West Hills Innovative Stormwater Demonstration

Final Report, Chapter 5: Implementing Recommended Practices
Depaving

What is it? The practice of removing any unnecessary areas of impervious pavement (aka hardscape) and replacing it with vegetation. (As mentioned at the beginning of this chapter, don't replace it with lawn or sediment export will increase.)

What pavement is considered unnecessary? At the FHHOA, depaving projects were adopted where resident parking would not be significantly impacted. Estimates of areas always leave two to four on-street parking spaces per house available and at least a 10 foot wide driving lane.

How does this reduce runoff and protect water quality? For every square foot of pavement removed in Portland, you can prevent, on average, 22 gallons of runoff/year. A small driveway depaving project, removing just the middle of the drive aisle can prevents about 5000 gallons of runoff/year.

Where can it be done?

  • Any unused impermeable surface. The center of a driveway or the 2’ overhang of existing parking spaces are two great opportunities for depaving. (The overhang area is where the front or back of a car hangs over the pavement, in front or behind the wheel axle. By city code, parking spaces must be a minimum of 8.5 feet wide and 16 feet long with an overhang of 2 feet. Driveways must have a minimum width of 9 feet, and roads must have a minimum width of 12 feet. Depaving should preserve these minimum areas.)
  • Steep slopes. On slopes steeper than 8%. (For slopes less than 8%, consider bioretention.)
  • Improve pedestrian safety. Locate them where pedestrians would like to be more insulated from traffic or where traffic calming/slowing is desired.

What’s the maintenance?

  • Weeding: Remove weeds twice a year, ideally in May and October (before weeds go to seed).
  • Compost application: Annually, replenish compost in gardens and under tree canopies to a depth of 2-3” and lawns 1/4".
  • Irrigation: Irrigate during the 2-3 year establishment period if using natives. (If not using natives, then permanent irrigation will be needed.) See the supplemental fact sheet including information on establishment irrigation.
Depaving: Example 1
Figure 5-9 On Miller Road at project 64, pedestrians are walking on curb-tight sidewalks. Vehicles in this area have more pavement than is needed especially since parking is prohibited here.
Figure 5-10 This depaving project removes pavement that far exceeds the 12 feet needed for a driving lane and where parking is not allowed anyway. (The existing "No Parking" signs here should be moved from the existing curb to the new curb.)
Depaving: Example 2

Figure 5-11 At project 57, these three cars show how a typical 28-foot wide road at the FHHOA can accomodate parking (on left) and still allow 2 cars to pass. Pedestrians must walk along a curb-tight sidewalk.
Fig 5-12 This area is adjacent to a natural area where there are no houses, so there is a generous length of road that is still 28 feet wide even though parking is not needed. This graphic shows how the road may be narrowed and still allow cars to pass. A video of this site explains additional site-specific information for implementing this project that might be considered at other depaving project sites.
Planting Strategy

Planting strategy. Plantings will be in highly visible areas adjacent to pedestrian areas. Ecologically, many native plants will work well in these areas.

Installing the plants. Hiring a professional landscape contractor who is experienced and will guarantee the plants through a two-year establishment period is ideal.

Locating Trees. Trees should only be planted in depaved areas where adequate soil volumes is provided (see pages 6 & 7 of this EPA document). As a rule of thumb, trees that will be healthy, safe, and low maintenance should only be planted in areas where:

  • The minimum width of the planting area is 6 feet
  • The minimum planting area is 330 square feet
  • The minimum depth of available soil is 3 feet

With a reasonable spacing of 25 to 30 feet, multiple trees may share the same soil.

Restoring the Soil. Restore the soil in depaved areas according to the information provided in Chapter 5, Restored Soils.

Stakeholder Considerations
Fig 5-13 NW Miller is a pleasing tree-lined street north of the commercial district. More areas of the HOA, where planters don't already exist could look similar if they were depaved.

Pedestrian safety : The traffic calming benefits of tree lined roads is well documented in numerous studies. In addition, people are more comfortable walking along a sidewalk separated by a few feet of landscape area.

Aesthetics: Shrubs and trees that flower at different times of the year might be carefully combined to be seen from the road.

Preserving Parking: Where parking is allowed, depave but leave two 20 foot long by 8 foot wide on-street parking spaces per house in the area.

Specifications

Restoring the soil: Soil restoration is essential to getting the plants established with minimal or no additional water. Soil underneath pavement has been highly compacted and will have a lower than optimal soil biology count or diversity. During any depaving project, follow the guidance provided in Chapter 5 on restoring soils.

Time of year: Shrubs and trees should be planted in the fall when they will have the longest oppportunity to establish during the rainy season.

Irrigation for 2-year establishment period: As evidenced by the landslide on SW Terrwilliger in 2008 that destroyed a number of homes, irrigation on steep slopes must be done with great care. For this reason, conventional piped irrigation systems are NOT recommended. Instead, as is commonly done, hire a landscape company with a low pressure water truck to deeply water plants in the root zone only (generally the area between the trunk and the dripline/canopy) according to guidance provided in the fact sheet provided on establishment irrigation.

Street Width Code Considerations

Road Widths. The road widths and parking requirements, whether public or private, are set by the city. In the recommended depaving areas, parking is likely in excess of what's required. Also, only parking areas, never road lanes, are proposed for depaving.

Per the city's website:

"City of Portland local street standards were adopted by City Council in 1991 to address many issues facing Portland. Streetscape design standards were developed to address environmental considerations, neighborhood speeding, traffic reduction, and neighborhood livability.

Local street widths range from 20 feet to 32 feet. Most local streets are 26 feet wide, which provides residents parking on both sides. In some cases, due to topography constraints, local streets will be constructed to a 20-foot width, which allows parking on one side only."

Fig 5-14 According to the City of Portland's standards, the typical roads widths of 28 feet and 35 feet at the HOA exceed the width needed to park cars on both sides of the and leave a driving aisle by 2 and 9 feet respectively. This road pictured here is 28 feet wide.

Local street widths at the HOA range from 28 feet to 35 feet wide, so they exceed the widths that the City require to pass two cars and park on both sides. (To confirm this, see the first table in "Creating Public Streets and Pedestrian Connections through the Land Use and Building Permit Process". The areas proposed to be depaved are in R10, which falls between the RF and R7 zoning per a conversation with city staff. Under section "A. RF-R7 Zoning", in the first table titled "Standard Through-Street OR Dead-end less than 300' in length", look for the rows labeled "Local Service Street" then "Two Lanes". Both indicate that the minimum roadway width is 26 feet.) Per city code Title 33.266, The standard parallel parking space width in the city is 8 feet, so this means that a only 10 foot driving lane (26 - 8 - 8 = 10) is required on a local road. What I've proposed will always leave at least 20 feet after depaving for one of two following conditions:

  • Parking on the opposite side of the road to the depaving project (8' required) and one car to pass (10' required), so 18 feet total is required, or
  • Two cars may safely pass each other (2 10-foot lanes), so a total of 20 feet is required.

Street Ownership. Public versus private ownership of roads was determined from a meeting with Jen Callaghan. One of the criteria is whether the road ends in a cul-de-sac (public) or a hammerhead (private).

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