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Practinl Approaches for Effxvve E R S Coouol - 8) <br /> <br />EFFECTIVENESS <br />AD Areal Density <br />Areas density relates to the percentage of the soil surface which is covered by <br />vegetative material, temporary mulch or a manufactured geotextile. It is the C <br />or "cover factor' described in the Universal Soil Loss Equation (LISLE). Area <br />density ranges from 096 for bare soil to up to 1009.0 for impervious materials <br />such as plastic sheering. <br />High areal density functions primarily to reduce rainfall impact, increase <br />infiltration of water and reduce runoff velocities. All of these attributes are <br />descriptive of the role played by mature vegetation on the soil surface, so it <br />should not be surprising that temporary surface mulches must have a percentage <br />of 'openness' which allows vegetation to emerge through them to effect a <br />permanent, stable erosion resistant ground cover. In this regard, I00% azea <br />density, achieved by materials or methods, which seal off the soil by making it <br />impervious to both water from above and plant penetration from within the soil, <br />are not always favorable when the ultimate goal is revegetation of a site. <br />Rp Volume of Runoff <br />This variable accounu for the amount of water leaving a treated area as a <br />percentage of the total amount applied. In theory, the runoff amount should <br />always be less than or equal to the total volume of water applied, once the <br />subject soil reaches field capacity. When an impervious surface material is <br />applied, runoff should approach 100%, as no free water enter the soil or is <br />bound up in a surface mulch. <br />Some surface treatmenu which utilized biodegradable materials or <br />photodegradable materials for temporary or permanent surface cover adsorb or <br />absorb water in the material matrix and by this manner decrease the percentage <br />of runoff. The amount of absorption or adsorption can be measured by <br />comparing the dry weight of a "mulch" material with iu saturated weight. <br />Volume of Infiltration <br />The ability of an erosion control material to increase water infiltration into <br />the soil and decrease runoff is an important characteristic. <br />. By deducting the volume of runoff and the amount of water adsorbed or <br />absorbed by an erosion control material (i.e. an erosion control blanket, <br />O 1995 lECA <br />