West Hills Innovative Stormwater Demonstration

Final Report, Chapter 5: Implementing Recommended Practices
Bioretention

Stormwater Planter: Example 1
Figure 5-15 Project 35 site as it exists today.
Figure 5-16 This is the schematic design for Project 35, a proposed stormwater planter. Many of the stormwater planters, like this one, are situated to include an existing catch basin. Designs such as this where runoff enters very close to existing catch basin should have a strategy for diverting and ponding water in the facility that prevents water from shortcutting directly into the existing catch basin. In this case, a 4" tall curb on two sides of the catch basin is suggested.

What is it? Bioretention is the practice of infiltrating runoff through soil to reduce pollution. Some variations include rain gardens, stormwater planters, swales, or vegetated filter strips. These facilities placed in the public right-of-way are called “green streets”. Bioretention facilities designed for challenging sites are lined to prevent runoff from infiltrating into the ground; however, water still passes through soil placed above the liner, within the facility.

To teach others in future projects how a variety of facility configurations might be implemented in different places throughout the HOA, the projects include different configurations that gather and store and/or convey runoff in different ways. See the schematic details below, which include a cross section image and descriptions of a stormwater planter, rain garden, swale, and vegetated filter strip.

How does this reduce runoff and protect water quality? As stormwater passes through the soil, pollutants are reduced through physical settling of large solids, filtering of small solids, and chemical and biological activity. Since the facility must be lined, runoff is reduced through evaporation from the top of the facility.

Where can it be done?

  • Since the facilities are lined to prevent infiltration into native soils, these facilities can be placed near steep slopes, but the slopes where the facilities themselves will be placed should be no greater than 8%.
  • Locate them where runoff from pavement can be directed into them. If there's a desire to locate a similar type of facility at the top of a hill, then depaving would more appropriate.

Where can it be done? (continued)

  • Locate them where pedestrians would like to be more insulated from traffic or where traffic calming/slowing is desired.
  • In Portland, avoid locating facilities where water lines run underneath.

What’s the maintenance?

 

Facility Sketches
Stormwater Planter: Example 2

Fig 5-17 Project 34 site as it exists today.
Fig 5-18 Accompanied by the stormwater planter detail below, this is the schematic design for Project 34. The homeowner in the background enthusastically supports this project.

Vegetated Filter Strip: Example 1

Fig 5-19 Project 43 site as it exists today.
Fig 5-20 Accompanied by the vegetated filter strip detail below, this is the schematic design for Project 43. This project will reduce maintenance, significantly reduce or eliminate sediment on the sidewalk and the ponding that probably occurs here during large storms when the catch basin inlet on the left is clogged (as it is in this photo).

Vegetated Filter Strip: Example 2

Fig 5-21 Project 39 site as it exists today.
Fig 5-22 Accompanied by the vegetated filter strip detail below, this is the schematic design for Project 39.

Swale: Example 1

Fig 5-23 Project 45 site as it exists today.
Fig 5-24 Accompanied by the swale detail below, this is the schematic design for Project 45.

Swale: Example 2 at Valley View Park

Fig 5-25 Project 32 site as it exists today.
Fig 5-26 Accompanied by the swale detail below, this is the schematic design for Project 32.
Planting Strategy

Consider pedestrian safety: Most bioretention facilities use short shrubs and groundcovers such as grasses and flowers/forbs to ensure that pedestrians can see and be seen as they walk along the sidewalk.

Consider pedestrian comfort and aesthetics. Grasses that will grow tall and flop over should be placed in the middle of a facility, but not on the edges where vegetation could grow over the sidewalk and road. (The City of Portland receives many calls/complaints about this condition in their green streets.)

Avoid trees. A somewhat recent study by the USDA found that most trees need at least 300 square feet of soil that is at least 3 feet deep to be healthy, safe, and low maintenance. Lined bioretention facilities usually only incorporate the minimum depth of soil needed for water quality treatment, which is 18" deep. Providing an additional 18" of soil would increase costs.

Plant densely to reduce weeding. Weed seeds blow in or are carried by birds and flowing stormwater. Sunny, bare spots of dirt that are moist are the ideal place for weeds, so plant and maintain a dense cover of vegetation to shade weeds out.

Material Specifications

Planting soil. For public right-of-way facilities, import the City of Portland standard "3-way mix" (click here for vendors), per the requirements of the City of Portland standard details for green streets.

For privately owned and located facilities, amended native clay soils has been found in the author's experience to improve plant establishment & will likely encourage increased runoff reduction through evaporation. Recipe: 2 parts native soil to 1 part compost plus Permamatrix at a rate of 50 lbs/500 square feet.

