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Admin Order 58-12-14
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Admin Order 58-12-14
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Last modified
11/7/2012 2:23:00 PM
Creation date
11/7/2012 2:09:00 PM
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City Recorder
CMO_Document_Type
Admin Orders
Document_Date
11/2/2012
Document_Number
58-12-14
Author
CRO
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SECTION 2 Study Area Characteristics <br />2.3.3 Surface Cover <br />Other than precipitation, surface cover is perhaps the single most influential factor that affects <br />the volume, quality, and velocity of stormwater runoff and the ability to treat runoff through <br />filtration and other natural processes. Pervious surfaces are undisturbed natural areas that retain <br />native prairie or forest vegetation or lands in developed areas that are typically covered with <br />lawn, agricultural fields, or pasture. In both cases, water is free to infiltrate into the ground. <br />Undisturbed natural areas provide significant beneficial stormwater functions. They help reduce <br />the volume and velocity of runoff by facilitating infiltration of precipitation into the <br />groundwater. Stormwater quality is best in undisturbed natural areas. The vegetative cover <br />associated with undisturbed natural areas is also important for stabilizing steep slopes and <br />streambanks. The infiltration capacity of undisturbed areas may be reduced during conversion to <br />urban lawns and agricultural crops. Stormwater quality may also be impacted by lawn care and <br />agricultural practices. Pervious surfaces in developed areas provide stormwater benefits, <br />although to a lesser degree than undisturbed natural areas. <br />In contrast, impervious surfaces are lands covered by hard surfaces such as rooftops, roads, and <br />parking lots and allow little or no infiltration of water. Impervious surfaces are unable to absorb <br />and infiltrate precipitation, which results in greater runoff volumes, higher but shorter duration <br />peak flows, and higher concentrations of pollutants. The transition from undisturbed to <br />developed land uses and densities involves a significant change from pervious to impervious <br />surfaces. As a consequence, adequate facilities must be planned, constructed, and maintained to <br />minimize drainage and flood problems and impacts to water quality and natural resources. <br />The purpose of this section is to describe existing surface cover conditions with data current to <br />2007, and as they are projected to exist at buildout of the River Road Santa Clara basin urban <br />growth boundary (UGB). <br />2.3.3.1 Impervious Surfaces <br />Total impervious surface area for the study area was calculated using a set of impervious surface <br />area factors (ISAF) that were applied to the existing and buildout land use data. To calculate <br />total impervious surface area, the ISAF percentages were multiplied by the total land area in each <br />of the land use categories. <br />The ISAFs used are provided in Volume I. These factors were derived through a process that <br />used existing developed properties in Eugene to generate typical impervious percentages. <br />Impervious surface area for residential, commercial, and industrial land uses had previously been <br />digitized as the basis for calculating stormwater user fees. By using this data source, the <br />resulting ISAFs have been calibrated specific to the City of Eugene and in some cases specific to <br />the basin. The ISAFs for land use categories that were not previously digitized were derived <br />through review of national standards and by calculating the impervious surface area on sample <br />sites. <br />The amount of existing impervious surface area in the UGB portion of the River Road Santa <br />Clara basin is estimated to be 2,277 acres or 37.5 percent of the basin's UGB area. [Note: <br />0:\25695978 Eugene RR -SC Final Basin P1an\Master P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 2-11 <br />
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