HomeMy WebLinkAboutAppendix D - Private Infrastructure
Barr Engineering Co. 4300 MarketPointe Drive, Suite 200, Minneapolis, MN 55435 952.832.2600 www.barr.com
Technical Memorandum
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Project: Edina Flood Risk Reduction Strategy Support (23271728.00)
Executive Summary
Barr was asked to review model-predicted flood impacts in the focal geography of the Morningside
neighborhood to evaluate the sensitivity of those impacts to the magnitude of stormwater storage within
the watershed. In particular, the focus was on underground storage methods within private property, the
right-of-way, or under streets. This evaluation was conducted as a result of Task Force discussions about
the potential benefits of requiring private homeowners to store stormwater on-site similar to
requirements for commercial development.
Barr reviewed the benefits achieved by storing the first 1-inch, 2-inches, and 3-inches of precipitation
from storm events of varying size, from the 20%-annual-chance storm event (5-year storm; 3.59 inches) to
the 1%-annual-chance storm event (100-year storm; 7.49 inches). For the private storage evaluation
(underground storage vaults under a portion of each of the 570 residential parcels), storage was assumed
for every parcel within the Morningside neighborhood. Barr found that storing the first 1-inch of storms of
this magnitude had a negligible impact on flood levels. Storing the first 2-inches and 3-inches showed a
more significant benefit with regards to reduction in peak flood levels. Depending on the storm event,
and depending on the location within in the neighborhood, the results varied anywhere from flood level
decreases of a few inches to decreasing nearly a foot and a half.
However, this apparent benefit comes at an initial cost of approximately $15,000 per inch of stormwater
stored, per residential parcel. To store 2-inches of runoff in the entire neighborhood (~570 residential
parcels) would cost approximately $17 million. In addition, while the flood levels may be lowered, the
number of homes that are removed from potential impacts from flood inundation is small. For example,
one home may potentially be removed from flood inundation at Weber Pond depending on the storm
event. Finally, the management and maintenance of these underground stormwater storage vaults
distributed throughout an entire neighborhood is expected to be complicated and unprecedented. This is
all to say, this solution would provide a moderate benefit for a very high cost. Additionally, a preliminary
look at the compounding effect of climate change suggests that improvements realized by implementing
additional private storage may eventually be negated by climate change (i.e., increased precipitation
amounts, see Appendix B on Climate Change Impacts Analysis).
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 2
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Private Infrastructure Analysis Details
A common example of private stormwater management infrastructure (infrastructure on a privately
owned parcel), is a rainwater garden (Figure 1). Rainwater gardens are typically designed to store the first
one inch of runoff generated from a storm, aimed at both reducing the volume of runoff and improving
water quality downstream.
Figure 1 Photo of a rainwater garden.
Other examples of private infrastructure for stormwater
storage can include tree trenches, cisterns, permeable
pavement, and underground storage vaults. Figure 2 shows
an example of an underground stormwater storage vault.
To simplify our analysis, we assumed that all parcels in the
Morningside neighborhood are approximately 60 feet wide
(along the road), and also assumed that every parcel would
have underground storage (below grade) that is 3 feet
deep. Then we determined how wide the underground
storage vault would need to be to contain 1 inch of runoff,
2 inches of runoff, or 4 inches of runoff. We found that
underground storage vaults on every parcel in the
Morningside neighborhood would need to be 5 feet wide
to store 1 inch of runoff, 10 feet wide to store 2 inches of
runoff, and 20 feet wide to store 4 inches of runoff. Figure 3
provides a graphic that shows the extent of underground
storage needed for sample parcels in Morningside.
Figure 2 Example of an underground storage vault (37th Avenue Greenway, Minneapolis).
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 3
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Figure 3 Private stormwater storage sizing examples for storing varying amounts of runoff.
Barr also analyzed using stormwater storage under streets and/or in the public right-of-way. Figure 4
provides a graphic that shows the approximate extent of underground storage available for a typical road
within the Morningside neighborhood. Assuming two 15-foot wide (and 3 feet deep) underground
storage vaults can be installed under all of the roads or right-of-way in the Morningside neighborhood,
3-inches of runoff could be stored in those vaults.
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 4
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Figure 4 Stormwater storage sizing (width) available for typical roads or right-of-way in the Morningside neighborhood.
