THE PLANNING PROCESS FOR LID - Low Impact Development and Sustainable Stormwater Management - page 79

that removes all existing natural features from a given planet planned for human
occupation and forms a desired habitat prior to settlement.
In a situation where the terrain varies significantly over a parcel, woodlands
and steep slopes, especially the combination, should be avoided for development.
Designs that attempt to terrace such locations usually destroy the natural land
system to the degree that soils erode rapidly and severely following regrading,
despite the application of our best erosion control measures.
Guideline 3: Manage Rainfall Where It Originates
This concept turns the standard site design approach on its head. For decades,
we have designed roof drainage systems, roadways, and grading surrounding
buildings to remove and drain rainfall away from all structures as quickly and
efficiently as possible. In fact, this was the beginning of the stormwater problem
for both quality and quantity, because the immediate runoff of rainfall from
impervious surfaces is not only the underlying cause of downstream flooding but
also the primary transport mechanism for pollutant transport.
Although we cannot allow rainfall to infiltrate back into the soil directly
beneath a building foundation, we can utilize the built surfaces that generally
surround the structures, including all pavements, such as driveways, parking lots,
local roadways, and the lands adjacent to these pavements that will be revegetated
and landscaped. The commercial “big box” is the best example of this oppor-
tunity, since the pavement created for customer parking is usually much larger
than the building footprint required (Figure 4-1) and offers ample opportunity
for infiltration design with porous pavements, as long as the grading does not
compact the soil mantle. One important consideration with the one-story retailing
center is the perceived notion that every store must be approached from a flat
pavement surface so the customers can move swiftly and easily from store to
car with a cart full of purchases. The end result is the desire for as flat a site as
possible, regardless of the natural landform.
Residential development is a more difficult setting in which to situate stormwa-
ter infiltration opportunities, since the associated pavements are smaller, with
the largest pavement surfaces comprised of shared driveways and roadways
(Figure 4-2). Landscaping elements that infiltrate rainfall, especially rooftop
runoff, find greater opportunity in residential settings (Figure 4-3) and have been
given the appealing label rain gardens . Some designs have used common site ele-
ments, such as cul-de-sacs (Figure 4-4), to infiltrate rainfall. A number of efforts
have also been made to build detention basins that also infiltrate, but most of
these have proven unsuccessful, with soil compaction and high water-table con-
straints limiting the long-term infiltration capacity and accumulating sediment
fines clogging the basin bottom.
Stormwater conveyance must also be rethought. The conventional storm sewer
system that carries runoff from rooftop and roadway in ever-larger pipes to the
nearest surface water body is no longer a sustainable solution. A storm sewer
system of perforated pipes that leak like sieves produces a very different end