12/9/2004; Reed College 1

Reed College Apartments 

5510-5522 SE 28th Avenue 

Project Summary 

Overview of the Stormwater System 

• Downspouts from two apartment buildings (a total of 7,900 sq. ft. of roof) have been diverted to new 

subsurface lateral pipes. The laterals connect with new stormwater trunk lines that direct runoff to 

the soakage trench. 

• Runoff from the parking lot (4,100 sq. ft.) drains to one of the new stormwater trunk lines which 

carries runoff to the soakage trench. 

• A silt basin provides pre-treatment for runoff draining to the soakage trench. 

 

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 a total of eleven projects. The projects were completed by July 1, 2003. 

Project Type: Retrofit of a multi-family residential property – demonstration project. 

Technologies: East side soakage trench 

Major 

Benefits: 

• Runoff from more than 12,000 sq. ft. of impervious surface - roof and parking lot - has been removed from 

the combined sewer. 

• The stormwater system removes more than 269,000 gallons of runoff from the sewer in a typical rain year, 

with corresponding reductions in runoff pollutants. 

Cost: $33,042 (unit cost of $2.75 per sq. ft. of impervious area managed). Environmental Services provided $30,000 

in grant funds for the project1. 

Constructed: Summer 2002 

Aerial photo of Reed College 

N Apartments, 2002 
12/9/2004; Reed College 2

Stormwater Capacity and System Components 

Stormwater Management Goal

The goal was to meet the Bureau of Development 

Services (BDS)2 standard for stormwater disposal. 

When BDS approved the project in 2002, the 

disposal standard was to infiltrate at least 3 in. of 

runoff in 24 hours, which is about the size of the 

10-year design storm. All design standards in this 

report were current in the year 2002. 

Geotechnical Evaluation/Infiltration Test

Geotechnical engineers tested soil infiltration 

rates at 3 locations on the site of the soakage 

trench. They augered test holes ranging from 2 to 

15 feet deep. Perforated pipes were inserted into 

the holes and filled with water before falling-head 

tests were conducted. The test results ranged from 

13 in. per hour to 27 in. per hour of infiltration. 

The engineers reported the soil texture as ranging 

from silty-sand to sandy-silt, the percentage of sand increasing with depth. 

The Natural Resources Conservation Service (NRCS) soil survey for Multnomah County classifies the 

soils as 51C-Urban Land-Latourell complex. The classification is described as highly developed land 

atop stratified soils and sometimes fill. The survey provides an estimated infiltration rate of 0.6 – 6.0 in. 

per hour.

System Components 

Soakage Trench 

(See Site Plan for details; Figure 1, pg. 8)

Catchment Area: 12,000 sq. ft. total (roofs and parking lot) 

Facility footprint3

: 625 sq. ft. Internal Volume: 765 cu. ft. 

Overflow: No overflow system is required – the soakage trench4

 meets City standards for stormwater 

disposal. 

Capacity: The design is consistent with the configuration from the SWMM for an “eastside soakage 

trench” and provides slightly more internal volume. 

 

2 BDS develops standards for stormwater disposal and inspects projects to confirm compliance with those standards. 3

The footprint has been calculated as the wetted (ponded) surface area when the facility reaches maximum capacity. 4 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 feet of trench per 1000 square feet 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.

Outline of soakage trench along the front of the Reed College

Apartments - SE 28th is to the left. Not to scale.
12/9/2004; Reed College 3

Additional Information: 

• The top of the soakage trench is approximately 2.5 

ft. below grade; it is covered with soil, and the 

ground surface is seeded with grass. 

• The trench is slightly deeper and wider than 

recommended by the SWMM: it is approximately 

125 ft. long, 5 ft. wide, and about 3.5 ft. deep. It is 

lined with filter fabric. 

