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ACZ Inc. Engineering and Analytical Services 1475 Pine Grove Road PO. Box 774078 Steamboat Springs, Colorado 80477 303-879-6260 June 2, 1989 Mr. Rick Mills Colorado Yampa Coal Company- 29588 Routt County Road 27 Oak Creek, Colorado 80467 Re: Channel Certifications - MLRD Comment Letter Dated April 7, 1989 Dear Rick: I have reviewed the MLRD letter and believe the following responses should put these questions to rest: Regarding the October 31, 1988 inspection, no stand alone inspection report was completed. The November 1, 1988 letter serves as the final inspection report. During the October 31, 1988 inspection, all previously noted deficiencies in channel construction had been corrected. With respect to the question of riprap specification, a little background on riprap design should be helpful. Riprap design is an empirical process and not one subject to rigorous numerical techniques. Therefore, as part of the riprap design and construction process, it is customary to allow a range of material size consist in recognition of the empirical nature of the design, the difficulty in securing and mixing quantities of large irregularly shaped rock and the difficulty in accurately sampling and measuring those rocks. It has been my practice to allow variations of plus or minus ten (10) percent from the design specification for riprap gradation sizes. I have found this to be a practice which is workable in the field and which will not compromise the actual design. In particular for construction of the drainages at Colorado Yampa Coal Company, I found the construction process to be exceptionally well supervised and implemented further reinforcing some measure of relief from precise design details. Therefore, I've found that the riprap delivered for both area 51 and area 41 was adequate to meet the reasonable expectations of the design specifications. The design of bedding materials is similarly an empirical process. Bedding functions to minimize the chances of the riprap material disappearing into the underlying unclassified materials, minimize the opportunity for the riprap to disappear into the bedding material itself and minimize the probablility of washing bedding material from beneath the riprap. The small deviations in the no. 4 and No. 200 size class for the bedding material will not materially affect the performance against criteria. Also, the quality and care of construction of tf is much more important to channel stability than strict/ numerical specifications. ~ence ,.~ ~ ~=1 i~~ ,~ ~ I ;h '~ J; \,p QJ. .i ~L% -.~~,~t~c,x%