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<br /> <br />; <br /> <br />fail even though they are not overtopped. The floods of May 1957 <br />did not overtop the highway north of the Hover Road bridge, but the <br />fill was eroded almost completely through (see figure 7). It is <br />likely that this erosion was caused by a strong eddy just upstream <br />from the road. <br />The effects of bridge openings being plugged by debris will be <br />to increase inundated areas, depths, and velocities on the flood <br />plain both upstream and downstream from the crossing and to increase <br /> <br />the chance of embankment failure. For example, the railroad bridge, <br /> <br />which is especially susceptible to plugging because of the piles <br />that extend above the streambed, will carry 8,200 cfs of the 50-year <br />flood if it is not plugged. Xf it becomes entirely plugged, part <br />of that 8,200 cfs will flow over the railroad embankment in the <br />vicinity of the bridge, but most of it will be deflected over the <br />levee onto the left flood plain. <br />Additional encroachment on the flood plain will reduce its <br />hydraulic capacity, thus increasing inundated areas and depths. <br />The overall mean velocity may be reduced, but in some areas velocities <br />will be increased. <br />Areas of inundation and depths and velocities of flow on the <br />flood plains will be reduced if there is improvement of the hydraulic <br />characteristics of the main channel and the bridge openings. <br /> <br />If flood control structures are constructed and properly oper- <br /> <br />ated on St. Vrain Creek or its tributaries, the discharge of floods <br /> <br />27 <br /> <br />