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<br />. " .. <br /> <br />, <br />, <br />I <br />, <br /> <br />) <br /> <br />",\", <br />" <br /> <br />INUNDATION MAPPING <br /> <br />Inundation mapping for the fair weather dam failure condition is <br />included herein as Appendix A, which consists of seven sheets at <br />a scale of approximately 1.3 inches equals a mile. Base mapping <br />is enlarged USGS 15 minute series quadrangles. Travel time and <br />inundation elevation for 13 downstream locations are included on <br />the maps. Travel times are for the leading front of the flood <br />wave; or the time since the breach began until water levels <br />begin to rise. For clarity, only the dam failure flood is <br />shown. <br /> <br />Available separately, and not intended to be published with the <br />EAP, is inundation mapping on orthophoto contour maps at a scale <br />of 1 inch equals 400 feet with a 5 foot contour interval. The <br />entire study reach, other than about 31 miles along Cow Creek <br />Canyon, is included on these maps. There are 24 sheets to this <br />mapping. The original reproducible maps are on file with the <br />Douglas County Water Resources Survey. One complete set has <br />been furnished to the Douglas County Sheriff's Office Emergency <br />Services Section, Douglas County Roadmaster, and the Oregon <br />State Highway Division District Maintenance Engineer. <br />Downstream communities have been furnished with the map sheets <br />which cover their area. These communities are: Glendale, <br />Riddle; Myrtle Creek; Winston; Roseburg. Travel time, <br />inundation elevation, and inundation above bankfull for the dam <br />failure flood are included on these maps for the same 13 <br />downstream locations as the maps in Appendix A. <br /> <br />The elevations and levels above bankfull for an unregulated <br />100-year flood (without dam failure) are also included on these <br />maps. The reasons for showing the unregulated 100-year flood <br />information are: this inundation line was already plotted on <br />the base mapping; Douglas County has had land development <br />ordinances in effect since the early 1970's relating to the <br />100-year flood, and consequently, considerable development has <br />occurred with this in mind; and it provides a frame of reference <br />for the dam failure flood. This 100-year flood elevation data <br />was derived as part of the National Flood Insurance study in the <br />County, and does not reflect the flood control feature of the <br />Galesville Dam; that is, it represents "without dam" <br />conditions. <br /> <br />For relation to historic flood levels, the January 1974 flood on <br />Cow Creek and the December 1964 flood on the South Umpqua River <br />were comparable to a 100-year flood. <br /> <br />B 6 of 11 <br />