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• Some of the segments of the permanent primary channels are flat enough that a vegetative liner <br />will provide sufficient erosion protection, even for the 100 year flow. This condition exists in <br />the upper reaches of the East Taylor East and East Taylor West Tributary ditches, above the <br />point where those ditches drop down in the much steeper and rip -rap lined channels at the corner <br />groins of the East Taylor Fill (— 25% slope). The upper reaches of the East Taylor East and East <br />Taylor West Tributary Ditches are therefore designed for a vegetative lining, while the steeper <br />segments adjacent to the face of the East Taylor Fill are designed with a rip -rap lining. <br />• <br />A similar situation exists in the upper reaches of the West Taylor West and West Taylor East <br />Tributaries, but in the West Taylor drainage, but the uppermost reaches are at slopes too steep <br />(7.9% to 15 %) for a vegetative liner. Although the middle reaches of the West Taylor East and <br />West Taylor East Tributary Ditches are flat enough that they too could be designed for a <br />vegetative lining, for construction expediency this design applies the rip -rap lining for the entire <br />length of both ditches because it is required at both ends of the ditches. The West Taylor West <br />and West Taylor East Tributary Ditches have therefore been designed with a rip -rap lining for <br />their entire respective lengths. <br />The general uniform section selected for these ditches is as follows: <br />• a trapezoidal channel with a 12 foot bottom width at the channel slopes described above, <br />• a total depth of 4.0 feet to ensure 1.0 feet of freeboard, <br />• 311:1 V side slopes, and <br />• a minimum D50 riprap diameter of 18 inches on all rip - rapped portions <br />• flow velocity checked against allowable velocity for vegetative lined channels <br />This design is adequate for the 100 year, 24 hour design event for these ditch segments. Table 1 <br />of this Part E summarizes the ditch design for these primary channels. <br />Pursuant to TR -81, the original designs for East and West tributaries of both the East Taylor and <br />West Taylor Ditches were modified to conform with the revised postmining topography <br />presented in TR -81. All four ditch segments have been shifted northward from their original <br />alignments, and each was lengthened, generally resulting in a slightly flatter channel in their <br />upper reaches. <br />For the redesign of the ditches under TR -81, the runoff curve numbers (CN) were selected in <br />accordance with the revisions to the methodology approved in TR -73. The ditch designs assume <br />the worst case situation where all areas are contributory but are not yet fully reclaimed. The <br />faces of the fill were assumed to have a curve number of 74 (3 year reclamation) while most of <br />the-remainder of the disturbed area has a curve number of 77 (1 to 2 year reclamation). The <br />uppermost (southerly) sub - watersheds are assumed to be at CN 83, reflecting a topsoiled and <br />seeded condition but vegetation not yet in place. Undisturbed portions, mainly to the west of the <br />West Taylor watershed, were set at CN 57. <br />Exh. 7 -20E -2 Revision Date: 6/6/11 <br />Revision No.: TR -81 <br />