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Pervious Pavement and Green Roofs
Pervious Pavement and Green Roofs require a higher level of technical skill to design and install than most systems presented in the South Portland Stormwater Manual. For these reasons, design specifications are not incorporated into the manual. However, they are generally considered effective approaches to stormwater, and should be looked upon favorably if included in a stormwater permit application.
Pervious Pavement (asphalt or concrete)
Pervious pavement (asphalt or concrete) looks very similar to normal paved surfaces, but uses larger aggregate which allows water to drain easily through the material. These materials can effectively eliminate runoff from most storm events, as water enters the underlying sand and gravel structure of a road, parking area, or sidewalk. The underlying sand can provide some degree of water quality treatment. Pervious pavement also requires much lower level of de-icing, which can reduce winter maintenance expenses. These surfaces must be protected from sedimentation, and may require annual vacuum-sweeping (similar to city streets) for maximum longevity.
Green roofs come in many types, but generally consist of a waterproof membrane on a flat roof supporting some type of specially-chosen vegetation. They are different from regular roofs in many ways, typically having stronger structural support, integrated drainage pipes, vegetation maintenance needs, and they may incorporate a patio or other form of access which allows building occupants to enjoy the space. Green roofs provide a modest stormwater benefit by slowing, retaining, and releasing through absorption and evapotranspiration some of the rain they receive. Often, they are installed for their heating and cooling benefits, which can be significant.
In South Portland, pervious pavement and green roofs are generally acceptable technologies for stormwater permit applications, even though specific design guidelines are not presented here. Potential applicants should work with an experienced professional (paving professional or architect, respectively) when designing these systems and quantifying the stormwater benefit.