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428 Chapter 12 Urban Runoff and Combined Sewer Overflow Management

Pervious concrete block or “paver” systems
Pavers with open surface spaces
filled with gravel or sand
Setting layer

Open-graded base material
Filter fabric

Subgrade, minimal compaction

Pervious (open-graded) concrete and asphalt mixes

Open-graded pavement mix

Open-graded base material

Filter fabric
Subgrade, minimal compaction

Figure 12.14 Porous Pavement Cross-Section
(Source: City of Portland Bureau of Environmental Services)

the underlying soils. Runoff should be completely drained within a maximum of 3 days
after the maximum design storm event to allow the underlying soils to dry, maintaining
aerobic conditions that improve pollutant removal.

Maintenance Requirements. Porous pavement can have extensive maintenance require-
ments. The pavement must be kept free of coarse particles that can clog the pavement and
prevent runoff from collecting. The pavement must, therefore, be regularly inspected and
cleaned with a vacuum sweeper and high-pressure jet. The state of Maryland, by reviewing
its porous pavement practices, found that after 4 years of use only 2 of the 13 systems were
functioning as designed. The 11 malfunctioning sites were affected primarily by clogging
and excessive sediment and debris.

Limitations on Use. Because porous pavement is expensive to replace or repair, it is gen-
erally only used on parking areas that receive moderate to low traffic. The area to be paved
also should be relatively flat with a depth of 2 to 4 ft (0.60 to 1.2 m) from the bottom of the
stone reservoir to the high-water table. In addition, the soils under the pavement must
allow for infiltration.

12.5.5 Vegetative Practices
Urbanization results in the elimination of vegetation and increases in impervious area.
Vegetative practices (Fig. 12.15) in urban areas decrease the impervious area and promote
runoff infiltration and solids capture. These practices generally provide moderate to low