West Hills Innovative Stormwater Demonstration
Chapter 3: Prioritizing the Practices

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 passing 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.

Analyzing the data from the individual projects proposed in the model, there appears to be only one clear "winner" where immediate consensus might be acheived: revegetating and stabilizing steep slopes. Partly, this is because Clean Water Services has committed to provide the FHHOA with about 14,500 bare root, native shrubs and trees, but also lawns have a very high rate of sediment export, so changing a landscape from lawn to forest is highly effective for addressing runoff reduction and sediment export.

Each surface (lawn, meadow, perennial garden, forest, or pavement) has a different runoff characteristic that changes from one to another after a particular project has been implemented. Likewise, each surface has a different amount of sediment it may export, but this is further complicated by pavement, which exports more sediment with more traffic. These variables have been accounted for in the model but add a relatively high degree of complexity that can make decision making processes arduous.

The remainder of this page is designed to step you through the recommended decision making process. To look at just my recommendation on the order in which best practices should be implemented based on environmental and cost-effective, click here to skip down to the bottom of the page and read the last two sections.

IMPORTANT! This data was specifically tailored to respond to the decision making needs of the FHHOA as well as local costs and conditions. Incorporating different best practices with a different set of stakeholders in a different part of the country is likely to yield very different conclusions!

Let's step through the logic together...

Runoff Reduction & Sediment Prevention Capacity

The following chart shows the volume of runoff prevented annually when the runoff reduction capacity of all the proposed projects for each practice are added up. One of the reasons that revegetating steep slopes has such a high value compared to other practices is that there were numerous opportunities and a larger physical area at the HOA to revegetate, while other practices had more limited areas. While knowing how much runoff is predicted to be prevented is useful, looking at this chart alone, we cannot make a reasonalbe decision about where to start working.

Similarly, the next chart shows the pounds of sediment prevented annually when all the proposed projects for each practice are added up.

Environmental Effectiveness: Runoff Reduction & Sediment Prevention

We can account for the area differences across practices by dividing the total pounds of sediment prevented or the total volume of runoff reduced for all the practices by the total area managed for all the practices, which gives us a measure of the environmental effectiveness of each practice.

In the case of the first eight rainfall practices, the area managed by the facility is the same as the area retrofitted. In the case of the bioretention practices, runoff is collected from a much larger area into a relatively small facility, so the area managed is much larger than the actual facility area. Graphing the environmental impact per square foot of area managed is the most meaningful way to compare "apples to apples".

Doing this analysis for sediment, we see that revegetating steep slopes is still the most effective practice.

Analyzing each practice for runoff reduction effectiveness below, we can see that from a purely runoff reduction perspective, removing pavement and replacing it with porous pavement generates the least amount of runoff of all the practices.

Converting existing conventional pavement to porous pavement, as you might expect, is a very expensive approach, so we obviously need to account for the design and construction costs of each practice.

Design & Construction Cost

The cost per square foot (aka unit cost) of design and construction costs were estimated for each practice (see Appendix B: Preliminary Cost Estimate for assumptions, sources of data, etc). The cost per square foot to design and construct the practice is of interest, but to the City of Portland, the cost per square foot to design and construct the practice relative to the area that's managed is more important (and is part of their % for Green Program application). Both unit costs are shown in the next chart.

In the case of the first eight rainfall practices, the area managed by the facility is the same as the area retrofitted, so the construction cost construction per square foot of the facility equals cost per square foot of area managed. In the case of the bioretention practices, runoff is collected from a much larger area into a relatively small facility. In this case the cost of construction per square foot is much higher than the cost per square foot managed.

Area of practice [sf]
Area Managed [sf]
Design & construction cost/square foot of area managed
Design & construction cost/square foot
Restored Soils from Lawn to Lawn 39,655 39,655 $2.45 $2.45
Restored Soils from Lawn to Meadow 8,955 8,955 $4.46 $4.46
Restored Soils from Lawn to Perennial Garden 28,444 28,444 $4.46 $4.46
Porous Walkway 6,189 6,189 $8.17 $8.17
Porous Roadway 6,251 6,251 $9.45 $9.45
Depaving (Low traffic road) 10,850 10,850 $19.21 $19.21
Depaving (Medium traffic road) 5,146 5,146 $19.21 $19.21
Revegetate/ Stabilize slopes 263,050 263,050 $0.32 $0.32
Bioretention Runoff Practices 6,394 80,197 $6.08 $76.29
Total
376,809 450,612

Cost Effectiveness of Practices

Using the unit cost only to decide on the best "best practice" does not account for the widely varying environmental effectiveness of each practice. In other words, the cheapest practice per square foot may not be the smartest way to spend a limited budget if the main goals are reducing runoff and preventing sediment export.

To analyze the true cost effectiveness of each practice, we should also account for how environmentally effective it is. The model calculates the cost per square foot per gallon of runoff prevented and the cost per square foot per pound of sediment prevented, but when the relatively small unit costs are divided by the relatively large gallon of runoff prevented or pounds of sediment prevented, the numbers are difficult to chart; therefore, a rank between 1 and 9 was assigned to each practice with the following logic:

Rankings
1 = worst = least cost effective considering the environmental benefit and should be the last practice to implement
9 = best = most cost effective considering the environmental benefit and should be the first practice to implement

The following chart shows the best practices ranked according to their cost effectiveness in reducing runoff, their cost effectiveness in preventing sediment export, and their overall cost effectiveness when the cost considerations for runoff and sediment are combined.

