12/9/2004; Liberty Centre Parking Garage 1

Aerial view of Liberty Centre parking garage, 

after project construction, 2002.

Liberty Centre Parking Garage 

633 NE Liberty Place 

(650 NE Holladay Street) 

Project Summary 

Overview of the Stormwater System 

• The stormwater planters were constructed in 

existing landscape areas bordering the parking 

deck. 

• Drainage from the parking deck is evenly divided; 

half is discharged to the eastern planter and half is 

discharged to the western planter. 

• Runoff enters each planter at two locations – at the 

north end and at the mid-point. 

• The floor of each planter slopes gently toward the 

south end, where runoff ponds to a depth of 11 in. 

before overflowing into a raised standpipe. Any 

overflow goes to the combined sewer. 

• The foundation is protected with an impervious 

seal and drainage panels. There is also a footing 

drain at the bottom of the foundation. 

 

1

 Portland’s Bureau of Environmental Services implemented the Willamette Stormwater Control Program in 2001. The 

Program offered financial grants and technical support for a series of projects to retrofit existing commercial properties with

stormwater controls incorporating green technologies. The Program recruited these demonstration projects to research the 

feasibility, cost and performance of commercial stormwater retrofits in the area served by the combined sewer. The 

Program provided grant funds for eleven projects. The projects were completed by July 1, 2003. 

Project Type: Commercial building retrofit - demonstration project 

Technologies: Stormwater planter (infiltration planter) 

Major Benefits: • Runoff from 36,155 sq. ft.(.83 acres) of parking deck enterstwo vegetated planters, reducing 

the volume of stormwater draining to the combined sewer and improving runoff water quality. 

• The landscaping includes a variety of native trees and vegetation, improving the urban 

environment and aesthetic appeal of the property and surrounding commercial area. 

Cost: $75,530 ($2.10 per sq. ft. of impervious area managed). Environmental Services granted $30,000 

for the project through the Willamette Stormwater Control Program1. 

Constructed: Summer 2002 
12/9/2004; Liberty Centre Parking Garage 2

Stormwater Capacity and System Components 

Stormwater Management Goal

The goal was to manage as much runoff as possible within the 

space available – within the existing landscape areas. The 

design maximizes the amount of runoff that is captured and 

infiltrated within the space. The design was influenced by the 

following factors: 

• The available landscape area was just 5% of the drainage 

catchment. 

• The configuration of the existing storm drains determined 

the points at which drainage could be brought into the 

planters (some flow enters at the mid-points of the planters). 

• The designers specified that runoff should not pond directly 

against the foundation, so the planter floors are trapezoidal 

and slope toward the south (similar to swales). 

Geotechnical Evaluation/Infiltration Test

Before construction, consulting engineers conducted infiltration 

tests at two locations in each planter. The four tests yielded 

infiltration rates of 0.5-1.0 in. per hour at depths of between 2 

and 3 ft. below grade. The engine eers incorporated these rates 

into their hydraulic simulations, which indicated the planters 

have enough capacity to retain a storm somewhat smaller than 

the 2-year storm (prior to overflowing). The simulations did not 

include assumptions about the effect of the drainage panels 

encasing the foundation, which may increase the infiltration 

rate. 

The Natural Resource Conservation Service (NRCS) Soil 

Survey for Multnomah County classifies the local soils as 50Curban land - highly developed lands with stratified soils and 

sometimes fill. The soil survey does not provide expected 

infiltration rates for the soil group. 

Excavated eastside infiltration planter 

under construction 2001 (prior to curbs)

Eastside infiltration planter; summer 

2003
12/9/2004; Liberty Centre Parking Garage 3

Facility components

Eastern Infiltration Planter

(See Site Plan for drainage details; Figure 1, pg.10)

Catchment Area: 18,000 sq. ft. 

Facility footprint2

: 1,155 sq. ft.

Internal Volume: 585 cu. ft. 

