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rya <br />H <br />ty N <br />t� O <br />H xl <br />� H �v <br />V1 H <br />� 0 <br />0 <br />H C7 <br />OH <br />HMO <br />9Hi <br />O H <br />otgqirn <br />Horn <br />a <br />�AW <br />precipitation of approximately 15' annually, which agrees with information presented in the <br />Pacific Northwest River Basins Commission, Vol. VI, Water Resources (1970). <br />Thus, th-re is ample water available for wetland creation, provided 12w permeability soils <br />are present to retard infiltration, and slopes are flat enough to retard surface runoff. This <br />conclusion is supported by the presence of the existing wetland on the northeast side of the site, <br />which has no surface inflow, and is therefore sustained by precipitation, and exists because of the <br />flat slope and the low permeability soils. Soil samples collected from the wetland indicated <br />vertical hydraulic conductivities ranging from 1 x 10*5 to 1 x 10'6 cm/s, based on soil <br />permeability tests conducted in the lab. <br />1.1.2.3 Wetlands Compensation Recommendations <br />The on -site wetland will be created by strategic placement of berms, terraces or dikes that <br />will temporarily impound overland and subterranean flow and placement or low permeability fill <br />soil. Dames & Moore staff have considered several alternate locations for placement of the <br />earthwork structures. <br />Placing berms and hydraulic structures, in the eastern gully was considered and ruled out <br />as a possible location. The gully is steep, resulting in the need for high berms (greater than 10 <br />ft) in order to capture enough area for wetland creation. The narrowness of the gully <br />signifiL , 'ly limits the area of capture, resulting in less created wetland. Also, soils in the area <br />appear to be well drained. In addition, storm runoff from parking lots and other developed areas <br />will be directed into the gully, increasing the peak flows that will need to be designed for. <br />The gully on the western edge of the property is more amenable to the development of <br />wetlands. The shallower slope allows for smaller earthen structures, while more area is captured <br />behind the structures. Also, additional runoff will not be directed into this gully, minimizing the <br />need for extensive spillways and erosion control measures. The soils in this area appear to be well <br />drained, similar to those encountered in the eastern gully. <br />The headlands or the western gully also show qualities amenable to the creation of <br />wetlands. Occurrence Li ponding on site in this region indicates low permeability soils near the <br />surface. Field observations indicated low permeability till material within 2 1/2 feet bgs. In <br />addition, the site is relatively flat, requiring less grading, and allowing for larger areas of <br />inundation by impoundments for a given berm height (allowing for lower berms). <br />The soils information indicates that native soils in the headland area may be suitable for <br />wetland creation, but that the native soils in the western gully may not be. Due to the uncertainty <br />regarding the suitability of soils, we recommend that imported soil, with recompacted hydraulic <br />conductivities of less than I x 10.6 cm/s, be used as fill for lining the bottom of the berms within <br />the western gully. At this time, we estimate that recompacted native soils in the headland area <br />should be adequate for wetland creation. <br />Estimates of peak runoff from a 100-year, 24-hour rainfall event were developed for the <br />DAMES & MOORE <br />