1988-09-26_ENFORCEMENT - M1977378 (2)

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' u11ur.�n4.�CrWiWILYi'il 111vaoWMifIf�GWlhdv ".•..•...••.'"•- ... ^. IYYYiuYr�YYi. ..vwva.�ae I Lf� 1' ui 111` I 1 Ir Q100 = 2. 5 Qa Q100 = 3450 cfs The average cross sectional area of the Animas River perennial channel i adjacent to the proposed new highway alignment is 114 square feet and the average channel slope is 0. 02 f t/ft. The perennial channel , bankfull , will pass a maximum of 1 ,000 cfs, assuming that the channel is not scoured j significantly during periods of peak discharge. The additional 2450 cfs for a 100-year flood will be passed as flood water over the flood plain. Using the above figures and the Manning equation, the estimated peak flood stage for a 100-year flood was determined to be approximately 2 feet along the proposed new highway alignment. It is our opinion that at a flood stage of 2 feet, damage to the highway embankment due to scour will be minor. However, as a precaution, protective riprap should be placed along the embankment as discussed below. To resist displacement by the action of flood waters, riprap stones having a minimum diameter of 9 inches will be required. This requirement was determined by using a Bureau of Public Roads chart for the sizing of riprap, a highway embankment slope of 1. 5:1 , and a design flood velocity of 9 fps. The riprap blanket should extend to a height of at least 3. 5 feet above the toe of i the highway embankment. It is recommended that the riprap be placed continuously" alongthe southeast side of the proposed highway embankment, starting at a point P P 9 Y 9 in the vicinity of the existing airstrip, and along that portion of the embankment parallel to the river. i CONSULTATION AND REVIEW I We would be glad to discuss the details of this report with you. The recommended locations of the diversion ditches were selected with the intent 26 � ,