LAND AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 70

Figure 3-18 Vegetated infiltration bed in a Portland street. (Courtesy of Portland Depart-
ment of Environmental Services).
surface, but should reduce the ambient temperature at street level significantly
by providing constant evapotranspiration cooling during the warmest months of
the year.
The habitat and aesthetic benefits of tree restoration in urban environments are
more obvious, but in terms of water balance, this type of street design will allow
the rainfall to soak into the soil mantle while the vegetation also returns it to
the atmosphere by evapotranspiration. The subsurface design of a typical green
street system must include a subsurface layer for rainfall storage and subsequent
infiltration, with an overflow feature to assure that the bed does not become
saturated and kill the root system. At the same time, because we are planting
trees with deep root systems, we must take care to avoid any threat to existing
structures, especially those with basements well below surface grade. Therefore,
the creation of green street vegetation must be done on a site-specific basis,
carefully fitted to the existing or reconstructed urban landscape.
While some forward-thinking cities such as Portland began developing green
street programs two decades or more ago, a number of other communities have
also initiated green streets and tree planting efforts, and in later sections we
describe in detail how some have approached the solution. They all share one
concept, however: The primary design goal is the reduction of runoff volume
from the urban landscape, by infiltration, evapotranspiration, and/or capture and
reuse.
End of the Sewer
Building beyond the urban fringe into the undeveloped countryside means that we
must create new infrastructure systems of water, sewer, and drainage, not even