Overflow: The standpipe at the southern end of the planter 

overflows to the adjacent street inlet to facilitate flow 

monitoring. 

Capacity: The planter has less internal volume than the comparable 

eastside soakage trench3

 that would be required for a catchment 

of 18,000 sq. ft. (the soakage trench would have a footprint of 

1,080 square feet and a volume of 1,134 cu. ft). 

Additional Information: 

• The planter is 175 ft. long and averages 6.6 ft. in width. A 6-in. 

curb along the outer edge results in an average total depth of 12 

in. 

• The planter floor has the configuration of a swale. It slopes 

gently toward the south (< 1%), it has trapezoidal side slopes 

(2:1 to 3:1), and it has four check dams to promote retention 

and infiltration. The side slopes ensure that runoff doesn’t pond 

directly against the foundation. 

• The standpipe inlet (overflow elevation) is 11in. above the 

floor of the planter. 

• The soils were excavated 6 in. at the center of the planter and 

amended with compost. No other changes were made to the 

soils.

• The slope beneath each scupper (inflow point) is lined with 

impervious HDPE fabric, and the entire cross section of the 

planter at those locations is covered with river rock to reduce 

erosion.

• The top of the drainage protection system along the foundation 

exceeds the height of maximum ponding within the planter – it 

was raised 6 in. during construction.

 

2

For the purpose of comparing the capacity of the facility with the standard eastside soakage trench, the footprint has been calculated as 

the wetted (ponded) surface area when the facility reaches maximum capacity. 3 The standard eastside soakage trench meets the City’s standard for complete stormwater disposal in soils, which infiltrate at least 2 in. 

per hour. The City requires 24 ft. of trench per 1000 sq. ft. of impervious area (drainage catchment). The trench is 3 ft. deep, 2.5 ft. wide, 

and filled with drainage rock. Flow enters the trench through a pervious pipe that travels the length of the top of the trench. Assuming a 

porosity of 35%, the trench provides an internal volume of 63 cu. ft. per 1,000 sq. ft. of catchment.

Overview of westside infiltration 

planter, spring 2004

Foundation protection system: 

impervious seal and drainage panel
12/9/2004; Liberty Centre Parking Garage 4

Western Infiltration Planter 

(See Site Plan for drainage details; Figure 1, pg.10)

Catchment Area: 18,000 sq. ft. 

Facility footprint2

: 1,155 sq. ft.

Internal Volume: 214 cu. ft. 

Overflow: An overflow standpipe, located at the southern end of 

the planter, drains to an existing subsurface pipe that connects 

to the combined sewer. 

Capacity: The swale has less capacity than the comparable eastside 

soakage trench3 that would be required for a catchment of 

18,000 sq. ft. The soakage trench would have a footprint of 

1,080 sq. ft. and a volume of 1,134 cu. ft. 

Additional Information: The planter is similar in configuration to 

the eastern planter with the following exceptions: 

• The western planter has a slightly larger footprint and greater 

stormwater capacity. 

• The overall slope of the floor is 1% over 165 ft. (slightly more 

than the eastern planter). 

Foundation Protection System 

The structure was built in 1996 with extensive drainage protection 

- waterproof seal, drainage panels, and a foundation drain (“toe 

drain”). 

• The waterproofing system consists of bentonite panels. 

• The drainage panels have a reported flow capacity of 10-16 

gallons per minute per lineal ft. They include a fabric cover to 

limit the passage of sediment. 

• The foundation drains are pervious pipes that run the length of 

the bottom of the foundations edge. They drain to the combined 

sewer.

Landscaping 

Construction of the planters required removal of most of the 

existing vegetation and excavation to a depth of 6 in. (center of the 

planters). Ashforth Pacific originally hoped to maintain the 

existing young maple trees, but there was uncertainty about the 

risks associated with excavating around the roots and inundating 

the trees with stormwater. To limit the risk, Ashforth decided to 

replace the Red Maples with 2-in. caliper Red Alder saplings. 

