characteristics allow for a much higher level of service due to the
introduction of a ballasted track surface, and it is proposed that
the drainage system be designed to avoid ponding on the ballast
up to the 100 year frequency. The actual level of service to be
provided in critical areas such as the toe of slopes, and where the
track will be fixed to a concrete slab surface, should be based on
operational and cost-benefit considerations at the preliminary
design stage. Appropriate major system design must be
provided for the 100 year event to avoid negative impacts on
operations and adjacent properties.
Guidelines suggest a 25 year level of service for depressed areas.
This event frequency is proposed as the basis of design for the
portal pump stations to limit surface drainage into the tunnel.
(Excess drainage that reaches the tunnel low point will be
combined with tunnel infiltration and pumped to an appropriate
outlet).
For the Maintenance and Storage Facility, a five (5) year level of
service is proposed with an engineered major system as described
above.
2.2 Water Quality
The future drainage system should maintain or improve on the
quality of runoff draining from the existing surfaces. As
described in the previous section, the nature of the project is
such that no significant impacts on water quality are
anticipated, in general. Conversion from diesel buses to electric
trains would be expected to result in somewhat improved runoff
quality. Nonetheless, the following approach to water quality
control is proposed:
•
Tunnel portals: Pumping facilities will be required at
the portals to manage runoff that would otherwise drain
into the tunnel. As described in a subsequent section of
this report, new outlets are anticipated for the station
discharges, and treatment to MOE guideline levels is
proposed for these small areas. The proposed outlet
1
•
•
3.0
2
locations would require Level 1 protection (80% TSS
removal).
Maintenance and Storage Facility (MSF): This 12 ha
site is highly impervious under existing conditions, and
the future use is expected to reduce imperviousness
somewhat. However, given that it is a practical location
to provide water quality control, a SWM facility has been
proposed for this site to improve on existing conditions.
Conceptual level sizing is based on Level 1 protection.
Maintenance and Storage Facility Connecting Tracks:
Roughly 1 km or 1.7 ha of new transit corridor is
required to link the Maintenance and Storage Facility to
the LRT corridor. Drainage from this corridor will be
treated in the proposed MSF SWF described above.
REQUIREMENTS OF THE NETWORK
For the purposes of functional design of the DOTT project, a
conceptual drainage plan has been prepared and is described in
this section.
Given the limited drainage related impacts, drainage
infrastructure needs will be driven by the need to relocate
infrastructure to accommodate the project, and identify
alternative outlets and/or pumping infrastructure where the
profile of the transit system is lowered to the extent that the
existing gravity outlet is no longer available. Specific drainage
infrastructure needs are addressed for various sections of the
project as follows.
3.1 Ballasted Track
As noted previously, the existing Transitway sewer system is
expected to be sufficient for most sections of the project.
However, existing inlets are expected to be insufficient to convey
runoff to the sewer system because the ballast which will overlay
the catchbasins will effectively reduce inlet capacities. On flat
and mild grades, the storage provided by the ballast will
compensate for the reduced inlet capacity. However, on steep
grades, additional capture capacity will be required. A
continuous grate-covered rectangular drainage channel located
under the ballast is proposed to interconnect the catchbasins to
provide the necessary inlet capacity.
3.2 Tunnel Drainage
Portals
The West and East Tunnel Portals are located below grade, as
they provide access and egress to the proposed tunnel by the
LRT vehicles. Thus existing grades will be substantially
increased in the vicinity of the portals. The general concept is
that the LRT corridor will transition from a ballasted track to a
concrete slab surface at the top of slope descending to each
portal. There are significant lengths of transit corridor that will
drain towards each portal. The drainage approaching the
portals must be managed appropriately to maintain the required
level of service and avoid negative impacts on transit operations.
A pumping station will be required at each portal to intercept
and pump drainage from the slope leading to the portal.2 The
pump station capacity is based on a 25 year storm. Excess flow
from very large storms will be routed through the tunnel in
drainage channels to the tunnel low point to be located at the
Downtown East transit station.
Tunnel
The International Tunneling Association (ITA)3 recommends a
maximum infiltration allowance of 0.082 L/m2/day (0.002
gal/ft2/day). For planning purposes, a design infiltration rate of
0.15 L/m2/day was selected. Based on a rough estimate for the
total tunnel surface area (two tunnels and four stations) of
165,000 m2, the design infiltration rate would be less than 1 L/s
for the entire tunnel.
