Bioretention


Costs

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

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Cost Guides

Residential Lot in a Subdivision

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.

  • planning phase $25
  • design phase $100
  • construction phase $950

TOTAL ESTIMATED COST: $1,075

Residential Rain Garden

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.

  • planning phase $95
  • design phase $340
  • construction phase $3225
  • closeout phase $130

TOTAL ESTIMATED COST: $3,790

Residential Single Lot

Costs are increased substantially due to the small scale of the project, and closeout is higher due to as-built requirements.

  • planning phase $200
  • design phase $875
  • construction phase $5750
  • closeout phase $950

TOT AL ESTIMATED COST: $7,775

Commercial - New

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.

  • planning phase $845
  • design phase $3600
  • construction phase $5237
  • closeout phase $675

TOTAL ESTIMATED COST: $10,357

Commercial - Retrofit

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.

  • planning phase $350
  • design phase $2410
  • construction phase $7943
  • closeout phase $1652

TOTAL ESTIMATED COST: $12,355

References


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://www.riverfriends.org/ 


4 The Bioretention Manual, Prince George's County Department of Environmental Resources Programs and Planning Division, Maryland, 2001.

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