2011-06-29_PERMIT FILE - C1981019A ‎(22)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 6/29/2011
• Doc Name: Permanent Channel Design
• Section_Exhibit Name: Exhibit 07 Item 20 Part E -ST
• Media Type: D
• Archive: Yes

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