2
There may be an opportunity to provide a gravity outlet for the West Portal, requiring the
construction of a very deep sewer discharging to the Tailrace. This option should be given
further consideration at the preliminary design stage.
3
http://www.fhwa.dot.gov/bridge/tunnel/pubs/nhi09010/01a.cfm
Key parameter estimates associated with the conceptual
drainage plan for the tunnel are summarized in Table 1. The
total dynamic head (TDH) given in Table 1 is based on the
assumption that a suitable outlet sewer is available (or will be
provided) a horizontal distance of 20 m from the station for the
West Portal and Tunnel Low Point stations. Due to a number
of constraints related to flooding issues in adjacent drainage
systems (Sandy Hill), topographic constraints, and nearby
contaminated soil conditions, this is unlikely to be the case for
the East Portal. For the East Portal, a 800m forcemain is
estimated to be required to a new gravity outlet sewer east of
Lees/Chapel.
Table 1: Conceptual Tunnel Drainage Plan Summary
Parameter
Drainage Area (ha)
Pump Station Capacity
(L/s)
Active Wet Well (m3)
Forcemain Size (mm)
Forcemain Length (m)
Total Dynamic Head (m)
West
Portal
0.35
50
East Portal
0.90
125
Tunnel Low
Point
25
50
250
30
10
130
400
800
12
25
200
60
40
3.3 East Portal to Rideau River
Drainage from about 425m beyond the East Portal along the
track alignment will drain to the East Portal Pump Station,
which is discussed in the previous section. This section covers
the area from 425m south of the East Portal to the high point
on the bridge crossing the Rideau River. Given the short, rising
grade from the East Portal to the Rideau River Bridge, it has
been assumed that the rail will be fixed directly to concrete
throughout the section. Drainage will be provided by the
existing Transitway drainage system. However, to limit carryover flows to the East Portal, an increase in storm capture in this
section is anticipated, with associated storm pipes oversized to
provide the storage necessary to avoid downstream impacts.
With grades ranging from 0.5% to 5%, ballasted track would
be of limited benefit from a surface storage perspective.
3.4 St. Laurent Station to Blair Station
This section is expected to be dominated by ballasted track, and
no change in imperviousness is anticipated. Thus the existing
Transitway drainage system would be expected to meet the
future LRT needs in this section of the project. However, the
profile of the system will drop several metres, starting from
roughly 350m west of Blair Station. The existing storm sewer
that serves the Transitway will need to be relocated. In order to
provide drainage for the new profile, the sewer will need to be
lowered. Preliminary investigations indicate that sufficient
grade is available to serve the area with the same outlet location
at the Cyrville Drain. However, more detailed analysis will be
required to verify that the required level of service can be
provided, considering flow levels in the Drain at critical
frequencies.
While the proposed drainage plan does not include provisions
for introducing upgraded SWM to this section of the project,
the feasibility of upgrading the existing Gloucester City Centre
Dry Pond (incorporating practical water quality management
features) and diverting the lowered sewer to the pond should be
investigated. (The design report for this pond is unavailable.)
3.5 Maintenance and Storage Facility
The Maintenance and Storage Facility will cover an area of
approximately 12 ha. Under existing conditions, the total
imperviousness of the site is about 85%, whereas a future
imperviousness in the order of 50% is anticipated. The site
development is expected to improve on existing conditions with
respect to runoff quality and quantity. Nonetheless, as discussed
in the section on drainage criteria, a SWM facility (wet pond) is
proposed for the site to manage runoff from the site, and
contribute to the mitigation of overall DOTT project impacts.
The ultimate outlet for the drainage area is the Rideau River,
and Level 1 protection was assumed for the basis for the
treatment volume.
Any major system issues associated with the existing site, and
the impact of the development on major system drainage will
need to be investigated at the preliminary design stage. To
account for additional SWM needs associated with major
drainage, the conceptual sizing of the SWM facility is based on
attenuation of the peak 100 year runoff from the site to the
existing 5 year rate, in addition to the water quality storage.
The estimated SWM facility characteristics are summarized in
Table 2.
