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Dave Zehner <br />Milner Pit No. 2 Expansion <br />Fooodplain Evaluation <br />Page 2 of 7 <br />February 3, 2009 <br />1. The gray area is the "floodplain" as currently shown on the Feb. 4, 2005 Flood <br />Insurance Rate Maps produced by FEMA'. In theory, this represents the limits <br />of land that would be flooded during a 100-year runoff event z. These limits are <br />determined by plotting calculated flood depths onto topographic mapping that <br />happened to be available to whomever did the flood modeling of the river, e.g., <br />FEMA's subcontractor. <br />2. The white areas shown within the gray area and marked "Zone V are areas <br />where at one time it was believed that the ground is higher than the surrounding <br />calculated flood waters, and more specifically, areas above the "500 year" <br />(p=0.002) base flood elevation. Areas marked "Zone V that are grey are <br />between the 100 year and 500 year floodplain elevations. <br />3. The areas that are cross-hatched denote the "Floodway." The floodway is the <br />main channel and a portion of the river banks on either side of the channel that <br />are needed to convey the "100-year flood" event without raising the water level in <br />the main channel (sic, floodway) by more than one foot 3. Per Federal, State and <br />County floodplain regulations, development cannot happen within the floodway <br />without first proving there will be no increase in the Base Flood Elevation, e.g., <br />the elevation of the "100-year flood." <br />' The Feb. 4, 2005 maps are the current regulatory maps approved by FEMA and Routt County. <br />2 The "100-year flood" concept is poorly understood by the general public, and poorly communicated by the <br />technical community. The general perception is that it is an event that will only happen once in a hundred years. <br />This is completely incorrect. It is an event that has a statistical probability of occurring, or being exceeded, in <br />any given year, of one percent (p=0.01). The concept arises from a mostly bygone era of risk management, as <br />opposed to a more prevalent current popular mindset of risk elimination. <br />3 The floodway is actually defined by modeling a "what if?" scenario that forces the majority of the water into the <br />main channel by artificially encroaching or blocking portions of the flood fringe. The factor that defines the <br />horizontal or lateral limits of the floodway is that the computed elevation of the flood event cannot increase by <br />more then one foot over what existed without the encroachment. <br />WATER RESOURCE CONSULTANTS, LLC