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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />purposes. In Colorado, the 100-year floodplain represents an <br />area of state interest as defined in H.B. 1041. <br /> <br />Between Logan Street and Ogden Street, Harvard Gulch flows through <br />the State Home Park. The reach consists of a grasslined open chan- <br />nel with a baffle chute drop structure and a 12 acre-foot detention <br />pond. Flood flows along this reach are contained within the park <br />property (Sheet 6). <br /> <br />The 10- and 50-year recurrence flood magnitudes have also been <br />calculated at various design points along each creek. The peak <br />flows for these events should be used in the planning and engi- <br />neering of improvements where the higher risk of failure or damage <br />is economically feasible and the hazard to life and property is <br />lower or non-existent. <br /> <br />The reach from Ogden Street to Downing Street consists of a <br />14.5'x 10' concrete box culvert. Only 2,000 cfs of the 3,600 <br />cfs lOa-year flood discharge will be passed by the culvert. The <br />remaining 1,600 cfs flows overland to an old channel and rejoins <br />the main channel at the State Home Park. This overland flow will <br />cause considerable flooding to residences in the area. Flood depths <br />are 3 to 5 feet (Sheet 7). <br /> <br />Flood magnitudes greater than the laO-year flood magnitude can <br />and will occur. Land improvement adjacent to the 100-year flood- <br />plain limit should consider the probabilities of flood damage. <br /> <br />Flooded Areas <br /> <br />The reach between Downing Street and Race Street is a straight, <br />concrete-lined open channel. Along its 2,500-foot length, the <br />channel has 12 bridge crossings. During the laO-year flood event, <br />the flood waters will overtop the concrete channel banks, particu- <br />larly at these bridge crossings. Ponding and shallow flooding will <br />occur along the reach, especially to the north side of the channel. <br />Many private residences in this area will be inundated with depths <br />of 3 to 5 feet (Sheets 7 and 8). <br /> <br />The laO-year floodplain limits and water surface profiles are <br />shown on the attached Flood Hazard Area Delineation Maps, <br />Sheets 1 through 15. The computed 100-year flood elevations <br />at each reference point are tabulated in Tables 3, 4 and 5. A <br />description of the extent of flooding is discussed in the follow- <br />ing paragraphs. <br /> <br />Harvard Gulch <br /> <br />The reach from the South Platte River to Logan Street consists <br />of a 14'x 9' concrete box culvert. This conduit is entirely below <br />grade, aligned under Wesley Avenue (Sheets 3 to 6). The inlet <br />structure for the culvert limits inflow to approximately 2,200 <br />cfs. At this point, the 100-year flood discharge is 4,860 cfs. <br />The remaining 2,660 cfs flows over Logan Street and continues <br />in a generally northwesterly direction to the South Platte River. <br />This overland flow will cause considerable flooding to residences <br />and businesses in the area. Flood depths are generally 1 to 2 <br />feet (Sheets 4 and 5), ponding occurs upstream of the Denver and <br />Rio Grande Western, and the Atchinson, Topeka and Santa Fe rail- <br />road embankments to depths of 2 to 4 feet. This ponding will <br />inundate several commercial buildings in the area (Sheet 5). <br /> <br />From Race Street upstream to Colorado Boulevard, Harvard Gulch <br />consists of an open grass lined channel. Along this reach, most <br />of the lOa-year flood flows are contained within the channel. A <br />few residences are affected by some shallow flooding that does <br />occur (Sheets 8, 9 and 10). <br /> <br />Dry Gulch <br /> <br />Along the entire study reach, Dry Gulch consists of two 43" x 68" <br />reinforced concrete pipe culverts. These culverts carry only 250 <br />cfs of the 1,330 cfs laO-year flood event. The remaining flood <br />waters flow overland through a park area and continue down Pearl <br />Street to the confluence with Harvard Gulch. Some shallow flooding <br />does occur, affecting several residences along the reach (Sheet 11). <br /> <br />-6- <br />