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426 Chapter 12 Urban Runoff and Combined Sewer Overflow Management
Design Considerations. The soil infiltration rate is probably the most important consid-
eration in the design of infiltration structures. The soils underlying the structure must be
tested to determine their suitability for infiltration. Some authorities specify the types of
soils acceptable for infiltration as noted above for infiltration basins.
Structure size is another primary consideration. The structures must be large enough to
handle the desired design storms. Also, the structures must be designed to allow larger
storms to bypass them. Because subsurface infiltration structures do not have outlets, they
usually have to be designed off line of the regular drainage system. Runoff can then enter
the infiltration structure until it is full; additional runoff is directed away from the structure.
A diversion structure upstream of the infiltration structure is normally part of the design.
The flow entering this structure (which could be a simple manhole) is directed to the subsur-
face infiltration structure until it is full; then additional flow is directed away from the struc-
ture and along the drainage system. A typical sizing rule for subsurface infiltration struc-
tures is that they should store the runoff from the first 0.5 in. (13 mm) of rainfall on the site.
Infiltration structures must also be designed to empty in a reasonable length of time.
The underlying soils, to remove pollutants from runoff effectively, must be allowed to dry
between rainstorms. Most experts specify that infiltration structures should contain a reser-
voir of runoff for no more than 3 days after rainfall.

Maintenance Requirements. Infiltration structures require periodic cleaning to remove
accumulated sediment and petroleum products. Often the need for this maintenance can be
reduced by incorporating into the design a pretreatment structure that removes sediments
and petroleum products from the runoff. These pretreatment structures can also minimize
the discharge to groundwater of some pollutants, such as solids. Although addressing these
issues in the design of infiltration structures can reduce routine maintenance requirements,
the design still should include an observation well that allows inspectors to determine sed-
iment deposition.
Limitations on Use. Subsurface infiltration structures can be used for end-of-pipe treat-
ment and can also be located at different points in the drainage system. If located at the
downstream end of a drainage system, infiltration structures can have large land require-
ments. Subsurface infiltration structures, because they are located underground, can be lo-
cated in areas such as parking lots and access roads.
The primary physical limitation to locating infiltration structures, other than land re-
quirements, is the suitability of soil, which must be neither too impermeable to run off (e.g.,
clay, silt, or till) nor too rapidly permeated (e.g., sand). Another potential physical limitation
is the depth to groundwater. To provide proper treatment and reduce the possibility of
groundwater contamination, a distance of at least 2 ft (0.60 m) should be maintained be-
tween the bottom of the infiltration structure and the mean high groundwater elevation.
12.5.4.3 Porous Pavement Paved roads and parking areas, because they increase water-
shed imperviousness, are major contributors to stormwater runoff problems in urban areas.
Porous pavement, however, allows water to flow through a porous asphalt layer and into an
underground gravel bed. Porous concrete pavement or cobblestone paving can also be used
(Fig. 12.13). Use of this porous pavement can thereby reduce runoff volume and pollutant
discharge. This practice, used in areas with gentle slopes, is generally designed into park-
ing areas that receive light vehicle traffic.
Pollutant Removal. Field studies have shown that porous pavement systems can remove
significant levels of both soluble and particulate pollutants. Porous pavement is primarily