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Pervious Concrete
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>Hydrologic Design of Pervious Concrete
Hydrologic Design of Pervious Concrete
Pervious concrete has been used in some areas for decades; however,
recent interest in sustainable development and recognition of pervious
pavements by the US Environmental Protection Agency (EPA) as a best
management practice for storm water management has heightened interest
in its use throughout North America. Its use supports national initiatives
such as EPA’s Heat Island Reduction Initiative and Low Impact
Development and provides a potential for credit in the LEED®
(Leadership in Energy and Environmental Design) rating system for
sustainable building construction.
Pervious concrete can be used to improve water quality by capturing
the “first flush” of surface runoff, reducing temperature
rise in receiving waters, increasing base flow, and reducing flooding
potential by creating short term storage detention of rainfall.
In order to fully utilize these benefits, the hydrological behavior
of the pervious concrete system must be assessed. The hydrological
performance is usually a key parameter in decisions to use this
material as a best management practice (BMP) for storm water management
and is usually the characteristic of most interest to permit-granting
agencies.
Passive and Active Mitigation Systems
In many situations, the use of pervious concrete to simply replace
an impervious surface may be considered a sufficient regulatory
standard to manage runoff. In other situations, the regulations
may require that runoff after development not exceed runoff, or
some percentage of runoff, prior to development. These two applications
are termed passive mitigation and active mitigation, respectively.
In the latter case, the pervious concrete pavement system must be
designed specifically to handle much more rainfall than that which
will fall on the pavement itself. For example, a parking lot can
be used to capture excess runoff from rain falling both on itself
and on surrounding areas, including, for instance, the rain collected
and discharged through roof drains of nearby buildings.
Passive mitigation. A “passive” mitigation
element is used only to reduce the quantity of impervious surface
in a given area by replacing impervious surface with pervious surface.
A passive mitigation element might also capture much, if not all,
of the “first flush,” providing additional hydrological
benefit, but is not intended to accommodate excess runoff from adjacent
surfaces.
Active mitigation. An “active” mitigation
system, on the other hand, is designed to maintain total runoff
at some specified level for a particular site with several types
of features. Pervious concrete used in an active mitigation system
must capture a sizeable portion of the runoff from other areas on
site as well as rain falling on its own “footprint.”
Typically, such areas include buildings, areas paved with conventional
(impervious) pavement (including delivery areas, trash pick up areas,
and bus lanes, all of which may carry significant, heavy truck traffic),
and traffic islands and buffer zones, which may or may not be vegetated
and which may or may not belong to the owner developing the property.
Active mitigation systems are particularly well suited to rehabilitating
existing impervious areas for remedial control of urban runoff since
they can be designed to capture runoff from adjacent areas. Depending
on the size, geometry, and porosity of the pervious concrete system,
the excess surface runoff from the site can be kept at or returned
to pre-development levels. Active pervious concrete pavement systems
can also be designed as boundary features used in conjunction with
conventional pavement to create a locally active, but sitewise passive
feature.
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| Figure 1. Pervious concrete sections in this
parking lot (Finley Stadium, Chattanooga, TN) help control runoff.
(Photo courtesy of L. Tiefenthaler.) |
For example, a pervious concrete system can be designed to capture
and temporarily store much, if not all, of the runoff from a conventionally
paved parking area by placing a relatively narrow strip of pervious
concrete over a deep, clean stone base along the edges of the parking
lot (see Fig. 1). Pervious concrete borders used for tree wells
or vegetated traffic islands can be designed as active elements,
helping maintain the net runoff from the entire parking area at
desired or permit constrained levels. An important benefit of pervious
concrete pavement systems with vegetated islands or tree wells is
that adequate moisture may be available with minimal, if any, need
for irrigation. This is particularly important when working with
minimum tree density requirements or when protecting large, existing
trees on the site. The active mitigation design approach is very
flexible and can be used for a variety of applications. Primary
applications of pervious concrete in an active mitigation role therefore
include commercial parking lots, boundary features of commercial
development sites, and containment features designed to intercept
at least a portion of overland surface runoff prior to entering
drainage channels.
The limiting factor for active mitigation applications are poorly
draining soils (generally those, with infiltration rates considerably
less than 0.3 cm/h (0.1 in./h.)), because the pervious concrete
system must be emptied and full storage capacity recovered in a
reasonable amount of time (typically 5 days or less). Recovery time
is typically not a major concern in passive mitigation applications
with these types of soils since the infiltration is slow and runoff
relatively high even with natural cover (see Fig. 2.)
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| Figure 2. Infiltration of rainfall into the
soil increases the effective storage capacity of the pervious
concrete pavement system. |
Design Storms
Another important consideration in design of a pervious concrete
system is the amount of stormwater to be stored: clearly a heavier
rain results in more runoff. Different sizes of storms will result
in different amounts of runoff and the selection of an appropriate
design storm is important. Larger storms occur less often on average
and storms are typically designated based on their return period.
For example, a storm which occurs on average once in 20 years is
designated a “20-year storm” and will be larger (more
rainfall is produced in the same period of time) than a “10-year
storm.” Often, the design storm is chosen by local authorities,
such as city or county water boards. Storms of interest in hydrologic
design of small watersheds are typically the 2-year storm and the
10-year storm. The 2-year storm is often used as the “service
load” storm for
the watershed for water quality purposes. The 10-year storm has
traditionally been used in the design of storm water collection
systems. Rainfall estimates for many areas are available online
at http://hdsc.nws.noaa.gov/hdsc/pfds/.
See Hydrologic
Design of Pervious Concrete for details on designing for
storms and active and passive mitigation. This resource covers the
fundamental hydrologic behavior of pervious concrete pavement systems,
basic design methodologies appropriate for a variety of sites and
circumstances and limitations of these methodologies.
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