West Hills Innovative Stormwater Demonstration
Final Report, Executive Summary

Project Objective

The objective of the stormwater demonstration is to to reduce runoff on challenging sites in the developed uplands of the Forest Heights Homeowners Association (FHHOA) on privately, commonly, and publically-owned property. The scope of work is to identify and prioritize a suite of best management practices (or simply "practices") and their potential locations and extents ("projects"). 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, and/or evaporation of runoff.

The Erosion & Sediment Problem

Erosion of the existing stream banks is caused by runoff conveyed quickly and in a much higher volume than historic rates as a result of development upstream from the stream itself. This erosion causes a high degree of maintenance in two ways:

1. Recent dredging to remove sediment accumulated in Mill Pond over time cost $585,000 with a projected cost in 2028 of $891,000. For this reason, this demonstration focuses on reducing runoff as a way to reduce sediment moving downstream to Mill Pond with the intent of extending the time period between dredgings. Since lawn has the highest capacity for exporting sediment and is much less permeable than most people think (see Chapter 2), many projects that address grassy areas are proposed.

2. Erosion at the upland sites themselves is generating a high level of maintenance costs. For instance, according to the landscape company contracted to perform work in common areas, one trouble spot, Valley View Park (at the intersections of NW Miller & NW Thompson Roads at the top and NW Miller Road and NW Murdock Street at the bottom) is a time consuming area for them to maintain compared to other common areas. Steep, compacted area make lawn difficult to keep established and runoff from those compacted soil areas, in addition to runoff from the highly compacted gravel walkways and a long private driveway at the top of the hill, generate a larger volume of runoff with a much faster rate of runoff than might be expected from landscape areas.

Practices and Projects

Information is provided based on "practices", which are general approaches that can be taken to reduce runoff and sediment export. Practices are prioritized in Chapter 3 & described in detail in Chapter 5, including specifications and preliminary details.

Within each practice are a variety of "projects". Projects are described in detail in Chapter 4.

Implementing any of the proposed projects will reap 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

Detailed information is provided in various sections of Chapter 5. The practices are briefly summarized as:

Revegetate/Stabilize slopes. Steep slopes generate faster and more erosive flows, so the grassy areas between and around existing trees should be planted with native trees & shrubs to intercept rainfall during our small, frequent rain events and to slow runoff by breaking up the flows during more intense storms.

Restored Soils. This is the practice of amending compacted soils by mixing compost and other helpful materials into them to restore their ability to manage rainfall instead of generate runoff. Once the area has been amended, future landscape types may include lawn (but only if it was lawn already), perennial garden, or meadow. Each has a different capacity to reduce runoff so the "Restored Soils" practice description always includes a description of the final proposed landscape, such as "Restored Soils (from Lawn to Perennial Garden)".

Depaving. The practice of removing any unnecessary areas of impervious pavement (aka hardscape) and replacing it with vegetation. Unnecessary areas most often include excess parking areas where houses are not fronting a street.

Porous Pavements. These are a stormwater facility that you can walk or drive on. This report proposes two different paving surfaces, retrofitting the gravel trails with porous gravel and permeable pavers. Mostly porous walkways and gathering spaces are proposed, but there is one roadway replacement included, should there be interest in implementing this practice on a wider scale.

Bioretention. Bioretention is the practice of infiltrating runoff through soil to reduce pollution. Some variations include rain gardens, stormwater planters, swales, or vegetated filter strips. When placed in the public right-of-way, any of these facilities are also be 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 providing a high degree of pollutant removal.

Modeling the Practices to Prioritize Projects

For modeling and analysis purposes, projects are also divided by rainfall projects and runoff projects. "Rainfall projects" reduce runoff by managing the rain that falls on them and include practices such as restoring soil and revegetating steep slopes. "Runoff projects" (also referred to as "Bioretention" in this report) reduce runoff by managing the runoff that flows into them from a much larger drainage area.

