DESIGN OF LID SYSTEMS - Low Impact Development and Sustainable Stormwater Management - page 137

most susceptible to failure difficulties during construction, and therefore it is
important that the construction be undertaken in such as way as to prevent:

• Compaction of underlying soil
• Contamination of stone subbase with sediment and fines
• Tracking of sediment onto pavement
• Drainage of sediment-laden waters onto pervious surface or into constructed
bed

Staging, construction practices, and erosion and sediment control must all be
taken into consideration when using pervious pavements.
When designed properly, pervious pavement systems provide effective man-
agement of stormwater volume and peak rates. The storage reservoir below the
pavement surface can be sized to manage both direct runoff and runoff gener-
ated by adjacent areas, such as rooftops. Because the stone bed provides storage,
outlet structures can be designed to manage peak rates with the use of weir and
orifice controls. A well-designed system can infiltrate the majority of frequent
small storms on an annual basis while providing peak-rate control for design
storms up to and including the 100-year-frequency storm event.
Studies have shown that pervious systems have been very effective in reduc-
ing contaminants, such as total suspended solids, metals, and oil and grease
(Table 7-2). Because pervious pavement systems often have zero net discharge
of stormwater for frequent small storms, they provide effective water quality

Table 7-2 Water Quality Benefits of Infiltration

System Component Mechanism(s) Contaminants Retained/Reduced

Porous pavement Filtration and adsorption Total suspended solids (TSS),

heavy metals, hydrocarbons,
COD, and deicing salt (less
required, more retained)
( Note: Maintenance by
vacuuming is required! )

Infiltration bed Filtration, adsorption, settling,

microbial biodegradation

TSS, metals, and hydrocarbons,
plus total organic carbon,
COD, nitrogen

Shallow soil Filtration, adsorption, ion

exchange, microbial
biodegradation, conversion,
and uptake (only with high
plant activity)

Metals and hydrocarbons,
including PAHs

Deeper soil Filtration, adsorption, ion

exchange, conversion, and
uptake (only with high plant
activity)

Metals and hydrocarbons, plus
organics and bacteria; very
low risk of groundwater
contamination