Page 469 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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12.5 Urban Runoff Control Practices 427

Figure 12.13 Permeable Cobblestone Paving
(Source: Wikipedia, http://en.wikipedia.org/wiki/Image:
Santarem_carfree.JPG)

designed to remove pollutants deposited from the atmosphere, because coarse solids can
clog the pavement pores. In these systems, pollutant removal occurs primarily after the
runoff has infiltrated the underlying soils. Pollutant removal is accomplished by trapping
of sediments, and infiltration through the underlying soils, which can remove pollutants
such as bacteria. The removal efficiency depends on the storage volume of the pavement,
the basin surface area, and the soil percolation rate.
Design Considerations. Porous asphalt pavement generally is designed with an upper
pavement layer 2 to 4 in. (50 to 100 mm) thick, a 1- to 2-in. (25- to 50-mm) layer of
coarse sand, a stone reservoir to provide storage, and a bottom filter fabric as shown in Fig.
12.14. Other types of porous pavement include poured-in-place concrete slabs, precast
concrete grids, and modular units of brick or cast concrete. The differences in pavement
design result in different ways that the collected runoff is discharged. Some systems let all
the runoff discharge through the underlying soils and into the groundwater. While these
systems provide good pollutant removal, they can result in groundwater contamination.
Other systems include perforated pipes to collect the runoff and discharge it directly to
surface water; although these systems protect the groundwater below the pavement, they
do not provide the same level of pollution removal as the full infiltration systems.
Porous pavement is designed so that a certain amount of runoff is collected and stored
in the stone reservoir. The design criteria, therefore, determine the depth of the stone reser-
voir. The maximum depth of the stone reservoir also is affected by the infiltration rate of