• There are three layers of material in the trench: 

o 12 inches of drain rock (top layer) 

o filter fabric 

o 18 in. of sand (bottom layer) 

• Flow enters the system through a perforated pipe 

that travels horizontally along the top of the layer 

of drain rock. 

• The trench is 15 ft. from the closest building foundation; it is 20 ft. from the public right of way. 

• Flow test: After completion of the project, City staff tested the facility’s ability to accept the 25-year 

storm. To mimic peak flows, a hose passing 380 gpm was introduced at the top of the silt trap after 5 

minutes of moderate flows. No problems were observed during the two-minute interval during which 

the peak rate was applied. The system received a total of 3,900 gallons in 13 minutes (note: the 

facility has an interior capacity greater than 5,000 gallons).

Piping 

• Fourteen downspouts on two buildings were disconnected from the sewer and connected to two new 

stormwater trunk lines. The laterals total 275 lineal feet of 4-in. ABS pipe. 

• The trunk lines total 300 lineal ft. of 6-in. ABS pipe. They run parallel to the building edges. 

• The parking lot catch basin, which drains the entire parking lot, was disconnected from the sewer and 

connected to one of the new stormwater trunk lines. 

• The new pipes were sized using the Rational method for a 10-year storm and a 5-minute initial time 

of concentration. 

Silt Basin 

Runoff travels through the silt basin prior to entering the soakage trench. 

Landscaping 

Four existing trees were removed on the west side of the property to allow for installation of the new 

trunk line pipes. 

Irrigation 

An irrigation system was not required. 

Looking into the silt basin - note debris in the bottom; 

spring 2004.
12/9/2004; Reed College 4

Budget 

The final project budget submitted by the Reed College property manager totaled $33,042, including 

design and construction. The final budget is shown below. 

I. Budget Elements

Non-construction Activities 

The total cost for overall project management, engineering design, and permitting was $7,392, comprising 

22% of the budget. This cost does not include construction management costs (see below). 

• Management (Project and Construction Management) 

Project management activities were very limited compared to many of the other grant projects. The 

plumbing contractor managed construction activities. All of his time was incorporated into the 

construction charges for the project. 

• Design 

Charges for engineering services including engineering design, surveying, hydraulic evaluations, plan 

drawings, and permit submittals totaled $6,600, comprising 20% of the budget. 

• Permitting 

The cost for the site development permit, which incorporates the plumbing permit, was $792, 

comprising 2% of the budget. 

 Item Item Cost Total Cost 

 Design $ 6,600.00 $ 6,600.00

 Demolition, excavation, grading 7,900.00 $ 

Concrete & asphalt demo, removal, export 2,820.00 $ 

Pipe work - excavate and haul soil 3,387.00 $ 

Soakage trench - excavate and haul soil 1,693.00 $ 

 Construction Management * - $ $ -

 Construction: 16,750.00 $ 

Soakage trench installation 6,380.00 $ 

Pipe installation: materials and labor 3,170.00 $ 

Pipe installation: backfill with 3/4 in. minus rock 2,258.00 $ 

Pipe installation: fill and replace asphalt 4,092.00 $ 

Pipe installation: replace concrete walk 850.00 $ 

 Landscaping (s.f.) 

Tree removal and turf re-seeding 1,000.00 $ $ 1,000.00

 Permitting 

 Site Development permit 792.00 $ $ 792.00

 TOTAL 33,042.00 $ 

Reed College Apartments Budget Summary 

* The plumbing contractor managed the project and didn't track management costs 

separately.
12/9/2004; Reed College 5

Construction Activities 

Demolition, excavation, construction, and 

landscaping costs totaled $25,650, comprising 78% 

of the budget. 

• Demolition, Excavation, and Grading 

The site preparation activities, including 

excavation for the pipes and the soakage trench, 

cost $7,900 (24% of the budget). 

• Construction 

Construction activities including downspout 

disconnection, pipe installation, and installation 

of the soakage trench totaled $16,750 (50% of 

the budget). 