Ashforth installed native plants throughout, including grasses, sedges and rushes. The plants were 

selected to provide water quality treatment and promote stormwater uptake. The careful selection and 

dense plantings of native vegetation has allowed Ashforth Pacific to eliminate the use of chemicals in 

the maintenance of the facilities. 

Westside infiltration planter and 

scupper; fall 2003

Coring through concrete wall for 

stormdrain and scupper, 2002
12/9/2004; Liberty Centre Parking Garage 5

Storm drains and Scuppers 

Four internal drains were re-directed to the exterior of the building. The new extensions (6-8 in. iron 

pipe) travel along the ceiling of the main floor, reaching as far as 50 ft. to the locations of the scuppers 

that drain into the planters. The pipes penetrate the foundation wall and terminate in attractive bronze 

scuppers. 

Emergency Overflow 

An overflow standpipe routes excess flows from the southern end of each planter underneath the 

sidewalk to the adjacent street inlet4

. The overflow is a small lynch-style catch basin with a metal 

grate. Each overflow grate is 11 in. above the floor of the planter, allowing stormwater to pond to a 

depth of 11 in. when the system reaches capacity. The City does not typically allow direct connection 

of private stormwater management facilities to public catch basins due to an increased risk of damage 

to private property in the event of a system failure. 

Budget 

Ashforth Pacific submitted a project budget of $75,530, which includes management, design, and 

construction. 

Table 1. 

 

4

 The City does not normally allow connections to street inlets. This connection was allowed to create easy access to the 

pipes for flow monitoring. 

Item Item Cost Total Cost

 Project Management $ 3,072.00

Design 12,135.00 $ 

Landscape design 1,835.00 $ 

Surveying, geotechnical & structural evaluation 10,300.00 $ 

Construction Management 1,695.00 $ 

Construction 33,015.00 $ 

Site plumbing/drainage systems 32,480.00 $ 

Moisture barrier -- labor and material 535.00 $ 

Landscaping (2300 s.f.) 24,140.00 $ 

labor and materials 22,350.00 $ 

Additional planting 1,790.00 $ 

Miscellaneous:

Permitting $ 1,473.00

Total 75,530.00 $ 

Liberty Centre Parking Garage Budget Summary
12/9/2004; Liberty Centre Parking Garage 6

Non-Construction Activities 

The estimated cost for management, design, and permitting was $18,375, comprising approximately 

25% of the budget. 

• Management (Project and Construction Management) 

The cost for both project and construction management was $4,767, comprising 7% of the budget. 

• Design 

Design costs were $12,135, comprising 16% of the budget. The cost of design attributable to the 

Landscape Architect was $1,835 or 2% of the budget. Engineering costs were $10,300 or 14% of 

the budget and consisted of surveying work, a geotechnical evaluation, and a structural analysis 

(due to siting the infiltration facilities next to the foundation). 

• Permitting 

The permit costs were $1,473, comprising 2% of the budget. Permits included a permit from 

Portland Department of Transportation (PDOT) for construction within the public right-of-way. 

Construction Activities

Construction and landscaping costs were $57,155 or 75% of the budget. 

• Demolition, Excavation, and Grading

The cost for excavation and grading is not provided as a separate figure in the budget. The project 

required only minimal excavation in the existing landscaped area; no demolition was required. 

• Construction 

The cost for construction activities was $33,015, comprising 44% of the budget. Two project 

components that notably contributed to construction costs were plumbing for the overflow pipes 

and plumbing to redirect runoff to the exterior of the building, an essential component to the 

feasibility of the project. 

• Landscaping 

The project included almost 2,300 sq. ft. of landscaping area, approximately 5% of the catchment 

area. The cost was $24,140, comprising 31% of the budget. Items included labor, materials, and the 

removal of ten 3 in. caliper trees. 

