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<br /> <br />CURRE:rr FLOOD S!~ATION <br /> <br />Obstructions to Floodflown <br /> <br /> <br />Obstructions, natural and man-~4de, within the flood- <br />ways L~ede floodflo~s, creating backwater and increased heights. <br />Debris washing downstream during flooGs often collect against <br />hridges or within restricted flow areas, reducing the waterway <br />openings and impeding the floodflow. This creates a damming effect <br />and, depending on the degree of clogging, causes increased backwater <br />depths with resultant overbank flooding. water pressure on embank- <br />ments and bridges can then result in damage or total destruction of <br />the structure involved. A pronounced increase in flow velocities <br />usually occurs downstre~ from obstructions, thus extending the flood <br />damage potential. <br />Natural obstructions to floodflows occur in streams <br />where trees, brush, and other vegetation encroach into channel areas <br />during peri~ds of low flows. ~he dengity of such growth is especially <br />pronounced at ct~nnel bends. Man-made obgtructions include land- <br />fills, sometimes originating a~ randen refuse dump~ or often as <br />leveeg constructed with the intent of protecting isolated proper~ <br />tien. Other man~lI\ade obstructions are the approach et:'.b<>,nkments for <br />the varieu:; bri<lg"s distributed throughout the study reaches. ~able <br />2 lists descriptive structur-al info"mation tor tn" bridge crQ~~i"yti <br />and representative photographS arc shown in Figure 4. <br /> <br />> <br />, <br /> <br />" <br />.,. <br />-- <br /> <br />- <br /> <br />Fiqura 2. UISTORIC FLOOD SCENBS~-Jun" 1965 <br />TopnA~&St' RlIilway-~Brid<Jes intaot but] ",iles of track <br />daroaged. <br />Sott~_--State Hwy 89 bridge over Arkansas River at Holly, <br />both appro..ches <lestroyed. <br /> <br />App.4 <br />, <br /> <br />,>,pp.4 <br />U <br />