• Landscaping 

Reed College staff removed 4 trees on the west side of the apartments to allow room for construction 

of the pipe system. The college staff also seeded the ground with turf grass upon completion of the 

project. Environmental Services staff estimated Reed’s costs for these activities at $1,000, about 3% 

of the project budget. 

II. Cost Elements 

Pipe Work 

The largest single cost was the installation of 575 lineal ft. of small-diameter pipe - 4 in. downspout 

laterals and a 6” stormwater trunk line. The cost of the pipe construction work was $16,577, including 

included the costs of asphalt and concrete cutting and removal, excavation, pipe installation, backfilling, 

and replacement of asphalt and concrete. 

Two-thirds of the piping was installed below hard surfaces, which added significantly to costs. The 385 

lineal ft. of piping under asphalt and concrete averaged $35 per lineal ft. The average unit cost for piping 

under grassy/landscaped areas was about $15 per lineal ft. The average cost for all of the piping was $29 

per lineal ft. 

Soakage Trench 

The second largest cost was from the construction of the trench for $8,073. 

Engineering Services 

The third largest cost was for the design provided by engineering services. As described above, the list 

of engineering activities is extensive for such a relatively simple project. 

III. Cost Comparisons

The relatively high cost of installing an extensive network of new collection pipes, particularly to install 

pipes under asphalt and concrete, is reflected in this project. Two thirds of the construction cost, 

including demolition and excavation, was for new pipes, and 80% of the cost for the new pipes was for 

installation under asphalt and concrete. 

Flow test - City staff simulated the peak of the 25-yr. storm

(hydrant water was injected into the silt basin); 2002
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The project could have been simplified by modifying the network of downspouts and reducing their 

number (with a corresponding reduction in lateral pipes). The average catchment per downspout is just 

560 square feet of roof, similar to the average for single-family residential homes. In contrast, 

catchments for downspouts on commercial buildings often exceed 1,500 sq. ft. 

The following changes might have reduced project costs. The roof on apartment building #2 is 3,900 sq. 

ft. in area and slopes to one side. The number of downspout could be reduced from 5 to 3 (average 

catchment size of 1,300 square feet per downspout). A reasonable cost to replumb the gutters and 

downspouts would be $1,000. The savings in pipe costs would be $2,600 (2 x 37 lineal ft. x $35 per 

lineal ft.). The estimated net costs savings would be about $1,600 (the savings in pipe work minus the 

cost of replacing the gutter and downspouts). It is unknown if the configuration of building #1 is 

conducive to gutter and downspout alterations. 

Maintenance and Monitoring 

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

monitor the performance of the soakage trench at the Reed College Apartments for at least five years. 

Confirming the hydraulic performance of the facility will be a primary focus. 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. 

Successes and Lessons Learned

The Cost of the New Collection and Conveyance System

Construction of the soakage trench seems relatively cost effective even though the new pipe system was 

more expensive than the facility. Although modification of the gutters and downspouts could have 

reduced the amount of pipe needed, the project illustrates the extent by which a pipe system installed 

under asphalt and concrete can increase costs. 

Stormwater Facilities in the Public Right of Way

The initial project plans were for construction of a swale in roughly the same location as the soakage 

trench. Since the configuration would have required part of the facility to sit in the public right-of-way – 

which the City does not allow – the soakage trench became a good alternative. Because the soakage 

trench could be buried more than 2 ft. below grade, it was possible to site the facility entirely on private 

property. Its depth is a function of the elevation of the pipe system delivering runoff to the facility. 

Flow Test

Flow tests have documented the ability of the soakage trench to accept the peak of the 25-year storm. 
12/9/2004; Reed College 

Figure 1 

COLLECTOR PIPEBLDG. 1 LATERAL PIPES 

SOAKAGE TRENCH 

COLLECTOR PIPE 

LATERAL PIPES 

REED COLLEGE APARTMENT

N

BLDG. 2