2015-09-09_PERMIT FILE - C1981019A ‎(33)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 9/9/2015
• Doc Name: Terrace Ditch Design
• Section_Exhibit Name: Exhibit 07 Item 20 Part G -ST
• Media Type: D
• Archive: Yes

EXHIBIT 7, ITEM 20, PART G
TERRACE DITCH DESIGN
A series of terrace ditches will be constructed across the face of the permanent fills and in the
upper portions of the permanent fill in the South Taylor Fill. The locations of these are shown
on Map 41A. The ditches in the upper portion of the permanent fill will originate near on the
even 100 foot postmining contours 7700, 7800, 7900, and 8000 foot elevations.
The function of the ditches is to direct overland flow runoff towards the channels of the ditches
denoted as the East and West Tributaries of the West Taylor Fill Ditch and the upper portions of
Taylor Tributary Ditch. The design basis for these permanent channels is provided in Volume 13
Exhibit 7, Item 20, Part E and in Volume 2C, Exhibit 7-14C.
The segment of the proposed Terrace Ditches with the largest watershed within the permanent
fill is a westward draining branch of the ditch originating at the 8000 foot elevation and
emptying into the Taylor Tributary Ditch in the NW/4 of Section 16. This segment is
approximately 2,000 feet in length and has a drainage area of 57.9 acres. It has been selected as
the representative segment for the design of all the terrace ditches. Terrace ditches with smaller
watersheds are thus conservatively overdesigned.
Using this length and a tributary area of 57.9 acres as a design basis for all of the terrace ditches,
the dimensions of the representative ditch for the terrace ditches was developed using SEDCAD
as described below. For a conservative design, the applicable storm was set as a 100 year, 24
hour event.
SEDCAD4 was used to determine the peak flow from this drainage area as detailed on the
following SEDCAD output pages. The 100 year, 24 hour design event was modeled using a CN
of 74 for reclaimed land with 1 to 2 years' growth in accordance with Table 1 in the Introductory
Text for Exhibit 7 (Volume 2D). This would be representative of a short term condition where
the upstream area has recently been topsoiled and seeded and has had only a year or two of
growth. The long term condition would have a CN of 62 for three year growth per the same
table and would produce somewhat less runoff. The modeled case produced a peak flow of
about 19.2 cfs in the 100 year event.
As shown in the following SEDCAD output, a grass lined asymmetric triangular ditch cross
section is assumed, with a channel slope of 0.015 ft/ft and 1.5H:1 V side slope on the uphill side
and 3.OH:1 V side slope on the downhill side. It would flow at a depth of 2.28 feet deep when
full retardance associated with mature grass lining for the long term is in place. In the short
term, with a shorter immature vegetative cover, it would flow slightly shallower and at higher
velocity, though still acceptable at 3.3 fps. A nominal freeboard of 1 foot has been added to the
calculated flow depth for the long term, resulting in a generic ditch section at least 3.3 feet deep
on a 1.5% slope for all terrace ditches on the fill face. This depth has been rounded up to an
even 4 feet, with other design parameters as summarized below:
Exh. 7-20G-1 Revision Date: 07/20/15
Revision No.: TR -105