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<br />4.0
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<br />HYDROLOGY
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<br />4.1
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<br />I NTRODUCTI ON
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<br />A hydrologic analysis was undertaken to establish peak flood rates and
<br />hydrographs for rare event hydrology as experienced in this region of Fountain
<br />Creek. Ten-year and laO-year return period floods reflecting existing and future
<br />watershed conditions were developed as baseline hydrology for use in the study.
<br />Hydrologic information generated as described herein was utilized primarily as
<br />input information for the hydraulic analysis of water as it flows in the Creek
<br />during these frequency events; that is, the elevations and lateral extent of
<br />flooding as a continuum along Fountain Creek. Hydrology and the related
<br />hydraulics also provided information for the geomorphologic fluival evaluation.
<br />The emphas is was to compute the streamflow at frequent enough i nterva 1 s to enabl e
<br />the evaluation of current streamflow characteristics in each and how these might
<br />impact or be affected by the various alternative creek improvement
<br />configurations.
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<br />As indicated previously, there is approximately 420 square miles of drainage area
<br />that contributes runoff to the Monument and Fountain Creek DBPS study reaches.
<br />Due to the size of this area and the interrelatedness of the two basins, it was
<br />determined that the hydrologic methodology utilized should be consistently
<br />developed and applied between the two studies.
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<br />To gUide the development of the hydrologic model, a Technical Hydrology Review
<br />committee was established consisting of representatives from the COE, Soil
<br />Conservation Service (SCS), Colorado Water Conservation Board (CWCB), FEMA,
<br />National Weather Service, and various City and County departments.
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<br />A seri es of techn i ca 1 meet i ngs were held duri ng the development of the hydro 1 ogi c
<br />model to discuss pertinent hydrologic principles, and consider how they might be
<br />appli ed to the Monument Creek and Fountai n Creek watersheds. Di scussions
<br />pertained to rainfall type, rainfall amounts, areal adjustment of rainfall and
<br />its applicability, contribution to flooding from land above 8,000 feet in
<br />elevation, storm tracking, average storm cell size, reservoir routing, stream
<br />gage analysis and its applicability to this basin, and historical flooding the
<br />basin and in the region.
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<br />4.2 EXISTING INFORMATION
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<br />The two U.S. Army Corps of Engineers reports for Monument and Fountain Creeks,
<br />(USACOE 1973) and (USACOE 1974) were used as the bas is for the exi st i ng
<br />hydrology. The flows presented within these reports estimate the 100-year flood
<br />event at 16,000 cfs, 20,500 cfs, and 45,000 cfs for the Manitou Gage, upstream
<br />of the confluence, and at the Tejon Gage, respectively. The Federal Emergency
<br />Management Agency Flood Insurance Study (FEMA 1990) used the same 100-year flood
<br />events as the USACOE reports and included 10-year flood event estimates of 3,700
<br />cfs, 4,400 cfs and 9,200 cfs occurring at the Manitou Gage, upstream of the
<br />confluence, and at the Tejon Gage, respectively. Table 4.2-1 shows a complete
<br />listing of the USACOE/FEMA 100-year flood event flow estimates along the study
<br />reach.
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<br />4.0-1
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