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 r unoff draining to the soakage trench.

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 commerc ial stormwater retrofits in the area serv ed 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 gall ons 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 project 1

. Constructed: Summer 2002

Aerial photo of Reed College Apartments, 2002 N12/9/2004; 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 stor mwater 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 test ed 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 te st 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 clas sification 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 footprint 3 : 625 sq. ft. Internal Volume : 765 cu. ft. Overflow : No overflow system is required – the soakage trench 4 meets City standards for stormwater disposal. Capacity: The design is consistent with the configur ation 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 conf irm compliance with those standards.

3 The footprint has been calculated as the wetted (ponded) surface ar ea when the facility reaches maximum capacity.

4 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 feet of trench per 1000 square feet of impervious area (drainage catchme nt). The trench is 3 ft. dee p, 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 volu me 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 fou ndation; 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 hos e passing 380 gpm was intr oduced at the top of the silt trap after 5 minutes of moderate flows. No problems were obs erved during the two-minut e 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 cap acity greater than 5,000 gallons).

Piping

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

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

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

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

connected to one of the ne w stormwater trunk lines.

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

of concentration.

Silt Basin Runoff travels through the si lt basin prior to entering the soakage trench.

Landscaping Four existing trees were removed on the west side of the property to allow fo r 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 co mpared to many of the other grant projects. The plumbing contractor managed construction activitie s. All of his time was incorporated into the construction charges for the project.

• Design Charges for engineering services including engineer ing design, surveying, hydr aulic evaluations, plan drawings, and permit submittals totaled $6,600, comprising 20% of the budget.

• Permitting The cost for the site development permit, wh ich 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 activitie s including downspout disconnection, pipe insta llation, 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 gr ass upon completion of the project. Environmental Services staff estimated R eed’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 inst allation of 575 lineal ft. of small-diameter pipe - 4 in. downspout

laterals and a 6” stormwater tr unk line. The cost of the pipe construction work was $16,577, including included the costs of asphalt and concrete cutting a nd 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 as phalt and concrete averaged $35 per linea l 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 th e 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, part icularly to install pipes under asphalt and concrete, is reflected in th is 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 aspha lt and concrete. Flow test - City staff simulated the peak of the 25-yr. stor m (hydrant water was injected into the silt basin); 200212/9/2004; Reed College 6 The project could have been simplified by modify ing the network of downs pouts and reducing their number (with a corresponding reducti on 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 co sts. 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 co sts 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 configur ation 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 fo cus. 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 modifi cation of the gutters and downspouts could have reduced the amount of pipe needed, the project illustra tes 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 ha ve required part of th e facility to sit in th e public righ t-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 soakag e trench to accept the peak of the 25-year storm.12/9/2004; Reed College7Figure 1COLLECTORPIPEBLDG. 1LATERALPIPESSOAKAGETRENCHCOLLECTOR PIPELATERAL PIPESREED COLLEGE APARTMENTSNBLDG. 2