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<br />DRAFT <br /> <br />French Creek <br /> <br />On French Creek, two major areas of flooding were identified. The <br />first is at Highway 9. The 6.5 foot diameter CMP culvert does not <br />have the capacity to pass the 100-year flood and the highway is <br />overtopped. This results in an area of flow breaking out over the <br />highway and to the north of the culvert. Damages would be confined <br />to the road and embankments. The crossing previously had a 10' x <br />10' box culvert which conveyed the 100-year flood without <br />overtopping. The other major area of flooding is at the Magnum <br />Bonum Drive crossing. The crossing is used as a pond with a <br />vertical riser type spillway. However, the embankment is not high <br />enough to permit the entire 100-year flood to enter the spillway. <br />A portion of the flow overtops the roadway and sheets across the <br />residential area. It is difficult to ascertain the exact extent <br />of this flooding since it will be a shallow sheet flow and greatly <br />influenced by small variations in the land surfaces. <br /> <br />Two potential overflow areas are located just upstream at cross <br />section 18 and cross section 20. These are referred to as <br />potential since the topographic mapping indicates that the flood <br />would be contained in the channel, but by less than one foot. <br />Actual field survey would be required to verify this. <br /> <br />A detailed study of French Creek was not performed by FEMA. <br />However, Leonard Rice included the reach from the Blue River <br />confluence to the pond upstream of Magnum Bonum Drive in the 1975 <br />"BreCkenridge Flood Plan Information". In comparing the 1975 study <br />with the current study, there are some major differences. <br /> <br />In 1975, the Highway 9 crossing was a 10' x 10 I box culvert. Under <br />this condition, the roadway is not overtopped and the flood levels <br />are four feet lower than current levels. The 1975 study did not <br />account for the backwater behind the 9' diameter CMP near station <br />10+00. At station 10+00 the current flood levels are 7 feet higher <br />than the 1975 study. Throughout the reach between the 9' culvert <br />and Magnum Bonum Drive, there are a number of gabion drop <br />structures. ,The 1975 study does not reflect the influence of these <br />structures. Consequently, at the drop structures, flood levels are <br />locally one to two feet higher based on this study. <br /> <br />At the upstream end of the 1975 study, there appears to be a survey <br />error on the order of 12 feet. Current surveys showed the spillway <br />to be at an elevation of 9565.5 where as the 1975 report lists an <br />elevation of 9577.1. The 9565.5 elevation agrees with existing <br />topography. <br /> <br />37 <br />