Infiltration trench and vegetated swale in road median (Source: LID Center)
Infiltration practices are designs that enhance water percolation through a media matrix that slows and partially holds stormwater runoff and facilitates pollutant removal.
Infiltration trenches are stone-filled excavated trenches that allow stormwater runoff to infiltrate into surrounding soils through the bottom and sides of the trench. Captured stormwater generally exfiltrates to surrounding soils within 48 hours and serves to recharge groundwater. Designs must include filter strips or other filtering mechanisms to prevent sediment from reaching and clogging the trench.
Dry wells typically are gravel or stone aggregate-filled pits located to catch stormwater from roof downspouts or paved areas. Most often used to treat stormwater from small impervious surfaces, dry wells act as an alternative to infiltration trenches and can be used on steep slopes where other infiltration practices are not as well-suited. Dry wells should not be installed in areas of high sediment loading.
Stormwater Management Objectives
Diverting runoff to the soil and encouraging infiltration has the ability to largely control volume from small storm events and reduce the overall volume of larger events. Infiltration retention volumes are typically equal to the first flush stormwater volume. The captured volume serves to recharge groundwater and help to maintain regional baseflows.
Infiltration practices have a small effect on peak discharge. Dependent upon the storage volume of the infiltration area and the porosity of surrounding soils, discharge stormwater flow rates will be modestly diminished with the use of infiltration techniques.
The filtering properties of the media and surrounding soils allow infiltration techniques to improve water quality. A wide suite of pollutants may be removed by various mechanisms: sorption, precipitation, filtering, and bacterial and chemical degradation. Estimated pollutant removals are 60 percent for nitrogen and phosphorus, 80 percent for TSS, and 90 percent for metals and pathogens.
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