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The [[filter media]] component of [[bioretention]], [[swales]] and [[green roofs]] must meet very specific design specifications related to texture in order for the BMP to achieve drainage and water treatment performance targets. If the media texture is too fine (i.e., contains more silt- and clay-sized particles than specified) it may have low permeability and [[Drainage time|drain too slowly]] or retain too much water for excessively long periods of time.
Soil texture is most accurately
characterized by submitting a representative sample to a soil laboratory for a particle-size distribution (PSD) test. Other commonly used terms for the PSD test by soil laboratories are “Particle-Size Analysis”, “Grain-Size Distribution” and “% Sand, % Silt, % Clay”.
For bioretention filter media and green roof growing media, “Sand Fraction Analysis” should also be requested.
These methods are recommended for use in inspection and testing of LID BMPs because they include assessment of the pebble-sized particles of the soil (i.e., particles that are greater than 2 mm in diameter).
Most soil laboratories will summarize PSD test results according to the proportions of the sample made up of pebble/gravel-, sand-, silt- and clay-sized particles. When Sand Fraction Analysis is requested, a more detailed breakdown of gradations of sand-sized particles is provided, which is important for evaluating the acceptability of filter media for bioretention and dry swales.
'''Figure 8.8 describes the Wentworth soil particle-size classification system (Wentworth, 1922) that should be used to classify pebble, sand, silt and clay fractions of a soil sample.'''