Green infrastructure can help you meet stormwater management requirements while providing financial and community benefits and in some cases lessening the permitting effort. The sample projects presented in this section compare development plans, costs and benefits of two hypothetical scenarios. One option (Option 1) shows green infrastructure being implemented extensively as an integral component of the landscaping plan; another option (Option 2) shows a more traditional “gray” approach, with green infrastructure implemented minimally or through a subsurface approach.
The three sample development projects represent commercial, residential and urban infill development project scenarios. Each scenario is approximately one acre in size. The Option 1 approach maximizes the use of green infrastructure practices such as rain gardens, blue roofs, green roofs, planters, and pervious pavement across the site to meet stormwater management water quality and quantity requirements. The Option 2 approach uses techniques such as an underground extended detention facility and a manufactured treatment device to demonstrate regulatory compliance. In all three scenarios, a subsurface management system would most likely be required to meet regulatory requirements for the green approach. However, the size of the system required for the green approach would be much smaller than for the gray approach.
For each sample scenario, there is no difference in useable development area (building area and parking spaces) between the green approach and the gray approach. A summary table is provided for each scenario that compares cost and other performance metrics. Performance metrics for sequestered carbon dioxide (CO2), urban heat island reduction and potential increase in property value were all calculated using the Green Infrastructure Co-Benefits Calculator described in the Benefits of Green Infrastructure section. The green approach also includes the additional aesthetic, permitting and economic benefits associated with green infrastructure described in the Benefits of Green Infrastructure section. Maintenance activities associated with surface green infrastructure are mainly geared toward landscaping and include weeding, watering, sediment and trash removal. Subsurface systems may require sediment removal.
Maintenance for manufactured treatment devices include sediment removal and periodic cartridge replacement. It is assumed that the cost to maintain green infrastructure is offset by the maintenance cost associated with a manufactured treatment device.
The estimated costs included in the summary tables are only provided for the design elements that differ between the green and gray scenarios. For example, no building costs or underground piping costs were included as these items were assumed to be the same for both approaches. Capital cost assumptions included the following:
- No additional cost was added for a blue roof versus the cost of a conventional roof.
- Porous pavement cost was calculated as the difference between porous pavement and standard impervious asphalt.
- Pervious paver cost was calculated as the difference between pervious pavers and standard concrete.
- Rain garden cost was calculated as special soil media, additional landscaping, and underdrains and was calculated as the difference between rain gardens and basic landscaping with trees and mulch.
- Underground detention/retention facility cost was calculated as the difference in size between the gray and green infrastructure approaches and includes a manufactured treatment device for water quality control for the gray approach.