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<br />Ms. Ann W. Jamison <br /> <br />-2- <br /> <br />September 19, 1984 <br /> <br />8. <br /> <br />Model floodplain <br />structures 1 i sted <br />sections (see HEC-2 <br /> <br />and floodway conditions <br />in Table 1 reflected in <br />output, Attachment 3). <br /> <br />with <br />cross <br /> <br />The encroach~ents used in modelling the natural condition are the same <br />encroachments used by the C.O.E. in the floodplain study. Mannings <br />"n" values and other coefficients are the same values used by Simons, <br />Li & Associates, Inc., in reproducing the C.O.E. water surface <br />elevations for the City's master flood\~<\y plan. The floodplain was <br />verified by SLA using HEC-2 because the original floodplain was <br />defined by the C.O.E. using an in-house backwater~ model no longer <br />used. <br /> <br />Gross sections were plotted using the C.O.E. data modified to reflect <br />existing structures and for the conditions that will exist when all <br />planned structures are completed. The cross section plots are given in <br />Attachments 4 and 5. <br /> <br />The floodway defined in the flood insurance study for the City of <br />Greel ey encroached each cross secti on to achi eve a 1.0 foot ri se of <br />I~ater surface el evati on. Thi s method does not account for backwater <br />conditions and is no longer considered appropriate. A floodway was <br />defi ned for thi sana lys is usi ng HEC-2 method four for the natural <br />condition. The results of the HEC-2 floodway analysis are very <br />s imil ar to the floodway defi ned in the flood insurance study. The <br />floodway determi ned for the natural condi ti on lias used in the future <br />conditior, analysis by encroaching each cross section to the limits <br />given in the natural condition. <br /> <br />The results of the floodway analysis indicate that the rise of the <br />100-year water surface elevation does not exceed 1.0 foot over the <br />natural floodplain. The reasons that significant increases in water <br />surface elevation do not occur are as follows: <br /> <br />1. Much of the Greeley reach of the Cache la Pouder River <br />is controll ed by bachlater condi tions caused by <br />undersized bridges and the obstructions caused by the <br />buildings have no effect. <br /> <br />2. Five of the structures are located at the fringe of the <br />floodway where conveyance (and velocities) are low. <br /> <br />3. Three of the buildings are actually additions to <br />existing structures and do not result in significant <br />additional blockage. <br /> <br />" <br />