West Hills Innovative Stormwater Demonstration
Final Report, Chapter 1: Stormwater Demonstration Overview

The objective of this demonstration effort is to identify and prioritize a suite of best management practices and their potential locations and extents, to reduce runoff on challenging sites in the developed uplands of the Forest Heights Homeowners Association (FHHOA) on privately, commonly, and publically-owned property (see Figure 1-1).

Figure 1-1, Best Management Practice (BMP) Priority Area
Challenging Sites

For the purposes of this report, a site is considered challenging because it cannot easily or safely infiltrate runoff, so other means of runoff reduction must be identified, which may include prevention, infiltration of rainfall (before it becomes runoff), evaporation of rainfall, or evaporation of runoff. The criteria that make a site challenging, all of which are known to be present at the FHHOA, include:

  • Steep slopes
  • High seasonal groundwater tables (i.e. seasonally flooded landscape areas)
  • Slowly draining & compacted, clayey soils
  • Tight setbacks or inadequate space between buildings
The Problem

Rainfall on hardscapes and on urban landscape areas tends to end up quickly in storm drains, which drain indirectly or directly to a waterway. This stormwater is called runoff and is laden with a whole host of pollutants, including soil.

While erosion is a natural process, the extent of erosion occurring at the FHHOA, caused by the past and current development pattern on its steep slopes, is unnaturally high. Oversaturated steep and compacted soils, as well as roofs, paved and gravel roads, driveways, sidewalks, and lawns are all contributing a volume of runoff that is much, much higher than the natural, pre-development conditions, to which the watershed is adapted. This additional volume of runoff not only scours soil and other pollutants from surfaces in the uplands, but also scours the stream banks in the valleys, contributing even more pollution and destabilizing slopes.

As a result, houses everywhere, both uphill and along the stream banks, are experiencing erosion on their property that one homeowner (project 52) believes threatens the property value of every home in the Forest Heights HOA area. Evidence of soils that have slid down the hill can be found throughout the HOA, and Mill Pond must be regularly dredged (map), at a recent cost of $585,000 with a projected cost in 2028 of $891,000. This cost and the associated landslide risk are spread collectively across all homeowners, and so, regardless of who is experiencing challenges, the problem is a collective one. So is the solution.

Maintenance is a costly issue as well. Excess runoff scours a variety of landscapes and sediment clogs storm systems.

Fig 1-2: Excess runoff from developed uplands (yellow areas of Fig 1-1) contribute to stream bank erosion.
Fig 1-3: Runoff is eroding the landscape next to Skyline Drive (right red arrow) and clogging a small area drain installed and maintained by the HOA (left red arrow) Figure 1-4: Runoff from compacted lawn area and compacted gravel walkways generate enough runoff to regularly erode the compost in the garden beds at Valley View Park causing an expensive regular maintenance issue
 
Figure 1-5: Future minor soil slides (like this one at the elementary school that can be seen from Miller Road) could be prevented by revegetating steep slopes with native trees & shrubs.
Overall Approach: Rainfall vs. Runoff

The city government has made reducing runoff one of their top priorities, with a strong emphasis on infiltration; however, the west hills of Portland, like so many other places in the world, have steep slopes, high groundwater tables, and clay soils, which make infiltration difficult if not, in some cases, dangerous.

To reduce landslides and flooding, there are a number of creative solutions that take a two-pronged approach to runoff (i.e. high volume flows often concentrated in downspouts and pipes from a particular area and directed to a particular area) and rainfall (i.e. low volume rain that falls on the facility itself before becoming concentrated):

  • Limit the volume of runoff actively directed into the ground by lining a facility, and instead use evaporation and uptake by plants.

  • In landscape areas, restore the historic hydrologic function by encouraging evaporation and/or infiltration of rainfall.
Multiple Benefits

By reducing runoff, we gain multiple benefits:

  • More stable slopes with less erosion and fewer landslides in both the developed uplands and the stream valleys
  • Reduced sediment in our waterways
  • Improved safety
  • Improved water quality
  • Improved air quality
  • Long-term savings to residents
  • If well implemented, reduced maintenance
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Updated 3 Sep 2014
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