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 5
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Figure 5 shows the subwatersheds in the Morningside neighborhood. Graphs are included below that
show the results and range of benefits of residential/private stormwater storage for Weber Pond
(subwatershed MS_40, Figure 6), for the area along Branson between Oakdale Avenue and Grimes Avenue
(subwatershed MS_48, Figure 7), and for the area along Crocker Avenue between West 42nd Street and
Morningside Road (subwatershed MS_2, Figure 8).
Figure 5 Map showing subwatershed divides in and around the Morningside neighborhood
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 6
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In Figure 6, the horizontal, maroon-dashed lines represent approximate low elevations based on structure
footprints for the four lowest homes around Weber Pond. They may or may not represent actual low entry
elevations of these homes. However, they give a good representation of the home elevations and how
close they are to the flood levels.
Figure 6 Peak water surface levels resulting from varying amounts of runoff stored using private infrastructure for varying storm events in the Weber Pond subwatershed (MS_40).
At first glance, the reductions shown in Figure 6 appear smaller than would be expected. There are
multiple other factors affecting the flood volume stored in Weber Pond. First, Weber Pond ultimately
receives water from Edina and also from St. Louis Park and Minneapolis. While private infrastructure is
overall beneficial, reducing the runoff to Weber Pond from Edina may allow more water from St. Louis
Park and Minneapolis to fill the pond back up during an event. Second, at the peak flood elevations
shown in Figure 6, stormwater flows out of Weber Pond both into Weber Park and over France Avenue to
the east to Minneapolis. When ponds rise high enough to overflow banks, additional water does not tend
to have a significant impact on the water level since water can start following natural overflow paths.
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 7
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Figure 7 Peak water surface levels resulting from varying amounts of runoff stored using private infrastructure for varying storm events in subwatershed MS_48.
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 8
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Figure 8 Peak water surface levels resulting from varying amounts of runoff stored using private infrastructure for varying storm events in subwatershed MS_2.
Barr commonly estimates that the cost per cubic foot of underground stormwater storage is
approximately $10 to $20. For one inch of runoff, for one 0.25-acre parcel, storage volume equals 900
cubic feet. This equates to a little under $15,000 (+/- $5,000) per parcel per inch of runoff stored. Figure 9
shows the approximate cost per parcel of underground storage using varying widths of underground
storage units and varying amounts of runoff stored. To put the cost of private underground storage into
perspective, Figure 10 shows a portion of the Morningside neighborhood (~180 parcels) and provides a
breakdown of an approximate cost to capture two inches of runoff from every parcel.
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 9
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Figure 9 Approximate cost per parcel of underground storage using varying widths of
underground storage units and varying amounts of runoff stored.
Figure 10 Cost breakdown for using private stormwater storage for a portion of the Morningside neighborhood.
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 10
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In total, there are approximately 570 residential parcels in the Morningside neighborhood watershed
drainage area, as shown in Figure 11.
Figure 11 Parcels in the Morningside neighborhood watershed/drainage area.
To: Jessica Wilson and Ross Bintner, City of Edina From: Sarah Stratton and Cory Anderson, Barr Engineering Co. Subject: Appendix D - Private Infrastructure Analysis Date: March 30, 2020 Page: 11
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The results of Barr’s private storage analysis are summarized in Table 1 below. Recall that storing 1-inch of
runoff from every parcel in Morningside had a marginal benefit in general on peak flood levels. Table 1
below shows that to store 2-inches of runoff in the entire neighborhood would cost approximately
$17 million. While storing 2-inches of runoff does reduce flood levels, the number of homes that are
removed from potential impacts from flood inundation is small. For example, as shown in Figure 6,
depending on the storm event, this level of effort may potentially remove only one home from flood
inundation at Weber Pond.
Table 1 Summary of costs and benefits of private stormwater storage for the whole Morningside neighborhood.
Inches of
Runoff
Stored
Cost for All Parcels to
Store the Runoff
Flood Level Reduction Benefit (in feet) for Weber Pond
Subwatershed (MS_40)
5-yr Storm
(3.59" of
precip)
10-yr Storm
(4.29" of
precip)
50-yr Storm
(6.39" of
precip)
100-yr Storm
(7.49" of
precip)
1 inch $ 8,550,000 0.1 <0.1 <0.1 0
2 inches $ 17,100,000 0.6 0.3 0.5 0.3
3 inches $ 25,650,000 0.7 0.9 0.6 0.5