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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />SECTION I <br />SUMMARY <br /> <br />The intense, short-duration thunderstorm which occurred in the <br />Frijole Creek basin southeast of Trinidad, Colorado, on July 2-3, 1981, <br />produced a flash flood resulting in the failure of a Colorado and <br />Southern Railway bridge. According to railway bridge maintenance <br />records, this was the only storm causing damage to the bridge since it <br />was constructed in 1909. <br /> <br />The rainfall over the 22.7 square mile area above the bridge <br /> <br /> <br />averaged approximately 11.8 inches ranging from less than 8 to in excess <br /> <br /> <br />of 16 inches. The average of 11.8 inches compares with a 100-year storm <br /> <br /> <br />precipitation of 3.1 inches. The peak discharge from the storm st the <br /> <br /> <br />railwsy bridge was approximately 50,000 cfs, or 4 times the peak runoff <br /> <br />estimated from a 100-yesr storm. The 100-year storm is nationally <br /> <br /> <br />recognized as a reasonable criteria for drainage design. The unit <br /> <br /> <br />discharge was 2200 cfs per square mile of tributary area. <br /> <br />) <br /> <br />I-I <br />