12/9/2004; Liberty Centre Parking Garage 1 Aerial view of Liberty Centre parking garage,

after p ro j ect 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 implemente d the Willamette Stormwater Control Program in 2001. The Program offered financial grants and technical support for a seri es of projects to retrofit ex isting commercial properties with stormwater controls incorporating green technologies. The Program recruited these demonstration projects to research the feasibility, cost and performance of commercial stormwater re trofits 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 prop erty 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 Program 1

. Constructed: Summer 200212/9/2004; 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 wa s 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 pl anter floors are trapezoidal and slope toward the south (similar to swales).

Geotechnical Evaluation/Infiltration Test Before construction, consulting e ngineers conducted infiltration

tests at two locations in each planter. The four tests yielded infiltration rates of 0.5-1.0 in. pe r 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 overf lowing). The simulations did not include assumptions about the e ffect of the drainage panels encasing the foundation, which ma y increase the infiltration

rate.

The Natural Resource Conservation Service (NRCS) SoilSurvey 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

200312/9/2004; Liberty Centre Parking Garage 3 F acility components

Eastern Infiltration Planter (See Site Plan for drainage details; Figure 1, pg.10) Catchment Area : 18,000 sq. ft. Facility footprint 2 : 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 trench 3 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 av erages 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 confi guration of a swale. It slopes gently toward the south (< 1%), it has trapezoidal side slopes (2:1 to 3:1), and it has four ch eck dams to promote retention and infiltration. The side slopes ensure that runoff doesn’t pond directly agains t 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 th e entire cross section of the planter at those locations is cove red with river rock to reduce

erosion.

• The top of the drainage prot ection 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 th e standard eastside soakage trench, the footprint has been cal culated as the wetted (ponded) surface area when the facility reaches ma ximum capacity.

3 The standard eastside soakage trench meets the City’s standard fo r complete stormwater disposal in soils, which infiltrate at l east 2 in. per hour. The City requires 24 ft. of trench per 1000 sq. ft. of impervious area (drainage catchm ent). The trench is 3 ft. deep , 2.5 ft. wide, and filled with drainage rock. Flow enters the trench through a pe rvious pipe that travels the length of the top of the trench. Assuming a porosity of 35%, the trench provides an internal vo lume 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 panel12/9/2004; 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 footprint 2 : 1,155 sq. ft.

Internal Volume : 214 cu. ft. Overflow : An overflow standpipe, located at the southern end of the planter, drains to an exis ting subsurface pipe that connects

to the combined sewer. Capacity : The swale has less capacity than the comparable eastside soakage trench 3 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 pa nels, and a foundation drain (“toe drain”).

• The waterproofing system consists of bentonite panels.

• The drainage panels have a re ported 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 re quired removal of most of the existing vegetation and excavation to a depth of 6 in. (center of the planters). Ashforth Pacific or iginally hoped to maintain the existing young maple trees, but th ere was uncertainty about the

risks associated with excavati ng 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, se dges and rushes. The plants were selected to provide water quality treatment and pr omote stormwater uptake. The careful selection and dense plantings of native ve getation 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, 200212/9/2004; Liberty Centre Parking Garage 5 Storm drains and Scuppers
Four internal drains were re-directed to the exteri or of the building. The new extensions (6-8 in. iron pipe) travel along the ceiling of the ma in floor, reaching as far as 50 ft. to the locations of the scuppers that drain into the planters. Th e 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 inlet 4 . The overflow is a small lynch- style catch basin with a metal grate. Each overflow grate is 11 i n. above the floor of the planter, allowing stormwater to pond to a depth of 11 in. when the system r eaches capacity. The City does not typically allow direct connection of private stormwater management f acilities 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 connect ions 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 Summary12/9/2004; 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 c onstruction 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 budge t. Engineering costs were $10,300 or 14% of the budget and consisted of surveying work, a geot echnical 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) fo r construction within th e public right-of-way.

Construction Activities Construction and landscaping cost s were $57,155 or 75% of the budget.

• Demolition, Excavation, and Grading The cost for excavation and gradi ng 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 contribut ed 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 landscap ing 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, c onduct the hydraulic analyses for the planters, conduct soil infiltration tests, and prov ide a structural evaluation for siting an infiltration facility next to the foundation.

Plumbing . Piping costs account for a subs tantial part of the constructi on 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 typi cal 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 th e 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 adequa tely sized replacement trees

.

Foundation Protection System. Despite minimal cost to increase the height of the existing foundation protection system, the project might have cost substa ntially 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 al ong the sidewalks (the foundation serves as the inside wall). Ma ny other planter projects would require more concrete construction work.

III. Cost Comparisons

Despite the relatively high co sts for engineering, piping, and landscaping, this project is an example of a relatively costeffective stormwater retrofit. Th e 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 respon sible 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 perf ormance of the facility will be a primary concern. Environmental Services wi ll also regularly evaluate the level of effort required to maintain the facility, the success of th e 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 basins12/9/2004; Liberty Centre Parking Garage 8 

### Successes and Lessons Learned

Good Design - the design successfully incorporates a stor mwater feature into a highly urban setting, adjacent to a heavily-used sidewalk.

Stormwater Capacity - Although the design does not provide total on-site di sposal, it makes the most of the limited landscape available for stormwater ma nagement. The walls of the planter substantially increase the internal volume of the stormwater system.

Landscaping - The project showcases the appropriate us e of trees and native plantings within stormwater planters. It also highlights the careful sc rutiny 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 ce rtification in the “Existing Building” category for sustainable design. The pr oject complied with the certification criteria for its use of native vegetation and a 25% minimum re duction in stormwater volume.12/9/2004; Liberty Centre Parking Garage 9 Figure 1.



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