Table 2: Maintenance and Storage Site SWM Facility
Parameter
Value
Surface Area
0.4 ha
Quality Storage Volume
3,600 m3
Total Storage Volume
5,800 m3
Peak 100 Year Outflow
2.6 cms
3.6 Maintenance and Storage Facility Connecting Tracks
The tracks that link the Maintenance and Storage Facility to the
LRT corridor east of Train Station will curve away from the
corridor and pass down through two tunnel portals located east
and west of Belfast Road, merging about 50 m south of
Tremblay Road, and rising back to existing ground elevation
about 575 m south of Tremblay at the west end of the
Maintenance and Storage Facility. Approximately 0.6 ha of
fully impervious area will drain to two low points located on the
east and west branches of the connecting tracks. These two low
points are located within about 30m of each other.
A single pumping facility and forcemain is proposed to drain the
connecting track low points, and discharge to a new sewer
draining to the proposed Maintenance and Storage Facility
SWM facility described in the previous section. For planning
purposes a pump station with capacity in the order of 150 L/s
(assuming a wet well of roughly 65 m3 of active storage), and a
400 mm forcemain, roughly 600 m in length will be required to
handle a 100 year storm, plus any adjustments to the
Maintenance and Storage Facility sewer system. The sizing of
the Maintenance and Storage Facility SWM facility will also
need to be adjusted to accommodate the forcemain discharge so
that treatment objectives and the estimated existing peak site
discharge rate of 2.6 cms can be maintained.
4.0
INFRASTRUCTURE RISKS AND RELOCATION
REQUIREMENTS
Potential infrastructure conflicts for the entire project corridor
were determined (considering pipes 400 mm and larger) based
on information provided by the City of Ottawa including GIS
databases for water and sewer, in addition to available as-built
drawings for pipes located in the study area. A number of data
gaps were identified as summarized later in this section of the
report. Further investigations will be required to confirm the
relocation and risk areas identified in this section.
4.1 Major Pipe Relocations
Major pipe relocation requirements are summarized in Table 3.
Only one known conflict has been identified, but several other
major pipes have been identified for reconstruction to carry the
proposed embankment loads in the vicinity of Hurdman Station.
Several other major pipes are within 2 m of the tunnel or LRT
surface. These latter pipes are described in the following section.
Table 3: Major Pipe Relocation Requirements
Service
Pipe
Diameter
(mm)
Blair Station to
Cyrville Drain
Storm
750,
900,
1050
Hurdman Station
Storm
1650
Hurdman Station
Storm
1200
Hurdman Station
Sanitary
1350
Hurdman Water
Pumping Station
Storm
1200
Location
Comment
~1km of sewer must be lowered
to drain new profile in vicinity of
Blair Station. Further
investigation to verify level of
service possible.
To be reconstructed to carry
embankment load
To be reconstructed to carry
embankment load
To be reconstructed to carry
embankment load
(Rideau River Collector)
To be reconstructed to carry
embankment load
3
4.2 Potential Infrastructure Risks and Issues
Potential infrastructure risks are identified in Table 4. Known
major infrastructure is identified where it intersects the corridor
and is within about 5 vertical metres from the tunnel or LRT
surface. Infrastructure 400mm and larger is highlighted where
it is within 2 m of the tunnel/LRT surface. Infrastructure that
must be relocated is excluded from Table 4 (see previous
section).
It should be noted that the conversion from bus to rail on the
West Transitway, west of Bayview, may conflict with existing
access points to the twin 3800 mm x 2000 mm box storm
sewers. Construction of new access points (e.g. offset
maintenance holes) to these sewers may be required. Further
investigation will be required at the preliminary design stage.
Table 4: Potential Infrastructure Risk and Issue Locations
Location
4
Service
Pipe
Diameter
(mm)
Twin 3800
x 2000
Tunney’s Pasture
Station to West Side
of Bayswater Bridge
West Side of
Bayswater Bridge
Bayswater
Water
2100,
2100, 750
1950
East of Bayswater
City Centre
City Centre
Broad Street
Broad to Booth
East of Booth
Commissioner
Templeton
Combined
Combined
Storm
Sanitary
Combined
Sanitary
Water
Sanitary
1200
1200
1800
950
1500
1800
1500
1800
Greenfield
Storm
750
Lees
Water
1200
Rideau Canal
Colonel By Drive at
Rideau
Rideau River
Combined
Storm
2300
1350 x
2025
1500
Storm
Storm
Sanitary
Comment
Existing box storm sewers are
less than 1m below LRT grade
Three storm sewers crossing
corridor near west end of bridge
Major high pressure
transmission main (HPTM)
from Lemieux Island Plant
Cave Creek Collector
Cave Creek Collector
Nepean Bay Storm Trunk Sewer
Status of pipe to be confirmed
Cave Creek Collector
Ottawa Interceptor Sewer
Fleet Street PS discharge main
Status of pipe to be confirmed.
