Admin Order 58-12-14 - Laserfiche WebLink

Home Browse Search

SECTION 2 Study Area Characteristics 2.8.1 Permeability Soil permeability measures the rate of water movement through the soil horizon. This factor is important in managing stormwater quantity and quality. Soils with slow permeability rates are more likely to result in higher stormwater runoff volumes than soils of high permeability. Under these conditions, larger and more extensive stormwater facilities are needed to accommodate new development where space permits. In more densely developed areas, slow permeable soils may be better suited to stormwater conveyance and storage facilities than infiltration facilities. Storage facilities could include detention ponds and treatment ponds where time is desired for settling and filtering purposes. Compared with other Eugene basins, soil permeability in the River Road Santa Clara basin within the UGB is relatively high with 81% being moderately slow and 17% being moderate to very rapid. The following table displays the distribution of soil permeability for the basin. Table 2 -6 Soil Permeability in the River Road Santa Clara Basin Location Permeability (percent) Very Moderately 'Moderate Moderately Slow Very No Data* Total Rapid Rapid Slow Slow Within 4% 3% 10 % 81% 0% 1% 1% 100% UGB Outside 8% 2% 3% 74% 3% 8% 2% 100% UGB Total 7% 3% 7% 78% 1% 1 3% 1% 100% Basin *Includes borrow pits and water features such as ponds Source: USDA Soil Survey ofLane County Area, Oregon, 1987. 2.8.2 Runoff Potential Soil groups have been rated according to their runoff potential under nonvegetated and saturated conditions without consideration of topographic conditions. Runoff potential measures a soil's capacity to permit infiltration and can be used to describe the degree of runoff expected during storm events. For example, soils rated with a low runoff potential are more likely to have high infiltration rates and, conversely, soils with a high runoff potential are more likely to have low infiltration rates. Hydrologic stormwater models often use this parameter in conjunction with slope and surface cover factors for estimating surface flows under undeveloped conditions. As shown on Map 7, the River Road Santa Clara basin within the UGB contains soil groups with runoff ratings ranging from moderately low (16 %), moderately high (71 %) to "high" (11 %). The following table displays the distribution of potential runoff qualities of the basin: 0:\25695978 Eugene RR -SC Final Basin P1an\Master P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 11 -26