LAND AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 68

Table 3-2 Portland CSO Program Costs, 1993-2010

Cornerstone projects 145 million

Columbia Sough 160 million

West Side CSO program 410 million

East Side CSO program 640 million

Total $1.4 billion

Note: Construction of the Cornerstone projects has included infiltration
sumps (3,000), downspout disconnections (50,000), local sewer separa-
tion efforts, and two stream diversions. The total annual reduction in
stormwater thus far is 2 billion gallons.

will not overflow. The idea of keeping the rain out of the sewer is simple but
has not been given serious consideration until the past 15 years, and only in a
limited number of communities. One community that has led the way in this
effort is the city of Portland, which under the leadership of the Department of
Environmental Services [3] began a comprehensive effort in the early 1990s to
both solve the problem of “conveyance” and to include program elements that
would reduce the inflow of rainfall into the system. Over the past two decades,
the city has invested over $1.4 billion in this program, most of which has been
spent on the “plumbing” (Table 3-2) but 10% of which has gone into the “green”
program elements. Of these investments, the least expensive and most successful
has been the disconnection of rooftop downspouts (Figure 3-16), as well as the
construction of two-manhole sewer inlets that infiltrate runoff, described as sinks
(Figure 3-17). These two measures have combined to prevent some 2 billion
gallons of runoff from entering the city sewers each year. More important, the
city's program after 2012 will be comprised entirely of such measures, including
an aggressive effort to infiltrate rainfall in vegetated designs (Figure 3-18), to
accommodate not only new development but also existing communities where
full stormwater mitigation has not yet been accomplished.
Urban streets that reduce or prevent rainfall from entering the sewer system
have been described as “green streets” (Figure 3-19), the most conspicuous of
which are streetscapes blended with planting beds or tree clusters, underlain by
infiltration beds. Other urban LID designs utilize a combination of LID tech-
nologies, from vegetated roof systems to enlarged downspouts to containment
and storage tanks to porous pavements, sidewalks, and walkways, all of which
are underlain by aggregate or other media to hold, infiltrate, and slowly release
rainfall from urban surfaces.
The reestablishment of surface vegetation comprised largely of trees is a major
element of all green streets efforts. Here the goal is not only to restore a significant
amount of pervious surface within the urban landscape, but also to create a
tree canopy that provides environmental benefits well beyond the infiltration of
rainfall, The direct cooling effect of a natural tree canopy, suggested to be on the
order of 40% at full development, is dramatic. Of course, the actual land surface
required to support this amount of canopy is more on the order of 15% of the land