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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 />IV-I <br /> <br />SECTION IV <br />HYDRAULIC ENGINEERING AND FLOODPLAINS <br /> <br />Stream <br />Leyden Creek <br /> <br />Reach <br />L-I <br />L-2 <br />L-3 <br /> <br />Locat i on <br />Ralston Creek to the city limit <br />City limit to Alkire Street <br />Alkire Street to Leyden Lake <br /> <br />I NTRODUCTI ON <br />The floodpl ains associ ated with the Van Bibber Creek, Leyden Creek and <br />Lower Ral ston Creek major drai nageways range from those in narrow canyons <br />in the mountainous regions to flat, slow moving sheet flow areas in the <br />lower regions. The upper reaches of the floodplain in this study area are <br />rural in character with 1 ittle natural water course interference as a <br />resul t of development. In the lower reaches of these drai nageways, the <br />channels have been significantly encroached upon and in some cases oblite- <br />rated, In some reaches there has been significant residential development, <br />and in other reaches commercial development, both of which, to one degree <br />or another, have caused flood hazards. <br /> <br />Van Bibber Creek <br /> <br />V-I <br /> <br />Confluence with Ralston Creek to <br />Kipling Street <br />Kipling Street to Oak Street <br />Oak Street to Ward Road <br />Ward Road to Indi ana Street <br /> <br />V-2 <br />V-3 <br />V-4 <br /> <br />TABLE IV-I <br />FLOODPLAIN REACHES <br /> <br />A hydraulic analysis was performed to define the flow profiles and flood <br />areas. This analysis was used to develop damage estimates for the economic <br />analysis of the alternatives during Phase A of this study. The hydraulic <br />analysis utilized the HEC-2 computer model developed by the U.S. Army Corps <br />of Engineers. The response of the hydraul ic conveyance system to various <br />storm events was simulated for the existing baseline condition. The results <br />were used to develop damage versus frequency data which were then used to <br />evaluate the alternatives. <br /> <br />Table IV-I lists the reaches studied in this report. These reaches are the <br />basis for the description of floodplains used in this report. Refer to <br />Sheet I in Appendix B for delineation of the reaches, <br /> <br />Stream Reach Location <br />Ralston Creek R-I Clear Creek to Pierce Street <br /> R-2 Pierce Street to confluence with Van <br /> Bi bber Creek <br /> R-3 Confl uence with Van Bi bber Creek to <br /> confl uence with Leyden Creek <br /> <br />During the Phase B Study, the original model was refined and a more de- <br />tailed hydrauliC analysis was performed. Only the IOO-year future flood <br />condition was simulated for the existing baseline condition. This is the <br />regulatory requirement commonly used for floodplain management. The <br />hydraulic model was then modified to reflect improvements proposed in this <br />plan and the residual floodplain for both the loo-year and the other design <br />frequency events were determined. In areas where the HEC-2 computer model <br />did not adequately simulate flood flow conditions, more detailed hand <br />analyses were performed. A floodway analysis was also performed. The <br />original contract required the definition of a floodway which resulted in a <br />one-half foot rise in the water surface as the result of encroachment. The <br />contract was amended to incl ude a one foot rise floodway in the City of <br />Arvada. <br />