Environmental Engineering Reference
In-Depth Information
Storage/Infiltration Bed Dimensions
The sizing of an infiltration bed is a function of the site layout and the opportuni-
ties presented. Unlike detention system designs, porous pavement design requires
that careful thought be given to potential locations as part of the initial site lay-
out, rather than as an afterthought based on the remaining vacant space after all
other structures have been situated.
Infiltration Area
The infiltration area is defined as the plan area of the storage
reservoir under the pervious pavement. The minimum infiltration area should be
no less than one-fifth the total contributing impervious surfaces, or a 5 : 1 ratio
of impervious surface to infiltration bed area. The storage bed below the pave-
ment provides runoff volume storage during storm events, while the undisturbed
subgrade allows infiltration of runoff into the underlying soil mantle. The storage
capacity of the stone bed can be calculated as follows:
storage volume
=
depth
(
ft
)
×
area
(
ft
2
)
×
void space
(
e.g.
,
0
.
40 for aggregate
)
where depth is the depth of the water stored during a storm event, depend-
ing on the drainage area, conveyance to the bed, and outlet control eleva-
tion/configuration.
The amount of runoff that a given bed can infiltrate is as follows:
infiltration volume
=
bed bottom area
(
ft
2
)
×
infiltration design rate
(
in./hr
)
×
infiltration period
(
hr
)
×
1
/
12
where infiltration period is the time when bed is receiving runoff and is capable
of infiltrating at the design rate, not to exceed 72 hours.
Peak-Rate Mitigation
Properly designed pervious pavement systems provide
effective management of peak rates. The infiltration bed below the pavement acts
as a storage reservoir during large storm events, even while runoff exfiltrates
through the soil mantle through the process of infiltration. Outlet structures can
be designed to manage peak rates with the use of weir and orifice controls, and
carefully designed systems may be able to manage peak rates for storms up to
and including the 100-year storm. For additional information relating to peak-rate
modeling and routing, refer to Chapter 6.
Construction Staging
The sequence of steps required during construction of a porous pavement system
is as follows:
1. Due to the nature of construction sites, pervious pavement and other infil-
tration measures should be installed toward the end of the construction period,
Search WWH ::
Custom Search