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<br /> ~i I <br />JI <br />, <br />ill <br />fl. <br />1)1 <br />\]. <br />I <br />I <br />;1. <br />I <br />1]1 <br />, <br />]. <br />, <br />, <br />t] I <br />,]1 <br /> :'11 <br /> ]1 <br /> 11 <br /> ]1 <br /> I <br /> I <br /> I <br /> I <br /> I <br /> <br />ASSESSHENT OF INTERIH FLOODWAY <br />REQUIREHENTS <br /> <br />This report has developed the 100-year storm runoff along Spring <br /> <br /> <br />Creek for the purpose of providing design flows at various points along <br /> <br /> <br />the creek. These design flows-can be used in the planning of future <br /> <br /> <br />development along Spring Creek. <br /> <br />In order to provide general guidlines for floodplain development <br /> <br /> <br />Spring Creek has been divided into seven design reaches. These reaches <br /> <br /> <br />are shown in Figure 7. A design discharge has been developed for each <br /> <br /> <br />reach, assuming importation of 25-year flows along the Larimer County #2 <br /> <br /> <br />Canal and the Pleasant Valley and Lake Canal as presented in Figure 6. <br /> <br />Based on meetings with the City. personnel a typical channel cross <br /> <br /> <br />section was developed for each reach. Three basic types of cross-sections <br /> <br /> <br />were developed as shown in Figure 8. The floodway type of cross-section <br /> <br /> <br />is a grassed floodway with a small channel provided for base flows. The <br /> <br /> <br />open channel and culvert type sections are concrete sections which will be <br /> <br /> <br />.used primarily where the existing floodplain has been encroached upon. <br /> <br /> <br />These sections are not intended for final design but for planning purposes <br /> <br /> <br />only and must be adapted for each site based on the topography and <br /> <br /> <br />existing conditions, etc. <br /> <br /> <br />Utilizing average slopes of Spring Creek fnr each reach the size of <br /> <br /> <br />typical cross-sections were determined. Table 6 shows the specific <br /> <br /> <br />dimensions for each reach for the type of cross-section which would pro- <br /> <br /> <br />bably predominate for that reach. A design average maximum velocity of 7.5 <br /> <br /> <br />fps was used as the criteria for sizing the floodway sections, (Soils <br /> <br /> <br />Conservation Service, 1965). It is recommended that the 7.5 fps criteria <br /> <br /> <br />should be used to limit the amount of erosion during a flood. The analysis <br /> <br /> <br />indicated that supercritical velocities would exist in the concrete channels <br /> <br /> <br />and therefore potentially large energy dissipation structures will be re- <br /> <br /> <br />quired when the channels discharge into a lower velocity floodway. In all <br /> <br /> <br />cases care should be exercised in the design of all transition sections so a <br /> <br /> <br />as not to create hydraulic jumps or erosive velocities. <br /> <br /> <br />-20- <br />