1. What if the stormwater cannot be infiltrated?
      1. 7.8 Urban Areas
      2. 7.8.1 Highly Impervious Urban Land

Pennsylvania Stormwater Best Management Practices Manual
Chapter 7
pollutant removal rates of over 90 percent for sediments, metals, bacteria and petroleum
hydrocarbons, and as high as 75 percent for total phosphorous. Pollutant removal effectiveness
increases with the amount of compost or compost/soil mixture the stormwater has to pass
through. Compost or soil/compost mixtures are not effective in removing chlorides such those
found in deicing salt. The
operation and maintenance plan for these BMPs should include
judicious or limited use of deicing salts in areas draining to the BMP.
Vegetated swales, bio-retention areas, infiltration trenches and basins should be constructed so
that stormwater must first pass through 18 to 36 inches of compost or a compost/soil mixture
before percolating into the ground. The type of vegetation planted in the compost or compost/soil
layer should be selected, in part, for its ability to replenish organic matter through seasonal leaf
fall or root die back. Maintaining a high percentage of organic material in the soil is of utmost
importance. It is the organic material (compost) that has the cation exchange capacity necessary
to capture pollutants in stormwater.
What if the stormwater cannot be infiltrated?
Infiltration is not the only way to reduce stormwater runoff volumes. Vegetated roofs can be used
effectively on brownfield sites to retain much of the rainwater that falls on the roof. Stormwater
can also be retained in basins or landscaped ponds and allowed to evaporate. Cisterns and
vertical storage units can be placed in corners of structured parking lots, inside buildings, on the
outside walls of buildings, in adjacent alleys, alongside elevator shafts, and other locations
deemed feasible by the designer. Vertical storage is very applicable to urban areas where space
is at a premium. The shape and location of this BMP requires very little land area. Water
collected this way can be re-used for things such as fire suppression, drip irrigation, lawn
sprinkling, cooling buildings, toilet flushing and recreational water. Chapter 6 of this manual
provides more detailed information on stormwater capture and reuse.
Urban Areas
7.8.1 Highly Impervious Urban Land
This land area of special consideration includes the most densely populated regions of the state.
The intensity of land development in most urban centers has resulted in a land use pattern that
could be considered fully developed, with an almost continuous impervious surface comprised of
multi-story structures surrounded by pavement. Beneath these paved areas lay a complex web
of; water, wastewater, stormwater, gas, electric, stream and communications infrastructure. In
the most densely developed urban communities, people also move beneath the surface in trains
and subways. Auto parking is largely provided in concrete boxes or below buildings. The few
“green areas” remaining are isolated parks and public spaces, many of which are also underlain
with auto parking levels extending 60 feet or more into the ground. Narrow planting strips along
many urban corridors support “street trees” that wage a constant battle to survive in a hostile
Beneath these urban landscapes lie the residue of prior development, which in older cities such
as Philadelphia can form a rubble layer many feet thick, comprised of bricks, blocks, concrete,
wood, and other building materials. All of these conditions severely limit the use of any BMPs that
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