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• A similar situation exists in The East Taylor Watershed except that there is no undisturbed land, <br />and the reclaimed lands consist of only two conditions, CN =74 for the fill face (3 year <br />revegetation) and CN = 77 for the remainder of the watershed (1 -3 year reclamation growth). <br />Attached in this Part E are four separate SEDCAD model runs to evaluate the peak flow and total <br />runoff volume for each of the four ditch segments under the 100 year, 24 -hour storm event. Map <br />41A shows the location of these ditches, and Map 33B presents the postmining channel profiles. <br />Where riprap is needed, all four ditches have been designed with a minimum D50 rip -rap <br />dimension of 18 inches as shown on Table 1. <br />The SEDCAD model indicates that riprap smaller than the 18 -inch diameter specified would be <br />adequate for most segments, but for construction expediency, the larger size has been selected <br />for all segments requiring riprap. <br />Finally, in cases where channel slopes are less than about 5 percent, the SEDCAD model shows <br />that a permanent vegetative lining will be adequate. <br />Secondary Channels <br />As shown on Map 41A, a system of eight secondary channels will also be provided. These <br />secondary channels are also summarized on Table 1. Profiles for these channels are presented on <br />Map 33B. Although Rule 4.09.2 only requires a 10 year 24 hour event, these have been designed <br />for a 100 year design event to produce a more conservative design. This results in all of these <br />channels requiring a riprap lining for erosion protection. <br />It should be noted that under the 10 year event, many of these ditches would not require riprap <br />assuming the entire area has been revegetated for at least 1 -2 years. Because other SEDCAD <br />runs provided in this submittal represent a particular period in time when only portions of the <br />area have been reclaimed, the selection here of the 100 year design basis and the need for erosion <br />protection is consistent with those analyses. <br />The Trib. G ditch, which has the largest drainage area and largest resultant peak flow has been <br />used as the representative example for design of all of the secondary ditches. Although <br />dimensions for all secondary ditches are presented in attached Table 1, the flow results are only <br />provided for Trib. G Ditch. Flows in the other ditches would be proportionately less. <br />The following SEDCAD4 run for Trib. G Ditch is provided for this long term case where the 100 <br />year, 24 hour design storm is used and the entire drainage area is assumed to be both <br />contributory and recently reclaimed (Curve Number 77). The resultant peak flow generated was <br />40.1 cfs, and requires a riprap size with D50 of 12 inches in its lowermost (steepest) reach. For <br />design simplicity and as an added conservative measure, a D50 of 12 inches has been selected <br />for all of the secondary ditches. As such, all of the secondary channels will be constructed as <br />trapezoidal ditches with a 12.0 foot bottom width, 2H:1 V side slopes, and a minimum D50 riprap <br />diameter of at least 12 inches. <br />Exh. 7 -20E -3 Revision Date: 6/6/11 <br />Revision No.: TR -81 <br />