II. Cost Elements

Engineering Design. Although the facilities appear rather simple in scope, substantial engineering 

support was required to design the plumbing and drainage system, conduct the hydraulic analyses for 

the planters, conduct soil infiltration tests, and provide a structural evaluation for siting an infiltration 

facility next to the foundation. 

Plumbing. Piping costs account for a substantial part of the construction budget, largely due to the rerouting of 4 existing interior drains to the exterior of the structure. More than 150 ft. of new iron pipe 

was hung from the ceiling to complete the project. 
12/9/2004; Liberty Centre Parking Garage 7

Landscaping. The unit cost for landscaping, almost $11 per sq. 

ft., is relatively high. More typical planting costs are less than $5 

per sq. ft. The costs are partly explained by the density of the 

plantings. Due to the time of year, plugs were used instead of 

seeds. The removal and replacement of the existing trees was 

probably the largest factor in the high unit cost. The replacement 

of the trees also affected the project management costs. Extra 

effort was expended to evaluate the risks associated with 

retaining the existing trees, research the associated permitting 

issues, and obtain adequately sized replacement trees.

Foundation Protection System. Despite minimal cost to increase 

the height of the existing foundation protection system, the 

project might have cost substantially more if the drainage 

protection system was not already in place. 

Planter Construction. The property owner saved money through 

the relatively simple design of the planter...it only required the 

addition of a 6-inch curb along the sidewalks (the foundation 

serves as the inside wall). Many other planter projects would 

require more concrete construction work. 

III. Cost Comparisons 

Despite the relatively high costs for engineering, piping, and 

landscaping, this project is an example of a relatively costeffective stormwater retrofit. The unit cost was $2.08 per square 

foot of impervious area, or catchment area. The design does not 

provide complete onsite disposal for all storm events, making the 

project costs a difficult basis for comparison with other projects 

that provide total disposal. 

Maintenance and Monitoring 

The owner of the property is responsible for all maintenance activities. 

Environmental Services will monitor the performance of the facilities at the Liberty Centre Parking 

Garage for at least five years, and perhaps longer. Confirming the hydraulic performance of the facility 

will be a primary concern. Environmental Services will also regularly evaluate the level of effort 

required to maintain the facility, the success of the planting regime, and comments from the owner. 

Eastside iInfiltration planter under 

construction, 2001

Stormdrain and bronze scupper, winter 

2003

Overflow standpipe at southern end 

of infiltration basins
12/9/2004; Liberty Centre Parking Garage 8

Successes and Lessons Learned 

Good Design - the design successfully incorporates a stormwater feature into a highly urban setting, 

adjacent to a heavily-used sidewalk. 

Stormwater Capacity - Although the design does not provide total on-site disposal, it makes the most 

of the limited landscape available for stormwater management. The walls of the planter substantially 

increase the internal volume of the stormwater system. 

Landscaping - The project showcases the appropriate use of trees and native plantings within 

stormwater planters. It also highlights the careful scrutiny required for existing trees in areas that are 

excavated or converted for stormwater management. Concern over the viability of the existing Red 

Maples led to their removal and replacement with Alder saplings. 

Re-direction of Internal Plumbing - The project provides helpful information about the feasibility and 

cost of re-directing internal storm drains to the exterior of a large commercial structure. For this 

project, the pipes were easy to access because the were not enclosed behind ceilings or walls. 

Infiltration Adjacent to a Structure - The project is a great example of how a landscape infiltration 

facility can be sited adjacent to a foundation. In this case adequate drainage protection was installed 

during the construction of the building in 1996. 

LEEDS Certification - Ashforth Pacific qualified for LEED certification in the “Existing Building” 

category for sustainable design. The project complied with the certification criteria for its use of native 

vegetation and a 25% minimum reduction in stormwater volume. 
12/9/2004; Liberty Centre Parking Garage 9

 

Figure 1. 

N