WATERSHEDS - Water Quality Engineering in Natural Systems - page 216

Porous Pavement. Porous pavement is an alternative
to conventional pavement whereby rainfall is
allowed to percolate through the pavement into
the subbase. Porous pavements are made either
from asphalt or (pervious) concrete in which fine
filler fractions are missing or are modular. Cementi-
tious permeable pavement has been shown to
reduce runoff, filter and treat infiltrating runoff,
reduce thermal pollution, and provide the load-
carrying capacity of conventional rigid concrete
pavement. Modular porous pavements usually
consist of concrete interlocking modules or plastic
reinforcing grid pavers. The primary benefit of
porous pavements is a significant reduction or
even complete elimination of surface runoff from
an otherwise impervious area, and most porous
pavements have been shown to perform similarly.
Aquifer recharge by infiltrated water is a second
benefit, and reduced need for storm drainage is a
third benefit. If subsoils are very permeable, there
may be no need for installing storm drains. Porous
pavement is most feasible when subsoils are per-
meable and ground slopes are relatively flat. In
areas with poorly draining subsoils or if the pave-
ment is installed over an existing impervious base,
a drainage system can be installed. Porous pave-
ment has an excellent potential for use in parking
areas, side streets, fire lanes, walkways, and drive-
ways. Clogging may occur during construction
and/or operation; however, this can be remedied
by flushing and sweeping. Inadequate siting and
maintenance of porous pavements can lead to
ineffective operation, and this consideration has
led some regulatory agencies to not accept porous
pavements as a SCM. An example of a porous
pavement used in a driveway is shown in Figure
6.4. The construction cost of porous pavement is
about the same or even less than for conventional
pavement when savings on storm drainage infra-
structure is included.

Figure 6.4. Porous pavement.

Figure 6.5. Rain barrel cistern. Source : Lake County, Illinois
(2012).

directly into the drainage system without passing
over pervious area. Roadways are the most
common example of DCIA, where runoff from
the pavement is routed directly to a stormwater
inlet, such as shown in Figure 6.6. It is widely rec-
ognized that minimization of DCIA is by far the
most effective method of runoff quality control
because it delays the peaks of flows into the sewers
and maximizes infiltration. Practices used to mini-
mize DCIA include disconnecting roof drains
from storm sewers, permitting surface runoff to
overflow onto adjacent pervious surfaces, and
directing stormwater runoff to infiltration struc-
tures, such as dry wells, infiltration basins, and
ditches.

Increasing Surface Storage. Rooftop storage on flat
roofs, temporary ponding, and restriction of storm-
water inlets are used for control of surface runoff.
Diverting rooftop-collected rainwater into storage
tanks and subsequent reuse for irrigation and
other nonpotable water supply has been practiced
in many countries for centuries and is a feasible
stormwater management and reuse alternative.
An example of such a practice is shown in
Figure 6.5.

Decreasing Directly Connected Impervious Area
(DCIA). Directly connected impervious area is
defined as the impermeable area that drains