All of the details below call for 18" of planting soil. This is a minimum and only applies to grasses. Shrubs will likely need 24" to 36" of planting soil.

Compost. Compost should be US Compost Council Seal of Testing Assured compost. Visit http://compostingcouncil.org/participants to find a participating supplier near you. The STA program is no guarantee of quality, only that the compost has been tested and those test results are available for the designer’s review.

OMRI certification is also desirable, as it will certify that the product is organic.

Clackamas Compost Products (503.557.1028) has "Premium Garden Mulch", which has both certifications.

Organic compost may NOT be peat moss, which is extracted- from wetlands and negatively impacts the watershed from which the peat moss was removed.

Separation/filter rock (to keep planting soil particles from filling voids in uniformly graded rock, used in place of a geotextile fabric) and Uniformly graded rock. Two layers of separation/filter rock have been specified. This rock should be delivered "clean" and have the following gradations:

The coarse sand portion shall meet the following gradation:

US standard
sieve size
Percent
passing
3/8"
100
#4
54-82
#10
34-56
#40
9-17
#100
0-3

The 1/2"-3/8" crushed gravel portion shall meet the following gradation (ASTM C-33):

US standard
sieve size
Percent
passing
1/2"
100
3/8"
85-100
#4
10-30
#10
0-10
#8
0-10
#16
0-5

Uniformly graded rock shall meet the following gradation (AASHTO No. 57):

US standard
sieve size
Percent
passing
11/2"
100
1"
95-100
1/2"
25-60
#4
0-10
#8
0-5

Impermeable liner (to prevent concentrated stormwater runoff from infiltrating into the soil of nearby steep slopes). Impermeable liner may be:

  • 45 mil EPDM (available from roofing supply stores such as Allied Building Products in NE Portland) = most environmentally sound.
  • Low density polyethylene (LDPE) pond liner.
  • PVC pond liner.

Check dams (used to slow water on steep slopes). These should be concrete or other non-polluted material. Avoid:

  • Galvanized steel and copper, which exports zinc and copper and is harmful to fish.
  • Clay, which is easily eroded and high maintenance.
  • Rocks, which get moved during large storms.
  • Treated wood (even the environmentally sound treated wood is infused with copper).
  • Untreated wood (except for cedar), which is not very durable.
Facility Schematic Details

All bioretention facilites should be designed by at least a team of two, a licensed landscape architect and a licensed engineer to ensure that native plants and soils will work together and that erosion from the facility, except during very large storms, is minimized.

Flow splitter to reduce erosion. A flow splitter device directs fast, erosive flows from large storms around the facility. These are not typically used in Portland because the majority of our storms have historically been very small; however, this may be, at the discretion of the licensed professional team, a desirable additional element to reduce erosion and prevent sediment export from these facilities.

Facilities in the public right-of-way. Design facilities according to the City of Portland standard details for green streets.

Privately owned facilities. The following schematic designs are suggested:

Stormwater Planter Schematic Detail
A stormwater planter is a structural container with soil and plants built to collect and slow runoff. Runoff is ponded and then treated as it passes through plants, roots, and soil. Treatment is provided through physical settling & filtration, biological breakdown or sequestering of pollutants by microbes and plants, and chemical treatment. Planters are very similar to rain gardens except they have vertical side slopes created by walls or curbs.
Swale Schematic Detail
A swale is long, planted, open channel that carries, slows and absorbs stormwater and filters out pollutants through settling. In most swales, infiltration into the planting or native soil is usually not the primary flow path for stormwater, but check dams, which will be needed to slow flows on steep slopes at the FHHOA, can hold water back and reduce erosion from the conveyance channel. Check dams will facilitate additional treatment in a similar fashion to stormwater planters and rain gardens, because when water ponds and passes through the soil, physical filtration, biological activity, and chemical processes will assist with pollutant treatment.
Vegetated Filter Strip Schematic Detail
A vegetated filter strip is a level facility that receives runoff in a distributed fashion along the length of one side of it and treats stormwater by settling out solids. More flat grades and dense vegetation across the top surface will encourage infiltration into the planting soil and improve treatment. A small densely vegetated berm on the surface of the left side could also be used, if desired, to pond water and improve runoff treatment.
Rain Garden Schematic Detail
A rain garden is a planted, bowl-shaped area designed to collect and absorb runoff and filter out pollutants. Runoff is ponded and then treated as it passes through plants, roots, and soil. Treatment is provided through physical settling & filtration, biological breakdown or sequestering of pollutants by microbes and plants, and chemical treatment. Rain gardens are very similar to stormwater planters except they have gentle sloping sides.
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