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Floodplain elevations developed in this report (Figure 12) were based on the NOAA river gauge <br />location adjacent to the EWEB steam plant and adjusted to account for the Willamette River slope <br />for locations upstream and downstream along the Riverfront property. The river slope was <br />determined by noting the location of topographical contours crossing the river using the current <br />LCOG GIS data then measuring the distance along the centerline of the river to calculate the rise <br />and run. Along the Riverfront site the Willamette River slope is approximately - 0.125% or lft drop <br />across 800 ft downstream. <br />FLOODPLAIN ELEVATIONS <br />o OLWv (3993re) m OHWM (49 ft) <br />5 yr Recurrence Interval (4wft -mefey <br />D <br />10yr Recurrence MervaO waft-410 <br />D 20yr Recurrence Interval (410f4- 410 .5ft) <br />D RrYer LeYd Sbpe (u ff p.,8W LF) <br />HISTORICAL 1910 FLOODPLAIN CURRENT 2010 FLOODPLAIN <br />50 300 <br />SCALE FEET <br />1 0 5 0 <br />Figure 12. 1910 and Current Floodplain Elevations. The historical Millrace channel, itself based upon an old <br />meander channel of the Willamette River is a prominent feature on the 1910 map. <br />The highest floodplain is listed on the maps and tables in green as a 20+ year recurrence interval <br />because 20 years of river gauge data only allows calculation of a 20 -year flood event. However, the <br />highest flood elevation onsite of 20.6 feet above 390 feet datum occurred during the winter of 1996, <br />which likely exceeded a 20 -year event. After construction of the Willamette River dams was <br />completed in the 1960s, the highest floods on record occurred in 1964 and 1996. The City of <br />12 <br />