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12.5 Urban Runoff Control Practices 423

Figure 12.10 Sample Infiltration Basin
(Source: Wikipedia, http://en.wikipedia.org/wiki/Image:Infilt_basin.jpg)

12.5.4.1 Infiltration Basins Infiltration basins are similar to dry ponds, except that infil-
tration basins have only an emergency spillway and no standard outlet structure. All flow
entering an infiltration basin (up to the capacity of the basin) is, therefore, retained and al-
lowed to infiltrate into the soil (see Fig. 12.11).

Pollutant Removal. Infiltration is the major pollutant removal mechanism. Infiltration
basins, like dry and wet ponds, receive stormwater runoff from drainage systems and pro-
vide storage up to a designed volume. Unlike dry detention ponds, which eventually re-
lease stored runoff through a drainage system, or wet ponds, which maintain a permanent
pool of water, infiltration basins release stored runoff through the basin’s underlying soil.
Infiltration basins provide stormwater pollutant removal through volume reduction and
filtration and settling. Infiltration basins are particularly effective in removing bacteria,
suspended solids, insoluble nutrients, oil and grease, and floating wastes.

Design Considerations. The most important consideration in the design of infiltration
basins is calculating the basin’s size for the drainage area and the soil type involved. Some
designers recommend off-line basins to capture and infiltrate the first 0.5 in. (13 mm) of
rainfall from the contributing drainage area. The appropriate amount of flow must be di-
verted to the system, and soil tests need to be performed to estimate the infiltration rates
and appropriately size the basin. Also related to the proper size of infiltration basins is the
amount of time necessary for the basin bottom to dry between rainstorms. Designers gen-
erally specify that infiltration basins should be designed to be dry for at least 3 days be-
tween storms. This interval allows the soil to dry, thereby increasing its pollutant removal
capacity. Basin shape is also important. It should have gently sloping sides to allow for
easy access to mow the bottom vegetation. An emergency spillway must also be incorpo-
rated into the basin design. Finally, some form of pretreatment is recommended to remove
suspended sediments from runoff before it is discharged to the basin. This pretreatment
will reduce the need for periodic removal of accumulated sediment, which can clog the soil
pores and reduce the level of infiltration.