California Department of Transportation

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Last Updated: Thursday, January 5, 2012 1:08 PM

Infiltration Basins and Trenches

An infiltration basin is a depression used to detain stormwater for short periods until it percolates to the groundwater table. It functions as a BMP through filtration of runoff and adsorption of pollutants using site vegetation and soils. These basins are designed as "off-line" configurations where flow is diverted to the basins until the basin reaches capacity. Infiltration basins are suitable for larger drainage areas and water is stored in a visible surface pond, while trenches are ideal for small drainage areas and store runoff out of sight within a gravel or aggregate matrix. Trenches are lined with filter fabric and a layer of fabric is also buried just under the rock surface to trap potentially clogging sediment. Basins and trenches are periodically cleaned to remove trapped sediments and restore permeability. Water and soil samples taken in the soil below the infiltration basin is monitored twice per year. This monitoring provides an estimate of the quantity of pollutants entering the groundwater and soil. Basin maintenance includes removing woody vegetation and hydroseeding bare soil.


Site Location: I-5/La Costa (west)

Directions: I-5 southbound to La Costa off ramp, 100 yards before exit off shoulder,


Site Location: Carlsbad Maintenance Station (east)

Directions: I-5 to eastbound Palomar Airport Road, right on Paseo Del Norte, left into station


Site Location: I-605/SR 91

Directions: Northbound I-605 to westbound SR 91 connector, off shoulder just beyond SR 91 overpass


Altadena Maintenance Station

Directions: I-210 to 212 North Windsor Avenue, on east side of I-210


Division of Environmental Analysis, Stormwater Unit
California Department of Transportation
P.O. Box 942874, MS-27
Sacramento, CA 94274-0001
Email: Division of Environmental Analysis, Stormwater Unit