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<br /> <br />FACTORS AFFECTING FLOODING AND ITS IMPACT <br />Obstructions to Floodflows <br /> <br />Natural obstructions to floodflow Include brush and other vege- <br /> <br /> <br />tlon growIng along streambanks. During floods, these obstructIons <br /> <br />Impede floodflows and cause backwater condItions that Increase flood <br /> <br /> <br />heights. Brush washed out durIng these floods are carried downstream <br /> <br />and may collect on bridges and In culverts, thus creating a damming <br /> <br /> <br />effect and overbank flow. As floodflows Increase, masses of debris may <br /> <br />break loose and a wal I of water and debris would surge downstream until <br /> <br /> <br />another obstruction Is encountered. <br /> <br />Man-made obstructions to floodflow In the study area consist of <br /> <br /> <br />an irrigation diversion structure, five brIdges, and two culverts. <br /> <br /> <br />Debris collecting on a brIdge may Increase to the point where struc- <br /> <br /> <br />tural capabIlIty is exceeded and the bridge Is destroyed, or it may <br /> <br /> <br />cause water levels to rise to the extent that abutments are eroded <br /> <br /> <br />and the overlying and approach roadbeds are damaged. <br /> <br />A major cause of flood damage wIthin the city Is the Pioneer Ditch <br /> <br /> <br />diversion structure on Rifle Creek just downstream from the bridge at <br /> <br /> <br />Third Street (See FIgure 1, page 8). The structure has caused si It to <br /> <br /> <br />accumulate upstreaM and has reduced the flow area under the bridge by <br /> <br /> <br />approximately 7C percent since 1930, thus causIng a backwater condition <br /> <br /> <br />that causes overbank flooding during high flow periods. <br /> <br /> <br />In general, obstructions restrIct floodflows and cause overbank <br /> <br /> <br />flows, unpredictable areas to be flooded, destruction of or damage to <br /> <br /> <br />culverts and brIdges, and Increased velocities of flow Immediately down- <br /> <br /> <br />stream. It Is ImpossIble to predict the degree or location of the accu- <br /> <br /> <br />mulation of debris; therefore, for the purposes of this report, It was <br /> <br /> <br />necessary to assume that there would be no accumulatIon of debris to <br /> <br /> <br />clog any of the bridge or culvert openings. Representative obstructive <br /> <br /> <br />bridges and culverts In the study area are shown In Figures 2-8. <br /> <br /> <br />Pertinent data on obstructive bridges and culverts are shown In Table 5, <br /> <br />page 24. <br /> <br />7 <br />