1. 7.8.2 Urban Water Quality
      2. 7.8.3 Other Urban Stormwater Management Considerations

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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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volumes eroding stream banks, pollutants conveyed with this runoff, and diminished stream base
flow. The pattern of degradation for urban streams shows a dramatic increase in magnitude and
intensity of runoff with a corresponding reduction in stream flow during much of the year, and
drought periods resulting in a transition from perennial to intermittent hydrology. In older urban
centers, where the impervious cover can reach 75% or more, the hydrologic cycle has been so
severely altered that full restoration seems to be impossible, especially in terms of restoring any
original stream networks that function as combined sewers beneath the city streets.
Physical pollutants of frequent concern in urban areas include suspended solids, bacteria,
phosphorus, nitrate, hydrocarbons, and metals. The runoff from streets is a significant source of
pollutants and concern in urban areas (Barrett, et al, 1995) and is the single greatest source of
water quality pollutants in the urban environment. In general, rooftop runoff is an order of
magnitude less in concentration for most pollutants, and only becomes a problem when it is
added to the surface flows, transporting the pollutant load accumulated on pavements. Such
street runoff is affected by hydrocarbon emissions including leaks from vehicles, nutrients and
organics from urban vegetation, bacteria and other pollutants from pet and other animal waste,
and the general mix of wastes discarded in urban environments. Street curb and gutter systems
are traditionally designed to convey, not trap, the fine particles associated with street runoff, and
will carry the litter and debris directly to surface inlets, the storm sewer system and finally the
receiving streams.
Increased temperature is a significant water quality issue in urban areas that can quickly pollute
receiving waters. Although interception or disconnection of stormwater flows (i.e., peak shaving)
to pervious areas may provide some limited reduction in temperature impact, opportunities for
disconnection are often limited. It should be noted that low dissolved oxygen levels in receiving
streams are related to the extreme temperature variability of runoff from impervious areas (as
temperature increases, dissolved oxygen levels decline with lethal consequences to aquatic life).
For fish and aquatic insects, temperature ultimately can be one of the most critical pollutants,
presenting especially difficult challenges in urban areas.
Many urban storm sewers are in fact buried streams, especially first and second-order streams
that were enclosed and buried as the urban center expanded in the late 19
century. These
buried streams still serve as storm runoff conduits with the natural movement of groundwater
along and into the stream channel. In some areas, the fill material above the original channel
may eventually wash away, creating subsidence problems and “cave-ins” in urban streets. In
other areas, the pipes serve to convey water more rapidly than the original stream would have
done, creating downstream flooding or surcharging of both the sub-surface culverts and surface
outlets. Deprived of both oxygen and sunlight, the original rate and water quality buffering
function of first and second-order streams has been lost.
One aggressive concept that has received considerable attention but little real implementation is
the idea of “daylighting” buried streams. This means that the original riparian channel is
uncovered and restored with new stream banks cut and revegetated as appropriate. While
representing a dramatic measure to restore an urban stream, the reality of fill removal and
possible loss of property values along the original channel alignment usually translates into an
unacceptable economic impact and disruption in the urban landscape. Even where substantial
redevelopment has occurred in older cities, little serious thought has been given to the restoration
of buried streams.
High levels of trash and debris, including concentrated areas of pet waste, characterize many
urban streets. A high degree of imperviousness, combined with a curb and gutter system
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designed to flush and convey debris, makes the urban landscape a significant source of
pollutants that are rapidly conveyed to receiving surface streams. The use of various devices to
intercept and contain these waste materials offers some measure of pollutant reduction, if
maintenance is performed on a regular basis. Street cleaning by vacuum units also presents a
very efficient method of pollutant removal, but purchase cost combined with operation and
maintenance makes this BMP a significant investment for any urban community. In one urban
center (Santa Monica, CA), the street gutters have been formed with porous concrete and
infiltrating underdrains, combined with traps at corner inlets. Less dense residential portions of
the urban community may utilize a variation of this approach, where shallow infiltration can be
Stormwater “hot spots
such as gas stations, industrial areas, vehicle service areas, and public
works storage areas are commonly found in urban communities, especially in the industrial
zones. Smaller facilities, such as fueling islands and dumpster pads, should be treated as
separate sources of pollution, and the runoff should be prevented or segregated from surface
runoff. On the larger scale, a block-by-block strategy may be appropriate in portions of the
community where pollutant-producing activities are concentrated.
7.8.3 Other Urban Stormwater Management Considerations
In many urban areas, local codes and regulations may require designs that are contrary to current
BMP design. For example, local codes may require that all roof leaders be connected directly to
a storm sewer, or that all streets have curbs and gutters. Local code officials may not be familiar
with on-going stormwater management efforts. In these instances, early review of local
requirements and communication to the appropriate officials is necessary to avoid BMP
construction delays or denials. Long-term, review and updating of local ordinances may be
warranted, with model urban guidelines developed by PADEP.
Redevelopment in depressed or blighted communities adds an additional dimension to
stormwater management. These conditions have led some states (such as New Jersey) to
exclude such communities from new stormwater regulations. The imposition of stringent
regulations that are not feasible may serve to direct redevelopment to undeveloped sites outside
the urban center. Brownfield parcels with significant residual contamination must be designed
carefully to assure that any residual pollutants are not mobilized by stormwater BMPs. Highly
contaminated sites may warrant excavation and removal of materials before any BMP can be
installed. Stormwater management must not be detrimental to the economic health of urban
areas, because this would ultimately be more damaging to the overall water resources of an area.
Most of the BMPs described in Chapter 6 can find some application in the urban environment, but
a number of seemingly small measures, not described in separate BMP sections, can have a
cumulative effect if applied to hundreds or thousands of individual residences or small buildings.
These types of measures include:
Reduce Parking Imperviousness -
New parking lots in urban areas can follow the guidelines set
out in Chapter 5 relating to reducing imperviousness, while rehabilitation of existing parking lots
can be designed with some areas of pervious paving, or even re-vegetated areas if the parking
spaces are under-utilized.
Rooftop Downspout Disconnection -
Roof leaders (gutters) in residential, urban areas can be
re-configured to drain into rain barrels or planter boxes, for example. Multiple, smaller stormwater
elements placed around the home/structure can be combined to form a flexible design applicable
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