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<br />MONTROSE FLOOD PLAIN DEFHlITION PROJECT <br />PROPOSED AGENDA ITEr4S: SEPTEMBER 15, 19713 MEETING <br /> <br />1. Backwater Calculations <br /> <br />a. Flow split analysis within the ~~jy of Montrose, <br /> <br />Before performing the backwater' cal cul ations through the <br />city, we need final approval on the proc€'dure to be used. As pre- <br />viously discussed, there are two options. <br /> <br />The fi rst i nvol ves defi ni ng the flood pl ai n thl'ough the city, <br />assuming that flows which spill at an upstream crossing do not return <br />to the channel to any greater extent than existing topography and <br />facil ities presently allow. This alterna.tive accuriltely reflects the <br />exi sti ng conditi on flood pl ai n and wi 1 show s.i gni fi cant 1'1 oodi ng of the <br />first few crossings within the city, and much less downstream. <br /> <br />The other alternative, which we recommend, follows the procedure <br />utilized in our flo()dplain definitiion studies for the U.D. & IF.C.D. <br />At each culvert-orCotner impediment the flood plain is defined based <br />upon the total stream flow quantity. The amount of flow passing and <br />spilling at a crossing sets the flood pla'in boundaries. Immedi,ately <br />downstream of each cross i ng all spi 11 ed f1 ow is consi dered to be back <br />in the channel, and any excess is again shown as spill ing at th,e next <br />inadequate crossing. Such procedure preVI!nts the analysis from <br />becoming superseded each time an upstream culvert is improved, and <br />illustrates the conservative situation. This alternative will show <br />more flooding for the downstream cul verts than the fi rst opti on. <br /> <br />b. Individual Stream Backwater Ana~ly~~ <br /> <br />We anticipate having preliminary data available on th,~ <br />resul ts of the backwater analysis for the three study streams for <br />discussion purposes. The extent of flood'ing in the non-urbanized <br />areas will be indicated. <br /> <br />c. Roughness Coeffi ci ents/Debri s B ]_~ckage Potenti al <br /> <br />The Grand Juncti on USBR and Sacl'amento Cor'ps of Enginl~ers <br />were contacted regarding 'n' determination. Neither was able to pro- <br />vide applicable backup data. <br /> <br />The'n' values need to be discussed. The preliminary backwater <br />analysis was performed using the 'n' values initia'lly supplied to CWCB. <br /> <br />Regarding debris blockage potential, we do not feel that it is <br />real i sti c to assume that blockage wi 1'1 occur only at bri dges and other <br />crossings. Certain reaches of stream, pal't'icularly those with dense <br />overbank growth, will, exhibit the blockage potential for the entire <br />reach, not just at the initial section. This needs to be discussed. <br /> <br />1 <br />