Simplfying this chart to only show the overall cost & environmental effectiveness ranking, we see that if we want to pick a family of practices to implement first, based on both their environmental and cost effectiveness, then we would implement them in this order:

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

Why Implement More Costly Practices

While the revegetation strategy is considered to be a "no brainer" by FHHOA stakeholders at the time of this report, more expensive practices may need further justification. Following are arguments for implementing more than just the "low hanging fruit":

1. Public safety can be improved. By implementing these practices, this study shows that reducing runoff and sediment export is possible and that through this strategy the upland hills and lowland stream corridors can be better stabilized from sliding. Even though the 64 projects proposed in Chapter 4 will not make a significant impact on dredging frequency, the criteria provided in Chapter 5 for each practice will help you and future professionals who assist you in identifying many more project sites to retrofit for this very important benefit.

2. Not all the benefits of all the practices have been quantified. For instance, all of the rainfall practices will improve groundwater conditions and reduce flooding, while lined bioretention facilities, which exclude groundwater entirely from their system, are not as effective at this. Flooding was not one of the criteria for this demonstration effort, but throughout the effort, properties have been identified that do have drainage issues that would be improved more effectively with rainfall practices than runoff bioretention facilities. These projects have been summarized in Chapter 4: Recommended Projects Summary.

3. Not all the benefts can be accurately quantified scientifically . The dynamics and interplay of stream bank erosion with uphill runoff is extremely complex and cannot be accurately predicted at this time; however, we do know from watershed scale studies that stream bank scouring will decrease with a decrease in runoff, so the amount of sediment prevented from these practices has likely been underestimated.

4. Maintenance costs are likely to be reduced. While a life cycle cost for maintenance is outside the scope of these recommendations, the maintenance of eroded landscapes is a known issue. For example, in Valley View Park (on NW Miller Road), monthly maintenance costs are $700. The recommendations for this area, reflected in 4 different projects, should significantly reduce erosion and the maintenance associated with this area.

5. All practices are not equally applicable to all existing sites. For example, you might ask if bioretention is so much more cost effective, why build another practice like porous walkways? The short answer is that the bioretention facilities chosen for this demonstration effort have been specifically chosen to be as cost effective as possible. Also, bioretention is suited for relatively flat areas (less than 8% slope) and the HOA has limited opportunities where drainage is concentrated in curbs or other drainages.

Let's compare installing bioretention against replacing gravel walkways with porous walkways. The porous walkways are proposed for woodland walkways where the area of the walkway and it's stormwater management would be one and the same. If we tried to used bioretention on these woodland paths, conveying runoff from the existing gravel path would be one challenge and finding an adequate place to install a bioretention facility adjacent to the path without needing to remove trees would be likely very difficult. Finally, those lined bioretention facilities would be far removed from the underground network of pipes where 77% of the annual rainfall would have to be conveyed as runoff. In this case, bioretention would be far more expensive due to excessive piping.

Another example might be comparing depaving against bioretention. According to Jennifer Devlin at the City of Portland, a water line beneath bioretention is a "deal killer", since it is perceived by the city's Water Bureau as a health hazard, so sometimes flatter areas that look suitable for bioretention are instead recommended to be depaved.

6. There are multiple benefits to implementing these practices, as described in Chapter 1.

Practices by the Numbers

The following chart summarizes all the projects by practice and was used to develop the ranking described above.

Rainfall Practices
Runoff Practices
Restored Soils (Lawn to Lawn)
Restored Soils (Lawn to Meadow)
Restored Soils (Lawn to Perennial Garden)
Porous Walkway
Porous Roadway
Depaving (Low traffic road)
Depaving (Medium traffic road)
Revegetate/ Stabilize slopes
Bioretention
Total for All Practices
# Proposed Projects
6
1
7
5
1
5
1
21
20
Area Managed [sf]
39,655
8,955
28,444
6,189
6,251
10,850
5,146
263,050
79,541
449,956
Area of practice [sf]
39,655
8,955
28,444
6,189
6,251
10,850
5,146
263,050
6,394
376,809
Runoff prevented annually [gal]
205,780
46,470
236,165
133,908
120,669
175,666
83,316
4,914,116
372,100
6,288,190
Sediment prevented annually [lbs]
295
150
397
193
173
302
140
25,971
1,257
28,879
Sediment reduction effectiveness
[lbs sediment prevented/sf area managed]
0.007
0.017
0.014
0.024
0.028
0.028
0.027
0.099
0.016
Runoff reduction effectiveness
[gal runoff prevented/sf area managed]
5.19
5.19
8.30
16.61
19.30
16.19
16.19
18.68
4.68
Design & construction cost/square foot
$2.45
$4.46
$4.46
$8.17
$9.45
$19.21
$19.21
$0.32
$68.67 to $76.29
Design & construction cost/square foot of area managed
$2.45
$4.46
$4.46
$8.17
$9.45
$19.21
$19.21
$0.32
$5.83
  Area Managed Rank by Cost
6
5
5
2
1
3
3
7
4
Cost/sf/lb sediment prevented x 100
$0.83
$2.98
$1.12
$4.23
$5.45
$6.36
$13.67
$0.0012
$0.49
  Sediment Prevented Rank by Cost
7
5
6
4
3
2
1
9
8
Total cost to implement per practice
$97,100
$40,000
$126,900
$65,900
$59,100
$208,400
$98,800
$83,000
$467,734
$1,267,000
Sum of Two Cost Rankings above
15
8
13
9
7
4
2
18
14
Overall Ranking (9 = best)
8
4
6
5
3
2
1
9
7
 
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