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_ ~ ~ III IIIIIIIIIIIII III STATE OF COLORADO Roy Romer, Governor DEPARTMENT OF NATURAL RESOURCES MINED LAND RECLAMATION DIVISION DAVID C. SH ELTON, Dlreetor DATE: April 28, 1987 T0: D. Matthews FROM: C. Thompson RE: YVCC (CYCC) Permit Revision The operator has proposed a revised post-mining topography for the central portion of the mine. This revised topography is proposed in part (as presented in the April 16th, meeting) to divert drainage from the present pit area away from drainage 9/10-2 (5IE). In doing so, an extreme meander has been placed in the channel on a relatively flat reach of the channel and drainage basin. This meander has the potential to be a point of instability in the channel in that overtopping of the channel could lead to the flow seeking a new channel. This possibility is further enhanced by the flat side slopes on the drainage basis in this area. The Division believes the radius of currative of this meander should be greatly increased or the meander eliminated totally. Several drainages proposed in the post-mining topography, as depicted in the shaded portions of Map 48A, show little to no relief on the side slopes of the drainage basins. As required by Rule 4.05.4(2)(a) "the floodplain shall be designed and constructed to remain stable" and from Rule 4.05.4(2)(b) "shall be adequate to pass safely the peak runoff of ... a 100 year, 24-hour precipi- tation event for permanent diversions." An analysis must be submitted demon- strating that the floodplains are capable of carrying a 100-year, 24-hour precipitation event. An alternative to performing this engineering analysis would be to revise the post-mining landscape to create better defined drainage basins with more relief between drainage side boundaries and the channel. In the "Comments and Responses" of March 5, 1987, it is stated that drainage 9/10-3 does not appear on the new mapping." The Division is unsure why the drainage originally was identified if it doesn't exist on the ground. Afield check must be accomplished to determine if channelization has occurred in this area. Channel 31-3 is not shown as continuing through the newly proposed post-mining topography. This channel reach must be depicted. The channel design drawings will need to be certified by a P.E. prior to Dermit aooroval being issued. /sak 9178E 423 Centennial Building, 1313 Sherman Street Denver, Colorado 80203-2273 Tel. (303) 866-3567