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<br />September 12, 2002 <br />Page 2 <br />recommend that a CMP arch pipe of an equivalent 60-inch diameter would better fit <br />the trapezoidal shape of the existing canal. The arch pipe that is equivalent to the <br />60-inch diameter round pipe is 71 inches wide and 47 inches tall. <br />The maximum capacity of the 60-inch CMP is about 48 cfs, as shown on the <br />attached printout sheet from the Haestad Flowmaster computer program. This <br />corresponds to slightly less than the peak 50-year snowmelt flood flow of 57 cfs. <br />It is our opinion that the 60-inch CMP will adequately protect the pond from <br />snowmelt flood events. The reasons for this opinion are summarized below: <br />11 The culvert installations will not be permanent. They will be removed after <br />the topsoil hauling is completed. <br />2) The culverts can be removed quickly if necessary. A snowmelt flood of a <br />50-year magnitude or larger would normally involve a snowpack that is <br />significantly in excess of the normal. This high snowpack would be <br />indicated by the Jones Pass and Glen Mar Ranch Snotel sites, allowing ample <br />time to remove the culverts prior to the peak of the snowmelt period. <br />3) The tailing pond has grown to a significant size, with a water pool surface <br />area of about 450 acres. This size allows for a significant volume of <br />surcharge storage space that would be available to store inflows from a <br />short-term breach of the bypass canal, if a breach were to occur. <br />4) In the Probable Maximum Flood analysis for the tailing impoundment, the <br />assumption is made that the bypass canal will fail and that all water tributary <br />to the canal spills into the pond. <br />As described above, we recommend that a 71x47 CMP Arch culvert be <br />installed at the haul road crossings, The culverts should be removed during the <br />snowmelt season when the Snotel readings indicate that the snowpack is <br />significantly above normal. <br />