Final Environmental Impact Statement - New Federal Courthouse

New Search

Water Resources Affected Environment Hydrologic Conditions Precipitation in the Eugene/Springfield area averages approximately 47 inches per year (Western Regional Climate Center 2000). The fate of precipitation under existing conditions depends on the ground surface conditions of the alternative sites. The developed sites are dominated by impervious surfaces such as parking lots, driveways, and rooftops (see Figures 3 and 5). Most of the precipitation that falls on these sites becomes surface water runoff and is routed offsite through. existing stormwater conveyance systems. A small percentage of the precipitation on these sites infiltrates permeable soils in unpaved areas and either recharges perched or shallow ground water that may be present under the site or evapotranspires through existing plant life. The alternative sites are located within the Willamette River drainage basin. Surface drainage from each of these sites and surrounding development flows through existing piped storm sewer systems to the Willamette River. The stormwater collection system includes all publicly and privately maintained pipes, culverts, gutters, catch basins, ditches, channels, ponds, wetlands, and related waterways. Stormwater runoff from the vicinity of the alternative sites represents a negligible fraction of the total flow in the Willamette River. The Alternative 2 site is located in the vicinity of the millrace in Eugene. The existing millrace represents the remains of what was once a small tributary system to the Willamette River. In the late nineteenth century, the millrace helped to facilitate the establishment of several industrial mills in the downtown Eugene area by providing a source of hydropower in the area. As industrial activity and development increased in the twentieth century, the millrace was ultimately converted from a natural system to a piped conveyance system. Today, the millrace is completely piped through the Alternative 2 site and serves primarily as a stormwater conveyance system. Flooding Based on the published Federal Emergency Management Agency flood insurance rate maps for the project areas (FEMA 1999), the proposed construction sites for Alternatives 1 and 2 are located well outside the 100-year and 500-year floodplain of the Willamette River (Figure 8). Stormwater Runoff Quality Existing land uses at the Alternative 1 and 2 sites consist primarily of uncovered parking lots and commercial buildings (see Figures 3 and 5). Because of greater landscaping and open spaces, the Alternative 1 site has slightly more pervious area than the Alternative 2 site, yet still consists of substantial parking, driveway, and rooftop areas. The majority of the stormwater runoff from New Federal Courthouse 45 Final EIS