PERMFILE69535

• Permit No: C1981071
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: PR2 ADEQUACY RESPONSE
• Media Type: D
• Archive: No

• MLRD states that the calculation of permissible velocity will not
identify short oversteepened stream segments because average slope was
used in the calculations. The procedure used to determine slope is a
standard method described by the U.S. Geological Survey in Manual for
Estimatin flood characteristics of Natural-Flow Streams in Colorado
McCain and Jarrett, 1976 Slope is measured between two points that
are located at 10 percent and 85 percent of the channel distance
upstream from the mouth of a basin. The method is widely accepted for
determining the average slope of a stream channel. Both approved and
existing topography were compared using the same method of slope
determination.
The stated objectives of the report (p.3) were: (1) to determine if
AOC had been achieved, and (2) to determine the relative stability of
the existing and approved topography. P1LRD comments concerning short
channel reaches of the existing topography miss the point of the
report.
The h1LRD letter tabulates five additional oversteepened stream segments
that appeared to have been overlooked by the report. The additional
segments were identified by MLRD from profiles in the report Appendix.
These segments have, in fact, been addressed in the Postmining
Topography channel stabilization permit revision submitted to the
Division. Specifically, segments 31-2 (Site C in NOV C-81-Ool) and 7-1
will be stabilized with riprap channel protection. Segments 31-1 (514!)
(NOV Site E) where similar stream channel improvements are
• recommended. These sites have been identified as potentially unstable
and mitigation measures have been determined. There is same question
as to which segment of channel 9710 - 3 is considered to be
oversteepened by MLRD. There is a nickpoint about 1400 feet upstream
of the mouth. However, the drainage area to this segment is so small
that the existing channel slope should not cause the segment to erode
excessively.
The geomorphic analysis in the report addressed the stability of
individual stream channel segments. The channel slope and drainage
area were determined for stream segments of the major drainages on the
mine and these values were plotted for existinc and approved topography
to identify those segments that may have an increased tendency for
erosion (see Figure 2.9 of the report). Five segments were identified
that plot significantly above data points for approved drainages.
These segments were addressed in the above referenced permit revision
and riprapping was proposed for these segments on channel relocation
plus riprapping.
MLRD COMMENT:
Hypsometric Analysis
The hypsometric curve derived in the study combines areas 31, i, 51, and
9/10. The contour interval which was planimetered ~,~as not specified in the
report. We are concerned that by grouping all the areas together a distinct
• smoothing of all slope irregularities resulted. The distinct Swale features
of area 9/10 (typified by a grade reversal) are averaged out by the lack of
such features in area 7 and by the less pronounced feature in area 31. The
smoothing could be further enhanced by the fact that the central axis of the