Laserfiche WebLink
8.7 Riprap Design <br />To design the riprap for the proposed modifications to Monument Dam, the wind speed and direction <br />are required. Data were not readily available for the wind speed or direction at the project location, <br />but information on the wind direction was obtained for Grand Junction, Colorado (State of Colorado, <br />2002). The wind direction is predominantly from the southeast with a component from the northwest. <br />This direction is such that the waves generated by a typical wind will not significantly impact the <br />proposed embankment. <br />A wind rose was also obtained for Telluride, Colorado (State of Colorado, 2002). The predominant <br />wind directions from this wind rose are essentially the same as the predominant directions for the <br />Grand Junction wind rose and the velocities are significantly less. <br />The wind speed for this area of Colorado was difficult to identify. The final wind speed of 90 mph <br />used in design was obtained from the building code, which requires this speed. This wind gust is for <br />a 3-second interval. The time required to establish the wave height on the reservoir is significantly <br />longer than 3 seconds. <br />Using the 90 mph wind speed and a fetch length of 700 feet computed from the northeast and east, <br />wave heights were generated. Also investigated was a case where the wind came from the critical <br />east-northeast direction, which provides a fetch length of approximately 1,400 feet. Because an <br />extreme wind event occurring in this exact direction is considered unlikely, the wind speed was <br />reduced to 50 mph. The resulting wave heights for the two conditions are 1.1 feet for the 50 mph <br />wind speed and 1.6 feet for the 90 mph speed. <br />The calculation of the wave heights assumed deep water conditions using the methodology of the <br />U.S. Army Corps of Engineers from the Devils Lake, North Dakota, wind induced impact study <br />(1996). The size of the stone calculated assumes that the side slopes on the dam are 3H:1V. The <br />stability coefficient was estimated to be 1.8. This stability coefficient is applicable to rounded <br />fieldstone such as may be obtained from streambeds. If crushed quarry stone was to be used for the <br />riprap, the stability coefficient would increase and the size of Keystone could be reduced. Using the <br />U.S. Army Corps of Engineers methods, the stone size was estimated to be 9.3 pounds and 16.3 <br />pounds; the larger size for the high wind speed from the shorter fetch distance. The conditions that <br />were chosen for the design of the riprap, including the wind speed and direction, are conservative and <br />a detailed data search or data acquisition would very likely result in significantly lower wind speeds <br />and a much lower probability of the wind approaching from the critical direction. <br />Using the U.S. Army Corps of Engineers Hudson equation, the diameter of the W50 stone was <br />computed. The results of this computation area 0.48 foot diameter stone and a 0.57 foot diameter <br />stone. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 37 <br />