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<br />roadway. <br /> <br />ORJ\f1 <br /> <br />At state Highway 9, the existing 6 foot diameter culvert creates <br />a considerable backwater. However, the major flooding problem in <br />this area, which extends from Highway 9 to the Boreas Pass Road <br />crossing, is caused by the lack of a well defined channel. A <br />combination of the channel being relocated and the flood plain <br />being flat results in most of the area between the roadway and the <br />hillside being mapped in the flood plain or at least subject to <br />sheet flow from upstream sources. Prior to development, an <br />adequate drainageway should be provided for this reach of Illinois <br />Gulch. <br /> <br />At Boreas Pass Road, the three existing 36" CMP culverts cannot <br />pass the IOo-year flood without the roadway overtopping. In <br />addition, a small portion of the flow will break out upstream of <br />Boreas Pass Road and flow to the northwest along the upstream side <br />of the roadway. <br /> <br />This study ended 300 feet upstream of Boreas Pass Road since, <br />upstream of above this point the channel has been relocated and <br />mapping of the relocated channel is not available. Field <br />inspection indicated that there are some erosion problems on this <br />channel as well as the channel being perched above the flood plain <br />area. It is recommended that flood plain mapping be developed for <br />this area to determine the adequacy of the relocated channel. <br />Since the channel is actively eroding, it is also contributing to <br />water quality problems in Illinois Gulch. <br /> <br />In comparing the flood plain elevations developed in this study <br />with FEMA, the differences are located in the lower half of the <br />area. Between the Maggie Pond and Columbine Road, the current <br />study shows a lower lOO-year flood level due to alterations which <br />have occurred to the channel. From Columbine Road to Broken Lance <br />Drive, there are significant differences in the topographic mapping <br />used to compile the two studies. Consequently, the current study <br />shows flood plain elevations up to five feet lower than FEMA. <br /> <br />Between Broken Lance Drive and State Highway 9, there is a two foot <br />difference at the middle of the reach. This is a result of the <br />current study taking into account the grade break that occurs in <br />this reach. <br /> <br />The current study has a lower flood level at the upstream end at <br />the Highway 9 culverts. The difference is nearly two feet and is <br />due to improving the entrance conditions at the culvert by adding <br />a concrete head wall. After 350 feet, the backwater from the <br />culvert dissipates and the two profiles coincide to Boreas Pass <br />Road. <br /> <br />40 <br />