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I ~ Prac7icil App~or ESective E & S Control - 87 <br />EF'F'ECTIVENESS <br />' AD AreallJeatity <br />Areal density relates to the percentage of the soil surface which is covered by <br />vegetative material, temporary mulch or a manufactured geoteztile. It is the C <br />or "cover factor" described in the Universal Soil Loss Equation (LISLE). Area <br />' densiry ranges from 096 for bare soil to up to 100% for impervious materials <br />such as plastic sheeting. <br />' High azeal density functions primarily to reduce rainfall impact, increase <br /> infiltration of water and reduce runoff velocities. AlI 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 regazd, 100% azea <br />density, achieved by materials or methods, which seal off the soil by malting 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 accounts 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 enters the soil or is <br /> bound up in a surface mulch. <br />' Some surface treatments 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 its saturated weight. <br /> <br /> I Volume of Infiltration <br />' <br /> The ability of an erosion control material io increase water infiltration into <br /> the soil and decrease runoff is an important characteristic. <br /> <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 IECA <br /> <br />