Ordinance No. 20513 - Laserfiche WebLink

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Floodplain and Riverbank Complexity In this section we compare the historical floodplain structure and complexity of the site circa 1910 to current conditions, and then compare current conditions to those proposed by the master planning design team. We have included: A) An assessment of the Ordinary High Water Mark (OHWM) boundary of the Willamette River along the EWEB Riverfront B) A comparison of historical (1910), current and proposed floodplain form based on NOAA Weather Service river gauge data from 1990 through 2009 through the following maps: 1) An overlay of a current aerial photograph with EWEB property boundary onto 1910 topographical map 2) An overlay of a current aerial photograph with EWEB property boundary onto LCOG 2ft contour data. 3) Three maps of historical, current, and proposed floodplains, each showing four flood recurrence intervals (2 yr, 5 yr, 10 yr, and 20 yr) 4) A table and narrative comparison of historical, current and proposed floodplains by acreage area, length, configuration and percentage of functional capacity. C) Narrative recommendations for improving the ecological function of the current design proposal that explore various interactions and tradeoffs among design and ecological function based on modifications of bank topography and river edge complexity. A & B Comparison of Historical, Current & Proposed Conditions and Function Assessment Evaluating historical riparian floodplain functions provides reference conditions necessary to assess ecological changes over time, including future development plans. A sequence of flood elevations increasing in magnitude have been calculated and then mapped to measure aquatic functions. This analysis of historical, current and proposed site conditions fulfills the request for a Habitat Inventory and Assessment made during public interviews and focus groups. The geomorphology of the current 2010 Riverfront floodplain is accurately mapped by two -foot elevation contours in the Lane Council of Governments Geographic Information System data. In contrast, the U.S. Geological Survey created the earliest topographic map (Figure 6) of Eugene using five -foot contour intervals, which provide the basis for the historical riverfront floodplain as reference conditions a century ago. Metro defines reference condition as "representing the optimal or best attainable conditions for habitats or ecosystems. " Through topographical analysis, current and historical floodplains are mapped to show the Willamette River Channel Migration Zone or lateral extent of likely channel movement over the past 100 -year period. Figure 6. 1910 USGS Topo Map. X7