Pipe is less than 0.5 m above
tunnel
Pipe is less than 1 m above
tunnel
Main transmission line to 1E
and 2E Pressure Zones
Rideau Canal Interceptor
DPW Tunnel
Rideau River Interceptor
Location
Interceptor
Hurdman Water
Pumping Station
East of Riverside
East of Riverside
East of Train Station
East of Train Station
Tremblay, East of
Train Station
Tremblay from East
of Train Station to
St. Laurent
Tremblay East of
Belfast
Maintenance and
Storage Facility
Connecting Tracks
at Tremblay
Maintenance and
Storage Facility
Connecting Tracks
at Tremblay
Maintenance and
Storage Facility
Connecting Tracks
at Tremblay
West end of HWY
417 Underpass
St. Laurent Station
St. Laurent Station
Service
Pipe
Diameter
(mm)
Water
1200
Storm
Storm
Sanitary
Storm
Water
1950
1500
600
1050
1200
Storm
1650
Water
1050
Water
1200
Storm
900
Comment
Two discharge transmission
mains serving pressure zones 1E
and 2E.
East side of bridge embankment
East side of bridge embankment
Pipe is within 1m of LRT grade
Status of pipe to be confirmed
Main transmission main
through Zone 1E
Storm sewer serving existing
Transitway
Major transmission main serving
Zone 1E
Main transmission main
through Zone 1E
Pipe is within 2 m of top of
MSF tunnel
Sanitary
600
Pipe is within 2 m of top of
Maintenance and Storage
Facility tunnel
Water
1050
Storm
1650
Major transmission main serving
Zone 1E
Drains Transitway. Obvert less
than 1 m below LRT grade (350
m length)
Siphon less than 1 m below LRT
grade
Sanitary
525
4.3 Tunnel Stations Shafts
A number of watermain and sewer relocations will be required
to accommodate construction of the tunnel station shafts. In
general, worst case locations were assumed for these shafts, or it
was confirmed that major infrastructure could be avoided. It
has been concluded that no major pipes are associated with these
shafts. However, relocation of minor infrastructure will be
required.
5.0 INFRASTRUCTURE DATA GAPS
A number of data gaps exist and require further investigation to
confirm complete scope of drainage and infrastructure risks
associated with the project. In general, current as-built
drawings and/or field investigations are needed. Gaps for the
project include:
•
•
•
•
•
Infrastructure in the vicinity of the West Portal. While
no conflict with the DOTT project is apparent, further
investigation is needed to confirm the alignment of pipes
that are of critical importance to the City’s water
distribution and wastewater collection systems.
Infrastructure in the vicinity of Rideau Station. While no
conflict with DOTT project is apparent, further
information is needed to confirm inverts of the Rideau
Canal Interceptor and the DPW Tunnel.
750 mm Storm Sewer at east end of Hurdman Station.
Storm outlet and main sewers to Rideau River from St.
Laurent to Train Station.
Transitway drainage infrastructure. No information on
transitway drainage infrastructure has been provided for
several sections of the project including: Lees Station to
Rideau River; St. Laurent to east of Cyrville Drain.
6.0 IMPACT MITIGATION PLAN
From a drainage perspective, the project is expected to have very
limited impacts. Stormwater Management Facilities are
proposed for specific drainage areas (tunnel portals and
Maintenance and Storage Facility) to improve on existing
conditions. Surface storage provided by the introduction of
ballast to the existing Transitway surface is also expected to
attenuate sewer flows in several areas.
The detailed design phase must include specifications for
sediment and erosion control to be complied with during
construction. These specifications will be prepared in accordance
with relevant Provincial and Regional legislation, guidelines and
by-laws.