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<br /> <br />CURRENT FLOOD SITUATION <br /> <br />Obstructions to Floodflows <br /> <br />.. <br /> <br />Obstructions, natural and man-made, within the flood- <br /> <br /> <br />ways impede floodflows, creating backwater and increased heights. <br /> <br /> <br />Debris washing downstream during floods often collect against <br /> <br /> <br />bridges or within restricted flow areas, reducing the waterway <br /> <br /> <br />openings and impeding the floodflow. This creates a damming effect <br /> <br /> <br />and, depending on the degree of clogging, causes increased backwater <br /> <br />depths with resultant overbank flooding. Water pressure on embank- <br /> <br /> <br />ments and bridges can then result in damage or total destruction of <br /> <br /> <br />the structure involved. A pronounced increase in flow velocities <br /> <br /> <br />usually occurs downstream from obstructions, thus extending the flood <br /> <br /> <br />damage potential. <br /> <br />Natural obstructions to floodflows occur in streams <br /> <br />where trees, ~rush, and other vegetation encroach into channel areas <br /> <br /> <br />during periods of low flows. The density of such growth is especially <br /> <br /> <br />pronounced at channel bends. Man-made obstructions include land- <br /> <br /> <br />fills, sometimes originating as random refuse dumps or often as <br /> <br /> <br />levees constructed with the intent of protecting isolated proper- <br /> <br /> <br />ties. Other man-made obstructions are the approach embankments for <br /> <br /> <br />the various bridges distributed throughout the study reaches. Table <br /> <br /> <br />2 lists descriptive structural information for the bridge crossings <br /> <br /> <br />and representative photographs are shown in Figure 4. <br /> <br />App.4 <br />11 <br />