Modeling, using an industry standard methodology applicable to early planning efforts such as this, was performed to estimate the average annual reduction in runoff and in sediment export for each project, and this was subsequently used to help the HOA decide the order the practices should be implemented. Unit costs for each practice were developed with the assistance of local landscape contractors, and were subsequently used to estimate the cost of individual projects. By analyzing the environmental effectiveness of practices and the design and construction cost of the practices and considering stakeholder preferences, practices were ranked as follows:

Ranking by Practice (from best to worst)
1st = Revegetate/Stabilize slopes (Ranking = 9)
2nd = Restored Soils (from Lawn to Lawn) (Ranking = 8)
3rd = Bioretention Runoff Practices (Ranking = 7)
4th = Restored Soils (from Lawn to Perrenial Garden) (Ranking = 6)
5th = Depaving (Low Traffic Road) (Ranking = 5)
6th = Porous Walkway ((Ranking = 4)
7th = Restored Soils (from Lawn to Meadow) (Ranking = 3)
8th = Porous Roadway (Ranking = 2)
9th = Depaving (Medium Traffic Road) (Ranking = 1)

Forty four rainfall projects and 20 runoff projects (aka bioretention projects) were identified and mapped, each falling into one of the above practice categories. These projects are listed with a photograph of most sites in Chapter 4 Recommended Projects Summary, and maps of all the projects together or projects individually mapped by practice can also be found in Chapter 4.

Fact sheets were created to give homeowners more information about a variety of practices and additional implementation information for each practice is included in Chapter 5.

Life Cycle Cost Analysis

Due to the cost of dredging, the area draining to Mill Pond was isolated as the target area on which to focus efforts. Modeling shows that by implementing all 64 recommended projects, sediment carried by runoff from the developed uplands to Mill Pond can be reduced by about 29,000 pounds or approximately 10 cubic yards annually. The current cost to implement all the projects, based on preliminary cost estimating (see Appendix B), is $1,231,600; however, if the City of Portland implements all the projects that could be considered capitol improvements (i.e. all the bioretention facilities that manage runoff from the public right-of-way), then the cost is reduced to $862,500.

Per a conversation with Jennifer Callaghan, FHHOA Manager, dredging was completed in 2012 and the quantity of sediment was known, so she estimates that about 700 cubic yards of sediment are captured in Mill Pond every year.She predicts dredging will be needed again in 16 years in 2028, costing $891,000.

In studying the life cycle costs of these practices, the 64 projects proposed here will not make a significant impact on the dredging frequency. Over a 32 year period, making some conservative assumptions, about $24,800 might be saved in two dredging events, based on the volume of sediment prevented.

So, why implement these projects? There are number of reasons:

1. Generally, for the multiple benefits already discussed above. A general benefit that is also a safety concern is stabilizing the slopes. Planting shrubs and trees and reducing runoff through these and other practices are powerful tools to improve public safety and serve as a kind of insurance policy that can reduce the liklihood of landslides.

2. The model estimates only the amount of sediment carried in runoff from developed areas. It does not estimate the amount of sediment scoured from stream banks as a result of that runoff being conveyed to it. If this sediment scouring could be modeled accurately, the projects would likely be found to be more effective at reducing sediment to Mill Pond, with additional cost savings.

3. Maintenance cost has not been factored in, and in a number of cases, is likely to be lower over time.

4. Significant water quality problems are present in Cedar Mill Creek, downstream of Mill Pond. Without the cooperation of partners like the FHHOA in implementing projects similar to the ones identifed, regulators are unlikely to ever meet our community's water quality goals. Runoff is estimated to be reduced about 2,299,000 gallons (307,322 cubic feet). Every gallon of runoff that is kept from draining rapidly out of the FHHOA stream network helps to improve overall water quality in Cedar Mill Creek, and ultimately the Tualatin, Willamette, and Columbia Rivers, which are the receiving waters for stream flow from the HOA.

A more detailed discussion of reasons to implement these practices can be found in Chapter 3.

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