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Bioretention - Cost
A general rule of thumb1 is that residential rain gardens average
about $3 to $4 per square foot, depending on soil conditions and the density
and types of plants used. Commercial, industrial and institutional site
costs can range between $10 to $40 per square foot, based on the need for
control structures, curbing, storm drains and underdrains. In any bioretention
cell design, the cost of plants varies substantially and can account for a
significant portion of the facility's expenditures. While these cost estimates
are slightly greater than those of typical landscaping treatment (due to the
increased number of plantings, additional soil excavation, backfill material,
use of underdrains etc.), those landscaping expenses that would be required
regardless of the bioretention installation should be subtracted when
determining the net cost.
Perhaps of most importance, however, the cost savings compared to the use of
traditional structural stormwater conveyance systems makes bioretention cells
quite attractive financially. For example, the use of bioretention can decrease
the cost required for constructing storm water conveyance systems at a site.
A medical office building in Maryland was able to reduce the amount of storm
drain pipe that was needed from 800 to 230 feet - a cost savings of
$24,000.2 And a new residential development spent a total of
approximately $100,000 using bioretention cells on each lot instead of nearly
$400,000 for the traditional stormwater ponds that were originally
planned.3 In addition, in residential areas, stormwater management
controls become a part of each property owner's landscape, reducing the public
burden to maintain large centralized facilities. Detailed cost estimates are
given below, as adapted from Prince George's County
Bioretention Manual.4
Cost Guides
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Residential Lot in a Subdivision
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This is applicable if the project is a shallow rain garden incorporating
in-situ soils and no underdrain system. Homeowner, garden group, or volunteers
provide the labor, and no heavy construction equipment is used (most of the
labor is done by hand). The disturbed area is small enough to avoid permits
and fees, and the rain garden is seen as a homeowner landscaping project.
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planning phase $25
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design phase $100
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construction phase $950
TOTAL ESTIMATED COST: $1,075
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Residential Rain Garden
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The cost information is the average cost per facility installed, assuming
a 100 lot subdivision. All of the facilities have an underdrain system, and
many of the facilities will be constructed simultaneously. Planning, designing,
and construction costs are all pro-rated as a portion of the overall site cost
work, and sediment control, permits, fees, and technical plan approval are
required.
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planning phase $95
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design phase $340
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construction phase $3225
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closeout phase $130
TOTAL ESTIMATED COST: $3,790
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Residential Single Lot
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Costs are increased substantially due to the small scale of the project,
and closeout is higher due to as-built requirements.
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planning phase $200
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design phase $875
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construction phase $5750
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closeout phase $950
TOT AL ESTIMATED COST: $7,775
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Commercial - New
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The facility construction costs are lower than that for a single
residential lot because of the increase in related site work. The storm
drainage discharge system is not included as part of the bioretention costs
since it is treated as a general site expense.
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planning phase $845
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design phase $3600
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construction phase $5237
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closeout phase $675
TOTAL ESTIMATED COST: $10,357
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Commercial - Retrofit
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Total retrofit costs are higher than those for new construction due to
economies of scale. Design costs are lower because the drainage conveyance
system is already in place.
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planning phase $350
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design phase $2410
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construction phase $7943
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closeout phase $1652
TOTAL ESTIMATED COST: $12,355
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1 Coffman, L.S., R. Goo and R.
Frederick, 1999: Low impact development: an innovative alternative approach to
stormwater management. Proceedings of the 26th Annual Water Resources
Planning and Management Conference ASCE, June 6-9, Tempe, Arizona. Get
Larry's paper from the WRPMD '99 CD.
2 EPA Office of Water, 1999. Storm
Water Technology Fact Sheet: Bioretention. EPA 832-F-99-012.
3 Growing Greener in your
Rappahannock River Watershed
http://for.communitypoint.org/pages/grogreen.html
4 The Bioretention Manual,
Prince George's County Department of Environmental Resources Programs and
Planning Division, Maryland, 2001. |
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