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Dave Zehner <br />Milner Pit No. 2 Expansion <br />Floodplain Evaluation <br />Page 4 of 7 <br />February 3, 2009 <br />the new maps were initially surveyed on the 1929 datum, and then a blanket shift <br />of +4.1' to 4.2' was applied to the maps to force the maps to more closely match <br />the 1988 NAVD datum. <br />4. Additionally, FEMA's survey subcontractor used a 1929 datum of 0.415' higher <br />than the local survey datum used previously in the Milner area. <br />5. Thus, to shift site data to match FEMA's elevations, subtract 0.415 feet from the <br />existing topographic maps (Map 2), then add 4.2' to obtain elevations that can be <br />compared with FEMA's regulatory floodplain maps. In other words, add 42- <br />0.415' = 3.78' to the topographic elevations shown on Map 2 to be able to <br />compare elevations with the FEMA's mapping. On Map 2, the zigzag lines with <br />numbers rounded to the nearest foot are the "Base Flood Elevations" shown on <br />FEMA's maps. For example, the line 6494 is what FEMA calculated the flood <br />elevation should be at that location. Looking at the site specific contours, add <br />3.78' to 6490 = 6493.8', would "imply" that at worst, flooding at this location would <br />only be a couple of inches deep (6494 - 6493.8 = 0.2 feet = 2.5 inches). <br />6. Unfortunately, this degree of accuracy is not within the realm of reason or <br />accuracy for floodplain analysis. The best accuracy that can be obtained from a <br />floodplain study is perhaps plus or minus one half foot. FEMA chooses to round <br />their numbers (on paper) to the nearest 0.1 feet, but when redoing models, <br />requires elevations to match within 0.5 feet. <br />Other Studies <br />In 2000, 1 prepared a floodplain map for the gravel pit application for Milner No. 2 Pit. A <br />site specific analysis was prepared because FEMA did not have detailed mapping that <br />covered this area. A reasonable question is why FEMA's mapping now shows the land <br />in the flood fringe, whereas the study I performed in 2000 did not. There are several <br />significant differences in the two different studies. <br />WATER RESOURCE CONSULTANTS, LLC