1. What if the stormwater cannot be infiltrated?
      1. 7.8 Urban Areas
      2. 7.8.1 Highly Impervious Urban Land
      3. 7.8.2 Urban Water Quality

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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Pennsylvania Stormwater Best Management Practices Manual
Chapter 7
are dependent on infiltration into the soil mantle for volume reduction, and so the use of other
BMPs is necessary.
One of the few “downtown” locations suitable for volume reduction is the roof of building
structures. European engineers and architects learned the importance of going “up on the roof”
for stormwater management several decades ago, and it has become the primary method in most
cities. In Germany local ordinances require the construction of vegetated roof systems on flat or
up to 20% sloping roofs. Failure to comply with these rules result in a “stormwater tax” being
levied that is sufficiently onerous to virtually assure compliance. This action was precipitated by
an increased awareness of the impacts of stormwater on “combined” sewers that convey both
runoff and raw sewage to the nearest stream, river or lake. Many of the German cities were
reduced to rubble during World War II, and in the rebuilding process it was recognized that
vegetated rooftops on all new buildings provided a solution to anticipated urban stormwater
Mandatory application of this BMP in existing urban centers, such as Philadelphia and Pittsburgh,
will require specific ordinances that guide both new and existing building efforts, a significant
capital program for any municipality. Without the opportunity for infiltration measures, however,
the available alternatives to vegetated roof systems are quite limited, and focus on various
capture and reuse efforts, most of which would require a significant re-plumbing effort for existing
In terms of appropriate Control Guidance for the urban center, the solution may have to be
tailored to fit the hydraulic capacity of the existing conveyance system. Where combined sewers
are the only drainage pipes available, the overflow and discharge from CSO outfalls is usually
triggered by frequent rainfall events of an inch or less. If the volume of runoff from a 1-inch storm
event can be reduced in these areas, many combined sewer overflows can be avoided and much
water quality benefits can be gained. Detailed computer modeling can develop the appropriate
volume control guidance for highly urban watersheds with single pipe sewers.
As development has extended out from urban centers into surrounding farmlands, the percentage
of impervious surfaces within a given land parcel has generally been regulated with the
assumption that less impervious cover (combined with height limitations) would result in a
community that did not have the negative aspects of the more dense urban environment. This
has proven not to be the case, especially for stormwater. The suburban commercial center or
office park can result in a highly impervious land parcel, equal to or greater than some older
communities, even though it exists on an isolated parcel. Suburban residential developments are
generally comprised of far less impervious cover than the urban streets, but still produce a
significant pollutant load (Bannerman et al., 1993). This suburban runoff is generated in large
measure from land that has been altered and then re-vegetated. The construction process has
compacted the soils in these grassed and landscaped areas such that runoff volume has
increased significantly. Thus a low-density suburban residential lot could degrade water quality as
severely as the row home in center city Philadelphia.
7.8.2 Urban Water Quality
Several studies (Schueler, 2003) have indicated that the amount of impervious cover in a
watershed is a good indicator of degraded water quality. The impacts of urbanization on a
watershed can be measured when the level of impervious cover reaches 5 percent. Water quality
in the watershed is severely degraded by the time the level of impervious cover reaches 20
percent. This reduction of water quality and stream habitat occurs from the increased runoff
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