11.5 Sustainable stormwater management  (Page 7/26)

A third way to address CSO issues off-site is to use the streets themselves to temporarily store stormwater by installing low speed bump-like structures at intersections and by restricting the streets' sewer intakes to the combined sewer system ( US EPA, 2000 ). This urban retrofit strategy would allow stormwater to flow from lots into the streets, which would flood up to their gutter tops during heavy storms, functioning as stormwater reservoirs. The stored stormwater would then slowly be discharged to the combined sewers through the restricted grates over a period of hours after the storm, reducing the amount of diluted sewage flow to a quantity that could be adequately treated by sewage treatment plants. The flooding of streets, impairing automobile access, and the possibility of stormwater overflowing the curbs and damaging parked cars and adjacent property during very heavy rainstorms may not make this a politically popular option, though.

Managing urban stormwater more sustainably

There is a fourth approach to dealing with CSO problems, which involves intercepting and delaying the discharge of precipitation from a parcel of land before it flows off-site to a separate or combined sewer system, or to an adjacent waterway. Encouraging on-site storage or infiltration reduces the stormwater contribution to a combined sewer's flow in developed areas, thereby reducing the amount of diluted wastewater being generated and enabling combined sewer systems to better handle their wastewater loads during rainstorms. These decentralized on-site approaches to managing stormwater could also be used to reduce the amount of conventional stormwater infrastructure needed in new developments using separate stormwater sewer systems. Because these on-site approaches are less resource-intensive and more cost-effective than conventional stormwater management approaches, they are also more sustainable investments.

On-site stormwater management techniques are also often known as "green infrastructure" ( Jaffe et al., 2010 ). Development projects using "green infrastructure" for urban stormwater management are commonly known as " Low Impact Developments ." Low Impact Development projects using green infrastructure usually allow stormwater to be managed at lower costs than by using conventional detention practices ( US EPA, 2007 ).

There are essentially three strategies for on-site stormwater management: (1) techniques that encourage the infiltration of stormwater into soils to reduce its volume before it reaches a sewer system, or which employ more selective grading and the planting of vegetation to reduce its rate of flow from the site; (2) techniques that encourage the temporary storage of stormwater on-site, instead of transporting it off-site for centralized detention within a development project or a municipality; and (3) techniques, such as the construction of artificial wetlands, which also allow some degree of longer-term retention and treatment of the stormwater by natural processes before it is discharged. Infiltration techniques might also provide some water treatment capabilities due to the longer retention times of groundwater before discharge, but the degree of such treatment would largely depend on soil characteristics, the amount of overlying vegetation and the depth of the soil's unsaturated zone.